The role of the trailing stratiform region in convective momentum transport and mesoscale convective system motion

Mesoscale convective systems (MCSs) are commonly characterized by a leading convective line followed by a region of stratiform precipitation. The trailing stratiform region is typically 50 – 200 km wide and features two main flow regimes: front-to-rear flow that ascends from middle to upper levels, and rear-to-front flow that descends from middle to low levels. Both airstreams are of dynamical and structural importance to the MCS itself; the ascending front-to-rear flow distributes hydrometeors into the stratiform region from the leading convective region, and the rear-to-front flow (or rear-inflow jet) is heavily impacted by the trailing stratiform region itself and may directly impact the surface cold pool. It is by this relationship with the rear-inflow jet and thus surface cold pool that the trailing stratiform region may have an effect on MCS motion.

Past studies have shown that convective momentum transport within an MCS may have a significant impact on MCS motion. The processes most important to this feedback are (i) the vertical advection of the storm perturbation wind, (ii) the vertical advection of the background wind, and (iii) the pressure gradient acceleration associated with the midlevel area of lower pressure. As the trailing stratiform region affects the vertical heating profile and buoyancy field, it is thus directly tied to both vertical motion and the perturbation pressure field in the trailing portion of the MCS. Therefore, it is reasonable to expect that gust-front-driven MCSs with stratiform regions of varying spatial extent and intensity may exhibit differences in the low-level momentum field and thus potentially the speed at which the entire system moves.

This study compares quasi-idealized simulations of varying trailing stratiform region horizontal extent to illustrate such differences and discusses implications for forecasting MCS speed. The challenges of simulating realistic trailing stratiform regions with current numerical models are also discussed.