Solar Radiation and Clouds

It has long been recognized that clouds play a crucial role in the determination of the earth’s energy budget. The spatial distribution of radiative heating is the fundamental energy source responsible for driving the dynamics of both atmosphere and oceans. Energy budget studies show that both the solar and infrared radiation fields are significantly altered by cloud cover, thereby affecting atmospheric heating/cooling profiles and surface temperature (Paltridge, 1974b). Furthermore, recent measurements by Cox et al. (1973) and Reynolds et al. (1975) show that direct absorption of solar radiation in clouds is an important tropospheric heat source.

Attenuation of atmospheric radiation occurs through absorption and scattering by atmospheric gases, cloud droplets and particulate matter. In the present investigation the influence of dust on atmospheric and cloud radiation fields will be neglected.

Measurements by Reynolds et al. (1975) show that direct absorption of solar radiation in clouds is an important tropospheric heat source. Their observations show cloud absorptance values as high as 30–52%. For a particular case study the measurements of Reynolds et al. showed an absorptance value of 36% while calculations in Chapter 2 showed a corresponding absorptance value of 17%, i.e., less than half. Twomey (1976) has shown that theory predicts a maximum absorptance value of 20%. Moreover, Twomey (1972) showed that the addition of particulates does not add significantly to cloud absorption. Therefore, there has been a disparity between theory and observations. Twomey (1976) mentioned that in order to obtain such large absorptance values as measured by Reynolds et al. there must either be total absorption of solar radiation at wavelengths > 0.7 µm or significant absorption of solar radiation at wavelengths < 0.7 µm. However, there has been no apparent mechanism postulated to satisfy either of these two conditions until now.

Numerous investigators have alluded to the potential importance of upper tropospheric clouds on a great variety of meteorological problems. Manabe and Strickler (1964) studied the response of a thermal equilibrium model which included radiation and a convective adjustment to the presence of cirrus clouds. Atmospheric radiative variations were shown to depend upon cloud height and infrared emissivity. Manabe and Strickler (1964), Möller (1943), Cox (1969) and Reynolds et al. (1975) show that clouds may be warmed by radiative convergence. Reynolds et al. show that direct absorption of solar radiation by clouds is an important tropospheric heat source.

The previous chapters have been concerned with the radiative characteristics of cloud layers which are horizontally homogeneous. Real clouds, of course, have definite geometries and inhomogeneous horizontal structure. The present chapter will consider the effect of cloud geometry upon cloud radiative characteristics.

The previous chapters have been primarily concerned with clouds of uniform cloud microstructure. However, in Chapter 2 vertical variations of liquid water content were analyzed. It was found that the cloud radiative characteristics were primarily a function of cloud optical depth rather than cloud vertical structure. Clouds with liquid water contents which decreased with increasing cloud depth gave essentially the same results as for clouds in which the liquid water content increased with increasing cloud depth; both cases gave essentially identical results to a homogeneous cloud with an average value of liquid water content. While the bulk cloud radiative properties were invariant to vertical cloud structure, local radiative characteristics within the cloud were highly sensitive to such variations. Local cloud heating rates were shown to vary substantially with variations in cloud microstructure.