Intra-seasonal and seasonal variability of convective properties of monsoon precipitation systems over West and Central Africa

Abstract

This study investigates the variability of convective properties during West and Central African monsoon season at both intra-seasonal and seasonal time scales based on 16-year Tropical Rainfall Measurement Mission (TRMM) precipitation features (PFs). Three rainforests (west coast, dry, Nigeria–Cameroon), two savannahs (Nigeria, Central Africa and South Sudan (CASS)), Sudano–Sahel and Sahel were classified. Convective properties were grouped according to six intra-seasonal periods defined as pre_monsoon, active_mam, break_mam, active_jjas, break_jjas and post_monsoon. Comparison of West (rainforest zone: south of 12° N) and Central (savannah zone: south of 12° N) African mean convective properties showed that West African rainforest zones often have higher numbers of PFs and mean convective properties than the CASS savannah at all the intra-seasonal periods except during the post-monsoon period. The convective properties are more frequent during pre_monsoon, active_mam and break_mam periods than during the intra-seasonal periods of JJAS (June–September) in the West African rainforest zone, and thus indicating the two distinct rainfall seasons in rainforest of West Africa, but this was not observed in Central Africa. Convective properties are found to have similar climatological pattern to precipitation climatology during March through May (MAM) and JJAS. Extreme cases of flashcounts (> 275), depressed 85-GHz polarisation-corrected temperature (49–65 K and 65–85 K) and maximum heights (> 17.59 km) of the 30 dBZ are frequent in the Nigeria–Cameroon rainforest during MAM, but extreme system sizes (≥ 80,000 km2) are found to be located in the CASS savannah zone during JJAS.