Lecture 8: Precipitation and Evaporation

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Lecture 8 Precipitation and Evaporation

• Reading Applied Hydrology Sec 3.5-3.6 on
  evaporation and evapotranspiration

2
NARR Energy Budget Revisited

• Net radiation, Rn, at the land surface is the
  energy provided by the difference between
  incoming and outgoing radiation
• This energy is consumed by snowmelt, and ground,
  sensible and latent heat fluxes

Rn DnSW DnLW UpSW - UpLw
LE
Rn LE SH G SPH 0
SH
G
SPH
Melting of snow cover
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Precipitation

• Precipitation requires air mass lifting. This
  can occur in 3 ways
• Convective cells
• Fronts
• Orographic (mountains)
• Or by a combination of the above

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Rising Air Cools

• As air rises, it cools adiabatically that is it
  does not exchange heat with its surroundings
  (they are cooling similarly)
• Lapse rate ranges
• 6C/km Wet
• 10C/km Dry

http//ess.geology.ufl.edu/ess/Notes/AtmosphericCi
rculation/lapserate.jpeg
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Water droplets form in clouds
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Water drops fall with a terminal velocity
Fg weight of drop Fb bouyancy force on
drop Fd drag force of air on drop At
equilibrium Fg Fb Fd 0
Generally D lt 3mm
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Terminal Velocity
8
Thunderstorms can be analyzed using a control
volume model
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Spatial Variability of Precipitation
3 hour forecast precipitation, July 2003
This is a very complex subject that the NARR does
a better job of depicting than books and maps.
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Forms of Precipitation
Liquid (Rainfall)
Precipitation
Snow (vapor condensed to ice)
Solid (Ice)
Hail (water condensed to ice)
We are going to concentrate on rainfall and not
worry about snowfall, snowmelt, etc (which are
very important subjects in the North and West of
the US)
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Rainfall Hyetograph

• The graph of rainfall vs time at a point is
  called the rainfall hyetograph

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Extreme Rainfall
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Evaporation

• Evaporation happens in several ways
• Open water evaporation
• Transpiration from leaves
• Evaporation from soil and land surface

Evapotranspiration
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Factors affecting evaporation

• Heat energy to supply latent heat of vaporization
  (Net radiation, air temperature)
• Capacity to transport vapor away from evaporative
  surface (wind, humidity)
• Water available to supply evaporative moisture
  (soil water content)
• Potential evaporation is evaporation when water
  supply is not limiting

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Evaporation Pan
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Effect of Vapor Transport