ASPECT EFFECTS

Photosynthetically-active radiation(spectral portion,0.3-0.4 CI)

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Figure 5.4 Top-of-atmosphere solar radiation on plane surfaces of 45 degree slope at 2.3°N, 77.0ºW, on Julian Day 352.

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Radiation receipt varies considerably inany mountainous environment

TOA K onA 45° slopeOn Julian Day352, 2.3N

Longwave Radiative Exchange

The atmosphere absorbs long-wave radiation (L) from the Earth, clouds and gases at all altitudes

Absorption greatest in lower portion of the atmosphere,where H20 and CO2 concentrations are highest

The atmosphere absorbs effectively from 3-100 m,except in the atmospheric window (8-11 m)

Most longwave loss to space occurs through this window, but clouds can partially close it

L = 0 (T0)4 + (1 - 0) L

Amount of L reflected(slight adjustment)

L is greater in magnitude and more variable than L

L* = L - L (usually negative)

NET ALL_WAVE RADIATION

DAYTIME: Q* = K - K + L - LQ* = K* + L*

NIGHT: Q* = L*

Radiation Measurements

L

L K

K (not visible)

UV-A

PAR

More radiation sensors…

Source: University of Colorado

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K in tropical forests of Colombia/Ecuador

Radiation Balance Components

Negative in Oke

Clouds

Reduce K because of absorption and reflectionfrom cloud tops (may eliminate S)

Increase D by scattering incoming solar radiation

Strongest K under partly cloudy skies with sunin clear patch

Absorb much of L and re-emit it as L(low cloud emits more)

Reduce diurnal temperature variation

Source: NOAA

GlobalEnergyBalance(SIMPLIFIED)

Q* - positive in daytime- almost always negative at night

Any Q* imbalance is accounted for byconvective exchange or conduction

Q* = QH + QE + QG + S

where QH = sensible heat fluxQE = latent heat fluxQG = conduction to or from ground

(See Figure 1.10)

Conduction, Convection and Advection

Conduction is the process through which heat is diffused to cooler materials as radiation is absorbed. Land surfaces heat quickly,while water bodies can mix and have higher heat capacity. Solids(land) are better conductors than gases (atmosphere).

Convection is physical mixing with a strong vertical motion ingaseous or liquid media. As heat is absorbed by the ground, the airimmediately above is heated. Warm air is less dense and, thus,rises, while cooler air falls.

Advection is the term used to describe lateral heat transfers. Windscarry heat from absorbed radiation from one area to another.

Recall the First Law of Thermodynamics

ENERGY IN = ENERGY OUT

Qin > Qout (flux convergence)Net storage gain leads to warming

Qout > Qin (flux divergence)Net storage energy loss leads to cooling

Qin = Qout

No net change in energy storage

Water: H2O•High heat capacity•Exists in all states at Earth’s temperatures

•Heat required/released during phase changes:

Latent heat of fusion (Lf = 0.334 MJ kg-1)Latent heat of vaporization (Lv = 2.45 MJ kg-1)Latent heat of sublimation (Ls = Lf + Lv)

Water Balancep = E + r + s

Where p is precipitationE is evapotranspirationr is net runoffs is soil moisture storage content

QE = Lv EQM = Lf M

Where E and M are in kg m-2 s-1See Fig. 1.13

Sensible and Latent Heat Fluxes

Eddy correlation (later)

•Sonic anemometer measurements of vertical velocity and temperature

•Krypton hygrometer measurements of water vapour density

Advection and Winds

Air flow at local scale can affect energy balanceas can air flow at scales larger than boundary layer

At the micro-scale, horizontal temperature variation causes horizontal pressure differences

Why ? Warm air is lighter than cold air

This leads to winds (kinetic energy)

Energy transferred to smaller and smaller scales before being dissipated as heat

DAYTIME:

• Both sides of equation are positive: surface radiative surplus

• Surplus partitioned into ground and atmosphere

Convection is the most important means of daytime heat transport from surface

QE is greater when soil moisture is highQH is greater when water is more restricted

NIGHT:

• Both sides of equation are negative:surface radiative deficit

• Deficit partitioned into heat gain from groundand atmosphere

Q* loss is partially replenished by QG

QE and QH of less importance as convectiveexchange is dampened by the night-timetemperature stratification