Diurnal variations and climatological rainfall contribution from extreme convective storms over the tropical belt between America and Africa

This study examines how extreme forms of convection affects the total precipitation and its diurnal variability over the near-equatorial belt containing South America, Africa, the Atlantic Ocean and East Pacific Ocean. We use 15 years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data (version 7) to identify cloud systems ("storms") containing extreme convective features in the form of deep convective cores (contiguous convective 40 dBZ echoes that reach at least 10 km in height), wide convective cores (contiguous convective 40 dBZ echoes with areas >1,000 km2), and broad stratiform regions (stratiform echo areas >50,000 km2). Cloud systems containing such elements represent the most extreme convective storms in terms of intensity, height and/or horizontal extent.

The frequency of occurrence of the three identified types of extreme echo structures occurring over the East Pacific and Central Atlantic oceans shows a weak diurnal variation, with storms contain echo elements in the form of wide convective cores and broad stratiform regions occurring almost every time of the day. In contrast, storms containing extreme echo elements that occur near and over the landmasses of Africa and America experience a marked diurnal variation. Over these continental regions, storms containing deep convective cores show the strongest diurnal variation with a peak in the late afternoon hours and clear minima around midday. Storms that contain wide convective cores experience a weak diurnal variation, with a broad maximum from midnight to early morning. On the other hand, storms with broad stratiform regions showed a peak from the early morning to midday and minimum values in the evening hours. The presence of echo elements peaking at successive times during the day in such a sequence is indicative of the various stages in the life cycle of the population of convective storms with mesoscale characteristics.

Over land, the relative contribution of extreme echo types to climatological rainfall is dominated by storms with wide convective cores, representing the largest part of the total rain across North Africa and northern South America. On the other hand, over the East Pacific Ocean and Central Atlantic Ocean, storms containing broad stratiform regions contribute to the largest part of the climatological rainfall. Storms with deep convective cores represent just a minor fraction of the climatological rainfall contribution for all of the aforementioned regions.

The results presented in this work are critical not only for assessing the diurnal distribution of storms that exhibit different convective character and their relative contribution to the climatological precipitation across different regions of the tropics, but also helpful in providing metrics for the evaluation of the output of both mesoscale and GCM modeling frameworks that simulate convective processes and precipitation in tropical latitudes.