The Impact on Simulated Bow Echoes of Changing Grid Spacing from 3 km to 1 km in WRF

Convection allowing models, such as the Weather Research and Forecasting (WRF) model, have shown some shortcomings when forecasting convective modes. This is especially true for the forecasting of bow echo events. Previous studies have suggested that finer grid spacing, specifically 1 km, does a better job of predicting bow echo events than 3 km grid spacing. However, these studies also suggest that the Morrison microphysical scheme is less affected by the grid spacing change, as it has less of an underprediction of bow echoes at 3 km than present when other schemes are used. The Thompson microphysical scheme, for instance, is more affected, showing more bow echo events at 1 km than at 3 km.

The current study expands upon these prior works by analyzing 10 observed bow echo events in the United States. These 10 events were simulated with the WRF model using 3 km and 1 km horizontal grid spacing. All 10 cases were simulated using both the Morrison and Thompson microphysics schemes. The simulated reflectivity and the observed reflectivity were used to classify convective mode and score the simulation based upon how similar it was to the observed system. Preliminary findings reflect those found in previous studies. For the events simulated using Morrison microphysics, if a bow echo was simulated, it was always present in the 1 km run. There have not been any instances where 1 km failed to forecast a bow while 3 km did. However, the Morrison microphysics scheme handled the 3 km bow echo simulations much better than Thompson, which saw greater improvement when changing from 3 km to 1 km. Most 1 km simulations had similar accuracy in bow echo depiction in both the Morrison and Thompson runs. It is hypothesized that finer grid spacing, especially in runs utilizing Thompson microphysics, may result in stronger microphysical tendencies that lead to stronger rear-to-front flow in the storms, making simulation of a bowing line segment more likely. Analyses of these microphysical tendencies and their contribution to the development of the cold pool are being performed to test this hypothesis. Additionally, the near storm environment and overall evolution of these storms is being analyzed to better understand why bow echo depictions changed as horizontal grid spacing changed.