Four Years of Global Cirrus Cloud Statistics Using HIRS, Revised

Abstract

Abstract : Trends in global upper tropospheric transmissive cirrus cloud cover are beginning to emerge from a four year cloud climatology using NOAA polar orbiting HIRS multispectral infrared data. Cloud occurrence, height, and effective emissivity are determined with the CO2 slicing technique on the four years of data (June 1989 - May 1993). There is a global preponderance of transmissive high clouds, 42% on the average; about three fourths of these are above 500 hPa and presumed to be cirrus. In the ITCZ a high frequency of cirrus (grater than 50%) is found at all times; a modest seasonal movement tracks the sun. Large seasonal changes in cloud cover occur over the oceans in the storm belts at mid-latitudes; the concentrations of these clouds migrate north and south with the seasons following the progressions of the subtropical highs (anticyclones). More cirrus is found in the summer than in the winter in each hemisphere. This paper reports on the investigation of seasonal changes with multispectral observations from polar orbiting HIRS (High resolution Infrared Radiation Sounder).