Multicase Assessment of Sub-Kilometer-Scale Processes in Atmospheric Bores Using WRF Large Eddy Simulations

Despite the prominent role of atmospheric bores in initiating and maintaining nocturnal convection in the U.S. Great Plains, typical operational model grid spacings of 3-4 km are too coarse to realistically resolve bores. A series of experiments are therefore conducted to better understand the consequences of this deficiency for prediction skill, the aggregate impacts of sub-km scale bore processes such as turbulence and gravity wave ducting, and the optimal model configurations for nocturnal convection prediction. These experiments will investigate the impacts of changing the forecast model horizontal grid spacing from 4-km to a range of values down to 50m, and the data assimilation model grid spacing from 4-km to 1-km. The impact of enhancing vertical resolution in certain atmospheric layers individually, and together will also be investigated. The bores simulated in this study will be taken from several intensively observed events from the Plains Elevated Convection At Night (PECAN) field experiment from June-July 2015. The PECAN observations allow the data assimilation system to initialize the simulations from more accurate analyses than are possible with currently operational observation networks. The PECAN data also allow for more detailed validation of the simulations.

The presented results will show the improved representation of bore speed and bore amplitude with increasing forecast resolution from 4-km to 1-km and from 1-km to 250-m, as well as increasing data assimilation model resolution from 4-km to 1-km. The 50-m grid spacing Large Eddy Simulations will be used to understand the aggregate impacts of sub-km scale processes and their implications for nocturnal convection prediction with km-scale models.