Deep convection initiation in the WRF model

The accurate prediction of deep convection initiation is difficult as initiation occurs due a variety of processes, such as boundary forcing, gravity waves, and variations in surface heating that are not observed well. Yet in many severe weather situations initiation is the most important forecast concern. If deep convection initiation occurs, then severe weather is sure to follow. Unfortunately, numerical forecast models do not always predict convection initiation in the observed location at the observed time. Thus, improvements in the numerical predictions of convection initiation are needed to assist forecasters. By studying the environmental conditions before and after isolated deep convection occurs in the Weather Research and Forecasting (WRF) model, a better understanding of how the model initiates convection can be obtained.

Seventeen cases of isolated deep convection initiation during April – September 2007-2010 are identified from hourly output of the 4-km WRF model run daily at the NOAA National Severe Storms Laboratory. Average conditions, or composites, are created that are centered on the initiation location for the initiation time and for the previous three hours and subsequent two hours from the initiation time. Composite fields from all 78 available archived model variables are examined to determine if there are readily identifiable features in the model fields before or after the model initiated isolated convection. Results indicate that the 700 hPa relative humidity field in the hours prior to initiation features a discernable and increasing maximum in the region near the initiation location. It is hypothesized that this maximum is due to a series of weaker convective cells that form and decay, resulting in a moistening and cooling of the environment between cloud base and cloud top. These cells continue to grow and decay until the environmental conditions are conducive for a strong, sustainable cell to develop. The composite fields show that once a surface outflow forms, the storms begin to intensify and develop a well-defined updraft and downdraft.