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Olli M. Turpeinen
and
A. A. Diallo

Abstract

Rainfall is estimated in Burkina Faso for a full year using the ESOC precipitation index (EPI), a statistical cloud indexing method based on satellite data from METEOSAT. The EPI is converted into rainfall with the linear regression calculated between the EPI and the observed rainfall from a dense network of rain gages. Only one regression line based on the largest possible sample is used. The purpose of the paper is to assess the accuracies of the yearly and seasonal rain estimates to find out whether a single EPI-rainfall relation can be applied.

The results show that the yearly precipitation can be estimated to a high degree of accuracy. On the other hand, the precision of the seasonal estimates exhibits large fluctuations. While the dry season estimates are reliable, the transition between the dry and rainy season is characterized by a considerable overestimation, caused by the abundance of cold, nonprecipitating cirrus on the northern side of the intertropical convergence zone. During the rainy season the method suffers from a slight underestimation.

To resolve the major problem, that of nonprecipitating cirrus, a lower temperature threshold of 220 K instead of 235 K is applied in the determination of the EPI. The rain estimates for the transition period do improve slightly, but the gain is offset by the deterioration of the rain estimates made for the whole year and the rainy season.

The results suggest that the rain estimates made with a single EPI-rainfall relation are useful, but that they could be improved with some type of seasonal adjustments.

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Wassila Thiao
and
Olli M. Turpeinen

Abstract

Diurnal variations of cold cloudiness during a 2-year period (October 1985–September 1987) over the tropical arm 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.

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