Tropical plumes and extreme precipitation in subtropical and tropical West Africa

Abstract

SUMMARYThree 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.

On the spatial coherence of rainfall over the Saloum delta (Senegal) from seasonal to decadal time scales

“…Note that because of our use of monthly mean values, the anomalous geopotential structures found both at seasonal and interannual timescales are not significant at the 95% level. Nevertheless, the dynamical elements are consistent with previous comprehensive studies of Knippertz and Martin (2005), Meier and Knippertz (2009) and Knippertz and Fink (2008), primarily based on single case-studies. To our knowledge, this is the first time that these mechanisms are described at lower frequencies.…”

“…At low-level, moist southerlies enter the area. This situation favors the creation of unstable air mass and convective dynamics (Knippertz and Martin, 2005). At interannual timescales, anomalous OLR is mostly significant over the oceanic band.…”

Airborne lidar observations in the inflow region of a warm conveyor belt

“…PV is destroyed by diabatic processes above the level of maximum latent heating, which leads to negative PV anomalies in the upper tropospheric WCB outflow region (Wernli, 1997;Pomroy and Thorpe, 2000;Grams et al, 2011). The upper tropospheric modification of PV at the level of the midlatitude jet stream can have a significant impact on the Rossby wave development and associated surface weather further downstream (Massacand et al, 2001;Knippertz and Martin, 2005).…”

Review Article: Atmospheric conditions inducing extreme precipitation over the eastern and western Mediterranean

“…For the western parts of the MR, the role of remote moisture sources for extreme rainfall events has been demonstrated by Knippertz and Martin (2005), for example. They discuss a case of extreme precipitation in the Atlas Mountains associated with a plume of moisture from low latitudes advected along the west African coastline.…”