Tropical plumes and extreme precipitation in subtropical and tropical West Africa

Abstract Three cases of extreme precipitation in October, January and March/April over subtropical north‐western and tropical West Africa have been selected in order to study the moisture transports, precipitation generation mechanisms and large‐scale dynamics involved. 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 using the University of Wisconsin–Nonhydrostatic Modeling System. Trajectory analyses show that moisture is transported from the tropical easterly mid‐level flow, the monsoonal southerlies and the north‐easterly trade winds to provide precipitable water and potential instability at mid‐tropospheric levels. The instability is released through ascent related both to weak quasi‐geostrophic forcing to the east of an upper‐level trough and to the strong, inertially unstable and highly divergent upper‐level subtropical jet (STJ) accompanying the TP. In the January and March cases the passage of a precursor upper‐level trough over the same location precedes the development and initiates the moisture transport from the deep Tropics. The associated north‐westerly cold advection supports the generation of precipitation by triggering convection over the tropical Atlantic Ocean, whose outflow feeds into the STJ over Africa, and by enhancing the vertical mass transport within the intertropical discontinuity over and near West Africa. In October, when the still active African monsoon facilitates the extraction of tropical moisture, a wave in the tropical easterlies and strong trade winds from the southern hemisphere are dominant factors. Low‐level cyclogenesis and frontogenesis only play roles in the late stages of the developments. The elongated, mostly positively tilted 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. The rapid amplification of the ridge is achieved through a combination of negative horizontal PV advection and diabatic reduction of upper‐level PV through latent heating within a cloud band that forms in connection with an explosive baroclinic development near the east coast of North America. Copyright © 2005 Royal Meteorological Society