Chapter 5: surface energy fluxes Flashcards
Radiative Fluxes
- Fs(down arrow): MAGNITUDE of the downward solar (shortwave) radiation that reaches the surface, integrated over all wavelengths in and near the visible spectrum
- Fs(up arrow): MAGNITUDE of the sunlight reflected back up by the surface
- FL(down arrow): MAGNITUDE of the longwave radiation emitted by the atmosphere that reaches the earths surface
- FL(up arrow): MAGNITUDE of the longwave radiation the surface emits upwards
Net radiation flux
(F*)
- the sum of the inputs to the surface minus the output
- Algebraic sum is nearly constant
- slightly negative during the night
- become positive with peak near solar noon (daytime)
- positive means input to the surface
𝐹∗ = 𝐹s ↓ −𝐹s ↑ +𝐹L ↓ −𝐹L ↑
In addition to the radiative fluxes at the Earth’s surface,…….. needs to be taken into account
the fluxes of sensible and latent heat
sensible heat flux
heats air in the BL directly
latent heat flux
the flux of water vapor times L, the latent heat of vaporization) is not converted to sensible heat and or potential energy until the water vapor condenses in clouds.
If we imagine the land surface as an infinitesimally thin surface that has…………… then ………………..
zero heat capacity
the heat flux coming in must balance the heat leaving
give the net radiation after concidering latent and sensible heat flux
energy gain or loss is partitioned among sensible heat flux, FHs, into the air (positive upward, for flux away from the surface),
- latent heat flux FEs into the air (positive upward), and
- the conduction of heat down into the ground, FGs (positive downward, away from the surface), where the extra subscript s denotes near the surface. Thus,
𝐹∗ = FHS + FES + FGS
Breifly describe the cases and fluxes
- case A: Daytime over moist vegetated surface
- case B: Nighttime over a moist vegetated surface
- case C: Daytime over a dry desert
- case D: oasis effect during the daytime
describe the cases
- case A: Daytime over moist vegetated surface
(sun’s energy goes to evaporation FES)- Positive
- FHS and FES because heat and moisture transport upward from the surface
- FGS because heat is conducted downward into ground from warm surface
- Positive
- case B: Nighttime over a moist vegetated surface
(F* often -ve due to net upward LW radiation cooling the space)- Negative
- FHS: due to downward heat flux from air
- FES: if dew or frost from
- FGS: conduction of heat from the warm ground up to the cool surface
- Negative
- case C: Daytime over a dry desert
- most of the sun’senergy goes into sensible heat flux (FHS)
- case D: oasis effect during the daytime
- during windy conditions if dry hot air is advected over a cool moist surface such as at a desert oasis
- F* and FHS combine to create very large evaporation and associated latent heat flux
- FHS can be downward from the warm air to the cool surface
- F*: solar heating of the surface
- F* and FHS combine to create very large evaporation and associated latent heat flux
- during windy conditions if dry hot air is advected over a cool moist surface such as at a desert oasis
Explain the change of flux magnitude and diurnal cycle of surface skin temperature change
The magnitude of flux into the ground, 𝐹GS , is ~10% of the net radiation magnitude during daytime, increasing to ~ 50% at night.
The amplitude of the diurnal cycle of surface skin temperature, Ts, is inversely proportional to the conductivity of the soil.
The sensible heat flux
- parametarized by the temperature difference between the surface and air
- if the surface skin temperature is known the sensible heat flux (in kinematic units of K/ms) from the ground to the air can be parametarized
- to convert from kinamatic to dynamic (W/m2) multiply by air density times the specific heat at constant pressure (pCp)
If the surface skin temperature, Ts, is known, then the sensible heat flux (inkinematic units of K ms-1) from the ground to the air can be parameterized as:
CH range
- stable conditions: decrease towards zero
- (CHN)neutral: 0.001 - 0.005
- unstable: 2 to 3 times as large as CHN
Latent Heat Flux
The latent heat flux (in kinematic units of K ms-1) at the surface is directly related to moisture flux (𝐹w) :
The moisture flux from the surface is calculated as:
Bowen Ratio
The ratio of sensible to latent heat fluxes at the surface is called the Bowen ratio:
B=FHS/FES
Bowen ration over land and ocean
- ocean
- decrease with increasing sea surface temp
- ice edge: 1 - 0.5
- tropical oceans: 0.1
- latent heat flux dominate due to the warmth of the sea surface
- decrease with increasing sea surface temp
- land
- bowen ratio (and the evaporation rate) depend on
- availability of water in soil
- the makeup of vegetation
- from soil: Osmosis
- release WV into air: transpiration through pores of leaves
- tropical oceans: 0.1
- irrigated crops: 0.2
- grassland:0.5
- semiarid regions: 5
- deserts: 10
- bowen ratio (and the evaporation rate) depend on
Soil Heat Flux
aka ground flux
represent heat flux into the ground measured at the top of the soil
- small but not insignificant in surface energy budget
- related to surface skin temperature
- what the atmosphere “sees” when it radiatively looks down at the bottom boundary.
- not measured directly
- need to parametarize for PBL forecast models
Simple Parameterizations
Averaged over a full 24-hour cycle, the net heat flux is often near zero
- little net change (assumed 0 in GCM) of heat in soil due to
- heating of the ground during the day is nearly balanced by cooling at night
How to calculate ground flux
Assuming that the ground flux is a percentage, X, of the net radiation:
FGS=XF*
where X= 0.1 during the daytime and X=0.5 at night
to assume the ground flux is a percentage of the turbulent sensible heat flux into the air
FGS=0.3 FHS
This method fails in an oasis situation
similarity in both equations that calculate ground flux
Both assume that the sign of the ground flux is always the same as that of the net radiation or sensible heat flux.
