CUAHSI WaterOneFlow Web Services By Tim Whiteaker CE 394K.2 Hydrology 1 February 2007.
CE 394K.2 Hydrology Atmospheric Water and Precipitation
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Transcript of CE 394K.2 Hydrology Atmospheric Water and Precipitation
CE 394K.2 HydrologyAtmospheric Water and Precipitation
• Literary quote for today:
“In Köhln, a town of monks and bones,And pavements fang'd with murderous stonesAnd rags, and hags, and hideous wenches;I counted two and seventy stenches,All well defined, and several stinks!Ye nymphs that reign o'er sewers and sinks,The river Rhine, it is well known,Doth wash your city of Cologne;But tell me, nymphs, what power devineShall henceforth wash the river Rhine?”
Samuel Taylor Coleridge, “The City of Cologne”, 1800Contributed by Eric Hersh
Questions for today
(1) How is net radiation to the earth’s surface partitioned into latent heat, sensible heat and ground heat flux and how does this partitioning vary with location on the earth?
(2) What are the factors that govern the patterns of atmospheric circulation over the earth?
(3) What are the key variables that describe atmospheric water vapor and how are they connected?
(4) What causes precipitation to form and what are the factors that govern the rate of precipitation?
(5) How is precipitation measured and described?
(Some slides in this presentation were prepared by Venkatesh Merwade)
Questions for today
(1) How is net radiation to the earth’s surface partitioned into latent heat, sensible heat and ground heat flux and how does this partitioning vary with location on the earth?
(2) What are the factors that govern the patterns of atmospheric circulation over the earth?
(3) What are the key variables that describe atmospheric water vapor and how are they connected?
(4) What causes precipitation to form and what are the factors that govern the rate of precipitation?
(5) How is precipitation measured and described?
(Some slides in this presentation were prepared by Venkatesh Merwade)
Heat energy
• Energy– Potential, Kinetic, Internal (Eu)
• Internal energy– Sensible heat – heat content that can be
measured and is proportional to temperature– Latent heat – “hidden” heat content that is
related to phase changes
fhg
Vyz
g
Vyz
22
22
22
21
11
Energy Units
• In SI units, the basic unit of energy is Joule (J), where 1 J = 1 kg x 1 m/s2
• Energy can also be measured in calories where 1 calorie = heat required to raise 1 gm of water by 1°C and 1 kilocalorie (C) = 1000 calories (1 calorie = 4.19 Joules)
• We will use the SI system of units
Energy fluxes and flows
• Water Volume [L3] (acre-ft, m3)
• Water flow [L3/T] (cfs or m3/s)
• Water flux [L/T] (in/day, mm/day)
• Energy amount [E] (Joules)
• Energy “flow” in Watts [E/T] (1W = 1 J/s)
• Energy flux [E/L2T] in Watts/m2
Energy flow of1 Joule/sec
Area = 1 m2
MegaJoules
• When working with evaporation, its more convenient to use MegaJoules, MJ (J x 106)
• So units are– Energy amount (MJ)– Energy flow (MJ/day, MJ/month)– Energy flux (MJ/m2-day, MJ/m2-month)
Internal Energy of Water
0
1
2
3
4
-40 -20 0 20 40 60 80 100 120 140
Temperature (Deg. C)
Inte
rna
l En
erg
y (
MJ
)
Heat Capacity (J/kg-K) Latent Heat (MJ/kg)Ice 2220 0.33Water 4190 2.5
Ice
Water
Water vapor
Water may evaporate at any temperature in range 0 – 100°CLatent heat of vaporization consumes 7.6 times the latent heat of fusion (melting)
2.5/0.33 = 7.6
Latent heat flux
• Water flux– Evaporation rate, E
(mm/day)
• Energy flux – Latent heat flux
(W/m2), Hl
Area = 1 m2
ElH vl = 1000 kg/m3
lv = 2.5 MJ/kg
)/)(1000/1(*)/)(86400/1(*/1)/(105.2)/(1000/ 632 mmmsdaydaymmkgJmkgmW
28.94 W/m2 = 1 mm/day
Radiation
• Two basic laws– Stefan-Boltzman Law
• R = emitted radiation (W/m2)
= emissivity (0-1) = 5.67x10-8W/m2-K4
• T = absolute temperature (K)
– Wiens Law = wavelength of
emitted radiation (m)
4TR
T
310*90.2
Hot bodies (sun) emit short wave radiationCool bodies (earth) emit long wave radiation
All bodies emit radiation
Net Radiation, Rn
Ri Incoming Radiation
Ro =Ri Reflected radiation
albedo (0 – 1)
Rn Net Radiation
Re
ein RRR )1(
Average value of Rn over the earth and over the year is 105 W/m2
Net Radiation, Rn
Rn Net Radiation
GLEHRn
Average value of Rn over the earth and over the year is 105 W/m2
G – Ground Heat Flux
LE – EvaporationH – Sensible Heat
http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/energy/radiation_balance.html
Energy Balance of Earth
6
4
10070
51
21
26
38
6
20
15
Sensible heat flux 7Latent heat flux 23
19
Energy balance at earth’s surfaceDownward short-wave radiation, Jan 2003
600Z
Energy balance at earth’s surfaceDownward short-wave radiation, Jan 2003
900Z
Energy balance at earth’s surfaceDownward short-wave radiation, Jan 2003
1200Z
Energy balance at earth’s surfaceDownward short-wave radiation, Jan 2003
1500Z
Energy balance at earth’s surfaceDownward short-wave radiation, Jan 2003
1800Z
Energy balance at earth’s surfaceDownward short-wave radiation, Jan 2003
2100Z
Latent heat flux, Jan 2003, 1500z
Questions for today
(1) How is net radiation to the earth’s surface partitioned into latent heat, sensible heat and ground heat flux and how does this partitioning vary with location on the earth?
(2) What are the factors that govern the patterns of atmospheric circulation over the earth?
(3) What are the key variables that describe atmospheric water vapor and how are they connected?
(4) What causes precipitation to form and what are the factors that govern the rate of precipitation?
(5) How is precipitation measured and described?
(Some slides in this presentation were prepared by Venkatesh Merwade)
Heating of earth surface• Heating of earth
surface is uneven– Solar radiation strikes
perpendicularly near the equator (270 W/m2)
– Solar radiation strikes at an oblique angle near the poles (90 W/m2)
• Emitted radiation is more uniform than incoming radiation
Amount of energy transferred from equator to the poles is approximately 4 x 109 MW
Hadley circulation
Warm air rises, cool air descends creating two huge convective cells.
Coriolis ForceCone is moving southward towards the pole
Camera fixed in the outer space (cone appears moving straight)
Camera fixed on to the globe (looking southward, cone appears deflecting to the right)
the force that deflects the path of the wind on account of earth rotation is called Coriolis force. The path of the wind is deflected to the right in the Northern Hemisphere and the to left in the Southern Hemisphere.
Atmospheric circulation
1. Tropical Easterlies/Trades
2. Westerlies
3. Polar easterlies
1. Intertropical convergence zone (ITCZ)/Doldrums
2. Horse latitudes
3. Subpolar low
4. Polar high
Ferrel Cell
Polar Cell 1. Hadley cell
2. Ferrel Cell
3. Polar cell
Latitudes
Winds
Circulation cells
Effect of land mass distribution
A) Idealized winds generated by pressure gradient and Coriolis Force. B) Actual wind patterns owing to land mass distribution
Uneven distribution of land and ocean, coupled with different thermal properties creates spatial variation in atmospheric circulation
Shifting in Intertropical Convergence Zone (ITCZ)
Owing to the tilt of the Earth's axis in orbit, the ITCZ shifts north and south.
Southward shift in January
Northward shift in July
Creates wet Summers (Monsoons) and dry winters, especially in India and SE Asia
Questions for today
(1) How is net radiation to the earth’s surface partitioned into latent heat, sensible heat and ground heat flux and how does this partitioning vary with location on the earth?
(2) What are the factors that govern the patterns of atmospheric circulation over the earth?
(3) What are the key variables that describe atmospheric water vapor and how are they connected?
(4) What causes precipitation to form and what are the factors that govern the rate of precipitation?
(5) How is precipitation measured and described?
(Some slides in this presentation were prepared by Venkatesh Merwade)
Structure of atmosphere
Atmospheric water
• Atmospheric water exists – Mostly as gas or water vapor– Liquid in rainfall and water droplets in clouds– Solid in snowfall and in hail storms
• Accounts for less than 1/100,000 part of total water, but plays a major role in the hydrologic cycle
Water vaporSuppose we have an elementary volume of atmosphere dV and
we want quantify how much water vapor it contains
Atmospheric gases:Nitrogen – 78.1%Oxygen – 20.9%Other gases ~ 1%
http://www.bambooweb.com/articles/e/a/Earth's_atmosphere.html
dV
ma = mass of moist airmv = mass of water vapor
dV
mvv Water vapor density
dV
maa Air density
Specific Humidity, qv
• Specific humidity measures the mass of water vapor per unit mass of moist air
• It is dimensionlessa
vvq
Vapor pressure, e
• Vapor pressure, e, is the pressure that water vapor exerts on a surface
• Air pressure, p, is the total pressure that air makes on a surface
• Ideal gas law relates pressure to absolute temperature T, Rv is the gas constant for water vapor
• 0.622 is ratio of mol. wt. of water vapor to avg mol. wt. of dry air
TRe vv
p
eqv 622.0
Dalton’s Law of Partial PressuresJohn Dalton studied the effect of gases in a mixture. He observed that the Total Pressure of a gas mixture was the sum of the Partial Pressure of each gas.
P total = P1 + P2 + P3 + .......Pn
The Partial Pressure is defined as the pressure of a single gas in the mixture as if that gas alone occupied the container. In other words, Dalton maintained that since there was an enormous amount of space between the gas molecules within the mixture that the gas molecules did not have any influence on the motion of other gas molecules, therefore the pressure of a gas sample would be the same whether it was the only gas in the container or if it were among other gases.
http://members.aol.com/profchm/dalton.html
Avogadro’s lawEqual volumes of gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties. This number (Avogadro's number) is 6.023 X 1023 in 22.41 L for all gases.
Dry airWater vapor
Dry air ( z = x+y molecules)
Moist air (x dry and y water vapor)
d = (x+y) * Md/Volume m = (x* Md + y*Mv)/Volume
m < d, which means moist air is lighter than dry air!
Saturation vapor pressure, es
Saturation vapor pressure occurs when air is holding all the water vaporthat it can at a given air temperature
T
Tes 3.237
27.17exp611
Vapor pressure is measured in Pascals (Pa), where 1 Pa = 1 N/m2
1 kPa = 1000 Pa
Relative humidity, Rh
es
e
sh e
eR Relative humidity measures the percent
of the saturation water content of the airthat it currently holds (0 – 100%)
Dewpoint Temperature, Td
e
Dewpoint temperature is the air temperatureat which the air would be saturated with its current vapor content
TTd
Water vapor in an air column
• We have three equations describing column:– Hydrostatic air pressure,
dp/dz = -ag– Lapse rate of temperature,
dT/dz = - – Ideal gas law, p = aRaT
• Combine them and integrate over column to get pressure variation elevation
Column
Element, dz
aRg
T
Tpp
/
1
212
1
2
Precipitable Water
• In an element dz, the mass of water vapor is dmp
• Integrate over the whole atmospheric column to get precipitable water,mp
• mp/A gives precipitable water per unit area in kg/m2
Column
Element, dz
1
2
Adzqdm avp
Area = A
Precipitable Water, Jan 2003
Precipitable Water, July 2003
January
July
Questions for today
(1) How is net radiation to the earth’s surface partitioned into latent heat, sensible heat and ground heat flux and how does this partitioning vary with location on the earth?
(2) What are the factors that govern the patterns of atmospheric circulation over the earth?
(3) What are the key variables that describe atmospheric water vapor and how are they connected?
(4) What causes precipitation to form and what are the factors that govern the rate of precipitation?
(5) How is precipitation measured and described?
(Some slides in this presentation were prepared by Venkatesh Merwade)
Precipitation
• Precipitation: water falling from the atmosphere to the earth.– Rainfall– Snowfall– Hail, sleet
• Requires lifting of air mass so that it cools and condenses.
Mechanisms for air lifting
1. Frontal lifting
2. Orographic lifting
3. Convective lifting
Definitions
• Air mass : A large body of air with similar temperature and moisture characteristics over its horizontal extent.
• Front: Boundary between contrasting air masses.
• Cold front: Leading edge of the cold air when it is advancing towards warm air.
• Warm front: leading edge of the warm air when advancing towards cold air.
Frontal Lifting• Boundary between air masses with different properties is
called a front• Cold front occurs when cold air advances towards warm air• Warm front occurs when warm air overrides cold air
Cold front (produces cumulus cloud)
Cold front (produces stratus cloud)
Orographic liftingOrographic uplift occurs when air is forced to rise because of the physical presence of elevated land.
Convective lifting
Hot earth surface
Convective precipitation occurs when the air near the Convective precipitation occurs when the air near the ground is heated by the earth’s warm surface. This warm ground is heated by the earth’s warm surface. This warm air rises, cools and creates precipitation. air rises, cools and creates precipitation.
Condensation
• Condensation is the change of water vapor into a liquid. For condensation to occur, the air must be at or near saturation in the presence of condensation nuclei.
• Condensation nuclei are small particles or aerosol upon which water vapor attaches to initiate condensation. Dust particulates, sea salt, sulfur and nitrogen oxide aerosols serve as common condensation nuclei.
• Size of aerosols range from 10-3 to 10 m.
Precipitation formation• Lifting cools air masses
so moisture condenses• Condensation nuclei
– Aerosols – water molecules
attach• Rising & growing
– 0.5 cm/s sufficient to carry 10 m droplet
– Critical size (~0.1 mm)
– Gravity overcomes and drop falls
Forces acting on rain drop
FdFd
Fb
Fg
D• Three forces acting on rain drop– Gravity force due to weight– Buoyancy force due to
displacement of air– Drag force due to friction
with surrounding air3
6DVolume
2
4DArea
3
6DgF wg
3
6DgF ab
242
22
2 VDC
VACF adadd
Terminal Velocity• Terminal velocity: velocity at which the forces acting on the raindrop
are in equilibrium.• If released from rest, the raindrop will accelerate until it reaches its
terminal velocity
32
23
6246
0
DgV
DCDg
WFFF
wada
DBvert
332
2
6624DgDg
VDC
WFF
wat
ad
BD
1
34
a
w
dt C
gDV
• Raindrops are spherical up to a diameter of 1 mm• For tiny drops up to 0.1 mm diameter, the drag force is specified by
Stokes law
FdFd
Fb
Fg
D
V
Re
24dCa
aVD
Re
At standard atmospheric pressure (101.3 kpa) and temperature (20oC), w = 998 kg/m3 and a = 1.20 kg/m3
Precipitation Variation
• Influenced by – Atmospheric circulation and local factors
• Higher near coastlines
• Seasonal variation – annual oscillations in some places
• Variables in mountainous areas
• Increases in plains areas
• More uniform in Eastern US than in West
Rainfall patterns in the US
Global precipitation pattern
Spatial Representation• Isohyet – contour of constant rainfall• Isohyetal maps are prepared by
interpolating rainfall data at gaged points.
Austin, May 1981 Wellsboro, PA 1889
Texas Rainfall Maps
Temporal Representation
• Rainfall hyetograph – plot of rainfall depth or intensity as a function of time
• Cumulative rainfall hyetograph or rainfall mass curve – plot of summation of rainfall increments as a function of time
• Rainfall intensity – depth of rainfall per unit time
Rainfall Depth and IntensityTime (min) Rainfall (in) Cumulative 30 min 1 h 2 h
Rainfall (in)0 05 0.02 0.0210 0.34 0.3615 0.1 0.4620 0.04 0.525 0.19 0.6930 0.48 1.17 1.1735 0.5 1.67 1.6540 0.5 2.17 1.8145 0.51 2.68 2.2250 0.16 2.84 2.3455 0.31 3.15 2.4660 0.66 3.81 2.64 3.8165 0.36 4.17 2.5 4.1570 0.39 4.56 2.39 4.275 0.36 4.92 2.24 4.4680 0.54 5.46 2.62 4.9685 0.76 6.22 3.07 5.5390 0.51 6.73 2.92 5.5695 0.44 7.17 3 5.5100 0.25 7.42 2.86 5.25105 0.25 7.67 2.75 4.99110 0.22 7.89 2.43 5.05115 0.15 8.04 1.82 4.89120 0.09 8.13 1.4 4.32 8.13125 0.09 8.22 1.05 4.05 8.2130 0.12 8.34 0.92 3.78 7.98135 0.03 8.37 0.7 3.45 7.91140 0.01 8.38 0.49 2.92 7.88145 0.02 8.4 0.36 2.18 7.71150 0.01 8.41 0.28 1.68 7.24Max. Depth 0.76 3.07 5.56 8.2Max. Intensity 9.12364946 6.14 5.56 4.1
Running Totals
Incremental Rainfall
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150
Time (min)
Incr
emen
tal
Rai
nfa
ll (
in p
er 5
min
)
Rainfall Hyetograph
Cumulative Rainfall
0
1
2
3
4
5
6
7
8
9
10
0 30 60 90 120 150
Time (min.)
Cu
mu
lati
ve R
ain
fall
(in
.)
30 min
1 hr
2 hr
3.07 in
5.56 in
8.2 in
Rainfall Mass Curve
Arithmetic Mean Method• Simplest method for determining areal average
P1
P2
P3
P1 = 10 mm
P2 = 20 mm
P3 = 30 mm
• Gages must be uniformly distributed• Gage measurements should not vary greatly about the mean
N
iiPN
P1
1
mmP 203
302010
Thiessen polygon method
P1
P2
P3
A1
A2
A3
• Any point in the watershed receives the same amount of rainfall as that at the nearest gage
• Rainfall recorded at a gage can be applied to any point at a distance halfway to the next station in any direction
• Steps in Thiessen polygon method1. Draw lines joining adjacent gages
2. Draw perpendicular bisectors to the lines created in step 1
3. Extend the lines created in step 2 in both directions to form representative areas for gages
4. Compute representative area for each gage
5. Compute the areal average using the following formula
N
iiiPAA
P1
1
P1 = 10 mm, A1 = 12 Km2
P2 = 20 mm, A2 = 15 Km2
P3 = 30 mm, A3 = 20 km2
mmP 7.2047
302020151012
Isohyetal method
P1
P2
P3
10
20
30
• Steps– Construct isohyets (rainfall
contours)– Compute area between each
pair of adjacent isohyets (Ai)– Compute average
precipitation for each pair of adjacent isohyets (pi)
– Compute areal average using the following formula
M
iii pAP
1
A1=5 , p1 = 5
A2=18 , p2 =
15
A3=12 , p3 =
25
A4=12 , p3 = 35
mmP 6.2147
35122512151855
N
iiiPAA
P1
1
Inverse distance weighting
P1=10
P2= 20
P3=30
• Prediction at a point is more influenced by nearby measurements than that by distant measurements
• The prediction at an ungaged point is inversely proportional to the distance to the measurement points
• Steps– Compute distance (di) from ungaged
point to all measurement points.
– Compute the precipitation at the ungaged point using the following formula
N
i i
N
i i
i
d
d
P
P
12
12
1ˆ
d1=25
d2=15
d3=10
mmP 24.25
101
151
251
10
30
15
20
25
10
ˆ
222
222
p
221
22112 yyxxd
Rainfall interpolation in GIS
• Data are generally available as points with precipitation stored in attribute table.
Rainfall maps in GIS
Nearest Neighbor “Thiessen” Polygon Interpolation
Spline Interpolation
NEXRAD
NEXRAD Tower
• NEXt generation RADar: is a doppler radar used for obtaining weather information
• A signal is emitted from the radar which returns after striking a rainfall drop
• Returned signals from the radar are analyzed to compute the rainfall intensity and integrated over time to get the precipitation
Working of NEXRAD
NEXRAD data
• NCDC data (JAVA viewer)– http://www.ncdc.noaa.gov/oa/radar/jnx/
• West Gulf River Forecast Center– http://www.srh.noaa.gov/wgrfc/
• National Weather Service Animation– http://weather.noaa.gov/radar/mosaic.loop/DS.p19r0/ar.us.conus.shtml