The Influence of Low-Level Stable Layers on Damaging Surface Winds within Bow Echoes

Observational and numerical modeling studies have shown that damaging surface winds within bow echoes are produced by a descending rear-inflow jet (RIJ) or by “mesovortices” formed on the bow echo gust front. Not all bow echoes, however, produce damaging surface winds. Recent observational studies have hypothesized that low-level stable layers may inhibit damaging winds from reaching the surface. The physical processes responsible for the suppression of damaging surface winds within bow echoes are generally not well understood.

In this study, the potential role of low-level stable layers in suppressing damaging surface winds in simulated bow echoes is investigated with the Advanced Research WRF (ARW) model. The model is uniformly initialized within an idealized sounding containing 2200 J Kg-1 of convective available potential energy and 20 ms-1 of vertical wind shear over the lowest 2.5 km.

The control run, with no stable layer, produces a well defined bow echo with a descending RIJ and numerous mesovortices on the gust front. Low-level stable layers are then introduced in two ways. In the first set of experiments, a near-surface stable layer 165 meters deep is initialized over the entire domain. The surface temperature perturbation from the base state is varied from 3K to 10.5K within the stable layer. In the second set of experiments, the stable layers are placed on the eastern side of the domain and propagated westward with the base state flow. They then interact with the developing bow echo at 2.5 hours into the simulation.

For both sets of experiments, it will be shown that the low-level stable layer appears to play a significant role in inhibiting strong winds from reaching or forming near the surface. In all experiments, well-defined bow echoes with system-scale features such as book-end vortices and the RIJ are observed. When a low-level inversion is present everywhere over the domain, the surface winds associated with the descending RIJ weaken and the number and strength of the mesovortices diminishes dramatically as the low-level inversion strength approaches that of the cold pool. In the experiments when the developing bow echo encounters the stable layer at 2.5 hours into the simulation, the number and strength of mesovortices decreases dramatically thereafter. Experiments with greater inversions depths have also been performed. The physical processes that inhibit the RIJ jet from reaching the ground and suppress the formation of the mesovortices will be presented at the conference. The forecasting implications of these results will also be discussed.