Infrared emissivity, short-wave albedo, and the microphysics of stratiform water clouds

Simultaneous measurements of the microphysics and solar and thermal radiation fields of stratocumulus clouds are reported. It is shown that the mass absorption coefficient a0 for window infrared radiation should be almost independent of position in these clouds because of the manner in which liquid water content and drop size distribution vary with height. Its theoretical value is of the order of 650 cm2 g−1. Measured values of total emissivity yield an a0 in the range 150–800 cm2 g−1. Such clouds must be at least 500 m thick in order to be optically black to longwave radiation. Neiburger's (1949) result that the two-stream approximation overestimates the shortwave albedo of thin clouds is not supported by the present work. His descrepancy between observation and theory can be largely eliminated by taking specific account of the increase in liquid water content with height.