Mechanisms for the production of severe surface winds in a simulation of an elevated convective system

Using idealized numerical simulations, we have been investigating the environments that are favorable for the production of severe surface winds from elevated convective systems. By "elevated," we mean convection in environments with stable layers at the surface, such that the highest equivalent potential temperature (θ_e) is above the surface (in this case, by more than 1 km). In this presentation, we focus our analysis on a simulation which was motivated by the IOP7 bow echo during BAMEX. Similar to the observations, the initial conditions for the idealized simulation has moderate CAPE (2,000 J/kg) and strong low-level shear (20 m/s over the 0-2 km layer).

During the first half of the simulation, a cold pool exists only in the elevated mixed layer at z = 2-4 km. After about 4 h, low θ_e air from mid-levels finally descends to the surface. At this time, strong diabatic cooling occurs in the 0-2 km layer, such that the cold pool extends downward, and now occupies the 0-4 km layer. Concurrently, the system develops a bow shape and produces severe winds at the surface. Passive tracers are used to diagnose the source of the strong system-generated horizontal winds. In this case, there is no evidence for an "up-down" trajectory, which has been discussed in previous studies (whereby it is hypothesized that near-surface parcels enter the system from the front and subsequently descend into the cold pool to create strong surface winds.) Rather, in this case, flow is accelerated into the back edge of the near-surface cold pool as it forms in the 0-2 km layer.