The Diurnal Cycle of Upper-Tropospheric Clouds Measured by GOES-VAS and the ISCCP

Abstract

A comparison of diurnal cycles in high clouds (<440 hPa) measured by the Geostationary Operational Environmental Satellite Visible Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (GOES-VAS) and the International Satellite Cloud Climatology Project (ISCCP) was made. The GOES-VAS longwave infrared data allow uniform detection of upper-tropospheric cirrus clouds from daylight into night without effects from solar reflections. It is sensitive to thin cirrus, which are difficult to detect. But it is not available globally because the sounder instrument is flown only on geostationary satellites operated by the United States. The ISCCP, however, is a global dataset using five to seven geostationary satellites. Large diurnal cycles were found in the Rocky Mountains and along the northern coast of the Gulf of Mexico mainly in the summer season. In the Tropics substantial diurnal cycles also were found in central Brazil and the Atlantic ITCZ. In the winter over the continental United States, diurnal cycles were very small or nearly absent. The ISCCP found similar diurnal cycles over land. The amplitudes of the cycles were about 1.5 times larger in the VAS data than the ISCCP (IR only) data over land because of the former's sensitivity to thin cirrus. The phase relationships were very similar. In the western tropical Atlantic ITCZ, the VAS found dual maxima in the diurnal cycle (morning and later afternoon), which the ISCCP could not detect. These changes in high cloud cover probably were driven by radiative cooling of the cloud tops over the ocean. Over land the obvious cause of diurnal cycles in high clouds is deep convection from solar surface heating.