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

qq

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

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1

wv

nr l

RE

Effect of Vapor Transport

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