The internal cloud radiation field and a technique for deter mining cloud blackness

The internal radiation fields of homogeneous and inhomogeneous stratocumulus model clouds under the same meteorological conditions were investigated at several wavelengths in the 11 µm window region. The results showed that a homogeneous cloud model may be used in place of an inhomogeneous one to deduce the upwelling radiances, but not the internal structure of the cloud radiation field. The resulting findings that in this window region the optical properties of a cloud at a shorter wavelength permit a greater penetration of the surface radiation and hence scattering by the cloud particles than those at longer wavelengths lead to the development of a technique for quickly assessing the “blackness” of a cloud.

The technique requires the use of two channels in the window region to establish a ratio of the observed upwelling radiances. The ratio is then compared with the reference ratio derived from a knowledge of the estimated surface temperature in what was termed the ratio test. It was shown that the estimated temperature need not be accurate. Examples are presented to demonstrate that the vertical emissivity of clouds is close to 0.95 or greater when the ratio test is satisfied. However, when the observed radiances convert to nearly the same temperature value, the cloud may be said to have an emissivity of unity and that temperature then becomes the cloud-top temperature. On the basis of this method, the temperature field generated by satellite infrared imagery may be accepted or rejected with reasonable confidence.