Precipitation systems in the most ‘continental’ of continents: Africa

Teferi Dejene, MS Candidate

The Tropical Rainfall Measuring Mission (TRMM) 2A25 radar reflectivity profiles and derived surface rainrates are used to describe the vertical structure of precipitation systems in Africa. The spaceborne radar has a wavelength of 2.2 cm, a surface footprint of 4.5 km, and a sensitivity of 17 dBZ. Five years of data are used in both the boreal and austral zenithal rain periods of Africa. A number of climate regions are isolated and compared. To place this description in context, it is contrasted against TRMM 2A25 observations over the Amazon basin.

In all of Africa, precipitation systems during the high sun tend to be deeper and more intense than those in the Amazon. Shallow warm rain events are rare, but many storms have their 20 dBZ echo top at or just above the freezing level. Especially in the Sahel and northern Savanna regions, storms have a more convective vertical echo structure with high hydrometeor loading aloft, barely a kink at the freezing level, and evidence of low-level evaporation. No region in Africa, including the Congo basin, comes close to the stratiform appearance of the composite reflectivity profile over the Amazon. The Amazon reflectivity profiles display a clear bright band signature and a rapid decay of reflectivity with height above the bright band.

Storms in Africa are most common, and most intense, in the late afternoon, although the diurnal modulation is regionally variable. It is weakest in the Congo basin (although still stronger than in the Amazon), where a secondary pre-dawn maximum occurs in both the frequency and intensity of rain events. Most regions exhibit a clear transition from shallow convection around local noon to deep convection in the afternoon and evening. This implies that the hydrometeor loading of early storms is more shallow than that of later storms.