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In the Congo Basin, a drying trend in the April–May–June rains prevailed between 1979 and 2014, accompanied by a decline in forest productivity. This article examines the subsequent years, in order to determine whether rainfall conditions have improved and to examine meteorological factors governing conditions in those years. It is shown that a wetter period, comparable to that of 1979–1993, spanned the years 2016–2020. However, the meteorological factors responsible for the wetter conditions appear to be significantly different from those related to the earlier wet period. The wetter conditions of 1979–1993 were associated with changes in the tropical Walker circulation, in moisture flux and flux divergence, and in Pacific sea-surface temperatures (SST), namely a warmer central and eastern Pacific and a cooler western Pacific, compared to the dry phase in 2000–2014. This resulted in a lower-than-average trans-Pacific SST gradient. In contrast, SSTs were almost ubiquitously higher in the 2016–2020 period than in either prior period. However, there was some reduction in the trans-Pacific gradient. The Walker circulation and moisture flux/flux divergence were not factors in this episode. The major factors provoking the return to wetter years appear to be an increase in convective available potential energy and in total column water vapor. This could be related to the general warming of the oceans and land.

 

Abstract This article examines the diurnal cycle of lake-effect rains over Lake Victoria and of rainfall in the surrounding catchment. The analysis focuses on four months, which represent the two wet seasons (April and November) and the two dry seasons (February and July). Lake-effect rains are strongest in April, weakest in July. In all cases there is a nocturnal rainfall maximum over the lake and a daytime maximum over the catchment, with the transition between rainfall over the lake and over the catchment occurring between 1200 and 1500 LST. During the night the surrounding catchment is mostly dry. Conversely, little to no rain falls over the lake during the afternoon and early evening. In most cases the maximum over the lake occurs at either 0600 or 0900 LST and the maximum over the catchment occurs around 1500 to 1800 LST. The diurnal cycle of Mesoscale Convective Systems (MCSs) parallels that of over-lake rainfall. MCS initiation generally begins over the catchment around 1500 LST and increases at 1800 LST. MCS initiation over the lake begins around 0300 LST and continues until 1200 LST. While some MCSs originate over the highlands to the east of the lake, most originate in situ over the lake. Maximum MCS activity over the lake occurs at 0600 LST and is associated with the systems that initiate in situ . 

Abstract The purpose of this article is to determine the meteorological factors controlling the lake-effect rains over Lake Victoria. Winds, divergence, vertical motion, specific humidity, Convective Available Potential Energy (CAPE), and Convective Inhibition (CIN) were examined. The local wind regime and associated divergence/convergence are the major factors determining the diurnal cycle of rainfall over the lake and catchment. The major contrast between over-lake rainfall in the wet- and dry-season months is the vertical profile of omega. This appears to be a result of seasonal contrasts in CAPE, CIN, and specific humidity, parameters that play a critical role in vertical motion and convective development.