Intraseasonal wind anomalies related to wet and dry spells during the ''long'' and ''short'' rainy seasons in Kenya

The largest part of Kenya exhibits two major rainy seasons, the March-May ''long rains'' and the October-December ''short rains'', both related to the passage of the ITCZ, but differing in the amount of rainfall recorded and its interannual variability. In order to investigate whether these differences also apply at intraseasonal time-scales, daily rainfall data for the peak month of each rainy season (April and November) were collected for 7 consecutive years (1982-1988). The network comprises 68 stations, from which a classification of the spatial patterns of daily rainfall anomalies has been performed. Wind anomalies corresponding to the various rainfall types and to specific regional rainfall departures were determined using four pilot balloon stations and one radiosonde station. They revealed that there exist significant differences between upper-air circulation anomalies exhibited in the ''long'' and ''short'' rainy seasons, especially as far as rain spells in the Eastern Highlands are concerned. In that region, easterly anomalies in the ''short rains'' period are associated with an increase in rainfall. During the ''long rains'', enhanced easterlies more generally coincide with an overall drop of convection in the country. In Western Kenya, wet conditions are more systematically associated to westerly wind anomalies.