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University of Nairobi (2007)

Inter annual variability of onset and cessation dates of the long rains in Kenya

Mwangi, R M

Titre : Inter annual variability of onset and cessation dates of the long rains in Kenya

Auteur : Mwangi, R M

Université de soutenance : University of Nairobi

Grade : Master of Science in Meteorology 2007

The overall objective of the study was to determine the inter-annual variability of onset and cessation dates of the long rains season in Kenya. The specific objectives that were addressed to achieve the overall objective included determination of the onset and cessation dates of long rains (MAM) during the period 1961-2001, determination of the seasonal rainfall performance during early, normal and late onset years, identification of the anomalous wet and dry years and examination of the circulation pattern associated with early, normal and late onset of the long rains. The data used in the study were daily rainfall, monthly rainfall, pentad rainfall, and wind data. The rainfall data was obtained from Kenya Meteorological Department while the wind data was obtained from the National Centre for Environmental Prediction (NCEP) website. In order to achieve the overall and specific objectives, the data was subjected to various analyses including quality control, correlation analysis, and principle component analysis. The core methodology lay in determination of onset, cessation and duration of the rainfall season using time series analysis, mass curve analysis and the quantitative determination of onset/cessation dates using instat program. Wind analysis for early, normal and late onset periods was performed. Stations located over the western and southern parts of Kenya received earlier onset than others. This was attributed to the fact that the rainfall during this season is associated with both the northward progression of the zonal arm of the ITCZ and the eastward extension of the meridional arm. Although onset progresses across the region, the mean onset date for the whole region is the 74th day of the year that is 14th of March. The average standard deviation, i.e. the interannual variability of the onset, is 24 days. The zone with the highest onset variability is zone 1 (Lodwar) with a standard deviation of 33 days and zone 10 (Narok) which has the lowest standard deviation of 15 days has the lowest variability. The study showed that in general the onset month is March while the cessation month is May. However, early onset will mostly occur in February and late cessation is June. It was quite evident that for years with early onset, rainfall withdrew late while for years of late onset, withdrew early. Thus on average, the years with early /Iate onset had longer/shorter rainfall duration. The analysis of the wind system in reference to early/late onset years indicated that during early onset there were stronger easterlies in the upper troposphere with relatively weak easterlies or westerlies in the mid/lower troposphere. Most of the late onset will be recovery to the normal flow pattern after experiencing a time delay in one phase. The high pressure in southern Africa helps in providing a mechanism for the advection of moisture towards the equator. In some cases westerlies develop from the incursions of the Congo air mass which are forced eastwards by high pressure over the eastern Atlantic Ocean. The study has demonstrated that there is both spatial and temporal variability of onset and cessation dates over Kenya. It may be concluded that when the onset of the rains is early (negative onset date anomaly), the rainfall tend to be heavier than normal. These findings may be used to develop a model for monitoring and forecasting of evolutions of anomalous rainfall over the region including the extreme events like droughts and floods. Such efforts would contribute enormously to regional/national early warning and preparedness fOL extreme events and food security. This would therefore contribute significantly to the effective management and sustainable development of the regional/national social-economic activities which are heavily raindependent


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