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

REVIEW

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

DAYTIME:

Both sides of equation are positive: surface radiative surplusSurplus 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 deficitDeficit 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

Temperature change resulting from QG

depends on:

1. Amount of heat absorbed or released

2. Thermal properties of the soilHeat capacity, C, in Jm-3K-1

Specific heat, c, in Jkg-1K-1

QS/ z = Cs Ts/ t(change in heat flux in a soil volume)

Sample Question

It is a hot, sunny day in the Sahara desert. Over a one hour period,the temperature of the top 0.1 m of the dry, sandy soil increased by2.3 ºC. Calculate the energy flux density that went into storage viasoil heating.

Exchange in Boundary Layers

1. Sub-surface Layer2. Laminar Boundary Layer3. Roughness Layer4. Turbulent Surface Layer5. Outer Layer

The first half of this course is concerned with energy exchange in the roughness layer, turbulentsurface layer and outer layer

1. Sub-surface layer

Heat flows from an area of hightemperature to an area of low temperature

QG = -HsCS T/z

Hs is the soil thermal diffusivity (m2s-1)(Hs and CS refer to the ability to transferheat energy)

2. Laminar Boundary Layer

Thin skin of air within which all non-radiative transfer is by molecular diffusion

Heat FluxQH = -cpHa T/z = -CaHa T/z

Water Vapour FluxE = - Va v/z

gradients are steep because is small

3. Roughness Layer

Surface roughness elements cause eddies and vortices (more later)

4. Turbulent Surface Layer

Small scale turbulence dominates energytransfer (“constant flux layer”)

Heat FluxQH = -CaKH T/z(KH is “eddy conductivity,” m2s-1)

Water Vapour FluxE = -KV v/ z

Latent Heat FluxQE = -LVKV v/ z(LV is the “latent heat of vaporization”)

5. Outer Layer

The remaining 90% of the planetaryboundary layer

FREE, rather than FORCED convection

Mixed layer

convective entrainment

Lapse Profile

DAYTIME: temperature usually decreases with height*negative gradient (T/ z)

NIGHT: temperature usually increases with heightnear the surface “temperature inversion”

*There are some exceptions and there is a lag time for the surface temperature wave to penetrate upward in the air.

Dry Adiabatic Lapse Rate ()A parcel of air cools by expansion or warms by compressionwith a change in altitude-9.8 x 10-3 ºCm-1

Environmental Lapse Rate (ELR)A measure of the actual temperature structure

If ELR> , the atmosphere is unstableIf ELR< , the atmosphere is stableIf ELR= , the atmosphere is neutral

Can you think of conditions likely to support each of thesethree cases ?

Moist adiabatic lapse rate:

The rate at which moist ascending air cools by expansion

m typically about 6C/1000m

Varies: 4C/1000m in warm airnear 10C/1000m in cold air

Latent heat of condensation liberated as parcel rises

Unstable conditionsELR > Rising parcel of air remains warmer and less dense than surrounding atmosphere 

Stable conditionsELR < m

Rising parcel of air becomes cooler and denser than surrounding air, eliminating the upward movement 

Conditionally unstable conditions >ELR> m

ELR = DALR =

Lifted parcel is theoretically cooler thanair around itafter lifting

Source: http://www.atmos.ucla.edu

ELR = DALR =

Lifted parcel is theoretically warmer thanair after lifting

Lifted parcel is the same temperature asair after lifting

Note: Conditionally-unstable conditions occur for m < < d