Large-Scale Diurnal Variations of Tropical Cold Cloudiness Based on a Simple Cloud Indexing Method

Diurnal variations of cold cloudiness during a 2-year period (October 1985–September 1987) over the tropical area covered by Meteosat (from 23°N to 23°S and from 60°E to 60°W) are studied using a simple precipitation index based on infrared data with a unique blackbody temperature threshold of 235 K. The index is calculated every three hours and accumulated over 5-day periods. The results indicate that the cold cloudiness undergoes a pronounced diurnal cycle only over the continent during the rainy season, with an afternoon maximum between 1500 and 2100 LST and a morning minimum between 0600 and 1200 LST. Over flat areas, diurnal variations are weaker than in the mountains. Over water surfaces, the diurnal variations of cold cloudiness are irregular, but along the coasts, morning maxima (0600–1200 LST) are the most frequent. The present study shows the potential utility of the easily manageable precipitation-index dataset in cloud climatology studies, as the results reported here are in agreement with earlier studies based on more complex datasets.