Signature of clouds over Antarctic sea ice detected by the Special Sensor Microwave/Imager

摘要

A method to detect the cloud signature (mainly the cloud liquid water) over the sea ice-covered Weddell Sea in the Austral summer season is presented. By using the polarization differences at the two high frequency channels (i.e., 37 and 85 GHz) of the Special Sensor Microwave/Imager (SSM/I), a new quantity called R-factor is defined. Using the R-factor, the atmospheric signal can be easily separated from the surface signal and, more importantly, the surface signal and its variation can be strongly suppressed, especially in regions with low ice concentrations. In regions with high ice concentrations, other sea ice parameters like snow cover play an important role as indicated by simulations using in situ measured sea ice emissivities and observed by the SSM/I. Under the assumption that the sea ice parameters remain sufficiently stable within a short period (e.g.. ten days), a method is proposed to determine the background term from SSM/I measurements, allowing the detection of the cloud signature. A comparison with a known SSM/I cloud liquid water algorithm over the open ocean shows a high degree of correlation (0.958) among the cloud signatures detected by the two algorithms. On January 2 and 3, 1996, a low pressure system moved into the sea ice-covered Weddell Sea. Its cloud signature detected using the R-factor method compares well with coincident observations from both visible and infrared sensors.