Seasonal and Diurnal Changes in Cirrus Clouds as Seen in Four Years of Observations with the VAS

Abstract

GOES VAS multispectral observations in the carbon dioxide absorption band at 15 ?m have been used to compile cloud-cover statistics over the continental United States for the past 4 years. The CO2 technique calculates both cloud-top pressures and effective emissivities and reliably distinguishes semitransparent cirrus from opaque clouds. The frequency of semitransparent cirrus clouds exhibits small seasonal variation; they are generally present 25%?30% of the time in all seasons. Diurnal variations of semitransparent cirrus are found only in the summer months and correspond to diurnal variations in convection in the Rocky Mountains and southeastern United States, increases of 20% in cirrus are noted subsequent to the convective cloud activity. In the winter months, no diurnal change in semitransparent cirrus is detected. Attempts to correlate cirrus with some common atmospheric features reveal that a majority of cirrus occurred where dynamic parameters indicate rising vertical motion but that considerable cirrus were also found where the dynamics was weak. Intercomparison with ground reports of cloud cover reveals that the satellite observations are corroborating or complementary 80% of the time; many of the disagreements come from the satellite identifying cold ground as low cloud or ground observations missing high thin clouds.