A subtropical rainstorm associated with a tropical plume over Africa and the Middle-East

Cloud bands that extend from the ITCZ along the subtropical jet toward the subtropics are known as 'tropical plumes'. At times rainstorms develop at their subtropical edges. One such rainstorm swept eastern North Africa and the Middle East on 23-24 December 1988, with rainfall comparable with the annual averages there. This study examines the storm using the ECMWF initialized data together with surface observations and satellite imageries. The analysis indicates that the storm developed at the inflection region ahead of a pronounced trough in the subtropical jet, with which a mid-latitude trough was merged. Two ageostrophic effects taking place along the jet ahead of the trough contributed to the intensity of the rainstorm. One was associated with acceleration at the jet entrance, located at tropical latitudes, which contributed to the enhancement of both tropical convection and the southerly wind component, which enhanced the moisture tropical transport toward the subtropics. The second was the enhanced near-tropospheric divergence associated with positive vorticity advection at the inflection region itself. Since both effects have a quadratic dependence on wind speed, the observed jet speed, 50% larger than its average value, explains the observed divergence at the inflection point at the 200 hPa level, over 6x10^-5 s^-1, and the vertical velocity at the 700 hPa level, about 10^-1 ms^-1. It is suggested here that the merging of a mid-latitude with the trough in the subtropical jet, with which the tropical plume is associated, is the cause for the intensification of the subtropical jet and hence of its related rainstorms.