MCS motion: The role of vertical momentum transport

Forecasting the motion of mesoscale convective systems (MCSs) remains a challenge to both human forecasters and numerical models. The movement of observed MCSs is often the combined result of several physical processes that occur on different temporal and spatial scales. The redistribution of momentum within MCSs has been examined by previous studies, mainly within the framework of feedbacks with the larger-scale environment and parameterization in large-scale numerical models. Fewer studies have focused on the downward branch of momentum transport in MCSs, and the ways in which both convective- and mesoscale downdrafts may affect the lower-tropospheric wind field. If such transports substantially alter the low-level momentum field, it is conceivable that MCS speed may be affected, either by the advective effect of increasing the mean cloud-bearing wind, or the propagative effect of increasing winds within the cold pool itself. Therefore, this study seeks to investigate the following question: What influence does the vertical momentum transport (of both large-scale and perturbation winds) by an MCS have on the ground speed of the MCS itself?

Toward answering this question, momentum budgets are computed using model output in order to quantify the contribution of specific processes to the low-level wind field in the system's surface-based cold pool and thus to MCS motion. Results show that the low-level momentum field toward the leading convective line of the MCS is most influenced by the vertical advection of the storm-induced perturbation wind. The mid-to-low-level wind field across the middle-to-rear portion of the system is largely determined by the pressure gradient acceleration and to a lesser extent, the vertical advection of the background environmental (base-state) wind. The prominence of the vertical advection terms in these storm regions, in addition to the strong correlation between these momentum tendencies and MCS acceleration, suggests that downward momentum transport by the MCS is a significant driver of MCS motion and potentially severe surface winds. Furthermore, the neglect or inadequate representation of these processes by numerical simulations utilizing convective parameterization schemes may lead to errors in MCS representation and a potentially negative bias in MCS speed.

Improving the forecast accuracy of MCS motion has important implications for the issuance of timely public watches and warnings and also for forecasting the type and timing of sensible weather such as strong surface winds, hail, heavy rainfall, and isolated tornados. Results also suggest a connection between the convective momentum transport process and the production of severe surface winds, particularly when in conjunction with regions of strong rear-inflow jet descent and mesovortices along the leading line of convection. Implications for forecasters and severe weather potential will be highlighted.