Cloud cover determinations with multispectral VAS observations: A two year study

Abstract

Multispectral GOES/VAS observations in the carbon dioxide absorption band at 15 microns have been used to calculate daily cloud statistics over North America for over two years. The CO 2 technique calculates both cloud top pressure and emissivity from radiative transfer principles. Cirrus clouds that are partially transparent to terrestrial radiation have been reliably separated from opaque clouds in the statistics of cloud cover. Intercomparisons with radiosonde, lidar, and stereo parallax measurements confirm these results. Observations since 1985 reveal that 20 to 30% of the United States is typically covered with thin clouds (radiative attenuation of less than 0.95), 45% is covered with thick opaque clouds, and 25 to 35% is clear or cloud free. It is likely that 5% of the opaque cloud should have been identified as semi-transparent cirrus. Opaque cloud cover is found to increase with increasing latitude while cirrus is found in all locations. Seasonal changes in mean cloud cover are found to be moderate with clear skies increasing from 24 to 35% from winter to summer, while cirrus and opaque decrease from 30 to 25% and 46 to 40%, respectively. Diurnal changes of all cloud types for six hour intervals are found to be consistent with opaque cloud development in the afternoon leaving cirrus behind at the end of the day.