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Forecasting the summer weather of the sudan and the rains that lead to the nile floods
Author(s) -
ElFandy M. G
Publication year - 1949
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49707532605
Subject(s) - anticyclone , climatology , trough (economics) , storm , latitude , equator , low pressure area , monsoon , atmospheric circulation , pressure system , geology , environmental science , atmospheric pressure , atmospheric sciences , oceanography , geodesy , economics , macroeconomics
In the Sudan, pressure‐distribution shows a characteristic seasonal variation. In summer (mainly June to August) a trough of low pressure (of the belt of the thermal equator) covers the Northern Sudan. An oscillatory barometric minimum is located over the NE. Sudan and is normally an area of almost cloudless skies and intense insolation; while over the greater part of Abyssinia and the SE. Sudan extends a barometric maximum (of the Indian Ocean anticyclone) which also suffers fluctuations in its strength and northward extent. “Haboobs” and local convectional rain over the Sudan usually accompany the northward oscillations of this barometric maximum. It is in tracing such oscillations of the main pressure systems that pressure becomes of some value in forecasting weather in these low latitudes. The present paper is devoted to a study of the disturbances in the average pressure distribution and the variations in the general wind circulation which are associated with the day‐to‐day outstanding changes of the weather of the Sudan. Four different types of summer disturbance have been distinguished as follows:— (1) Sand‐storms, or “haboobs”, with a modification of the general circulation by the onset of strong S. winds. These storms are common over the desert areas of the central and northern Sudan. Marked strengthening (and deepening) of the S. to SW. monsoon usually accompanies the northward extension of the barometric maximum of the SE. Sudan together with a northward oscillation of the intertropical convergence zone (I.C.Z.). The origin of this modification of the general circulation appears to lie in a pressure surge which first shows itself by a rise of pressure and fall of temperature in the S. Sudan‐Uganda area and spreads northwards rapidly. (2) Dust‐storms, or “haboobs”, with “induced cold fronts”, or line squalls set up by thunderstorms. (3) Low‐level (or local instability) thunderstorms, which form, under favourable conditions of temperature and humidity, by direct convection within the S. to SW. monsoon. These generally occur when the I.C.Z. is at its farthest north, the depth of the monsoon current being then greatest and there is marked increase of moisture content within the lowest layers. (4) High‐level (or widespread) thunderstorms followed by fairly widespread outbreaks of rain. The term “wide‐spread rain” has been proposed to describe the rainfalls which cover a wide area and may persist for about 24 hours in places.