Convection Initiation within Convection-allowing Numerical Weather Prediction Model Forecasts

While we are beginning to understand the fine-scale processes that produce convection initiation using data from field experiments, we have not yet fully explored how convection initiation occurs in numerical weather prediction models. Hourly output is not sufficient to discriminate how deep convection forms, and even 15-minute output fails to capture fully the processes involved. In this study, 5-minute model output will be used to explore the physical processes of convection initiation in convection allowing model forecasts.

The Weather Research and Forecasting (WRF) Advanced Research version will be used to produce forecasts of 10 to 20 deep convective events encompassing various regions within the United States during the warm season and using 4 km horizontal grid spacing and 51 vertical layers. Five-minute output files will be produced during the hour prior to convection initiation and the physical processes that lead to the initiation of deep convection studied. One unexpected example is that initiation of deep convection occurred due to the transition of a layer of stratus into roll-like circulations; cloud-top entrainment instability is suggested. Other examples and a summary of the pathways by which initiation occurs in convection allowing model forecasts will be provided. If the research progresses well, comparisons between the processes leading to initiation from 4 km and 1 km horizontal grid spacing will be presented.