Predictability and Dynamics of Warm-Core Mesoscale Vortex Formation with the 8 May 2009 "Super Derecho" Event

Abstract

The predictability and dynamics of the warm-core mesovortex associated with the northern-flank of the 8 May 2009 “Super Derecho” event are examined by coupling the Advanced Research Weather Forecasting Model with the Data Assimilation Research Testbed facility. A 50-member convection-allowing EnKF ensemble was produced with 6 hourly-cycled analysis and assimilated observations. Cycled analysis started five days prior to 1200 UTC 7 May 2009, at which time the 36 h ensemble forecasts were launched. The ensemble forecasts all attempted to produce a mesoscale convective system (MCS) but only fourteen percent produced a warm-core mesovortex-like feature similar to the intensity of the observed mesovortex. Ensemble sensitivity analysis was conducted to analyze the environmental differences between ensemble members. Six member composites were also created by selecting the members with the strongest and weakest 850 hPa circulation associated with the mesovortex during the 1000 UTC to 1400 UTC 8 May 2009 timeframe. It is found that a more amplified upstream upper-level trough a few hours prior to peak strength in the simulated mesovortex is associated with a stronger 850 hPa circulation. Cascading effects on the mesoscale from the amplification of the trough occur as the low-level jet and frontal zone magnitudes increase. More moisture is able to be transported poleward into western Kansas in the stronger 850 hPa circulation members leading to convection initiation (CI). We hypothesize that CI must occur early enough in order for the characteristic airstreams of a MCS to converge the background cyclonic absolute vorticity and the vorticity contributions from the eddy and tilting vorticity terms of the local circulation tendency equation.