3 REVIEW Q* - positive in daytime - almost always negative at night Any Q* imbalance is accounted for byconvective exchange or conductionQ* = QH + QE + QG + Swhere QH = sensible heat fluxQE = latent heat fluxQG = conduction to or from ground
4 Recall the First Law of Thermodynamics ENERGY IN = ENERGY OUTQin > Qout (flux convergence)Net storage gain leads to warmingQout > Qin (flux divergence)Net storage energy loss leads to coolingQin = QoutNo net change in energy storage
6 DAYTIME:Both sides of equation are positive:surface radiative surplusSurplus partitioned into ground and atmosphereConvection is the most important means ofdaytime heat transport from surfaceQE is greater when soil moisture is highQH is greater when water is more restricted
7 NIGHT:Both sides of equation are negative:surface radiative deficitDeficit partitioned into heat gain from groundand atmosphereQ* loss is partially replenished by QGQE and QH of less importance as convectiveexchange is dampened by the night-timetemperature stratification
8 Temperature change resulting from QG depends on: Amount of heat absorbed orreleased2. Thermal properties of the soilHeat capacity, C, in Jm-3K-1Specific heat, c, in Jkg-1K-1QS/ z = Cs Ts/ t(change in heat flux in a soil volume)
10 Sample QuestionIt 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.
11 Exchange in Boundary Layers Sub-surface LayerLaminar Boundary LayerRoughness LayerTurbulent Surface LayerOuter LayerThe first half of this course is concerned withenergy exchange in the roughness layer, turbulentsurface layer and outer layer
12 Sub-surface layerHeat flows from an area of hightemperature to an area of low temperatureQG = -HsCS T/zHs is the soil thermal diffusivity (m2s-1)(Hs and CS refer to the ability to transferheat energy)
16 2. Laminar Boundary Layer Thin skin of air within which all non-radiative transfer is by molecular diffusionHeat FluxQH = -cpHa T/z = -CaHa T/zWater Vapour FluxE = - Va v/zgradients are steep because is small
22 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 thesurface temperature wave to penetrate upward in the air.
23 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-1Environmental Lapse Rate (ELR)A measure of the actual temperature structureIf ELR> , the atmosphere is unstableIf ELR< , the atmosphere is stableIf ELR= , the atmosphere is neutralCan you think of conditions likely to support each of thesethree cases ?
25 Moist adiabatic lapse rate: The rate at which moist ascending air cools byexpansion m typically about 6C/1000mVaries: 4C/1000m in warm airnear 10C/1000m in cold airLatent heat of condensation liberated as parcel rises
26 Conditionally unstable conditions ELR > Rising parcel of air remains warmer and less dense than surrounding atmosphereStable conditionsELR < mRising parcel of air becomes cooler and denser than surrounding air, eliminating the upward movementConditionally unstable conditions >ELR> m
27 ELR = DALR = Lifted parcel is theoretically cooler than air around itafter liftingELR = DALR = Source:
28 ELR = DALR = Lifted parcel is theoretically warmer than air after liftingELR = DALR =
29 Lifted parcelis the sametemperature asair after liftingNote: Conditionally-unstable conditionsoccur for m < < d