Convection over Tropical Africa and the Atlantic Ocean during Northern Summer. Part I: Interannual and Diurnal Variations

Some aspects of the interannual and the diurnal variations of the convection over the tropical Africa and the Atlantic Ocean are derived using Meteosat data. The study is based on four summer months (June, July, August and September) of three years from 1983 to 1985, for regions of 2.5°×2.5° extending from 5°S to 25°N and from 50°E to 50°W. Using ECMWF (European Centre for Medium Range Weather Forecasts) analyses, mean cloud fields and interannual changes are interpreted in terms of dynamical forcing and feedback. Anomalies in the thermal wind at 700 mb between 1985 (relatively wet year) and the two other years are consistent with previous results based on more contrasted wet and dry years. Defining the high cloudiness by a threshold in the infrared signal, the amplitude of the diurnal variation is maximum over land with larger values over regions of large mean fractional cloudiness corresponding generally to regions of highlands. The diurnal cycle of high clouds is generally not sinusoidal and the period of development is shorter than the period of dissipation. Over land the maximum high cloud coverage occurs between 1800 LST and midnight and the minimum between 0900 LST and noon. Over oceanic coastal region the maximum high cloudiness is maximum around local noon. The phase, however, becomes more noisy far from land areas. The diurnal composite of the infrared histogram of selected regions gives additional information. A striking result is the general presence of a large concentration of midlevel cloud with tops typically near 500 mb. These clouds have a maximum development near sunrise and a minimum in the afternoon. This particular diurnal phase modifies their radiative forcing, giving an enhancement of their greenhouse effect at the expense of their albedo effect. Another striking result is the existence of a strong coherent diurnal cycle of the cloudiness over all oceanic convergence areas. This diurnal behavior of the cloudiness is basically the same over all ocean regions studied and is compatible with results obtained for regions of the tropical Pacific.