Exploring the Sensitivity of Convection Initiation to Grid Spacing in Convection-Allowing NAM Simulations

Forecasting the initiation of convection often is a difficult problem, and this timing can affect both storm type and storm mode, especially when the environment is evolving.  As atmospheric models use increasingly finer grid spacing, processes involved in convection initiation (CI) might be expected to be better represented, leading to increased skill.  In this study, we compare observations to rare 5-minute output from the operational NAM at both 4- and 1.33-km grid spacing.  Two severe weather cases (28 April 2014and 6 May 2015) are simulated. On 28 April 2014, convection initiates along and ahead of a cold front that traverses across Mississippi, causing over 450 severe weather reports (153 of which were tornado reports). On 6 May 2015, CI occurs in the vicinity of a dryline across Oklahoma, resulting in nearly 200 severe weather reports (65 of which were tornado reports). The high temporal resolution of the model output allows for the identification of mesoscale mechanisms responsible for CI. The analysis focuses on identifying key differences in environmental evolution between the two model resolutions and the observations. In particular, we compare gravity wave propagation, moisture advection, and vertical thermodynamic profiles, as well as the strength and orientation of fronts, drylines, and storm outflow boundaries. This research is a part of the Next Generation Global Prediction System (NGGPS) Project.