Bowing convective systems in a popular operational model: Are they for real?

Bowing, propagating precipitation systems appear as prominent features in output from the NAM (formerly called the Eta model) and other models that use the Betts-Miller-Janjic (BMJ) convective parameterization, especially during the spring. These features often catch the eye of forecasters because their physical dimensions and general appearance are similar to bow echoes, which can be very destructive phenomena. Yet, an explanation for their origin, mechanism of upscale growth, and meteorological significance has never been offered. Such an explanation is provided in this study, based on diagnostic analysis of a simulated bowing, propagating system that appeared in Eta model output on the same day that a damaging bowing mesoscale convective system (MCS) plowed through the western Great Lakes region in June of 2001. The systems are shown to be associated with an unusual convective heating profile generated by the BMJ scheme in certain environments. A key component of this profile is a deep layer of cooling in the lower to middle troposphere. This strong cooling tendency induces circulations that favor expansion of convective activity into nearby grid columns, which can lead to growing, self-perpetuating mesoscale systems under certain conditions. The propagating characteristics of these systems are examined and three contributing mechanisms of propagation are identified. Although these systems often appear in meteorological regimes that support real bowing systems, their correspondence with observed strongly propagating systems is typically poor, especially on the time and space scales that are important for regional weather prediction. Furthermore, forecasters often view them as spurious features, since they appear in model output much more frequently than bow echoes develop in reality. Thus, it appears that they have limited value as predictors of real bowing MCSs.