On the Diurnal Cycle of Rainfall and Convection over Lake Victoria and Its Catchment. Part I: Rainfall and Mesoscale Convective Systems

摘要

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 and 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 local standard time (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. Inmost cases the maximum over the lake occurs at either 0600 or 0900 LST and the maximum over the catchment occurs around 1500-1800 LST. The diurnal cycle of mesoscale convective systems (MCSs) parallels that of overlake 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.