Atmospheric Analysis Lecture 2

REVIEW Q* - positive in daytime - almost always negative at night Any Q* imbalance is accounted for by convective exchange or conduction Q* = QH + QE + QG + S where QH = sensible heat flux QE = latent heat flux QG = conduction to or from ground

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 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 high QH is greater when water is more restricted

NIGHT: Both sides of equation are negative: surface radiative deficit Deficit partitioned into heat gain from ground and atmosphere Q* loss is partially replenished by QG QE and QH of less importance as convective exchange is dampened by the night-time temperature stratification

Temperature change resulting from QG depends on: Amount of heat absorbed or released 2. Thermal properties of the soil Heat capacity, C, in Jm-3K-1 Specific heat, c, in Jkg-1K-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 by 2.3 ºC. Calculate the energy flux density that went into storage via soil heating.

Exchange in Boundary Layers Sub-surface Layer Laminar Boundary Layer Roughness Layer Turbulent Surface Layer Outer Layer The first half of this course is concerned with energy exchange in the roughness layer, turbulent surface layer and outer layer

Sub-surface layer Heat flows from an area of high temperature to an area of low temperature QG = -HsCS T/z Hs is the soil thermal diffusivity (m2s-1) (Hs and CS refer to the ability to transfer heat energy)

2. Laminar Boundary Layer Thin skin of air within which all non- radiative transfer is by molecular diffusion Heat Flux QH = -cpHa  T/z = -CaHa  T/z Water Vapour Flux E = - Va  v/z gradients are steep because  is small

Roughness Layer Surface roughness elements cause eddies and vortices (more later) Turbulent Surface Layer Small scale turbulence dominates energy transfer (“constant flux layer”)

(LV is the “latent heat of vaporization”) Heat Flux QH = -CaKH  T/z (KH is “eddy conductivity,” m2s-1) Water Vapour Flux E = -KV  v/ z Latent Heat Flux QE = -LVKV  v/ z (LV is the “latent heat of vaporization”)

Outer Layer The remaining 90% of the planetary boundary 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 height near 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 compression with a change in altitude -9.8 x 10-3 ºCm-1 Environmental Lapse Rate (ELR) A measure of the actual temperature structure If ELR> , the atmosphere is unstable If ELR< , the atmosphere is stable If ELR= , the atmosphere is neutral Can you think of conditions likely to support each of these three cases ?

Moist adiabatic lapse rate: The rate at which moist ascending air cools by expansion  m typically about 6C/1000m Varies: 4C/1000m in warm air near 10C/1000m in cold air Latent heat of condensation liberated as parcel rises

Conditionally unstable conditions ELR >  Rising parcel of air remains warmer and less dense than surrounding atmosphere   Stable conditions ELR <  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 than air around it after lifting ELR =  DALR =  Source: http://www.atmos.ucla.edu

ELR =  DALR =  Lifted parcel is theoretically warmer than air after lifting ELR =  DALR = 

Lifted parcel is the same temperature as air after lifting Note: Conditionally-unstable conditions occur for m <  < d