The Mean Dynamical and Thermodynamic Fields for July 1989 over Tropical North Africa and Their Relationship to Convective Storm Activity

Abstract

Dynamic and thermodynamic fields from ECMWF analyses have been presented for the tropical north African region for July 1989. Except for the region equatorward of 10°N, it has been shown that the pattern of mean low-level ?e has a striking correspondence with the propagating storm statistics obtained from Meteosat in that region, with mean high ?e corresponding to more propagating storm activity, and vice versa. Considerable correspondence is also found with the cold cloud duration pattern and observed rainfall. The low-level ?e pattern poleward of 13°N has been shown to be strongly influenced by the low-level meridional wind v. The v = 0 contour at low levels, for example, has been shown to have a marked northwest-to-southeast tilt between about the zero meridian and 25°E. Superimposed on this is a marked sinusoidal variation in the latitudinal position of the contour associated with the orography in this region. The low-level ?e pattern is consistent with these variations with higher values of ?e associated with southerlies and lower values with northerlies. Especially low ?e values found around 25°E appear to be associated with low-level northerlies emanating from a region of descent in the eastern Mediteranean. A comparison of ECMWF-analyzed mean vertical velocity at 500 mb. a measure of mean model diabatic heating, with satellite-derived variables and rainfall indicates two 1ocations of significant model discrepancy: a region of ascent close to the Air Mountains that was not consistent with observations and a region near the Dafur Mountains with no significant ascent but where the observations indicate the opposite.