Organization of Rainfall by an Unstable Jet with an Application to African Waves

Abstract

African waves are believed to originate as shear instabilities, although in certain cases rainfall is organized so that latent heating contributes to wave growth. What determines whether the shear instability can organize rainfall is considered here; in particular, why African waves organize rainfall mainly during the late summer, despite the regular occurrence of shear instability and rainfall throughout the season.During GATE, moisture convergence by the waves was also largest toward the late summer. It is assumed that an African wave will organize rainfall if it converges moisture?as measured by the ascent at the top of the moist layer?with sufficient amplitude. The wave amplitude is specified at some level beneath the 600-mb African jet, whose instability is a plausible source of the wave. The ascent is calculated using the quasigeostrophic potential vorticity and thermodynamic equations, and depends on the zonal wind separating the unstable jet from the top of the moist layer.Before turning to the example of the African jet, the more general behavior of the model is considered. In the absence of shear, a wave can arrive at the moist layer with undiminished amplitude. However, the ascent corresponding to this wave is small?less than the estimated ascent for Phase I of GATE when rainfall remained unorganized. For larger values of the shear, this threshold can be exceeded, although the ascent decays beneath the jet. Thus, the question arises whether a wave source can organize rainfall from an arbitrarily large distance above the moist layer. It is suggested that organization can only occur if the unstable jet is within a few kilometers of the moist layer and separated by large shear, although exceptions are noted.The calculation is applied to a wind profile resembling the observed 600-mb African jet. The wave amplitude decays beneath the jet so that the ascent at the top of the moist layer increases as the separation of the jet and moist layer decreases. Evidence is presented that the waves are closer to the moist layer during the late summer, resulting in larger ascent at this time.Large variations in the ascent can also occur even if the separation of the jet and moist layer remains constant. It is shown that the ascent can vary greatly as a result of small changes in the jet that are within its observed summer variability.