Sandy desert

Modifications of the surface radiation budget

LLKK*Q

mao ZcosIK

The downward short-wave radiation is a function of the solar zenith angle (day of the year, latitude, hour of the day), the absorption (water, O3,CO2, clouds) and scattering (Raleigh, Mie) in the atmosphere

Except for dust, the desert atmosphere is very clear (low water vapor and no clouds). It is common that nearly 80% of the solar radiation at the top of the atmosphere reaches the surface.

High K

However, the albedo of sandy desert is usually high

KK

High K

Low L

The water content of the atmosphere is low, and clouds are nearly absent. The downward long-wave radiation is low

As a result of soil factors, the desert surface is very hot.

Since

High L

4TL

The upward long wave radiation is high

As a result, the net all-wave radiation Q* is not particularly high. Despite the large solar radiation input, values for tropical deserts (20-30O Lat.) in mid-summer are comparable with mid-latitude (40-50O Lat.) values for others surfaces (grass, crop, water) in the same season.

35 N Lat, desert

49 N Lat, short grass

Surface energy budget

GQEQHQ*Q

The water available in the soil is very low, so the latent heat flux is almost negligible.The available radiant energy is used to heat the soil and the atmosphere.

QE negligible

No water

Dry soil and sand have very low ground thermal diffusivity.

The soil layer heated is very shallow. By day there is strong heat flux convergence with very strong heating.

The surface becomes very hot.

By night, there is a strong heat flux divergence, with very strong cooling

The surface becomes very cold

In the early morning and during night QH is small because the wind is usually weak (weak turbulence). The radiant energy off-set is mainly counterbalanced by QG

As the day progress, the radiation forcing becomes stronger, and surface temperature much hotter than air. QH starts to increase, the atmosphere becomes strongly unstable, convective motions mix momentum down to the surface. Surface wind speed increases, which in turn increases QH. QG decreases and the off-set of radiant energy is counterbalanced mainly by QH

As a result of lack of moisture and the concentration of heat in the uppermost shallow sand layer, the daytime surface temperature over a desert is high

However it decreases sharply with depth in the soil, and with height in the atmosphere.

During daytime, the atmosphere is strongly unstable above desert. This leads to many different phenomena, like:

Miniature whirlwinds known as “dust devils”

Optical effects (mirage, etc.).

The strong variations in air temperature close to the surface, induces strong variations in air density, and consequently in the refraction index

During night, the strong surface radiative cooling, induces a strong cooling of the atmosphere close to the surface. There is a large diurnal temperature variation (40 C are common). During night-time the atmosphere

is strongly stable close to the surface.

T1

T2

T3

T1 << T2 <<T3

In a desert winds are stronger during day (turbulence mix down momentum from upper levels), and weaker at night (the strong stability prevents the mixing of momentum from above).