lecture 8: precipitation and evaporation reading: applied hydrology sec 3.5-3.6 on evaporation and...
TRANSCRIPT
Lecture 8: Precipitation and Evaporation
• Reading: Applied Hydrology Sec 3.5-3.6 on evaporation and evapotranspiration
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
UpSW
UpLWDnSW
DnLW
Rn = DnSW + DnLW – UpSW - UpLw
SH
LE
SPHMelting of snow coverG
Rn + LE + SH + G + SPH = 0
Precipitation
• Precipitation requires air mass lifting. This can occur in 3 ways:– Convective cells– Fronts– Orographic
(mountains)– Or by a combination of
the above
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:– ~6°C/km Wet– ~10°C/km Dry
http://ess.geology.ufl.edu/ess/Notes/AtmosphericCirculation/lapserate.jpeg
Water droplets form in clouds
Water drops fall with a terminal velocity
Generally D < 3mm
Fg = weight of drop
Fb = bouyancy force on drop
Fd = drag force of air on drop
At equilibrium Fg – Fb – Fd = 0
Terminal Velocity
1
3
4
a
w
dt C
gDV
Thunderstorms can be analyzed using a control volume model
2
211
1
4 11
v
vv
w
a
q
D
zVi
Spatial Variability of Precipitation
3 hour forecast precipitation, July 2003
This is a very complex subject that theNARR does a better job of depicting thanbooks and maps.
Forms of Precipitation
Precipitation
Liquid (Rainfall)
Solid (Ice)
Snow (vapor condensedto ice)
Hail (water condensedto ice)
We are going to concentrate on rainfall and not worry aboutsnowfall, snowmelt, etc (which are very important subjects in theNorth and West of the US)
Rainfall Hyetograph
• The graph of rainfall vs time at a point is called the rainfall hyetograph
Extreme Rainfall
Evaporation
• Evaporation happens in several ways– Open water
evaporation– Transpiration from
leaves– Evaporation from soil
and land surface
Evapotranspiration
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
Evaporation Pan
vvsn mlGHR
GHRl
E snwv
1
wv
nr l
RE
Effect of Vapor Transport
aasa eeBE