Multiscale Organization in Simulated Shear Parallel Mesoscale Convective Systems

A major research issue in organization tropical convection is a good understanding of the way moist convection interacts with the large scale flow and waves in particular. Organized mesoscale systems are a good medium through which small scale convective cells interact with the ambient shear. They are also important to study in their own right as they induce a great deal of climate variability especially in the monsoon regions and in the eastern pacific ITCZ. They are also believed to strongly interact with convectively coupled waves and the MJO. In this talk we will discuss new numerical simulations using the Research and Weather Forecasting (WRF) model of mesoscale systems evolving in a background shear. We are particularly interested in long time integrations allowing the systems to develop and die and essentially interact with the background shear. Starting with a typical African and equatorial jet-shear, the simulation goes through various phases or stages. First, a transient state, consisting of scattered squall -like systems that are aligned perpendicular to the background shear which then evolve onto multiscale mesoscale systems with large stratiform anvils. During this period the background wind goes through a major transformation through the effect of both up scale and down scale convective momentum transport (CMT), resulting in a first baroclinic flow structure resembling the MJO zonal wind. At this stage, the mesoscale systems turn around and become aligned parallel to the shear and are characterized by elongated stratiform anvils in which meso-beta scale convective systems evolve and propagate parallel to the shear direction, with much slower speeds compared to the main stratifrom envelope, which moves at a much faster speed consistent with an upper level steering level wind. This is reminiscent of some recent results obtained by Khouider and Mocrieff (2015, J. Atmos. Sci., 72, 3073–3096) using a simple multi-cloud model and as such it has important implications for the parameterization of CMT in climate models. The three-way interactions between the meso-beta convective cells, the meoscale stratiform systems and the background wind shear will be discussed and particular attention will be given to the transport of momentum and moisture across scales, which are found to be the key drivers of these disturbances.