School of Meteorology (Defense)

Evaluation of 1- and 4-km WRF-ARW Forecasts of Storm Motion during the 2010 VORTEX2 Convective Season

Michael Vandenberg
OU School of Meteorology

02 December 2013, 9:00 AM

National Weather Center, Room 5820
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

The goal of this study is to assess the ability of the WRF-ARW model runs to predict storm motion over a period of time in the spring of 2010. To accomplish this, we first utilized RUC soundings to compute 2D fields of Bunkers╩╝ motion, Rasmussen-Blanchard motion, the 850-300 mb mean wind, and the 0-6 km vertical shear vector to
characterize the background environment. We then used a tracking algorithm to determine the motion of storm cells based on observed reflectivity images at 5-minute intervals. The deviation of these observed storm tracks from the four environment-based storm motion predictors was then computed. We repeated the process using WRF-ARW modeled environments and simulated reflectivity fields at grid spaces of 4-km and 1-km. In addition, we smoothed the 1-km data to a 4-km grid to see if removing the smallest
cells had any impact on the interpretation of the performance of the 1-km run.

The raw 1-km model forecasts were found to more closely match the observed distribution of storm track direction and speeds in both absolute and environment relative terms, but not when smoothed to a 4-km grid. We hypothesize that the 1-km
model performed better than the 4-km model because internal processes responsible for storm propagation may be better resolved on the 1-km grid. In addition, it appears as though the reason for the improvement in the 1-km forecasts is a more accurate prediction of the rightward deviation of storms (the 1-km forecasts reduce a rightward bias in the 4-km forecasts), with a secondary reason being a more realistic depiction of the less frequent moderate left-moving storms.

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