Tropical plumes and extreme precipitation in West Africa: Moisture transports, precipitation generation and large-scale dynamic and diabatic processes

Abstract

Three cases of extreme precipitation in October, January, and March/April over (sub-) tropical West Africa have been selected to study the involved moisture transports and precipitation generating mechanisms. All cases show strong tropical-extratropical interactions and elongated cloud bands extending from the Tropics into the subtropics, usually referred to as tropical plumes (TPs). Investigations are based on observational data and high-resolution output from simulations with the University of Wisconsin-Nonhydrostatic Modeling System (UW-NMS).

The cases involve a moisture export from the African Tropics, potential air mass instability, quasi-geostrophic (QG) forcing for ascent to the east of an upper-level trough, and a strong, dynamically unstable and highly divergent upper-level subtropical jet (STJ). In the January and March cases the actual precipitation event is preceded by the passage of a precursor upper-level trough over the same location that initiates the moisture export. In October, a wave in the tropical easterlies and strong trade winds from the SH are important factors. A second focus is on the role of large-scale dynamic and diabatic processes leading to the generation of the involved deep upper-level troughs. It will be shown that the potential vorticity (PV) streamers associated with the observed troughs form as a result of an equatorward transport of high-PV air downstream of a large ridge over the North Atlantic. A crucial point is the amplification of the ridge through diabatic reduction of upper-level PV in association with an explosive baroclinic development near the east coast of North America.