Cloudysky fluxes

Clouds are an important determinant of atmospheric radiation as they have lower temperatures then the surface and hence reduce the thermal emission of the planet. Optically thick clouds, like the low-level and middle-level clouds, emit essentially as blackbodies with emissivity close to 1, where ecX = 1 — e~TcA and rcX is the cloud optical depth at wavelength A. High-level clouds, are usually not ideal blackbodies and can have an emissivity well below unity. In Fig. 4.19 we see that the low-emissivity clouds are mainly located in the tropics. Here, highlevel clouds with low emissivity play an important role in the Earth's radiation budget as they allow more of the thermal radiation from the surface to escape to space and usually reflect relatively more of the incoming solar radiation (Chapter 6). The fraction of thermal flux transmitted by the clouds is 1 — ecX, assuming no cloud reflection in the infra-red, which in any case would be incorporated in the cloud emission by measurements. This high-level cloud transmission plays a very important role in the Earth's climate since the outgoing thermal flux at TOA has a very significant component of radiation from the surface of the planet that is transmitted to space. Thus, although one would expect that as the emissivity of the high-level clouds increases their contribution to the outgoing flux to increase, they essentially block the larger thermal flux from the warmer Earth's surface from escaping to space. Thus, an increase in high-level cloud emissivity results in reduced outgoing thermal flux at TOA.

fx (0) = Bx(Ts)t(rgX, 0) + (1 - egX)f-(rgX)t(rgX, 0)

fx (0) = Bx(Ts)t(rgX, 0) + (1 - egX)f-(rgX)t(rgX, 0)

, High cloud emissivity (January 1983)

, High cloud emissivity (January 1983)

FlG. 4.19. High-level cloud thermal emissivity for January 1988, based on ISCCP data.

4.9.1 Outgoing flux above a cloud layer

The upwelling flux at TO A above the cloud layer has seven components, as shown in Fig. 4.20, arising from; the cloud-top surface, the atmosphere above the cloud layer directly and via reflection from the Earth's surface, the Earth's surface itself, the atmosphere below the cloud layer directly and via reflection from the Earth's surface, and the cloud-base surface via reflection from the Earth's surface. For optically thick clouds, we can use the expressions for clear-sky fluxes but with the emissivity and surface temperature replaced by that of the cloud surface. For optically thin clouds we need to compute all seven components.

4.9.2 Downwelling flux at the Earth's surface

The downwelling flux at the Earth's surface has three components. For a partly cloudy sky we can use the fraction of the sky covered by a particular cloud i, the cloud-cover fraction aci, to compute the various fluxes from aci/c + (1 — aci)/s, where /c corresponds to the flux for a cloudy sky and /s is that for a clear sky. One can extend this idea to many non-overlapping clouds. For overlapping clouds one needs to invoke some cloud-overlapping procedure (see Chapter 7).

i

, i

5

L

L i

i 1 i 1 2

L

cloud layer

4

1 '

1 '

7

FlG. 4.20. The components of outgoing thermal flux at the top of the atmosphere and downwelling flux at the surface.

Earth's surface

FlG. 4.20. The components of outgoing thermal flux at the top of the atmosphere and downwelling flux at the surface.

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