Properties of austral winter clouds derived from radiometric profiles at the South Pole


  • Robert S. Stone


Although clouds are known to radiatively modulate the energy budget of Antarctica, particularly during the polar night, very little is known about their physical, radiative, or microphysical properties. A unique set of measurements consisting of infrared flux profiles (radiometersonde data) collected at the south pole is analyzed in conjunction with ancillary meteorological observations to gain a better understanding of austral winter clouds. Distinct radiometric features associated with cloud boundaries are used to estimate the heights, thicknesses, and temperatures of selected cloud systems. The physical cloud properties are combined with infrared flux measurements to formulate the thermal energy budgets of the cloud layers and to derive their bulk radiative properties. By comparing these derived radiative properties with theoretically computed values for model clouds varying in microphysical characteristics the clouds' ice contents and effective particle sizes are also inferred. Austral winter clouds are found to be moderately thick and on average extend to heights greater than 3 km above the top of the surface-based inversion layer. They are optically thin, however, and nonblack, having mean effective emissivities of about 0.6. Because they are cold (≤ −40°C) these clouds are composed of small ice particles, have low total ice contents, and contain little or no liquid water. Overall, they are similar to high-level cirrus clouds observed in the mid-latitudes. The characteristic radiometric features that distinguish overcast from clear sky conditions at south pole are discussed; mean profiles of infrared fluxes, temperatures, vector winds, and heating rates are presented; and a summary is provided of cloud properties that may be of use to climate modelers.