Transmission of Solar Radiation by Clouds over Snow and Ice Surfaces.Part II: Cloud Optical Depth and Shortwave Radiative Forcing fromPyranometer Measurements in the Southern Ocean

摘要

Downward solar irradiance at the sea surface, measured on several voyages of an icebreaker in the Southern Ocean, is used to infer transmittance of solar radiation by clouds. Together with surface albedo estimated from coincident hourly sea ice reports, instantaneous cloud radiative forcing and effective cloud optical depth are obtained. Values of 'raw cloud transmittance' (trc), the ratio of downward irradiance under cloud to downward irradiance measured under clear sky, vary from 0.1 to 1.0. Over sea ice, few values of trc were observed between 0.8 and 1.0, possibly due to the threshold nature of the aerosol-to-cloud-droplet transition. This sparsely populated region of transmittances is referred to as the Koehler gap. The instantaneous downward shortwave cloud radiative forcing is computed, as well as the time-averaged net forcing. The net forcing at a solar zenith angle of 60 degree is typically -250 W m-2 over open ocean, but only half this value over sea ice because of the higher surface albedo and less frequent occurrence of clouds. 'Effective' optical depths tau (for a radiatively equivalent horizontally homogeneous cloud) are classified by season and surface type. The frequency distributions of tau are well fitted by decaying exponentials, giving a characteristic optical depth of 15 at 47 degree S, increasing to 24 in the region of maximum cloud cover at 58 degree S, and decreasing to 11 at 67 degree S near the coast of Antarctica.