Cumulus parametrization and mid-tropospheric African waves

Abstract

Infrared (IR) satellite photographs of 1982 (August, September) and 1983 (July, August, September) have been used to study African waves. Prominent wave formations were identified as clbu.d clusters, and their wavelength was estimated by measuring the distance between the clusters. The phase velocity of the disturbances was estimated by tracking the movement of the cloud· clusteis using consecutive images at 3h intervals. Streamline and isotach analyses of the wind field at 850mb were performed to supplement data on the position and movement of perturbations. Using upper air sounding from Dakar, Abidjan, Bamako, Khartoum and Nairobi, the heights of cloud base and tops were computed by three methods, namely: (a) The Normand Point from tephigram; (b) the Dew-point depression method and (c) the equivalent potential temperature. The bouyancy of the atmosphere at the above radiosonde stations was evaluated by. computing the parcel excess, or log (potential temperature) to investigate whether there was any inhibition to convection. Kuo's (1965, 1974) model was applied to determine the contribution by cumulus convection. The model was run over a horizontal area extending from 200W to 200E longitude, and from 00 to 200N latitude. The average surface heat flux (Wm-2 ) generated by the model was found to be more pronounced in areas of significant. convection and large-scale low-level convergence. The vertical heating profiles depict a maximum in the mid-troposphere (650 - 500mb) a region where the African wave perturbations are most pronounced. The model also simulated convective rainfall rates which were comparable to those observed by Thompson et ale (1979) during the third phase of the Global Atlantic Tropical Experiment (GATE). We do not attempt in the present study to parameterize these heating rates in terms of the large-scale meteorological flow, but simply assert that as maximum heating by convection occurs in regions of maximum wave activity (mid-troposphere), cumulus convection has a significant role to play ln the genesis and dynamics of Africa Waves. This should be incorporated in the mathematical models used to simulate the properties and structural characteristics of these waves