Gravity waves as a mechanism of convection triggering

Gravity waves may often play a role in the initiation of deep convection, especially over tropical ocean, where both processes are expected to be active. Here, we explore this role in a 5km-resolution numerical simulation of the triggering of a squall line that occurred on 11 december 1992 during Toga-Coare. Satellite data show that the disturbance seems to be excited by a previous mesoscale convective system (MCS) about 200km to the north-east 6 hours before. The simulated initial MCS is seen as a source of convection. A spectral analysis of the source reveals a broad spectrum of scales. On the contrary, away from the source, only a few pure modes prevail : the first one, characterized by a compensating subsidence in the free atmosphere and an ascent in the PBL, evolves slowly ; the second one is a deep convective mode travelling at 50-55m/s, with a period of 1 hour ; the third one is a less deep mode, probably excited by stratiform precipitation of previous MCS. The remarkable simplicity of spectral analysis could be a consequence of a scale selection, due to the trapping of short waves which cascade upscale and permit the two propagative modes to be radiated into the far field without significant loss. Budgets of humidity and temperature fields are then performed to analyze the influence of gravity waves on the triggering process. At the beginning, PBL ascent and turbulent eddies force shallow convection, capped by the compensating subsidence. Then a modulation by the fast mode alternatively induces favourable and unfavourable conditions for the shallow layer deepening. Helped by horizontal humidity advection and cloud detrainment, the gravity wave, during its ascent phase, allows the mid-layer moistening, up to the deep convection triggering instant. Thus, deep convection initiation is mainly driven by a coupling between the PBL dynamics and gravity waves travelling in the free atmosphere.