Abstract
Using a 10-year daily data thunderstorm and dry thunder (thunder without precipitation) that spans from 1991-2000, the spatial, seasonal and inter-seasonal variations of thunderstorm frequency have been investigated with results which include the production of seasonal and inter-seasonal maps of dry thunder (thunder without precipitation) and thunderstorm frequencies. Results showed that there exists a latitudinal belt of reduced thunderstorm activity between 8 and 10°N. Also, due to absence of little dry season (LDS) over the south-eastern part of the country, they do not experience double maxima of thunderstorm activity. Hence, the weather over the country has been observed as not following the latitudinal divides. Also, the thunderstorm activities over the country are more prominent over the coast, but gradually decrease towards the inland with anomaly over Ilorin and Yola. Though, several authors have carried out series of researches on the thunderstorm activities but none have been able to look at the seasonal and inter-seasonal variations of thunderstorm frequency over Nigeria. Hence, the need to look into this aspect came as a result of the fact that thunderstorm activities is one of the most contributing systems to total precipitation in Nigeria. Key words: Thunderstorm, seasonal, frequency, latitude, Nigeria.
Highlights
Studies have shown that the greatest proportion of the annual rainfall of West Africa countries comes from deep convective systems (Adelekan, 1998; Omotosho, 1985)
While the updraft flows arise as a result of the formation of a gust front ahead of a thunderstorm cell, the low-level downdrafts arise from the flow of low theta-e towards the cumulonimbus cloud, at the level of the African Easterly Jet (AEJ)
Thunderstorm activities were only recorded in the Sahelian region of Maiduguri between July and August (Figures 2g and h) which coincides with the period of northernmost position of the Inter-tropical Discontinuity (ITD) and the time of Little Dry Season (LDS) in the south
Summary
Studies have shown that the greatest proportion (with about 70%) of the annual rainfall of West Africa countries comes from deep convective systems (Adelekan, 1998; Omotosho, 1985). As a weather system, thunderstorm has been observed to usually comprise fluctuating ensemble of “cells” and produces a contagious precipitation area of about ~100 km in horizontal scale in at least one direction (Houze, 1993) Within each of these cells are; updraft flows, precipitation showers and downdraft flows. While the updraft flows arise as a result of the formation of a gust front ahead of a thunderstorm cell, the low-level downdrafts arise from the flow of low theta-e (low θe which is dry and cold) towards the cumulonimbus cloud, at the level of the African Easterly Jet (AEJ) This air is driven by melting, evaporation and hydrometeor drag (Srivasta, 1985 and Knupp, 1988)
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