Chapter 5: surface energy fluxes

Question 1

Radiative Fluxes

Answer 1

• F_s(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
• F_L(down arrow): MAGNITUDE of the longwave radiation emitted by the atmosphere that reaches the earths surface
• F_L(up arrow): MAGNITUDE of the longwave radiation the surface emits upwards

Question 2

Net radiation flux

Answer 2

(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∗ = 𝐹_s ↓ −𝐹_s ↑ +𝐹_L ↓ −𝐹_L ↑

Question 3

In addition to the radiative fluxes at the Earth’s surface,…….. needs to be taken into account

Answer 3

the fluxes of sensible and latent heat

Question 4

sensible heat flux

Answer 4

heats air in the BL directly

Question 5

latent heat flux

Answer 5

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.

Question 6

If we imagine the land surface as an infinitesimally thin surface that has…………… then ………………..

Answer 6

zero heat capacity

the heat flux coming in must balance the heat leaving

Question 7

give the net radiation after concidering latent and sensible heat flux

Answer 7

energy gain or loss is partitioned among sensible heat flux, F_Hs, into the air (positive upward, for flux away from the surface),

• latent heat flux F_Es into the air (positive upward), and
• the conduction of heat down into the ground, F_Gs (positive downward, away from the surface), where the extra subscript s denotes near the surface. Thus,

𝐹∗ = F_HS + F_ES + F_GS

Question 8

Breifly describe the cases and fluxes

Answer 8

• 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

Question 9

describe the cases

Answer 9

• case A: Daytime over moist vegetated surface
  (sun’s energy goes to evaporation F_ES)
  
  • Positive
    
    • F_HS and F_ES because heat and moisture transport upward from the surface
    • F_GS because heat is conducted downward into ground from warm surface
• case B: Nighttime over a moist vegetated surface
  (F* often -ve due to net upward LW radiation cooling the space)
  
  • Negative
    
    • F_HS: due to downward heat flux from air
    • F_ES: if dew or frost from
    • F_GS: conduction of heat from the warm ground up to the cool surface
• case C: Daytime over a dry desert
  
  • most of the sun’senergy goes into sensible heat flux (F_HS)
• 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 F_HS combine to create very large evaporation and associated latent heat flux
      
      • F_HS can be downward from the warm air to the cool surface
      • F*: solar heating of the surface

Question 10

Explain the change of flux magnitude and diurnal cycle of surface skin temperature change

Answer 10

The magnitude of flux into the ground, 𝐹_GS , 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.

Question 11

The sensible heat flux

Answer 11

• 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 (pC_p)

Question 12

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:

Answer 12

Question 13

CH range

Answer 13

• stable conditions: decrease towards zero
• (C_HN)neutral: 0.001 - 0.005
• unstable: 2 to 3 times as large as C_HN

Question 14

Latent Heat Flux

Answer 14

The latent heat flux (in kinematic units of K ms-1) at the surface is directly related to moisture flux (𝐹_w) :

Question 15

The moisture flux from the surface is calculated as:

Answer 15

Question 16

Bowen Ratio

Answer 16

The ratio of sensible to latent heat fluxes at the surface is called the Bowen ratio:

B=F_HS/F_ES

Question 17

Bowen ration over land and ocean

Answer 17

• 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
• 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

Question 18

Soil Heat Flux

Answer 18

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

Question 19

Simple Parameterizations

Answer 19

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

Question 20

How to calculate ground flux

Answer 20

Assuming that the ground flux is a percentage, X, of the net radiation:

F_GS=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

F_GS=0.3 F_HS

This method fails in an oasis situation

Question 21

similarity in both equations that calculate ground flux

Answer 21

Both assume that the sign of the ground flux is always the same as that of the net radiation or sensible heat flux.