Liquid cloud droplets and solid ice-crystals in ice clouds (cirrus) are minor constituents of the atmosphere with major impact. A water cloud with 500 m thickness, a typical stratus deck, reaches an optical depth 5 of about 25 in the solar spectral range, although the liquid water content is only 0.2 g m-3 or 100 g m-2 liquid water column content, which is equivalent to a 0.1 mm water layer. Finely dispersed liquid water in air together with very low absorption of liquid water in the visible creates clouds as effective backscatterers that can reach albedo values up to 0.8, nearly as brilliantly "reflecting" as fresh powder snow.

Calculating spectral optical depth 5x of a 2 km thick water cloud extending from height z1 to height z2 gives z2 r2

$x = U Qtt xnr2 N (r, z )dr dz «100 (in the visible spectral range) (2. 2)

zi ri

We realize that the cloud's impact on the radiation budget is mainly a function of its droplet size distribution N(r, z), which also varies with height z and not only with droplet radius r, but is also proportional to the cross section n r2 of the droplet and the spectral extinction efficiency Qest x of a Mie-scatterer. Qest, x is also close to 2 for cloud droplets in most parts of the solar radiation range with X < 2^m. Direct solar radiation transmission of many clouds Tr = e~Sx «0, i.e. we cannot see the sun's disk. As our eye can distinguish radiance Lx differences of about 2 percent the sun's disk disappears for our eye at 5 ~ 10, when Lx(sun) e"10 «Lx(sky).

In the thermal infrared (terrestrial) radiation range at wavelength > 4 ^m liquid water is a strong absorber, hence already thin clouds with 5 > 3 (50 m fog layer) fully decouple long-wave radiation transfer above and below clouds, while we can still read a newspaper under a cloud with 5 ~ 100 at high solar elevation because very low absorption by liquid water allows scattered solar radiation to reach the ground.

Therefore the following feedbacks of clouds exist (as also seen in table 2.4):

1. High, optically thin ice clouds enhance the greenhouse effect of the atmosphere, most contrails from airplanes belong to this category.

2. Low, optically thick clouds counteract the greenhouse effect of the atmosphere; especially stratocumulus decks off the western subtropical coasts of the American and African continents belong to this cate-

3. Many clouds will exist neither enhancing nor reducing the greenhouse effect; the sign of their feedback will depend on subtle changes of cloud height and thus temperature, mean droplet radius, crystal shape, geometrical extent, three-dimensional structure and even surface albedo and - of course - solar zenith angle.

Table 2.4:

The Climate System Cloud cover change and its feedback on the greenhouse effect

Table 2.4:

The Climate System Cloud cover change and its feedback on the greenhouse effect

Cloud type

Feedback sign and strength

Key influencing factors, attempt to rank

stratus 3 stratocumulus ground fog

- strong

- strong

- very strong

shortwave albedo small temperature difference between cloud top and surface

- medium

cloud cover height of top

cumulus congestus

- medium

cloud cover cloud top temperature


- or +

cloud top temperature optical depth cloud cover

altostratus thin cirrus cirrostratus cumulonimbus


cloud top temperature optical depth optical depth cloud top temperature optical depth cloud top temperature vertical extent

Overall, clouds lead to a reduced greenhouse effect of the atmosphere at present, compared to the cloudless regions. Their effect on the radiation budget amounts to about -15 Wm-2, which is equivalent to a planetary albedo change of about 6 percent. Whether they loose or gain in their cooling capacity when a further surface warming occurs is not clear yet. To find out what will happen to their distribution, lifetime, microphysical properties, etc. is a key research question.

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