repetition geo3020/4020 lecture 1: meteorological elements
TRANSCRIPT
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Repetition
GEO3020/4020
Lecture 1: Meteorological elements
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is determined by the energy and mass transport at the surface:
Weather
Meteorological variables are used to describe the weather and to calculate the components of the energy and water balance equation.
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• Precipitation• Radiation• Air temperature• Air humidity• Wind• Air pressure
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Meteorological variables
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Radiation
Why do we want to calculate the radiation budget at the land surface?
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Daily clear-sky solar radiation
• Important contributor to the energy balance at the earth surface;
• Difficult to measure;• Method for estimating it on a horisontal and slope
surface at an arbritary place is given in Appendix E.
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Summary
= Extraterrestrial Radiation on a horizontal plane
= Extraterrestrial Radiation on a sloping plane
= Total daily clear sky incident radiation on a horizontal
plane at the earth surface
= global short wave radiation at the earth surface
= backscattered radiation (= )
and
'ETK
ETK
'csK
'gK
''''' 5.0 ETETsdirdifg KKKKK
'bsK
'''bsgsc KKK
'5.0 gs K
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Empirical adjustments
• The total daily clear sky radiation flux at the surface , are derived from
• Empirical relationships (adjusting for the effect of clouds and vegetation) have been developed, e.g.
where global short wave radiation on the surface and , Extraterrestrial Radiation, n = actual sunshine hour and N= max sunshine hour (can be read from table for a given location and season).
'csK
21)-(E ...
'''
bsgcs KKK
'gK
N
nbaKK ETg
''
'ETK
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Relation between K, Kin, Kcs, KET
• KET = Extraterrestrial (potential) solar radiation• Kcs = clear sky short wave radiation flux on a horizontal surface on earth• Kin = adjusted Kcs for slope, aspect, clouds and vegetation• K = net flux of solar energy entering the surface, e.g. snowpack
• Normally K < Kin < Kcs < KETKET = extraterrestrial (potential) solar radiation
Kin = measured solar radiation
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Structure of the atmosphere– Composition– Vertical structure
• Pressure-temperature relation (Ideal gas law)• Adiabatic lapse rate (dry & wet)
Vapour – Vapour pressure, ea
– Sat. vapour pressure, ea*– Absolute humidity, ρv – Specific humidity, q = ρa/ρv – Relative humidity, Wa = ea/ea*– Dew point temperature, Td
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• Precipitation• Radiation• Air temperature• Air humidity• Wind• Air pressure
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Meteorological variablesMeasurements
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Lena M. Tallaksen
Chapter 5.4.2 & 7.3.4, Appendix D.4; Dingman
GEO3020/4020
Lecture 2: I. Energy balance
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Albedo is the
shortwave reflectance
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Shortwave radiation input, K Net solar radiation, K
K = Kin - Kout = Kin·(1-) (5-27)
• Where K is net flux of solar energy entering the body
• Kin – flux of solar energy incident on the surface (= global radiation)
• Kout – reflected flux
• – albedo, depends on the properties of the surface
K can be measured by pyranometers, but more common to estimate
Kcs represents the clear-sky shortwave radiation fluxKin, adjusted for the slope and aspect:
Λ, latitude, J day of year, , slope inclination angle, , slope azimuth angle
C, fraction of sky covered with cloudsF, fraction of sky obscured by forest canopy,
are functions
29)-(5 )1()()(),,(),( 321 FfCffJKK cs
inK
321 ,, fff
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Shortwave radiation input, K
• The function is derived using the concept of equivalent latitude,
• The function , the effect of cloud cover can be estimated using empirical relations like,
or
• The function effect of forest canopy, an example for pine,
• The albedo, , changes with location, season, vegetation, ….
,,1 f)2.( EAppendixAeq
Cf2
N
nbaCf 2 CbaCf 12
Ff3
FFf 91.3exp3
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Practical considerations
• Empirical equation for Shortwave radiation
C = fraction of sky covered with clouds
KCS = total daily clear sky incident radiation on a horizontal plane at
the earth surface
29)-(5 )-(1)1()()(),,(),( 321 incs KFfCffJKK
26)-(7 168.0355.0 csin KCK
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Longwave radiation exchange, L Net longwave radiation – equals to incoming atmospheric longwave radiation
minus the portion of that reflected and the radiation flux emitted by the surface
L = Lin – Lout = Lat – (1-s)Lat - Ls (7-28)
where:
Lat - incoming atmospheric longwave radiation flux,
Ls - outgoing radiation from the surface
s - emissivity of surface
L, Lat, Ls can be measured by pyranometers, but more common to estimate using equations like:
where:
at - emissivity of the atmosphere and canopy
s - emissivity of the surface
σ - Stefan-Boltzmann constant
Tat ,Ts – temperature of the atmosphere and water surface
(5.35) )2.273( 4 atatin TL
(5.36) )1()2.273( 4insssout LTL
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Longwave radiation exchange, L
Combine the two equations (5.35 and 5.36) we get:
where, Stefan-Boltzmann constant (4.90×10-9 MJ m-2 day-1 K-4)
Tat, effective radiating temp of atmosphere and canopy (˚C)Ts, temperature of surface (˚C)
29)-(7 )2.273()2.273( 44 ssatsat TTL
Table D-1 for s values, the question remains how to calculate at.
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Table d1
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Longwave radiation exchange, L
• Clear sky, no forest canopy
• Cloudy sky, no forest canopy
• Cloudy sky, forest canopy (general equation)
where ea is near surface saturation atmospheric vapor pressure
and Ta is air temperature in °C, F is the ratio of the horizontally projected area of forest canopy to the total area of interest, C is degree of cloud cover
38)-(5 2.273
72.17
1
a
aat T
e
39)-(5 )C0.22(1 2.273
72.1 27
1
a
aat T
e
40)-(5 (missing) F)C0.22(1 2.273
72.1)1( 27
1
a
aat T
eF
7)-(D 2.273
3.17exp611.0
71
a
aa T
Te
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Sensible heat, H
Sensible-heat exchange by turbulent transfer, H:
and from equation (D-49)
where
a = density of air;
Ca = heat capacity of air;
k = 0.4;
zd = zero plane displacement
height
52)-(D saaH TTvKH
50)-(D
ln
2
0
2
z
zz
kcK
da
aaH
z0 = surface-roughness height;
za = height above ground surface
at which va & Ta are measured;
va = windspeed,
Ta = air temperatures and
Ts = surface temperatures.
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Zveg
Zd
Z0
velocity
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Latent heat, LE
Latent heat exchange by turbulent transfer, LE
and from equation (D-42)
where
a = density of air;
λv = latent heat of vaporization;
P = atmospheric pressure
k = 0.4;
zd = zero plane displacement
height
45)-(D saaLE eevKLE
43)-(D
ln
622.02
0
2
zzz
k
PK
da
aVLE
z0 = surface-roughness height;
za = height above ground surface
at which va & ea are measured;
va = horisontal windspeed,
ea = air vapor pressure
es = surface vapor pressure (measured at z0 + zd)
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Exchange of (sensible) heat with the ground, G
• Positive or negative depending on the temperature of the air
and soil surface (often negligible compared to other terms).
• If soil temperature is increasing downward (due to thermal energy stored during the summer) heat is transferred upwards at a rate:
• kG is the thermal conductivity of the soil (E L-1 T-1 -1), depends on soil texture, soil density, and moisture content and vary widely with season and place.
48)-(5 dz
dTkG GG
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Water-advected energy, Aw (lakes)
Net water-advected energy Aw [E L-2 T-1] is found from:
where:
w density of water [M L-3]cw – specific heat of water [E M-1 θ-1]w – average precipitation rate [L T-1]SW and GW – surface water and ground water inflows and outflowsTs – temperatures of the respective inflows and outflows [θ]
out in out in gwoutgwinswoutswinawww TGWTGWTSWTSWTwcA
Heat input by rain, R (snowpack)
i.rainwater is first cooled to the snow temperature (Eq. 5-47a)ii.if the snow is below zero, then freezing may occur and latent heat realised (Eq. 5-47b)
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Change in stored energy, ΔQ
Energy can be stored in e.g. a snowpack, lake or soil
• Snow (warming phase)
• Lake
where: hm snow water equivalentci heat capacity of iceV lake volume, TL average lake temperature, AL lake area.subscripts 1 and 2 designate values at the beginning and end of t
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)32.7(1122 LLL
ww TVTVA
cQ
)19.5(12 TThcQ mwi
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Energy balance equation
29
0/ tQAGLEHLK w
where:
K net shortwave radiationL net longwave radiationLE latent heat transferH sensible heat transferG soil fluxAw advective energyΔQ/Δt change in stored energy
Units: [EL-2T-1]
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Calculation of evaporation using the energy balance method
Evaporation can be calculated solving for LE:
where:
22)-(7 /
vw
w tQAHGLKE
15)-(7 / tQAHGLKLE w
Latent Heat of Vaporization :v= 2.501 - (2.361 × 10-3) Ta
LE has units [EL-2T-1]
E [LT-1] = LE/ρwλv
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Lena M. Tallaksen
Chapter 7. – 7.1.2, 7.3.6; Dingman
GEO3020/4020
Lecture 2: II. Evapotranspiration- Definitions- Governing factors- Measurements
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Global water balance
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114
1255
1141
275 490
765
mm/year
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Water balance for Norway, 1931 -60
33Otnes & Ræstad, 1978
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Evaporation - Norway
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Global distribution of evaporation (cm)
35Encyclopedia Britannica Inc.
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Temporal variation – bare soil
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Temporal variation – Lake Ontario
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Evaporation and evapotranspirationSummary
– Evapotranspiration is a collective term for all the processes by which water in the liquid or solid phase at or near the earth’s surface (rivers and lakes, bare soils, and vegetative surfaces) becomes atmospheric water vapor.
– Evapotranspiration is a second largest term in the global water balance; about 62% of precipitation that falls on the continents is evapotranspired.
– Evapotranspiration is the term that links earth surface’s water balance and energy balance.
– It is much more difficult to measure evapotranspiration than to measure precipitation and streamflow.
– There are numerous methods/models available in calculating evapotranspiration, of which the most well-known methods will be discussed in the class.
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Governing factors of evaporation
I. Meteorological situation• Energy availability• How much water vapour can be received
– Temperature– Vapour pressure deficit– Wind speed and turbulence
Optimal conditions: ?
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Governing factors of evaporationII. Physiographic and plant characteristics• Characteristics that influence available energy
– albedo– heat capacity
• How easily can water be evaporated– size of the evaporating surface– surroundings– roughness (aerodynamic resistance)– salt content– stomata
• Water supply– free water surface (lake, ponds or intercepted water)– soil evaporation– transpiration
The wind speed immediately above the surface. • The humidity gradient away from the surface.
– The rate and quantity of water vapor entering into the atmosphere both become higher in drier air.
• Water availability. – Evapotranspiration cannot occur if water is not available.
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EvapotranspirationMeasurements
Free water evaporation- Pans and tanks- Evaporimeters
Evapotranspiration (includes vegetation)- Lysimeters- Remote sensing
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Definitions• Potential evapotranspiration, PE, is the rate at which
evapotranspiration would occur from a large area completely and uniformly covered with growing vegetation which has access to an unlimited supply of soil water and without advection or heat-storage effects (i.e. the rate is depedent on the vegetation)
• Actual evapotranspiration, ET, is the rate at which evapotranspiration occurs (i.e. describes all the processes by which liquid water at or near the land surface becomes atmospheric water vapor).
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Pan evaporation methods
Pan evaporationEpan = W – [V2-V1]
where
W = precipitation during t
V1 = the storage at the beginning of t
V2 = the storage at the end of t
For American Class-A pan, Kohler et al. (1955) developed an empirical equation to account for energy exchange through sides of a pan, and adjust daily pan evaporation, Epan, to free water evaporation, Efw [mm day-1] (Equations 7-41 and 7-42).
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Pan evaporation methods
Pan coefficient
Elake/Epan = kp
where
k is a coefficient that varies with seasons and lake.
Its annual average over the US is about 0.7
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Pan evaporation methods
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4646
Pan evaporation methods
Example of pan coefficient in the Yangtze River catchment in China
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Lysimeter
One of the most reliable way of measuring potential or actual evapotranspiration is to use large containers (sometimes on the order of several metres across) called lysimeters;
Evapotranspiration is calculated by subtraction considering the different components of the water balance.
A lysimeter is most accurate when vegetation is grown in a large set up which allows the rainfall input and water lost through the soil to be easily calculated from the difference between the weight before and after a given period.
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input (Rainfall R and Additional water A) and output (Percolated water P) collected in the receiver, then PE can be estimated from the equation:
PE = R + A – P
Lysimeter for measuring potential evapotranspiration
R A
P
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49Figure. Schematic of a weighable gravitation lysimeter.
Lysimeter for measuring actual evapotranspiration
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Estimation of evapotranspiration by remote sensing
Remote sensing has two potentially very important roles in estimating evapotranspiration (Engman, 1995).
First, remotely sensed measurements offer methods for extending point measurements or empirical relationships to much larger areas, including those areas where measured meteorological data may be sparse.
Secondly, remotely sensed measurements may be used to measure variables in the energy and moisture balance models of ET, such as as radiometric surface temperature, albedo, and vegetation index.