Chapter 9 Radrar Measurements

8/2/2019 Chapter 9 Radrar Measurements

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radar MeasureMents

9.1 general

This chapter is an elementar discussion o meteor-ological microwave radars the weather radar usedmostl to observe hdrometeors in the atmosphere.It places particular emphasis on the technical andoperational characteristics that must be consideredwhen planning, developing and operating radarsand radar networks in support o Meteorologicaland Hdrological Services. It is supported b asubstantial list o reerences. It also brief mentionsthe high requenc radar sstems used or observa-

tion o the ocean surace. Radars used or verticalproles are discussed in Part II, Chapter 5.

9.1.1 twd

Meteorological radars are capable o detectingprecipitation and variations in the reractive indexin the atmosphere which ma be generated b localvariations in temperature or humidit. Radar echoesma also be produced rom airplanes, dust, birds orinsects. This chapter deals with radars in commonoperational usage around the world. The meteoro-

logical radars having characteristics best suited oratmospheric observation and investigation trans-mit electromagnetic pulses in the 310 GHrequenc range (103 cm wavelength, respec-tivel). The are designed or detecting and mappingareas o precipitation, measuring their intensitand motion, and perhaps their tpe. Higher requen-cies are used to detect smaller hdrometeors, suchas cloud or even og droplets. Although this hasvaluable applications in cloud phsics research,these requencies are generall not used in opera-tional orecasting because o excessive attenuationo the radar signal b the intervening medium. At

lower requencies, radars are capable o detectingvariations in the reractive index o clear air, andthe are used or wind proling. Although thema detect precipitation, their scanning capabili-ties are limited b the sie o the antenna requiredto achieve eective resolution.

The returned signal rom the transmitted pulseencountering a weather target, called an echo, hasan amplitude, a phase and a polariation. Mostoperational radars worldwide are still limited toanalsis o the amplitude eature that is related to

the sie distribution and numbers o particles in the(pulse) volume illuminated b the radar beam. The

amplitude is used to determine a parameter calledthe refectivit actor (Z) to estimate the mass oprecipitation per unit volume or the intensit oprecipitation through the use o empirical relations.A primar application is thus to detect, map andestimate the precipitation at ground level instanta-neousl, nearl continuousl and over large areas.

Some research radars have used refectivit actorsmeasured at two polariations o the transmittedand received waveorm. Research continues todetermine the value and potential o polariation

sstems or precipitation measurement and targetstate, but operational sstems do not exist atpresent.

Doppler radars have the capabilit o determiningthe phase dierence between the transmitted andreceived pulse. The dierence is a measure o themean Doppler velocit o the particles therefectivit weighted average o the radial compo-nents o the displacement velocities o thehdrometeors in the pulse volume. The Dopplerspectrum width is a measurement o the spatial

variabilit o the velocities and provides someindication o the wind shear and turbulence.Doppler radars oer a signicant new dimensionto weather radar observation and most new sstemshave this capabilit.

Modern weather radars should have characteristicsoptimied to produce the best data or operationalrequirements, and should be adequatel installed,operated and maintained to utilie the capabilit othe sstem to the meteorologists advantage.

9.1.2 rd,md

The selection o the radar characteristics, andconsideration o the climate and the application,are important or determining the acceptable accu-rac o measurements or precipitation estimation(Tables 9.1, 9.2 and 9.3).

9.1.3 Mpp

Radar observations have been ound most useul orthe ollowing:

(a) Severe weather detection, tracking andwarning;

CHaPTEr 9


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ParT II. OBSErvING SYSTEMSII.92

(b) Surveillance o snoptic and mesoscaleweather sstems;

(c) Estimation o precipitation amounts.

The radar characteristics o an one radar will notbe ideal or all applications. The selection criteria oa radar sstem are usuall optimied to meet severalapplications, but the can also be specied to bestmeet a specic application o major importance.The choices o wavelength, beamwidth, pulselength, and pulse repetition requencies (PRFs) haveparticular consequences. Users should thereorecareull consider the applications and climatologbeore determining the radar specications.

Severe weather detection and warning

A radar is the onl realistic surace-based means o

monitoring severe weather over a wide area. Radarecho intensities, area and patterns can be used toidenti areas o severe weather, including thun-derstorms with probable hail and damaging winds.Doppler radars that can identi and provide ameasurement o intense winds associated withgust ronts, downbursts and tornadoes add a newdimension. The nominal range o coverage isabout 200 km, which is suicient or localshort-range orecasting and warning. Radarnetworks are used to extend the coverage (Browningand others, 1982). Eective warnings require eec-

tive interpretation perormed b alert andwell-trained personnel.

tbl 9.1. r quny bn

Radarband

Frequency Wavelength Nominal

UHF 3001 000 MHz 10.3 m 70 cm

L 1 0002 000 MHz 0.30.15 m 20 cm

S 2 0004 000 MHz 157.5 cm 10 cm

C 4 0008 000 MHz 7.53.75

cm

5 cm

X 8 00012 500MHz

3.752.4cm

3 cm

Ku 12.518 GHz 2.41.66cm

1.50 cm

K 1826.5 GHz 1.661.13cm

1.25 cm

Ka 26.540 GHz 1.130.75cm

0.86 cm

W 94 GHz 0.30 cm 0.30 cm

a Most common weather radar bands.

tbl 9.2. sm mlgil pm

n uni

Symbol Parameter Units

Ze

Equivent o eective

eectivity

mm6 m3 o

BZ

Vr Men i veocity m s1

v Spectum with m s1

Zdr dieenti eectivity B

CDRCicu epoiztiontio

B

LDRline epoiztiontio

B

kdp Popgtion phse degee km1

Coetion coefcient

tbl 9.3. Phyil pm n uni

Symbol Parameter Units

c Spee o ight m s1

Tnsmitte equency Hz

ddoppe equencyshit

Hz

Pr

receive powe mW o Bm

Pt

Tnsmitte powe kW

PRFPuse epetition

equencyHz

T Puse epetition time(=1/PrF)

ms

antenn ottion te

degee s1 opm

Tnsmitte

wveength

cm

azimuth nge degee

Bemwith betweenh powe points

degee

Puse with s

Eevtion nge degee

Surveillance o synoptic and mesoscale systems

Radars can provide a nearl continuous monitor-

ing o weather related to snoptic and mesoscalestorms over a large area (sa a range o 220 km,


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CHaPTEr 9. radar MEaSurEMENTS II.93

area 125 000 km2) i unimpeded b hills. Owingto ground clutter at short ranges and the Earthscurvature, the maximum practical range orweather observation is about 200 km. Over largewater areas, other means o observation are otennot available or possible. Networks can extendthe coverage and ma be cost eective. Radarsprovide a good description o precipitation.Narrower beamwidths provide better resolutiono patterns and greater eectiveness at longerranges. In regions where ver heav and exten-sive precipitation is common, a 10-cm wavelengthis needed or good precipitation measurements.In other areas, such as mid-latitudes, 5 cm radarsma be eective at much lower cost. The 3 cmwavelength suers rom too much attenuation inprecipitation to be ver eective, except or verlight rain or snow conditions. Development work

is beginning on the concept o dense networks o3 cm radars with polarimetric capabilities thatcould overcome the attenuation problem ostand-alone 3 cm radars.

Precipitation estimation

Radars have a long histor o use in estimating theintensit and thereb the amount and distributiono precipitation with a good resolution in time andspace. Most studies have been associated with rain-all, but snow measurements can also be taken with

appropriate allowances or target composition.Readers should consult reviews b Joss andWaldvogel (1990), and Smith (1990) or a compre-hensive discussion o the state o the art, thetechniques, the problems and pitalls, and the eec-tiveness and accurac.

Ground-level precipitation estimates rom tpicalradar sstems are made or areas o tpicall 2 km2,successivel or 510 minute periods using lowelevation plan position indicator scans withbeamwidths o 1. The radar estimates have beenound to compare with spot precipitation gauge

measurements within a actor o two. Gauge andradar measurements are both estimates o a contin-uall varing parameter. The gauge samples anextremel small area (100 cm2, 200 cm2), while theradar integrates over a volume, on a much largerscale. The comparabilit ma be enhanced badjusting the radar estimates with gaugemeasurements.

9.1.4 Mpd

A radar can be made to provide a variet o meteor-

ological products to support various applications.The products that can be generated b a weather

radar depend on the tpe o radar, its signal process-ing characteristics, and the associated radar controland analsis sstem. Most modern radars automati-call perorm a volume scan consisting o a numbero ull aimuth rotations o the antenna at severalelevation angles. All raw polar data are stored in athree-dimensional arra, commonl called thevolume database, which serves as the data sourceor urther data processing and archiving. B meanso application sotware, a wide variet o meteoro-logical products is generated and displaed asimages on a high-resolution colour displa moni-tor. Grid or pixel values and conversion to x-coordinates are computed using three-dimensionalinterpolation techniques. For a tpical Dopplerweather radar, the displaed variables are refectiv-it, rainall rate, radial velocit and spectrum width.Each image pixel represents the colour-coded value

o a selected variable.

The ollowing is a list o the measurements andproducts generated, most o which are discussed inthis chapter:(a) The plan position indicator: A polar ormat

displa o a variable, obtained rom a singleull antenna rotation at one selected eleva-tion. It is the classic radar displa, used prima-ril or weather surveillance;

(b) The range height indicator: A displa o a vari-able obtained rom a single elevation sweep,

tpicall rom 0 to 90, at one aimuth. It isalso a classic radar displa that shows detailedcross-section structures and it is used or identi-ing severe storms, hail and the bright band;

(c) The constant altitude plan position indicator(CAPPI): A horiontal cross-section displao a variable at a specied altitude, producedb interpolation rom the volume data. It isused or surveillance and or identicationo severe storms. It is also useul or monitor-ing the weather at specic fight levels or airtrac applications. The no data regions asseen in the CAPPI (close to and awa rom the

radar with reerence to the selected altitude)are lled with the data rom the highest andlowest elevation, respectivel, in another ormo CAPPI, called Pseudo CAPPI;

(d) Vertical cross-section: A displa o a vari-able above a user-dened surace vector (notnecessaril through the radar). It is producedb interpolation rom the volume data;

(e) The column maximum: A displa, in plan, othe maximum value o a variable above eachpoint o the area being observed;

() Echo tops: A displa, in plan, o the height

o the highest occurrence o a selectablerefectivit contour, obtained b searching in


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the volume data. It is an indicator o severeweather and hail;

(g) Verticall integrated liquid: An indicator o theintensit o severe storms. It can be displaed,in plan, or an specied laer o the atmos-phere.

In addition to these standard or basic displas,other products can be generated to meet the partic-ular requirements o users or purposes such ashdrolog, nowcasting (see section 9.10) oraviation:(a) Precipitation-accumulation: An estimate o

the precipitation accumulated over time ateach point in the area observed;

(b) Precipitation subcatchment totals: Area-inte-grated accumulated precipitation;

(c) Velocit aimuth displa (VAD): An estimate

o the vertical prole o wind above the radar.It is computed rom a single antenna rotationat a xed elevation angle;

(d) Velocit volume processing, which usesthree-dimensional volume data;

(e) Storm tracking: A product rom complex sot-ware to determine the tracks o storm cells andto predict uture locations o storm centroids;

() Wind shear: An estimate o the radial andtangential wind shear at a height specied bthe user;

(g) Divergence prole: An estimation o diver-

gence rom the radial velocit data romwhich divergence prole is obtained givensome assumptions;

(h) Mesocclone: A product rom sophisticatedpattern recognition sotware that identiesrotation signatures within the three-dimen-sional base velocit data that are on the scaleo the parent mesocclonic circulation otenassociated with tornadoes;

(i) Tornadic vortex signature: A product romsophisticated pattern recognition sotwarethat identies gate-to-gate shear signatureswithin the three-dimensional base velocit

data that are on the scale o tornadic vortexcirculations.

9.1.5 rdm

The accurac requirements depend on the mostimportant applications o the radar observations.Appropriatel installed, calibrated and maintainedmodern radars are relativel stable and do notproduce signiicant measurement errors. Externalactors, such as ground clutter eects, anoma-lous propagation, attenuation and propagation

eects, beam eects, target composition, partic-ularl with variations and changes in the vertical,

and rain rate-relectivit relationship inadequa-cies, contribute most to the inaccurac.

B considering onl errors attributable to theradar sstem, the measurable radar parameterscan be determined with an acceptable accurac(Table 9.4).

tbl 9.4. auy quimn

Parameter DefnitionAcceptableaccuracya

azimuth nge 0.1

Eevtion nge 0.1

Vr

Men doppeveocity

1.0 m s1

Z reectivity cto 1 BZ

vdoppe spectumwith

1 m s1

a These gures are relative to a normal Gaussian spectrum with

a standard deviation smaller than 4 m1. Velocit accurac

deteriorates when the spectrum width grows, while refectivit

accurac improves.

9.2 raDartechnology

9.2.1 Ppdmm

The principles o radar and the observation oweather phenomena were established in the1940s. Since that time, great strides have beenmade in improving equipment, signal and dataprocessing and its interpretation. The interestedreader should consult some o the relevant textsor greater detail. Good reerences includeSkolnik (1970) or engineering and equipment

aspects; Battan (1981) or meteorologicalphenomena and applications; Atlas (1964; 1990),Sauvageot (1982) and WMO (1985) or a generalreview; Rinehart (1991) or modern techniques;and Doviak and zrnic (1993) or Doppler radarprinciples and applications. A brie summar othe principles ollows.

Most meteorological radars are pulsed radars.Electromagnetic waves at xed preerred requenciesare transmitted rom a directional antenna into theatmosphere in a rapid succession o short pulses.

Figure 9.1 shows a directional radar antennaemitting a pulsed-shaped beam o electromagnetic


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energ over the Earths curved surace andilluminating a portion o a meteorological target.Man o the phsical limitations and constraints othe observation technique are immediatel apparentrom the gure. For example, there is a limit to theminimum altitude that can be observed at ar rangesdue to the curvature o the Earth.

A parabolic refector in the antenna sstem concen-trates the electromagnetic energ in a conical-shapedbeam that is highl directional. The width o thebeam increases with range, or example, a nominal1 beam spreads to 0.9, 1.7 and 3.5 km at ranges o50, 100, and 200 km, respectivel.

The short bursts o electromagnetic energ areabsorbed and scattered b an meteorologicaltargets encountered. Some o the scattered energ is

refected back to the radar antenna and receiver.Since the electromagnetic wave travels with thespeed o light (that is, 2.99 108 m s1), b measur-ing the time between the transmission o the pulseand its return, the range o the target is determined.Between successive pulses, the receiver listens oran return o the wave. The return signal rom thetarget is commonl reerred to as the radar echo.

The strength o the signal refected back to the radarreceiver is a unction o the concentration, sie andwater phase o the precipitation particles that make

up the target. The power return, Pr, thereoreprovides a measure o the characteristics o themeteorological target and is, but not uniquel,related to a precipitation rate depending on theorm o precipitation. The radar range equation

relates the power-return rom the target to the radarcharacteristics and parameters o the target.

The power measurements are determined b thetotal power backscattered b the target within avolume being sampled at an one instant thepulse volume (i.e. sample volume). The pulse volumedimensions are dependent on the radar pulse lengthin space (h) and the antenna beam widths in thevertical ( b) and the horiontal (b). The beam width,and thereore the pulse volume, increases with range.Since the power that arrives back at the radar isinvolved in a two-wa path, the pulse-volume lengthis onl one hal pulse length in space (h/2) and isinvariant with range. The location o the pulsevolume in space is determined b the position o theantenna in aimuth and elevation and the range tothe target. The range (r) is determined b the time

required or the pulse to travel to the target and to berefected back to the radar.

Particles within the pulse volume are continuouslshufing relative to one another. This results inphase eects in the scattered signal and in intensitfuctuations about the mean target intensit. Littlesignicance can be attached to a single echo inten-sit measurement rom a weather target. At least 25to 30 pulses must be integrated to obtain a reasona-ble estimation o mean intensit (Smith, 1995).This is normall carried out electronicall in an

integrator circuit. Further averaging o pulses inrange, aimuth and time is oten conducted toincrease the sampling sie and accurac o the esti-mate. It ollows that the space resolution is coarser.

9.2.2 tdpp

Meteorological targets consist o a volume o moreor less spherical particles composed entirel o iceand/or water and randoml distributed in space.The power backscattered rom the target volume isdependent on the number, sie, composition, rela-

tive position, shape and orientation o the scatteringparticles. The total power backscattered is the sumo the power backscattered b each o the scatteringparticles.

Using this target model and electromagnetic theor,Probert-Jones (1962) developed an equation relatingthe echo power received b the radar to theparameters o the radar and the targets range andscattering characteristics. It is generall accepted asbeing a reliable relationship to provide quantitativerelectivit measurements with good accurac,

bearing in mind the generall realistic assumptionsmade in the derivation:

Antennaheight

PPI mode

Antenna elevation 0parallel to tangentof the Earth

h/2

H

R

Pulsevolume

Target

Radarantenna

ha

Antennabeamwidthsqb, fb

Antennaelevation angle

Antennabeam

igu 9.1. Ppgin lmgni wvhugh h mph pul wh

; ha i h high h nnn bv heh u, R i h ng, h i h lngh

h pul, h/2 i h mpl vlum ph nHih high h pul bv h eh u


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PP hG K Z

rr

t b b=

3 2

2

2 18

21024 2

10

ln(9.1)

where Pr is the power received back at the radar,averaged over several pulses, in watts;P

t

is the peakpower o the pulse transmitted b the radar inwatts; h is the pulse length in space, in metres (h =c/2 where cis the speed o light and is the pulseduration); G is the gain o the antenna over anisotropic radiator; b and b are the horiontal andvertical beamwidths, respectivel, o the antennaradiation pattern at the 3 dB level o one-watransmission, in radians; is the wavelength o thetransmitted wave, in metres; |K|2 is the reractiveindex actor o the target; ris the slant range romthe radar to the target, in metres; and Zis the radarrefectivit actor (usuall taken as the equivalent

refectivit actor Ze when the target characteris-tics are not well known), in mm6 m3.

The second term in the equation contains the radarparameters, and the third term the parametersdepending on the range and characteristics o thetarget. The radar parameters, except or the transmit-ted power, are relativel xed, and, i the transmitteris operated and maintained at a constant output (asit should be), the equation can be simplied to:

PC K Z

r

r =

2

2

(9.2)

where C is the radar constant.

There are a number o basic assumptions inherentin the development o the equation which havevaring importance in the application and inter-pretation o the results. Although the are reasonablrealistic, the conditions are not alwas met exactland, under particular conditions, will aect themeasurements (Aoagi and Kodaira, 1995). Theseassumptions are summaried as ollows:(a) The scattering precipitation particles in the

target volume are homogeneous dielectricspheres whose diameters are small comparedto the wavelength, that is D < 0.06 orstrict application o Raleigh scatteringapproximations;

(b) The pulse volume is completel lled withrandoml scattered precipitation particles;

(c) The refectivit actor Zis uniorm through-out the sampled pulse volume and constantduring the sampling interval;

(d) The particles are all water drops or all iceparticles, that is, all particles have the same

reractive index actor |K|2

and the powerscattering b the particles is isotropic;

(e) Multiple scattering (among particles) isnegligible;

() There is no attenuation in the interveningmedium between the radar and the targetvolume;

(g) The incident and backscattered waves arelinearl co-polaried;

(h) The main lobe o the antenna radiationpattern is Gaussian in shape;

(i) The antenna is a parabolic refector tpe ocircular cross-section;

(j) The gain o the antenna is known or can becalculated with sucient accurac;

(k) The contribution o the side lobes to thereceived power is negligible;

(l) Blockage o the transmitted signal b groundclutter in the beam is negligible;

(m) The peak power transmitted (Pt) is the actual

power transmitted at the antenna, that is, allwave guide losses, and so on, and attenuationin the radar dome, are considered;

(n) The average power measured (Pr) is averagedover a sucient number o pulses or inde-pendent samples to be representative o theaverage over the target pulse volume.

This simplied expression relates the echo powermeasured b the radar to the radar refectivitactor Z, which is in turn related to the rainallrate. These actors and their relationship are crucial

or interpreting the intensit o the target and esti-mating precipitation amounts rom radarmeasurements. Despite the man assumptions,the expression provides a reasonable estimate othe target mass. This estimate can be improved burther consideration o actors in theassumptions.

9.2.3 bwd

The basic weather radar consists o the ollowing:(a) A transmitter to produce power at microwave

requenc;

(b) An antenna to ocus the transmitted micro-waves into a narrow beam and receive thereturning power;

(c) A receiver to detect, ampli and convert themicrowave signal into a low requenc signal;

(d) A processor to extract the desired inormationrom the received signal;

(e) A sstem to displa the inormation in anintelligible orm.

Other components that maximie the radar capa-bilit are:

(a) A processor to produce supplementardisplas;


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(b) A recording sstem to archive the data ortraining, stud and records.

A basic weather radar ma be non-coherent, that is,the phase o successive pulses is random andunknown.

Almost exclusivel current sstems use comput-ers or radar control, digital signal processing,recording, product displas and archiving.

The power backscattered rom a tpical radar is othe order o 108 to 1015 W, covering a range oabout 70 dB rom the strongest to weakest targetsdetectable. To adequatel cover this range o signals,a logarithmic receiver was used in the past. However,modern operational and research radars with linearreceivers with 90 dB dnamic range (and other

sophisticated eatures) are just being introduced(Heiss, McGrew and Sirmans, 1990; Keeler, Hwangand Loew, 1995). Man pulses must be averaged inthe processor to provide a signicant measurement;the can be integrated in dierent was, usuall ina digital orm, and must account or the receivertranser unction (namel, linear or logarithmic). Inpractice, or a tpical sstem, the signal at theantenna is received, amplied, averaged over manpulses, corrected or receiver transer, and convertedto a relectivit actor Z using the radar rangeequation.

The relectivit actor is the most importantparameter or radar interpretation. The actorderives rom the Raleigh scattering model andis deined theoreticall as the sum o particle(drops) diameters to the sixth power in thesample volume:

Z= volD6 (9.3)

where the unit oZis mm6 m3. In man cases, thenumbers o particles, composition and shape are notknown and an equivalent or eective refectivit

actor Ze is dened. Snow and ice particles must reerto an equivalent Ze which represents Z, assuming thebackscattering particles were all spherical drops.

A common practice is to work in a logarithmic scaleor dBz units which are numericall dened as dBz= 10 log10 ze.

Volumetric observations o the atmosphere arenormall made b scanning the antenna at a xedelevation angle and subsequentl incrementing theelevation angle in steps at each revolution. An

important consideration is the resolution o thetargets. Parabolic refector antennas are used to

ocus the waves into a pencil shaped beam. Largerrefectors create narrower beams, greater resolutionand sensitivit at increasing costs. The beamwidth,the angle subtended b the line between the twopoints on the beam where the power is one halthat at the axis, is dependent on the wavelength,and ma be approximated b:

ed

=70 (9.4)

where the units o e are degrees; and d is theantenna diameter in the same units as . Goodweather radars have beamwidths o 0.5 to 1.

The useul range o weather radars, except orlong-range detection onl o thunderstorms, is othe order o 200 km. The beam at an elevation o,

or example, 0.5 is at a height o 4 km above theEarths surace. Also, the beamwidth is o the ordero 1.5 km or greater. For good quantitative precipi-tation measurements, the range is less than 200 km.At long ranges, the beam is too high or groundestimates. Also, beam spreading reduces resolutionand the measurement can be aected b underll-ing with target. Technicall, there is a maximumunambiguous range determined b the pulse repeti-tion requenc (equation 9.6) since the range mustbe measured during the listening period betweenpulses. At usual PRFs this is not a problem. For

example, with a PRF o 250 pulses per second, themaximum range is 600 km. At higher PRFs, tpi-call 1 000 pulses per second, required or Dopplersstems, the range will be greatl reduced to about150 km. New developments ma ameliorate thissituation (Joe, Passarelli and Siggia, 1995).

9.2.4 Dppd

The development o Doppler weather radars andtheir introduction to weather surveillance provide anew dimension to the observations (Heiss, McGrewand Sirmans, 1990). Doppler radar provides a meas-

ure o the targets velocit along a radial rom theradar in a direction either towards or awa rom theradar. A urther advantage o the Doppler techniqueis the greater eective sensitivit to low refectivittargets near the radar noise level when the velociteld can be distinguished in a nois Zeld.

At the normal speeds o meteorological targets,the requenc shit is relativel small comparedwith the radar requenc and is ver dicult tomeasure. An easier task is to retain the phase o thetransmitted pulse, compare it with the phase o

the received pulse and then determine the changein phase between successive pulses. The time rate


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o change o the phase is then directl related tothe requenc shit, which in turn is directlrelated to the target velocit the Doppler eect.I the phase changes b more than 180, thevelocit estimate is ambiguous. The highest unam-biguous velocit that can be measured b a Dopplerradar is the velocit at which the target moves,between successive pulses, more than a quarter othe wavelength. At higher speeds, an additionalprocessing step is required to retrieve the correctvelocit.

The maximum unambiguous Doppler velocitdepends on the radar wavelength (), and the PRFand can be expressed as:

VPRF

max =

4(9.5)

The maximum unambiguous range can be expressedas:

rc

PRFmax =

2(9.6)

Thus, Vmax and rmax are related b the equation:

V rc

max max =

8

(9.7)

These relationships show the limits imposed b theselection o the wavelength and PRF. A high PRF isdesirable to increase the unambiguous velocit; alow PRF is desirable to increase the radar range. Acompromise is required unti l bettertechnolog is available to retrieve the inormationunambiguousl outside these limits (Doviak andzrnic, 1993; Joe, Passarelli and Siggia, 1995). Therelationship also shows that the longer wavelengthshave higher limits. In numerical terms, or a tpicalS-band radar with a PRF o 1 000 H, Vmax = 25 ms1, while or an X-band radar Vmax = 8 m s

1.

Because the requenc shit o the returned pulseis measured b comparing the phases o the trans-mitted and received pulses, the phase o thetransmitted pulses must be known. In a non-co-herent radar, the phase at the beginning osuccessive pulses is random and unknown, so sucha sstem cannot be used or Doppler measurements;however, it can be used or the basic operationsdescribed in the previous section.

Some Doppler radars are ull coherent; their trans-mitters emplo ver stable requenc sources, in

which phase is determined and known rom pulse topulse. Semi-coherent radar sstems, in which the

phase o successive pulses is random but known, arecheaper and more common. Full coherent radarstpicall emplo klstrons in their high-poweroutput ampliers and have their receiver requenciesderived rom the same source as their transmitters.This approach greatl reduces the phase instabilitiesound in semi-coherent sstems, leading to improvedground clutter rejection and better discrimination oweak clear-air phenomena which might otherwisebe masked. The microwave transmitter or non-co-herent and semi-coherent radars is usuall amagnetron, given that it is relativel simple, cheaperand provides generall adequate perormance orroutine observations. A side benet o the magnet-ron is the reduction o Doppler response to secondor third trip echoes (echoes arriving rom beondthe maximum unambiguous range) due to theirrandom phase, although the same eect could be

obtained in coherent sstems b introducing knownpseudo-random phase disturbances into the receiverand transmitter.

Non-coherent radars can be converted relativeleasil to a semi-coherent Doppler sstem. Theconversion should also include the more stablecoaxial-tpe magnetron.

Both refectivit actor and velocit data are extractedrom the Doppler radar sstem. The target is tpi-call a large number o hdrometeors (rain drops,

snow fakes, ice pellets, hail, etc.) o all shapes andsies and moving at dierent speeds due to the turbu-lent motion within the volume and due to their allspeeds. The velocit eld is thereore a spectrum ovelocities the Doppler spectrum (Figure 9.2).

Two sstems o dierent complexit are used toprocess the Doppler parameters. The simpler pulsepair processing (PPP) sstem uses the comparisono successive pulses in the time domain to extractmean velocit and spectrum width. The secondand more complex sstem uses a ast Fourier trans-orm (FFT) processor to produce a ull spectrum o

velocities in each sample volume. The PPP sstemis aster, less computationall intensive and betterat low signal-to-noise ratios, but has poorer clutterrejection characteristics than the FFT sstem.Modern sstems tr to use the best o bothapproaches b removing clutter using FFT tech-niques and subsequentl use PPP to determine theradial velocit and spectral width.

9.2.5 Pdd

Experiments with polariation diversit radars

have been under wa or man ears to determinetheir potential or enhanced radar observations o


9/30


10/30


As an electromagnetic wave propagates through amedium with oblate particles, the phase o the inci-dent beam is altered. The eect on the vertical andhoriontal phase components depends on theoblateness and is embodied in a parameter termedthe specic dierential phase (K

DP

). For heav rain-all measurements, KDP has certain advantages(zrnic and Rhkov, 1995). English and others(1991) demonstrated that the use oKDPor rainallestimation is much better than Zor rainall ratesgreater than about 20 mm hr1 at the S-band.

Propagation eects on the incident beam due tothe intervening medium can dominate target back-scatter eects and conound the interpretation othe resulting signal. Bebbington (1992) designed aparameter or a circularl polaried radar, termedthe degree o polariation, which was insensitive to

propagation eects. This parameter is similar tolinear correlation or linearl polaried radars. Itappears to have value in target discrimination. Forexample, extremel low values are indicative oscatterers that are randoml oriented such as thosecaused b airborne grass or ground clutter (Holtand others, 1993).

9.2.6 gdj

Echoes due to non-precipitation targets are knownas clutter, and should be eliminated. Echoes caused

b clear air or insects which can be used to map outwind elds are an exception. Clutter can be theresult o a variet o targets, including buildings,hills, mountains, aircrat and cha, to name just aew. Good radar siting is the rst line o deenceagainst ground clutter eects. However, clutter isalwas present to some extent. The intensit oground clutter is inversel proportional to wave-length (Skolnik, 1970), whereas backscatter romrain is inversel proportional to the ourth power owavelength. Thereore, shorter wavelength radarsare less aected b ground clutter.

Point targets, like aircrat, can be eliminated, i theare isolated, b removing echoes that occup asingle radar resolution volume. Weather targets aredistributed over several radar resolution volumes.The point targets can be eliminated during thedata-processsing phase. Point targets, like aircratechoes, embedded within precipitation echoes manot be eliminated with this technique dependingon relative strength.

Distributed targets require more sophisticated signaland data-processing techniques. A conceptuall

attractive idea is to use clutter maps. The patternso radar echoes in non-precipitating conditions are

used to generate a clutter map that is subtractedrom the radar pattern collected in precipitatingconditions. The problem with this technique is thatthe pattern o ground clutter changes over time.These changes are primaril due to changes inmeteorological conditions; a prime example isanomalous propagation echoes that last severalhours and then disappear. Micro-changes to theenvironment cause small fuctuations in the patterno ground echoes which conound the use o cluttermaps. Adaptive techniques (Joss and Lee, 1993)attempt to determine dnamicall the clutterpattern to account or the short-term fuctuations,but the are not good enough to be used exclu-sivel, i at all.

Doppler processing techniques attempt to removethe clutter rom the weather echo rom a signal-

processing perspective. The basic assumption is thatthe clutter echo is narrow in spectral width and thatthe clutter is stationar. However, to meet these rstcriteria, a suicient number o pulses must beacquired and processed in order to have sucientspectral resolution to resolve the weather rom theclutter echo. A relativel large Nquist interval isalso needed so that the weather echo can beresolved. The spectral width o ground clutter andweather echo is generall much less than 12 m s1and greater than 12 m s1, respectivel. Thereore,Nquist intervals o about 8 m s1 are needed.

Clutter is generall stationar and is identied as anarrow spike at ero velocit in the spectral repre-sentation (Figure 9.2). The spike has nite widthbecause the ground echo targets, such as swaingtrees, have some associated motions. Time domainprocessing to remove the ero velocit (or DC)component o a nite sequence is problematic sincethe ltering process will remove weather echo atero velocit as well (zrnic and Hamidi, 1981).Adaptive spectral (Fourier transorm) processingcan remove the ground clutter rom the weatherechoes even i the are overlapped (Passarelli andothers, 1981; Croier and others, 1991). This is a

major advantage o spectral processing. Stripped oclutter echo, the signicant meteorological param-eters can be computed.

An alternative approach takes advantage o theobservation that structures contributing toground clutter are ver small in scale (less than,or example, 100 m). Range sampling is carriedout at a ver ne resolution (less than 100 m) andclutter is identied using refectivit and Dopplersignal processing. Range averaging (to a nalresolution o 1 km) is perormed with clutter-ree

range bins. The philosoph is to detect and ignorerange bins with clutter, rather than to correct or


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the clutter (Joss and Lee, 1993; Lee, Della Brunaand Joss, 1995). This is radicall dierent romthe previousl discussed techniques and itremains to be seen whether the technique will beeective in all situations, in particular in anoma-lous propagation situations where the clutter iswidespread.

Polariation radars can also identi clutter.However, more work is needed to determine theiradvantages and disadvantages.

Clutter can be reduced b careul site selection (seesection 9.7). Radars used or long-range surveil-lance, such as or tropical cclones or in a widelscattered network, are usuall placed on hilltops toextend the useul range, and are thereore likel tosee man clutter echoes. A simple suppression tech-

nique is to scan automaticall at several elevations,and to discard the data at the shorter ranges romthe lower elevations, where most o the clutterexists. B processing the radar data into CAPPIproducts, low elevation data is rejected automati-call at short ranges.

9.3 ProPagationanDscatteringoraDarsignals

Electromagnetic waves propagate in straight lines,in a homogeneous medium, with the speed o light.The Earths atmosphere is not homogeneous andmicrowaves undergo reraction, absorption andscattering along their path. The atmosphere isusuall verticall stratied and the ras changedirection depending on the changes in height othe reractive index (or temperature and moisture).When the waves encounter precipitation andclouds, part o the energ is absorbed and a part isscattered in all directions or back to the radar site.

9.3.1 rmpThe amount o bending o electromagnetic wavescan be predicted b using the vertical prole otemperature and moisture (Bean and Dutton, 1966).Under normal atmospheric conditions, the wavestravel in a curve bending slightl earthward. Thera path can bend either upwards (sub-reraction)or more earthward (super-reraction). In either case,the altitude o the beam will be in error using thestandard atmosphere assumption.

From a precipitation measurement standpoint, the

greatest problem occurs under super-reractive orducting conditions. The ra can bend sucientl

to strike the Earth and cause ground echoes notnormall encountered. The phenomenon occurswhen the index o reraction decreases rapidlwith height, or example, an increase in temperatureand a decrease in moisture with height. Theseechoes must be dealt with in producing aprecipitation map. This condition is reerred to asanomalous propagation (AP or ANAPROP).

Some clear air echoes are due to turbulent inho-mogeneities in the reractive index ound in areaso turbulence, laers o enhanced stabilit, windshear cells, or strong inversions. These echoesusuall occur in patterns, mostl recogniable, butmust be eliminated as precipitation elds (Gossardand Strauch, 1983).

9.3.2 amp

Microwaves are subject to attenuation owing toatmospheric gases, clouds and precipitation babsorption and scattering.

Attenuation by gases

Gases attenuate microwaves in the 310 cm bands.Absorption b atmospheric gases is due mainl towater vapour and oxgen molecules. Attenuationb water vapour is directl proportional to the pres-sure and absolute humidit and increases almost

linearl with decreasing temperature. The concen-tration o oxgen, to altitudes o 20 km, is relativeluniorm. Attenuation is also proportional to thesquare o the pressure.

Attenuation b gases varies slightl with the climateand the season. It is signicant at weather radar wave-lengths over the longer ranges and can amount to 2to 3 dB at the longer wavelengths and 3 to 4 dB at theshorter wavelengths, over a range o 200 km.Compensation seems worthwhile and can be quiteeasil accomplished automaticall. Attenuation canbe computed as a unction o range on a seasonal

basis or ra paths used in precipitation measurementand applied as a correction to the precipitation eld.

Attenuation by hydrometeors

Attenuation b hdrometeors can result rom bothabsorption and scattering. It is the most signicantsource o attenuation. It is dependent on the shape,sie, number and composition o the particles. Thisdependence has made it ver dicult to overcomein an quantitative wa using radar observationsalone. It has not been satisactoril overcome or

automated operational measurement sstems et.However, the phenomenon must be recognied and


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the eects reduced b some subjective interventionusing general knowledge.

Attenuation is dependent on wavelength. At 10 cmwavelengths, the attenuation is rather small, whileat 3 cm it is quite signicant. At 5 cm, the attenua-tion ma be acceptable or man climates,particularl in the high mid-latitudes. Wavelengthsbelow 5 cm are not recommended or good precipi-tation measurement except or short-rangeapplications (Table 9.5).

tbl 9.5. on-wy nuin linhip

Wavelength (cm) Relation (dB km1)

10 0.000 343 R0.97

5 0.00 18 R1.05

3.2 0.01 R1.21

Ater Burrows and Attwood (1949). One-wa specic attenua-

tions at 18C.R is in units o mm hr1.

For precipitation estimates b radar, some generalstatements can be made with regard to the magni-tude o attenuation. Attenuation is dependent onthe water mass o the target, thus heavier rainsattenuate more; clouds with much smaller massattenuate less. Ice particles attenuate much less

than liquid particles. Clouds and ice clouds causelittle attenuation and can usuall be ignored. Snowor ice particles (or hailstones) can grow muchlarger than raindrops. The become wet as thebegin to melt and result in a large increase inrefectivit and, thereore, in attenuation proper-ties. This can distort precipitation estimates.

9.3.3 sddpp

The signal power detected and processed b theradar (namel, echo) is power backscattered b the

target, or b hdrometeors. The backscatteringcross-section (b) is dened as the area o an isotropicscatterer that would return to the emitting sourcethe same amount o power as the actual target. Thebackscattering cross-section o spherical particleswas rst determined b Mie (1908). Raleigh oundthat, i the ratio o the particle diameter to thewavelength was equal to or less than 0.06, a simplerexpression could be used to determine the backscat-ter cross-section:

s

b

K D

=

5 2 6

4

(9.8)

which is the justication or equation 9.3. |K|2, thereractive index actor, is equal to 0.93 or liquidwater and 0.197 or ice.

The radar power measurements are used to derivethe scattering intensit o the target b using equa-tion 9.2 in the orm:

z

C P r

K

r=

2

2

(9.9)

The method and problems o interpreting the refec-tivit actor in terms o precipitation rate (R) arediscussed in section 9.9.

9.3.4 s

In regions without precipitating clouds, it has beenound that echoes are mostl due to insects or tostrong gradients o reractive index in the atmos-phere. The echoes are o ver low intensit and aredetected onl b ver sensitive radars. EquivalentZe values or clear air phenomena generall appearin the range o 5 to 55 dBz, although these arenot true Zparameters, with the phsical processgenerating the echoes being entirel dierent. Forprecipitation measurement, these echoes are aminor noise in the signal. The can usuall beassociated with some meteorological phenomenon

such as a sea breee or thunderstorm outfows. Clearair echoes can also be associated with birds andinsects in ver low concentrations. Echo strengthso 5 to 35 dBz are not unusual, especiall duringmigrations (Table 9.6).

tbl 9.6. typil bk -in viu g

Object b (m2)

aict 10 to 1 000

Humn 0.14 to 1.05

Wethe boon 0.01

Bis 0.001 to 0.01

Bees, gonies, moths 3 x 106 to 105

2 mm wte op 1.8 x 1010

Although normal radar processing would interpretthe signal in terms oZorR, the scattering propertieso the clear atmosphere are quite dierent romthat o hdrometeors. It is most oten expressed in

terms o the structure parameter o reractive index,Cn

2. This is a measure o the mean-square fuctuations


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o the reractive index as a unction o distance(Gossard and Strauch, 1983).

9.4 velocityMeasureMents

9.4.1 tDpppm

Doppler radars measure velocit b estimating therequenc shit produced b an ensemble o movingtargets. Doppler radars also provide inormationabout the total power returned and about the spec-trum width o the precipitation particles within thepulse volume. The mean Doppler velocit is equalto the mean motion o scatterers weighted b theircross-sections and, or near horiontal antennascans, is essentiall the air motion towards or awa

rom the radar. Likewise, the spectrum width is ameasure o the velocit dispersion, that is, the shearor turbulence within the resolution volume.

A Doppler radar measures the phase o the returnedsignal b reerencing the phase o the receivedsignal to the transmitter. The phase is measured inrectangular orm b producing the in-phase (I)and quadrature (Q) components o the signal. TheI and Q are samples at a xed range location. Theare collected and processed to obtain the meanvelocit and spectrum width.

9.4.2 Dppm

To detect returns at various ranges rom the radar,the returning signals are sampled periodicall,usuall about ever s, to obtain inormationabout ever 150 m in range. This sampling cancontinue until it is time to transmit the next pulse.A sample point in time (corresponding to a distancerom the radar) is called a range gate. The radialwind component throughout a storm or precipita-tion area is mapped as the antenna scans.

A undamental problem with the use o an pulseDoppler radar is the removal o ambiguit inDoppler mean velocit estimates, that is, velocitolding. Discrete equi-spaced samples o a time-varing unction result in a maximum unambiguousrequenc equal to one hal o the samplingrequenc (fs). Subsequentl, requencies greaterthan fs/2 are aliased (olded) into the Nquistco-interval (fs/2) and are interpreted as velocitieswithin fs/4, where is the wavelength o trans-mitted energ.

Techniques to dealias the velocities include dualPRF techniques (Croier and others, 1991; Doviak

and zrnic, 1993) or continuit techniques (Eiltsand Smith, 1990). In the ormer, radial velocitestimates are collected at two dierent PRFs withdierent maximum unambiguous velocities andare combined to ield a new estimate o the radialvelocit with an extended unambiguous velocit.For example, a C band radar using PRFs o 1200and 900 H has nominal unambiguous velocitieso 16 and 12 m s1, respectivel. The amount oaliasing can be deduced rom the dierencebetween the two velocit estimates to dealias thevelocit to an extended velocit range o 48 m s1(Figure 9.3).

Continuit techniques rel on having sucientecho to discern that there are aliased velocities andcorrecting them b assuming velocit continuit(no discontinuities o greater than 2Vmax).

There is a range limitation imposed b the use ohigh PRFs (greater than about 1 000 H) asdescribed in section 9.2. Echoes beond the maxi-mum range will be aliased back into the primarrange. For radars with coherent transmitters (e.g,klstron sstems), the echoes will appear withinthe primar range. For coherent-on-receivesstems, the second trip echoes will appear as noise(Joe, and Passarelli and Siggia, 1995; Passarelli andothers 1981).

9.4.3 vpmm

In principle, a Doppler radar operating in theverticall-pointing mode is an ideal tool orobtaining accurate cloud-scale measurements overtical wind speeds and drop-sie distributions(DSDs). However, the accurac o verticalvelocities and DSDs derived rom the Dopplerspectra have been limited b the strongmathematical interdependence o the twoquantities. The real dicult is that the Dopplerspectrum is measured as a unction o thescatterers total vertical velocit due to terminal

hdrometeor all speeds, plus updrats ordowndrats. In order to compute the DSD rom aDoppler spectrum taken at vertical incidence, thespectrum must be expressed as a unction oterminal velocit alone. Errors o onl 0.25 m s1in vertical velocit can cause errors o 100 percent in drop number concentrations (Atlas,Scrivastava and Sekhon, 1973). A dual-wavelengthtechnique has been developed (termed the Ratiomethod) b which vertical air velocit ma beaccuratel determined independentl o the DSD.In this approach, there is a trade-o between

potential accurac and potential or successulapplication.


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9.4.4 Mmd

A great deal o inormation can be determined in realtime rom a single Doppler radar. It should be notedthat the interpretation o radial velocit estimatesrom a single radar is not alwas unambiguous. Colourdisplas o single-Doppler radial velocit patterns aidin the real-time interpretation o the associated refec-tivit elds and can reveal important eatures notevident in the refectivit structures alone (Burgessand Lemon, 1990). Such a capabilit is o particularimportance in the identication and tracking o severestorms. On tpical colour displas, velocities between Vmax are assigned 1 o 8 to 15 colours or more.Velocities extending beond the Nquist intervalenter the scale o colours at the opposite end. Thisprocess ma be repeated i the velocities are aliasedmore than one Nquist interval.

Doppler radar can also be used to derive verticalproiles o horiontal winds. When the radarsantenna is tilted above the horiontal, increasingrange implies increasing height. A prole o windwith height can be obtained b sinusoidal curve-t-ting to the observed data (termed velocit aimuthdispla (VAD) ater Lhermitte and Atlas, 1961) i thewind is relativel uniorm over the area o the scan.The winds along the ero radial velocit contourare perpendicular to the radar beam axis. The colourdispla ma be used to easil interpret VAD data

obtained rom large-scale precipitation sstems.Tpical elevated conical scan patterns in widespread

precipitation reveal an S-shaped ero radial velocitcontour as the mean wind veers with height (Woodand Brown, 1986). On other occasions, closedcontours representing jets are evident.

Since the measurement accurac is good, divergenceestimates can also be obtained b emploing theVAD technique. This technique cannot be accuratelapplied during periods o convective precipitationaround the radar. However, moderatel powerul,sensitive Doppler radars have successull obtainedVAD wind proles and divergence estimates in theopticall clear boundar laer during all but thecoldest months, up to heights o 3 to 5 km aboveground level. The VAD technique seems well suitedor winds rom precipitation sstems associatedwith extratropical and tropical cclones. In theradars clear-air mode, a time series o measurements

o divergence and derived vertical velocit isparticularl useul in nowcasting the probabilit odeep convection.

Since the mid-1970s, experiments have been madeor measuring three-dimensional wind elds usingmultiple Doppler arras. Measurements taken at agiven location inside a precipitation area ma becombined, b using a proper geometrical transor-mation, in order to obtain the three windcomponents. Such estimations are also possiblewith onl two radars, using the continuit equa-

tion. Kinematic analsis o a wind eld is describedin Browning and Wexler (1968).

Actual velocity (m s1)

Measuredvelocityorv

elocitydifference(ms

1)

48 32 16 0 16 32 48

32

16

0

16

32

igu 9.3. sli n h lin hw dppl vliy mumn kn wih w in pulpiin quni (1 200 n 900 Hz c bn ). sp g hn h mximum unm-

biguu vlii li. th in ( lin) bwn h dppl vliy im iin n n b u iniy h g liing.


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9.5 sourcesoerror

Radar beam flling

In man cases, and especiall at large ranges romthe radar, the pulse volume is not completel lledwith homogeneous precipitation. Precipitationintensities oten var widel on small scales; at largedistances rom the radar, the pulse volume increasesin sie. At the same time, the eects o the Earthscurvature become signicant. In general, measure-ments ma be quantitativel useul or ranges oless than 100 km. This eect is important or cloud-top height measurements and the estimation orefectivit.

Non-uniormity o the vertical odistribution oprecipitation

The rst parameter o interest when taking radarmeasurements is usuall precipitation at groundlevel. Because o the eects o beam width, beamtilting and the Earths curvature, radar measure-ments o precipitation are higher than average overa considerable depth. These measurements aredependent on the details o the vertical distributiono precipitation and can contribute to large errorsor estimates o precipitation on the ground.

Variations in the Z-R relationship

A variet oZ-R relationships have been ound ordierent precipitation tpes. However, rom the radaralone (except or dual polaried radars) these varia-tions in the tpes and sie distribution o hdrometeorscannot be estimated. In operational applications, thisvariation can be a signicant source o error.

Attenuation by intervening precipitation

Attenuation b rain ma be signicant, especiallat the shorter radar wavelengths (5 and 3 cm).Attenuation b snow, although less than or rain,

ma be signicant over long path lengths.

Beam blocking

Depending on the radar installation, the radar beamma be partl or completel occulted b the topog-raph or obstacles located between the radar andthe target. This results in underestimations o refec-tivit and, hence, o rainall rate.

Attenuation due to a wet radome

Most radar antennas are protected rom wind andrain b a radome, usuall made o breglass. The

radome is engineered to cause little loss in the radi-ated energ. For instance, the two-wa loss due tothis device can be easil kept to less than 1 dB at theC band, under normal conditions. However, underintense rainall, the surace o the radome canbecome coated with a thin lm o water or ice,resulting in a strong aimuth dependent attenua-tion. Experience with the NEXRAD WSR-88D radarsshows that coating radomes with a special hdro-phobic paint essentiall eliminates this source oattenuation, at least at 10 cm wavelengths.

Electromagnetic intererence

Electromagnetic intererence rom other radars ordevices, such as microwave links, ma be an impor-tant actor o error in some cases. This tpe oproblem is easil recognied b observation. It ma

be solved b negotiation, b changing requenc, busing lters in the radar receiver, and sometimes bsotware.

Ground clutter

The contamination o rain echoes b ground clut-ter ma cause ver large errors in precipitation andwind estimation. The ground clutter should rstbe minimied b good antenna engineering and agood choice o radar location. This eect ma begreatl reduced b a combination o hardware

clutter suppression devices (Aoagi, 1983) andthrough signal and data processing. Ground clut-ter is greatl increased in situations o anomalouspropagation.

Anomalous propagation

Anomalous propagation distorts the radar beampath and has the eect o increasing ground clut-ter b reracting the beam towards the ground. Itma also cause the radar to detect storms locatedar beond the usual range, making errors in theirrange determination because o range aliasing.

Anomalous propagation is requent in someregions, when the atmosphere is subject to strongdecreases in humidit and/or increases in temper-ature with height. Clutter returns owing toanomalous propagation ma be ver misleading tountrained human observers and are more dicultto eliminate ull b processing them as normalground clutter.

Antenna accuracy

The antenna position ma be known within 0.2

with a well-engineered sstem. Errors ma also beproduced b the excessive width o the radar beam


16/30


or b the presence o sidelobes, in the presence oclutter or o strong precipitation echoes.

Electronics stability

Modern electronic sstems are subject to small varia-tions with time. This ma be controlled b using awell-engineered monitoring sstem, which will keepthe variations o the electronics within less than 1dB, or activate an alarm when a ault is detected.

Processing accuracy

The signal processing must be designed to optimiethe sampling capacities o the sstem. The variancesin the estimation o refectivit, Doppler velocitand spectrum width must be kept to a minimum.Range and velocit aliasing ma be signiicant

sources o error.

Radar range equation

There are man assumptions in interpretingradar-received power measurements in terms o themeteorological parameter Z b the radar rangeequation. Non-conormit with the assumptionscan lead to error.

9.6 oPtiMizingraDarcharacteristics

9.6.1 sd

A radar is a highl eective observation sstem. Thecharacteristics o the radar and the climatologdetermine the eectiveness or an particular appli-cation. No single radar can be designed to be themost eective or all applications. Characteristicscan be selected to maximie the procienc to bestsuit one or more applications, such as tornadodetection. Most oten, or general applications,

compromises are made to meet several user require-ments. Man o the characteristics are interdependentwith respect to perormance and, hence, the needor optimiation in reaching a suitable specica-tion. Cost is a signicant consideration. Much othe interdependence can be visualied b reerenceto the radar range equation. A brie note on some othe important actors ollows.

9.6.2 W

The larger the wavelength, the greater the cost o

the radar sstem, particularl antenna costs orcomparable beamwidths (i.e. resolution). This is

due both to an increase in the amount o materialand to the dicult in meeting tolerances over agreater sie. Within the bands o weather radarinterest (S, C, X and K), the sensitivit o the radaror its abilit to detect a target is strongl dependenton the wavelength. It is also signicantl related toantenna sie, gain and beamwidth. For the sameantenna, the target detectabilit increases withdecreasing wavelength. There is an increase insensitivit o 8.87 dB in theor and 8.6 dB in prac-tice rom 5 to 3 cm wavelengths. Thus, the shorterwavelengths provide better sensitivit. At the sametime, the beamwidth is narrower or better resolu-tion and gain. The great disadvantage is that smallerwavelengths have much larger attenuation.

9.6.3 a

Radar ras are attenuated most signicantl in rain,less in snow and ice, and even less in clouds andatmospheric gases. In broad terms, attenuation atthe S band is relativel small and generall not toosignicant. The S band radar, despite its cost, isessential or penetrating the ver high refectivitiesin mid-latitude and subtropical severe storms withwet hail. X-band radars can be subject to severeattenuation over short distances, and the are notsuitable or precipitation rate estimates, or even orsurveillance, except at ver short range whenshadowing or obliteration o more distant storms

b nearer storms is not important. The attenuationin the C band lies between the two.

9.6.4 tmpw

Target detectabilit is directl related to the peakpower output o the radar pulse. However, there arepractical limits to the amount o power output thatis dictated b power tube technolog. Unlimitedincreases in power are not the most eective meanso increasing the target detectabilit. For example,doubling the power onl increases the sstem sensi-tivit b 3 dB. Technicall, the maximum possible

power output increases with wavelength.Improvements in receiver sensitivit, antenna gain,or choice o wavelength ma be better means oincreasing detection capabilit.

Magnetrons and klstrons are common powertubes. Magnetrons cost less but are less requencstable. For Doppler operation, the stabilit o kls-trons was thought to be mandator. An analsisb Strauch (1981) concluded that magnetronscould be quite eective or general meteorologicalapplications; man Doppler radars toda are based

on magnetrons. Ground echo rejection techniquesand clear air detection applications ma avour


17/30


klstrons. On the other hand, magnetron sstemssimpli rejecting second trip echoes.

At normal operating wavelengths, conventionalradars should detect rainall intensities o the ordero 0.1 mm h1 at 200 km and have peak poweroutputs o the order o 250 kW or greater in theC band.

9.6.5 P

The pulse length determines the target resolvingpower o the radar in range. The range resolution orthe abilit o the radar to distinguish between twodiscrete targets is proportional to the hal pulselength in space. For most klstrons and magnetrons,the maximum ratio o pulse width to PRF is about0.001. Common pulse lengths are in the range o 0.3

to 4s. A pulse length o 2s has a resolving powero 300 m, and a pulse o 0.5s can resolve 75 m.

Assuming that the pulse volume is lled with target,doubling the pulse length increases the radar sensitiv-it b 6 dB with receiver-matched ltering, whiledecreasing the resolution; decreasing the pulse lengthdecreases the sensitivit while increasing the resolu-tion. Shorter pulse lengths allow more independentsamples o the target to be acquired in range and thepotential or increased accurac o estimate.

9.6.6 Pp

The PRF should be as high as practical to obtain themaximum number o target measurements per unittime. A primar limitation o the PRF is theunwanted detection o second trip echoes. Mostconventional radars have unambiguous rangesbeond the useul range o weather observation bthe radar. An important limit on weather targetuseul range is the substantial height o the beamabove the Earth even at ranges o 250 km.

For Doppler radar sstems, high PRFs are used to

increase the Doppler unambiguous velocit meas-urement limit. The disadvantages o higher PRFsare noted above.

The PRF actor is not a signiicant costconsideration but has a strong bearing on sstemperormance. Brief, high PRFs are desirable toincrease the number o samples measured, toincrease the maximum unambiguous velocitthat can be measured, and to allow higherpermissible scan rates. Low PRFs are desirable toincrease the maximum unambiguous range that

can be measured, and to provide a lower dutccle.

9.6.7 am,mwd,dpdd

Weather radars normall use a horn ed antennawith a parabolic refector to produce a ocusednarrow conical beam. Two important considera-tions are the beamwidth (angular resolution) andthe power gain. For common weather radars, thesie o the antenna increases with wavelength andwith the narrowness o the beam required.

Weather radars normall have beamwidths in therange o 0.5 to 2.0. For a 0.5 and 1.0 beam at a Cband wavelength, the antenna refector diameter is7.1 and 3.6 m, respectivel; at S band it is 14.3 and7.2 m. The cost o the antenna sstem and pedestalincreases much more than linearl with refectorsie. There is also an engineering and cost limit.

The tower must also be appropriatel chosen tosupport the weight o the antenna.

The desirabilit o having a narrow beam tomaximie the resolution and enhance thepossibilit o having the beam lled with target isparticularl critical or the longer ranges. For a 0.5beam, the aimuthal (and vertical) cross-beamwidth at 50, 100 and 200 km range is 0.4, 0.9 and1.7 km, respectivel. For a 1.0 beam, the widthsare 0.9, 1.7 and 3.5 km. Even with these relativelnarrow beams, the beamwidth at the longer ranges

is substantiall large.

The gain o the antenna is also inversel propor-tional to the beamwidth and thus, the narrowerbeams also enhance sstem sensitivit b a actorequal to dierential gain. The estimates o refec-tivit and precipitation require a nominalminimal number o target hits to provide anacceptable measurement accurac. The beammust thus have a reasonable dwell time on thetarget in a rotating scanning mode o operation.Thus, there are limits to the antenna rotationspeed. Scanning ccles cannot be decreased with-

out consequences. For meaningul measurementso distributed targets, the particles must havesucient time to change their position beore anindependent estimate can be made. Sstemsgenerall scan at the speed range o about 3 to6 rpm.

Most weather radars are linearl polaried with thedirection o the electric eld vector transmittedbeing either horiontal or vertical. The choice isnot clear cut, but the most common polariationis horiontal. Reasons or avouring horiontal

polariation include: (a) sea and ground echoes aregenerall less with horiontal; (b) lesser sidelobes


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in the horiontal provide more accurate measure-ments in the vertical; and (c) greater backscatterrom rain due to the alling drop ellipticit.However, at low elevation angles, better refectiono horiontall polaried waves rom plane groundsuraces ma produce an unwanted range-depend-ent eect.

In summar, a narrow beamwidth aects sstemsensitivit, detectabilit, horiontal and verticalresolution, eective range and measurementaccurac. The drawback o small beamwidth ismainl cost. For these reasons, the smallest aord-able beamwidth has proven to improve greatlthe utilit o the radar (Croier and others,1991).

9.6.8 tpwd

The characteristics o tpical radars used in generalweather applications are given in Table 9.7.

tbl 9.7. spifin ypil mlgil

Type Z only Doppler Z only DopplerMicro-Doppler

Bn C C S S C

Fequency(GHz) 5.6 5.6 3.0 2.8 5.6

Wveength(cm)

5.33 5.33 10.0 10.7 5.4

Pek powe(kw)

250 250 500 1 000 250

Puseength( s)

2.0 0.5, 2.0 0.25,4.0

1.57,4.5

1.1

PrF (Hz) 250300

2501 200

200800

3001 400

2352 000

receivelog log/lin log log/lin log/lin

MdS (Bm) 105 105 110 113 106

antennimete(m)

3.7 6.2 3.7 8.6 7.6

Bemwith()

1.1 0.6 1.8 1.0 0.5

Gin (B) 44 48 38.5 45 51

Poiztion H H H H H

rottionte (pm)

6 16 3 6 5

As discussed, the radar characteristics and parame-ters are interdependent. The technical limits on theradar components and the availabilit o manuac-tured components are important considerations inthe design o radar sstems.

The Zonl radars are the conventional non-co-herent pulsed radars that have been in use ordecades and are still ver useul. The Dopplerradars are the new generation o radars that adda new dimension to the observations. Theprovide estimates o radial velocit. Themicro-Doppler radars are radars developed orbetter detection o small-scale microbursts andtornadoes over ver limited areas, such as orair-terminal protection.

9.7 raDarinstallation

9.7.1 opmm

Optimum site selection or installing a weatherradar is dependent on the intended use. When thereis a denite one that requires storm warnings, thebest compromise is usuall to locate the equipmentat a distance o between 20 and 50 km rom thearea o interest, and generall upwind o it accord-ing to the main storm track. It is recommended that

the radar be installed slightl awa rom the mainstorm track in order to avoid measurement prob-lems when the storms pass over the radar. At thesame time, this should lead to good resolution overthe area o interest and permit better advance warn-ing o the coming storms (Leone and others,1989).

In the case o a radar network intended prima-ril or snoptic applications, radars atmid-latitudes should be located at a distance oapproximatel 150 to 200 km rom each another.The distance ma be increased at latitudes closer

to the Equator, i the radar echoes o interestrequentl reach high altitudes. In all cases,narrow-beam radars will ield the best accuracor precipitation measurements.

The choice o radar site is inluenced b maneconomic and technical actors as ollows:(a) The existence o roads or reaching the

radar;(b) The availabilit o power and telecommu-

nication links. It is requentl necessarto add commerciall available lightning

protection devices;


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(c) The cost o land;(d) The proximit to a monitoring and mainte-

nance acilit;(e) Beam blockage obstacles must be avoided.

No obstacle should be present at an anglegreater than a hal beamwidth above thehorion, or with a horiontal width greaterthan a hal beamwidth;

() Ground clutter must be avoided as much aspossible. For a radar to be used or applicationsat relativel short range, it is sometimes possi-ble to nd, ater a careul site inspection andexamination o detailed topographic maps, arelativel fat area in a shallow depression, theedges o which would serve as a natural clutterence or the antenna pattern sidelobes withminimum blockage o the main beam. In allcases, the site surve should include a camera

and optical theodolite check or potentialobstacles. In certain cases, it is useul to emploa mobile radar sstem or conrming the suit-abilit o the site. On some modern radars,sotware and hardware are available to greatlsuppress ground clutter with minimum rejec-tion o weather echoes (Heiss, McGrew andSirmans, 1990);

(g) When the radar is required or long-rangesurveillance, as ma be the case or tropicalcclones or other applications on the coast,it will usuall be placed on a hill-top. It will

see a great deal o clutter, which ma notbe so important at long ranges (see section9.2.6 or clutter suppression);

(h) Ever surve on potential sites should includea careul check or electromagnetic inter-erence, in order to avoid as much as possi-ble intererence with other communicationsstems such as television, microwave linksor other radars. There should also be conr-mation that microwave radiation does notconstitute a health haard to populationsliving near the proposed radar site (Skolnik,1970; Leone and others, 1989).

9.7.2 tmmdmdp

Recent developments in telecommunications andcomputer technolog allow the transmission oradar data to a large number o remote displas.In particular, computer sstems exist that arecapable o assimilating data rom man radars aswell as rom other data sources, such as satellites.It is also possible to monitor and to controlremotel the operation o a radar which allows

unattended operation. Owing to these technical

advances, in man countries, nowcasting iscarried out at sites removed rom the radarlocation.

Pictures ma be transmitted b almost anmodern transmission means, such as telephonelines (dedicated or not), bre optic links, radio ormicrowave links, and satellite communicationchannels. The most widel used transmissionsstems are dedicated telephone lines, becausethe are easil available and relativel low in costin man countries. It should be kept in mind thatradars are oten located at remote sites whereadvanced telecommunication sstems are notavailable.

Radar pictures ma now be transmitted in a ewseconds due to rapid developments in communica-

tion technolog. For example, a product with a100 km range with a resolution o 0.5 km ma havea le sie o 160 kBtes. Using a compression algo-rithm, the le sie ma be reduced to about 20 to 30kBtes in GIF ormat. This product le can be trans-mitted on an analogue telephone line in less than30 s, while using an ISDN 64 kbps circuit it matake no more than 4 s. However, the transmissiono more refectivit levels or o additional data, suchas volume scans o refectivit or Doppler data, willincrease the transmission time.

9.8 calibrationanDMaintenance

The calibration and maintenance o an radarshould ollow the manuacturers prescribed proce-dures. The ollowing is an outline.

9.8.1 c

Ideall, the complete calibration o refectivit usesan external target o known radar refectivit actor,such as a metal-coated sphere. The concept is to

check i the antenna and wave guides have theirnominal characteristics. However, this method isver rarel used because o the practical dicultiesin fing a sphere and multiple ground refections.Antenna parameters can also be veried b sun fuxmeasurements. Routine calibration ignores theantenna but includes the wave guide and transmit-ter receiver sstem. Tpicall, the ollowing actionsare prescribed:(a) Measurement o emitted power and waveorm

in the proper requenc band;(b) Verication o transmitted requenc and

requenc spectrum;


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(c) Injection o a known microwave signal beorethe receiver stage, in order to check i thelevels o refectivit indicated b the radar arecorrectl related to the power o the input;

(d) Measurement o the signal to noise ratio,which should be within the nominal rangeaccording to radar specications.

I an o these calibration checks indicate anchanges or biases, corrective adjustments need tobe made. Doppler calibration includes: the verica-tion and adjustment o phase stabilit using xedtargets or articial signals; the scaling o the realand imaginar parts o the complex video; and thetesting o the signal processor with known arti-ciall generated signals.

Levelling and elevation are best checked b track-

ing the position o the sun in receive-onl modeand b using available sun location inormation;otherwise mechanical levels on the antenna areneeded. The presence or absence o echoes romxed ground targets ma also serve as a crude checko transmitter or receiver perormance.

Although modern radars are usuall equipped withver stable electronic components, calibrationsmust be perormed oten enough to guarantee thereliabilit and accurac o the data. Calibrationmust be carried out either b qualied personnel, or

b automatic techniques such as online diagnosticand test equipment. In the rst case, which requiresmanpower, calibration should optimall beconducted at least ever week; in the second, it mabe perormed dail or even semi-continuousl.Simple comparative checks on echo strength andlocation can be made requentl, using two or moreoverlapping radars viewing an appropriate target.

9.8.2 M

Modern radars, i properl installed and operated,should not be subject to requent ailures. Some

manuacturers claim that their radars have a meantime between ailures (MTBF) o the order o a ear.However, these claims are oten optimistic and therealiation o the MTBF requires scheduled preven-tive maintenance. A routine maintenance plan andsucient technical sta are necessar in order tominimie repair time.

Preventive maintenance should include at least amonthl check o all radar parts subject to wear, suchas gears, motors, ans and inrastructures. The resultso the checks should be written in a radar logbook b

local maintenance sta and, when appropriate, sentto the central maintenance acilit. When there are

man radars, there might be a centralied logisticsuppl and a repair workshop. The latter receivesailed parts rom the radars, repairs them and passesthem on to logistics or storage as stock parts, to beused as needed in the eld.

For corrective maintenance, the Service should besucientl equipped with the ollowing:(a) Spare parts or all o the most sensitive compo-

nents, such as tubes, solid state components,boards, chassis, motors, gears, power supplies,and so orth. Experience shows that it is desir-able to have 30 per cent o the initial radarinvestment in critical spare parts on the site. Ithere are man radars, this percentage ma belowered to about 20 per cent, with a suitabledistribution between central and local main-tenance;

(b) Test equipment, including the calibrationequipment mentioned above. Tpicall, thiswould amount to approximatel 15 per cento the radar value;

(c) Well-trained personnel capable o identiingproblems and making repairs rapidl andecientl.

Competent maintenance organiation should resultin radar availabilit 96 per cent o the time on aearl basis, with standard equipment. Betterperormances are possible at a higher cost.

Recommended minimum equipment or calibra-tion and maintenance includes the ollowing:(a) Microwave signal generator;(b) Microwave power meter;(c) MH oscilloscope;(d) Microwave requenc meter;(e) Standard gain horns;() Intermediate requenc signal generator;(g) Microwave components, including loads,

couplers, attenuators, connectors, cables,adapters, and so on;

(h) Versatile microwave spectrum analser at the

central acilit;(i) Standard electrical and mechanical tools and

equipment.

9.9 PreciPitationMeasureMents

The measurement o precipitation b radars has beena subject o interest since the earl das o radar mete-orolog. The most important advantage o usingradars or precipitation measurements is the coverage

o a large area with high spatial and temporal resolu-tion rom a single observing point and in real time.


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Furthermore, the two-dimensional picture o theweather situation can be extended over a ver largearea b compositing data rom several radars. However,onl recentl has it become possible to take measure-ments over a large area with an accurac that isacceptable or hdrological applications.

Unortunatel, a precise assessment o this accuracis not possible partl because no satisactor basiso comparison is available. A common approach is touse a network o gauges as a reerence against whichto compare the radar estimates. This approach hasan intuitive appeal, but suers rom a undamentallimitation: there is no reerence standard againstwhich to establish the accurac o areal rainall meas-ured b the gauge network on the scale o the radarbeam. Nature does not provide homogeneous, stand-ard rainall events or testing the network, and there

is no higher standard against which to compare thenetwork data. Thereore, the true rainall or the areaor the accurac o the gauge network is not known.Indeed, there are indications that the gauge accuracma, or some purposes, be ar inerior to what iscommonl assumed, especiall i the estimates comerom a relativel small number o raingauges (Ne,1977).

9.9.1 Ppdmm:

tZ-R

Precipitation is usuall measured b using the Z-Rrelation:

Z=A Rb (9.10)

whereA and b are constants. The relationship is notunique and ver man empirical relations havebeen developed or various climates or localitiesand storm tpes. Nominal and tpical values or theindex and exponent areA = 200, b = 1.60 (Marshalland Palmer, 1948; Marshall and Gunn, 1952).

The equation is developed under a number oassumptions that ma not alwas be completelvalid. Nevertheless, histor and experience haveshown that the relationship in most instancesprovides a good estimate o precipitation at theground unless there are obvious anomalies. Thereare some generalities that can be stated. At 5 and 10cm wavelengths, the Raleigh approximation isvalid or most practical purposes unless hailstonesare present. Large concentrations o ice mixed withliquid can cause anomalies, particularl near themelting level. B taking into account the reractive

index actor or ice (i.e., |K|2

= 0.208) and b choos-ing an appropriate relation between the refectivit

actor and precipitation rate (Ze againstR), precipi-tation amounts can be estimated reasonabl well insnow conditions (the value o 0.208, instead o0.197 or ice, accounts or the change in particlediameter or water and ice particles o equal mass).

The rainall rate (R) is a product o the mass contentand the all velocit in a radar volume. It is roughlproportional to the ourth power o the particlediameters. Thereore, there is no unique relation-ship between radar refectivit and the precipitationrate since the relationship depends on the particlesie distribution. Thus, the natural variabilit indrop-sie distributions is an important source ouncertaint in radar precipitation measurements.

Empirical Z-R relations and the variations romstorm to storm and within individual storms have

been the subject o man studies over the past ortears. A Z-R relation can be obtained b calculatingvalues oZandR rom measured drop-sie distribu-tions. An alternative is to compare Zmeasured alotb the radar (in which case it is called the equiva-lent radar refectivit actor and labelled Ze) withRmeasured at the ground. The latter approachattempts to refect an dierences between theprecipitation alot and that which reaches theground. It ma also include errors in the radar cali-bration, so that the result is not strictl a Z-Rrelationship.

The possibilit o accounting or part o the varia-bilit o the Z-R relation b stratiing stormsaccording to rain tpe (such as convective, non-cellular, orographic) has received a good deal oattention. No great improvements have beenachieved and questions remain as to the practical-it o appling this technique on an operationalbasis. Although variations in the drop-sie distribu-tion are certainl important, their relativeimportance is requentl overemphasied. Atersome averaging over time and/or space, the errorsassociated with these variations will rarel exceed a

actor o two in rain rate. The are the main sourceso the variations in well-dened experiments atnear ranges. However, at longer ranges, errorscaused b the inabilit to observe the precipitationclose to the ground and beam-lling are usualldominant. These errors, despite their importance,have been largel ignored.

Because o growth or evaporation o precipitation,air motion and change o phase (ice and water inthe melting laer, or bright band), highl variablevertical relectivit proiles are observed, both

within a given storm and rom storm to storm.Unless the beam width is quite narrow, this will


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lead to a non-uniorm distribution o refectivitwithin the radar sample volume. In convectiverainall, experience shows that there is less di-cult with the vertical prole problem.

However, in stratiorm rain or snow, the verticalprole becomes more important. With increasingrange, the beam becomes wider and higher abovethe ground. Thereore, the dierences between esti-mates o rainall b radar and the rain measured atthe ground also increase. Relectivit usualldecreases with height; thereore, rain is underesti-mated b radar or stratiorm or snow conditions.

At long ranges, or low-level storms, and especiallwhen low antenna elevations are blocked b obsta-cles such as mountains, the underestimate ma besevere. This tpe o error oten tends to dominate all

others. This is easil overlooked when observingstorms at close ranges onl, or when analsing stormsthat are all located at roughl the same range.

These and other questions, such as the choice othe wavelength, errors caused b attenuation,considerations when choosing a radar site orhdrological applications, hardware calibration oradar sstems, sampling and averaging, and mete-orological adjustment o radar data are discussed in

Joss and Waldvogel (1990), Smith (1990) andSauvageot (1994). The ollowing considers onl

rainall measurements; little operational experienceis available about radar measurements o snow andeven less about measurements o hail.

9.9.2 Mmpd

The basic procedure or deducing rainall rates rommeasured radar refectivities or hdrological appli-cations requires the ollowing steps:(a) Making sure that the hardware is stable b

calibration and maintenance;(b) Correcting or errors using the vertical refec-

tivit prole;

(c) Taking into account all the inormation aboutthe Ze-R relationship and deducing the rain-all;

(d) Adjustment with raingauges.

The rst three parts are based on known phsicalactors, and the last one uses a statistical approachto compensate or residual errors. This allows thestatistical methods to work most ecientl. In thepast, a major limitation on carring out these stepswas caused b analogue circuitr and photographictechniques or data recording and analses. It was,

thereore, extremel diicult to determine andmake the necessar adjustments, and certainl not

in real time. Toda, the data ma be obtained inthree dimensions in a manageable orm, and thecomputing power is available or accomplishingthese tasks. Much o the current research is directedtowards developing techniques or doing so on anoperational basis (Ahnert and others, 1983).

The methods o approach or (b) to (d) above andthe adequac o results obtained rom radar precipi-tation measurement greatl depend on the situation.This can include the speciic objective, thegeographic region to be covered, the details o theapplication, and other actors. In certain situations,an interactive process is desirable, such as thatdeveloped or FRONTIERS and described inAppendix A o Joss and Waldvogel (1990). It makesuse o all pertinent inormation available in modernweather data centres.

To date, no one method o compensating or theeects o the vertical refectivit prole in real timeis widel accepted ((b) above). However, threecompensation methods can be identied:(a) Range-dependent correction: The eect o the

vertical prole is associated with the combi-nation o increasing he

Chapter 9 Radrar Measurements

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