Diurnal Variability of Regional Cloud and Clear-Sky Radiative Parameters Derived from GOES Data. Part I: Analysis Method

Abstract

A hybrid bispectral threshold method (HBTM) is developed for hourly regional cloud and radiative parameters from geostationary satellite visible and infrared radiance data. The quantities derived with the HBTM include equivalent blackbody temperatures for clear skies, for the total cloud cover and for the cloud cover at three levels in the atmosphere; the total fractional cloud cover and the fractional cloud amounts at three altitudes; and the clear-sky and total cloud reflectance characteristics. Geostationary satellite data taken during November 1978 are analyzed. A minimum reflectance technique is used to determine clear-sky brightness. A visible bidirectional reflectance model is derived for clear ocean areas. Clear-sky radiative temperature is found with a bispectral clear radiance technique during daylight hours. An empirical model is derived to predict clear-sky temperature at night. A combination of previously published infrared threshold and bispectral techniques is used to determine the remaining parameters. Sources of uncertainty are discussed and means to minimize them are proposed. Monthly mean, regional fractional cloudiness determined with this method agrees well with more conventional subjective techniques. On the average, the present results are approximately 0.05 less than corresponding surface observations; this is consistent with previous comparisons of satellite- and surface-based nephanalyses. Comparisons between subjective analyses of satellite photographs and the HBTM yielded average differences in mean regional cloudiness, mean hourly cloudiness and instantaneous cloud amounts of 0.04, 0.05 and 0.11 respectively. Root-mean-square differences in these same quantities derived by two satellite data analysts were 0.03, 0.04 and 0.08 respectively.