Convective momentum transport and multiscale organization in simulated shear parallel mesoscale convective systems

The way in which moist convection interacts with large scale flows is a major contemporary research issue. Organized mesoscale systems are, in particular, important for the interactions between convection and the ambient shear. Here we present numerical simulations of mesoscale systems evolving in a background shear using the Research and Weather Forecasting model. We are particularly interested in the long time integration, allowing the systems to repeatedly develop and die and effectively interact with the background shear. Starting with a typical African and equatorial jet-shear, the simulated solution goes through various phases or stages. First, a transient state, consisting of scattered squall-like systems that are aligned perpendicular to the background shear, develops and then evolves into a regime of multiscale mesoscale systems with large stratiform anvils. During the latter period the background wind changes substantially through the effect of both up scale and down scale convective momentum transport. At this stage, the systems become aligned parallel to the wind shear, with elongated stratiform anvils in which meso-beta scale convective cells evolve and propagate in the shear direction, relative to the stratiform anvils. These results are reminiscent of the development of shear parallel mesoscale convective systems observed for instance in the Eastern Pacific ITCZ and corroborate recent theoretical results obtained with a simple multi-cloud model. As such they have important implications for the parameterization of CMT in climate models.