A seasonal air transport climatology for Kenya

Abstract

Abstract. A climatology of air transport to and from Kenya has been developed using kinematic trajectory modeling. Significant months for trajectory analysis have been determined from a classification of synoptic circulation fields. Five-point back and forward trajectory clusters to and from Kenya reveal that the transport corridors to Kenya are clearly bounded and well defined. Air reaching the country originates mainly from the Saharan region and northwestern Indian Ocean of the Arabian Sea in the Northern Hemisphere and from the Madagascan region of the Indian Ocean in the Southern Hemisphere. Transport from each of these source regions show distinctive annual cycles related to the northeasterly Asian monsoon and the southeasterly trade wind maximum over Kenya in May. The Saharan transport in the lower troposphere is at a maximum when the subtropical high over northern Africa is strongly developed in the boreal winter. Air reaching Kenya between 700 and 500 hPa is mainly from Sahara and northwest Indian Ocean in the months of January and March, which gives way to southwest Indian Ocean flow in May and November. In contrast, air reaching Kenya at 400 hPa is mainly from southwest Indian Ocean in January and March, which is replaced by Saharan transport in May and November. Transport of air from Kenya is invariant, both spatially and temporally, in the tropical easterlies to the Congo Basin and Atlantic Ocean in comparison to the transport to the country. Recirculation of air has also been observed but on a limited and often local scale and not to the extent reported in southern Africa. Findlater demonstrated the gradual wind change from northeasterlies in January to easterlies in March, southeasterlies in July, and back to easterlies again in November. The change was observed to occur first at low levels before rising to midlevels. The northeasterly monsoon was noted to divide into two streams on entering Kenya. One stream curves to become easterly and flows between the highlands of Kenya and Ethiopia. The other becomes northerly and flows over the relatively flat areas to the east of 38øE. Both of these branches were found to coexist on any one day, but often, one is more strongly developed than the other. In northwestern Kenya, persistent easterly winds prevail throughout the year at all levels in the lower troposphere.Prior to determining actual aerosol transport, it will be useful to establish an air transport climatology for Kenya, as has been done for southern Africa [Tyson et al., 1996a]. In this paper, such a climatology is developed at the synoptic scale using Lagrangian kinematic trajectory modeling.The

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