Atmospheric InstrumentationM. D. Eastin Atmospheric Sounding Systems.

Atmospheric Instrumentation M. D. Eastin

Atmospheric Sounding Systems


Outline

Sounding Systems

• Review of Atmospheric Soundings

• “Upsondes” (Weather Balloons)• Balloon• Sensor Package• Navigation / Telemetry System• Wind Estimation• Exposure Errors

• “Downsondes” (GPS Dropsondes)

• Experimental Sounding Systems


Definition and Concepts:

Goal: Obtain a near-vertical profile of atmospheric pressure, temperature, humidity,and wind structure for use in both nowcasting and forecasting of convection,as well as the assimilation into global and regional numerical forecast models

Methods: Balloon-based radiosondes **Aircraft-based radiosondes **Commercial AircraftRemote Sensing **

** Instrument: Radiosonde

Review of Atmospheric Soundings


Receiving Antenna

Balloon Inflation Area

HeliumTanks

Review of Atmospheric SoundingsComponents of a Radiosonde Sounding System:

1. Balloon (with gas) / Parachute

2. Atmospheric Sensor Package

3. Navigation / Telemetry (GPS, transmitter, receiver)

4. Analysis Software

Latex Balloon(helium)

Parachute

SensorPackage


Review of Atmospheric SoundingsCommon Data Collected by Radiosondes:

1. Parameters: PressureTemperatureHumidityAltitude / Time (determined from navigation satellites)Latitude / Longitude (determined from navigation satellites)Wind Speed / Direction (calculated from sequential locations)

2. Mandatory Levels: Atmospheric altitude/pressure levels at which all radiosondesare required to report current observations

Tropospheric Levels (12) → Surface, 1000-mb, 925-mb, 850-mb, 700-mb, 500-mb, 400-mb, 300-mb, 250-mb, 200-mb, 150-mb, 100-mb

3. Significant Levels: Additional pressure levels at which a parameter experiencesa vertical gradient change (Ex: lapse rate changes sign)

Temperature begins to increase / decrease with heightHumidity begins to increase / decrease with heightWind speed begins to increase / decrease with height


Review of Atmospheric SoundingsDisplay / Analysis of Radiosonde Sounding Data:

1. Skew-T Log-P diagram

•Temperature plotted vs. pressure(usually in red)

•Dewpoint plotted vs. pressure(usually in blue or green)

•Wind speed and directionplotted vs. pressure (using barbs or a hodograph)


Review of Atmospheric SoundingsDisplay / Analysis of Radiosonde Sounding Data:

2. Lifted Parcels and Stability Indices

a. Surface Layerb. Mixed Layer

• Lifted Condensation Level (LCL)• Level of Free Convection (LFC)• Equilibrium Level (EL)• Convective Inhibition (CIN)• Convective Available Potential Energy (CAPE)

TTd

LCL

LFC

Tp < Te

Tp > Te

ELTp < Te

Tp < Te

CIN

CAPE

Thermodynamic“path” of alifted parcel

from the surface

• Lifted Index (LI)• Showalter Index (SI)• K-Index (K)• SWEAT Index

Td, w

P = 1000 mb

T, wswP = 800 mb

θw

TLCL

θepθ

Tw

PLCL


Review of Atmospheric Soundings

• Temperature at the LCL (TLCL)• Pressure at the LCL (PLCL)• Wet-bulb potential temperature (θw)• Pseudo-adiabatic equivalent potential temperature (θe)

Given:

P = 800 mb T = +9.5ºC Td = –8.0ºC


Review of Atmospheric SoundingsUses of Radiosonde Sounding Data: Severe Weather Forecasting


Review of Atmospheric SoundingsUses of Radiosonde Sounding Data: Assimilation into Numerical Models

Analysis of Radiosonde Observations Numerical Model Field


The Balloon:

•Expandable latex weighing 200 – 1000 g•Inflated with a “lift gas” less dense than air:

1. Helium (inert but becoming rare)2. Hydrogen (explosive but common)

•Balloon (with a radiosonde package attached) ascends and expands (volume increases) until it reaches burst altitude

(in the stratosphere) and then the radiosonde package descends by parachute → very few are recovered

Balloon Diameter during Ascent:

•Balloon volume, diameter, and lift gas density are inversely proportional air density

where: VBZ = balloon volume at height z (m3)

VBO = balloon volume at the surface (m3)

ρZ = air density at height z (kg m-

3) ρO = air density at the surface (kg

m-3) ρBZ = lift gas density at height z

(kg m-3) ρBO = lift gas density at surface (kg

m-3) DBZ = balloon diameter at height z

(m) DBO = balloon diameter at the

surface (m)

Upsondes – Weather Balloons

Latex Balloon(helium)

Parachute

InstrumentPackage

OBOZBZ VV 3

6 BB DV

31

Z

OBOBZ DD

O

ZBOBZ


Balloon Ascent Rate:

•Balance between upward-directed free lift force (FL) and downward-directed drag force (FD)

where: DBZ = balloon diameter at height z (m) ρZ = air density at height z (kg m-3) ρBZ = density of lift gas at height z (kg

m-3) mB = balloon mass (kg) mP = sensor package mass (kg)

g = gravity (m s-2) CD = drag coefficient (none) wB = balloon vertical velocity (m s-1)

•If we solve for the balloon’s vertical velocity

• If the vertical profile of air density is known (or can be reasonably estimated), then thethe balloon’s expected diameter and vertical velocity can be calculated as a functionof altitude (in a future homework)


gmm

gDF

PB

BZZBZL

3

6

22

8 BBZZDD wDCF

21

2

3

8/

6/

BZZD

PBBZZBZB DC

gmmgDw


Balloon Ascent Rate:

•Tropospheric ascent rates are typically 5-6 m/s

•The balloon’s vertical velocity deviates slightlyfrom this expectation for several reasons:

1. The balloon material is not perfectly elastic2. A small portion of the lift gas will leak out3. The internal lift gas temperature will become

warmer than then ambient air4. Balloon shape is not perfectly spherical5. Vertical air motions will impact ascent rate6. Drag coefficient is not constant

Standard U.S. Weather Balloon Specifications:

Manufacturer / Model: Vaisala RS-92Lift Gas: HeliumBalloon Mass: 250 gSensor Package Mass: 280 gOperating Time: 135 minutes


Sensor Package: Vaisala RS-92

Barometer: Aneroid silicon capsule

Accuracy: ±0.4 mbResolution: 0.1 mbDynamic Range: 3 – 1080 mbResponse Time: < 1 s

Thermometer: Platinum resistance thermistor

Accuracy: ±0.3°CResolution: 0.1°CDynamic Range: –90°C to +60°CResponse Time: < 1 s

Hygrometers: Twin-heated film-capacitance hygristors

Accuracy: ±5.0%Resolution: 1.0%Dynamic Range: 0 – 100%Response Time: < 20 s (due to heating cycle)



TemperatureSensor

HumiditySensors

PressureSensor(inside)

Navigation / Telemetry System: Vaisala RS-92

•Sonde location is determined through simultaneoustriangulation with at least four GPS satellites(more satellites increase position accuracy)

•The sonde transmits the detected GPS signalsdown to the ground station (which also receivesGPS signals directly from the satellites)

•The ground station then computes the sondeposition relative to the ground station position

GPS Antenna: Code-correlating receiver

Channels: 12 (maximum)Position Accuracy: ±10 m (horizontal)

±20 m (vertical)Wind Accuracy: 0.2 m/s

Telemetry Antenna: Synthesized digital multiplexer

Frequency: 403 MHzData Downlink: 2400 bits/sData Cycle: 1 s



GPSAntenna

TelemetryAntenna


Wind Profile Estimation:

• Since balloons are characterized by large surface area and small mass, they usually followthe mean wind flow, and thus horizontal winds can be determined by tracking the

balloon’sposition during its ascent relative to the ground station’s position

where: x = east-west distance (m) y = north-south distance (m) z = altitude (m) ϕ = elevation angle (degrees) θ = azimuth angle (degrees)

Δt = time interval (s) u = zonal wind component (m s-1) v = meridional wind component (m s-1)

•The vertical winds are rarely computedsince vertical position errors are largerand precise ascent rate is unknown


z

R

Balloon

GroundStation

ϕ

Elevation

y

x

θ

Azimuth

x = y = z = 0t = 0

t = 1

t = 2 t = 3

t = 4

sincotzx

coscotzy

txxu tt /1

tyyv tt /1


Exposure Errors: Radiation

• Affects thermistors • Direct exposure to solar radiation can

produce a warm temperature errordepending on the following:

1. Sensor Diameter (D)2. Sensor Reflectivity3. Ascent Rate (wB)

• The RS-92 platinum resistance thermistoris highly reflective with a 0.2 mm diameter

Exposure Errors: Wet-bulbing

• Affects thermistors and hygristors• Sensors remain wet for some time after emerging from a cloud.• Cool temperature error due to evaporation• Moist humidity error until heating cycle removes the excess moisture

• The RS-92 sensors are coated with a thin film of water-shedding material (like Rain-ex)that limits any sensor wetting (both in cloud and after emerging from cloud)



Downsondes – GPS DropsondesConcept:

•Special radiosondes launched fromhigh-altitude aircraft and descend ata controlled rate using a parachute

•Only deployed over oceans

•Only used during “targeted” observingefforts designed to improve high-

impactforecasts for severe weather events ornatural disasters

Parachute

InstrumentPackage

DropsondeLaunch

Tubein a

NOAAResearchAircraft

The Parachute:

•Square-cone chute deployed upon exit from launch tube•Provides controlled and stable sonde descent without the

“pendulum motion” common with circular parachutes

•Descent rate varies from 11–20 m/s in the troposphere

Standard Dropsonde Specifications:

Manufacturer / Model: Vaisala RD-94Sensor Package Mass: 350 gTransmitter frequency: 400-406 MHzMax Telemetry Range: 325 kmMax Deployable Airspeed: 250 knots


Downsondes – GPS Dropsondes

Transmitter AntennaGPS Antenna PTH Sensors

TemperatureSensor

HumiditySensors

PressureSensor

Standard Dropsonde Specifications:

Sensor Type Accuracy Resolution Range Response

Barometer Silicon Aneroid ±0.4 mb 0.1 mb 3 to 1080 mb< 1 s

Thermistor Platinum Resistance ±0.2 °C 0.1 °C -90°C to +60°C< 2 s

Hygristors Heated Capacitance ±2.0 % 1.0 % 0% to 100%< 20 s

Winds GPS Positions ±0.5 m/s 0.1 m/s 0 to 150 m/sN / A

Data Cycle: 2 Hz (PTH) Position Accuracy: ±5 m (horizontal)4 Hz (winds) ±10 m

(vertical)

Descent Time: 15 min (from 14 km) 1. Wind Estimation → as with radiosondes, but 8 min (from 7 km) starting

at drop location2. Exposure Errors → same as for

radiosondes


Downsondes – GPS Dropsondes

Transmitter AntennaGPS Antenna PTH Sensors

TemperatureSensor

HumiditySensors

PressureSensor


Experimental Sounding SystemsRocket Sondes:

•Used to launch radiosondes into the upper level atmosphere(up to 120 km) where balloons cannot ascend

•Once the rocket reaches its maximum altitude, the sensorpackage descends using a square-cone parachute

•Used to study:Ozone Volcanic EmissionsRadioactivity Earth’s Magnetic FieldCosmic Rays Mesospheric Chemistry


Glider Sondes:

•Weather balloon transports instrumented gliderto burst altitude and then the plane glides backto a designated point using an onboard GPSand flight navigation computer

•Reusable•Low-cost alternative to the standard weather

balloon packages often lost after each launch(total cost of $115,000 per year)

“DataBird” manufactured by GPS-Boomerang:

•Still in development (field tests with prototypes)•Expected to be useable for 50 flights•Expected cost → $1000 per glider•Expected savings → $100,000 per year

•Ascends to 20-35 km altitude in ~1 hour•Descends in ~30 minutes at 3-4 m/s•Measures pressure, temperature, humidity, winds

using proprietary sensors•Major remaining “hurdle” is air-space restrictions http://www.gpsboomerang.com/content/view/34/42/

Experimental Sounding Systems


Student Sonde:

•Still in development (field tests in progress)•Inexpensive (volunteers – no health insurance)•Major remaining “hurdle” is getting the student

to survive at upper altitudes in the cold andlow-oxygen environment

Experimental Sounding Systems


Summary

Sounding Systems

• Review of Atmospheric Soundings

• “Upsondes” (Weather Balloons)• Balloon• Sensor Package• Navigation / Telemetry System• Wind Estimation• Exposure Errors

• “Downsondes” (GPS Dropsondes)

• Experimental Sounding Systems


References

Boire, G., D. C. Sutter, S. P. Pryor, and A.K. Brown, 1993: A low cost GS rawinsonde system. Preprints 8th Symposium on Meteorological Observations and Instrumentation, Anaheim, CA, American Meteorological Society, 23-24.

Brock, F. V., and S. J. Richardson, 2001: Meteorological Measurement Systems, Oxford University Press, 290 pp.

Finger, F. G., and F. J. Schmidlin, 1991: Upper-air measurements and instrumentation workshop. Bulletin of the American Meteorological Society, 72, 50-55.

Harrison, R. G., 2015: Meteorological Instrumentation and Measurements, Wiley-Blackwell Publishing, 257 pp.

Hock, T. F., and J. L. Franklin, 1999: The NCAR GPS dropwindsonde. Bulletin of the American Meteorological Society, 80, 407-420.

Leurs, J.K., 1991: Estimating the temperature error of the radiosonde thermistor under different environmental conditions.Journal of Atmospheric and Oceanic Technology, 7, 882-895.

Olsen, R. O., R. J. Okrasinski, and F. J. Schmidlin, 1991: Inter-comparison of upper air data derived from variousradiosonde systems. Preprints 7th symposium on Meteorological Observations and Instrumentation, New Orleans, LA, American Meteorological Society, 232-236.

Pratt, R. W., 1985: Review of radiosonde humidity and temperature errors. Journal of Atmospheric and Oceanic Technology, 2, 404-407.

Reynolds, R. D., 1966: The effect of atmospheric lapse rates on balloon ascent rates. Journal of Applied Meteorology, 5, 537-541.

Atmospheric InstrumentationM. D. Eastin Atmospheric Sounding Systems.

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