Explaining low convective echo top heights during a strong DYNAMO westerly wind burst

A strong MJO associated with an intense westerly wind burst was observed in November 2011 during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign. Radar observations show that the event began with deep convection and then transitioned to predominantly stratiform rain with embedded, shallow convection for nearly two days. A westerly wind burst ensued, which featured 20 m/s sustained surface winds and many embedded westward moving squall lines. These embedded squall lines exhibited highly tilted updrafts that frequently failed to reach the melting layer in an environment that would otherwise favor deep tropospheric convection.

In order to ascertain the dynamics that contributed to this convective behavior, high-resolution convection permitting numerical simulations were conducted with the weather research and forecasting model (WRF) with ERA-Interim initial, lateral boundary conditions, and assimilated DYNAMO upper air observations. Preliminary analysis of these simulations suggests an imbalance between strong cold pool induced vorticity from horizontal buoyancy gradients and only moderate vorticity associated with westerly vertical wind shear. This resulted in sub-optimal lifting along outflow boundaries, which potentially explains the unusually weak and shallow nature of this westerly wind burst convection.