Diurnal Cycle of Rainfall and Convective Properties over West and Central Africa

The need to investigate diurnal weather cycles in West Africa originates from the fact that complex interactions often result between mesoscale and synoptic weather processes. This study investigates diurnal cycles of rainfall and convective properties using six (6) hour interval data from the ERA-Interim and derived products from the Tropical Rainfall Measurement Mission (TRMM). Results showed that the land-ocean warming contrast is more strongly sensitive to the seasonal cycle, being very weak during March-May (MAM) but clearly spelled out during June-September (JJAS). Dipoles of wind convergence/divergence and wet/dry precipitation, between CASS and Nigeria Savannah zones, were identified in the morning and evening hours of MAM, whereas distinct night and day anomalies, same location in CASS, were found to be consistent during the JJAS season. The locations of flash count and system sizes agree with the climatology of convective properties, that morning and day-time hours are dominated by stratiform precipitation and small system sizes. Most results clearly showed that the eastern locations of Sudano and Sahel are consistently dry because rainfall and precipitation features are predominantly few. Very unique results about the dipole of wind and precipitation between two zones and the unusual dry zones of Sudan and Sahel have been found. Results presented had shown the importance of diurnal variation in understanding precipitation, flash count, system sizes patterns at diurnal scales, and understanding land-ocean contrast, precipitation and wind field anomaly at diurnal scales.

Local Impacts and Surface Characteristics of Cold Pools Wind Gust Observed from Thunderstorm

Cold pools and associated wind storms are frequent occurrences in Southwestern Nigeria, especially during the early monsoon phase. The associated surface wind gust frequently destroys properties resulting in economic losses. Two case events were investigated in this study;one event occurred in May 2019 and the other occurred in March 2020, both in southwestern Nigeria. The National Oceanic and Atmospheric Administration (NOAA) National Center for Environmental Prediction (NCEP)/Climate Prediction Center (CPC) Infrared brightness temperatures and CPC Morphing technique (CMORPH) rainfall products were analysed alongside in-situ observations from the Nigerian Meteorological Agency (NiMET). Other data sources analysed are the National Aeronautics and Space Administration (NASA) Prediction Of Worldwide Energy Resources (POWER) and the World Wide Lightning Location Network (WWLLN). Cold pools were identified in the impacted communities as indicated by surface characteristics investigated from the in-situ observations. There was a sudden change in wind direction, with a simultaneous drop in temperature accompanied by increasing wind speed. Pressure and humidity were observed to change in the same period. Thunderstorms were also present in the impacted communities, as observed by the in-situ observations, in both case events. The presence of lightning as observed by WWLLN agrees with the in-situ thunderstorms. The cloud characteristics showed the presence of cloud shields, by their brightness temperature, over the impacted communities during the period of the cold pools in both case events. The systems were raining at the time of the observations in both cases, consistent with the in-situ thunderstorm observations. The communities were heavily impacted with several properties destroyed in the events. These early monsoon seasonal windstorms require a forecasting tool for their prediction and this study presents an eye-opener for further investigation and innovative research to address the menace.

Distribution of Convective Intensities in West Africa Using Reflectivity and Ice Scattering Characteristics from TRMM Precipitation Features

An evaluation of Radar Precipitation Feature (RPF) characteristics and distribution of convective intensity is performed across 12 regions in West Africa. Results presented in this study have shown that these characteristics over West Africa revealed interesting results which were not observed on a larger spatial scale. The ice scattering characteristics and heights attained by the 15, 20, 30, and 40 dBZ echoes show patterns that agree with the season and movement of the Inter-Tropical Discontinuity (ITD). Some locations in the Western-coast rainforest, Nigeria/Cameroon rainforest and South Sudan savannah had strong potential for convective intensity during MAM, JJA, and SON as shown by their 37-GHz and 85-GHz PCT which fell below 250 K and 225 K respectively while the maximum height attained by their 20 dBZ, 30 dBZ and 40 dBZ are well above the freezing level in those locations. One result revealed a location on the eastern part of south-central Sahel (SC Sahel) where the maximum height attained by the 30 dBZ reflectivity is above 12 km and the maximum height attained by the 40 dBZ reflectivity is above 10 km during SON. The 37-GHz and 85-GHz PCT for this particular location are below 215 K and 150 K respectively indicating a very strong potential for intense convection and hence destructive storms. The distribution of convective intensity, considering only the 85-GHz PCT ice scattering signature, revealed that the percentage of convective intensity increases, especially in the rainforest and savannah, as the ITD shifts northwards during MAM, JJA and during its retreat in SON.