This study was primarily focused on Nigeria and adjacent areas, a region in the tropics bounded by latitudes 40N – 140N and Longitudes 30E – 140E. It investigated the rain bearing convective systems over the region, using conventional and satellite derived precipitation data. The period of study was 1998, with special emphasis on the month of July, a period during which Nigeria normally fell within the most convectively active zones in the West African region.
Precipitation data, derived from the Tropical Rainfall Measuring Mission (TRMM) satellite was used to study the migration of the precipitation zone over West Africa. The result from the study confirmed the observation from conventional data as regards the meridional migration of the convective precipitation zone in tandem with the Inter Tropical Discontinuity (ITD). However, this study established that the northward advance of the convective cloud band was about 2.30 latitude per month, while the southward retreat, after attaining the northernmost advancement in the month of August was 3.70 latitude per month.
This study also revealed the existence of super-cells within slow moving cloud clusters. These super-cells, sighted on the Meteosat visible imagery, were found to be almost stationary and randomly distributed within the cluster, not aligned as in the case of fast moving squall lines. Such clusters produced heavy rainfall as was observed at the Benin station.
Vorticity and Divergence computations in the general areas of cloud build ups showed values ranging from –48.9 x 10-6s-1 to 96.3 x 10-6s-1 in the case of vorticity, and –63.3 x 10-6 s-1 to 57.0 x 10-6 s-1 in the case of divergence. Contrary to conventional wisdom, it was observed that divergence and negative vorticity characterized the regions of precipitating systems at low levels in the tropics. This study attributed this observation to be due to wide spread down draughts and diverging air at the low levels.
The satellite derived rainfall over Nigeria, was compared with the rain gauge measurements observed at 31 different meteorological stations located within the country, at the same period. The agreement between the two different sets of data varied between 31% and 97%. Rainfall amounts from rain gauges in 28 out of 29 stations (97%) showed agreement with rainfall estimation from the TRMM satellite in January, whereas rainfall records from 9 out of 29 stations (31%) agreed with the satellite derived precipitation estimates in the month of September. In January, rainfall was limited to southern stations and came mainly from cumulus and stratocumulus clouds. The disagreement in rainfall amounts, which was greatest for the months of June to September, was attributed to the inability of the TRMM satellite to capture the existence of super-cells in the numerous cloud clusters that were prevalent over the West African region during those months of the year. Such systems, which were very predominant in the tropics, were short lived, and produced very heavy precipitation, which were not detected by the TRMM satellite, due to its poor frequency of observation. The satellite flew over the same area only twice a day.
|Advisor:||Balogun, Ekundayo E.|
|Commitee:||Jegede, Olughenga O.|
|School:||Obafemi Awolowo University (Nigeria)|
|Source:||MAI 82/4(E), Masters Abstracts International|
|Subjects:||Meteorology, Remote sensing, Applied physics|
|Keywords:||Convective Systems, Ground Validation of TRMM, ITD Movement, Meteosat Imagery, Tropical Rainfall|
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