Storm occurrence over Dar Es Salaam, Tanzania
Sumner, G. N.
PublisherKing's College London (University of London)
MetadataShow full item record
Abstract Data from a dense network of autographic gauges has yielded a record of 5-minute interval rainfall over sixty square kilometres at Dar as Salaam on the Indian Ocean coast of Tanzania, permitting detailed analysis of storm development and movement for a 29-month period. A total of 671 storms of varying form, intensity and size was analyzed to determine morphology, mode of development, movement and rainfall characteristics, the inter-relationships between these parameters and their relationship to prevailing wind fields an a seasonal and diurnal basis. Models of storm development and form have been evolved which may equally be applied to other, similar, tropical coastal locations. Two basic storm types have been identified with either a linear or elliptical shape. Further subdivisions of each have also been identified but in general linear storm are associated with land- or sea-breeze features, are aligned approximately parallel to the coast and the gradient wind at 850 millibars, and move to the left of the storm axis in a direction dictated by the relative strengths of surface winds on either side of the axis. Elliptical storms on the other hand are more closely related to overall air motion and move, and are aligned, on a majority of occasions, in the same direction as the general wind field. With both major storm types in situ development can occur according to prevailing atmospheric conditions. Generally high storm incidence is associated with on- or offshore gradient winds. Diurnal variation in storm occurrence is also very closely related to gradient wind, and therefore also varies seasonally with changes in the trade wind systems; storms occurring earlier when gradient winds are onshore.