Convective Meteorology (Mesoscale Dynamics)

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

Michael Vandenburg
OU School of Meteorology

08 November 2013, 3:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

In this study, we compare the ability of WRF-ARW model simulations at grid spaces of 1- and 4-km to predict observed storm motion from the 2010 VORTEX2 convective season. In addition, the 1-km model data is smoothed to the 4-km model grid in order to determine if smoothing has any effect on the interpretation of the 1-km results. To facilitate all of this, cells are tracked using an object-based tracking algorithm with 5-minute reflectivity data as input. The resulting observed and modeled storm tracks are compared to four environment-based storm motion predictors: Bunkers’ and Rasmussen-Blanchard right-moving storm motions, 850-300 mb mean wind, and the 0-6 km vertical shear vector. These variables are derived from soundings computed by the RUC and WRF-ARW for the observed and modeled environments, respectively.
Differences in both the raw observed and modeled storm motions as well as differences in their deviations from the environmental shear are computed. In addition, analogous right-moving storms between the observations and the models are identified manually; each analogous storm track is divided into thirds in order to determine how each model’s ability to predict storm motion changes throughout the lifecycle of a storm.
From comparison of the storm motion deviations, it is apparent that the 4-km and smoothed 1-km model data under-predict the relative proportion of left-moving storms and storms that do not deviate much from the mean 0-6-km shear. Raw output from the 1-km simulation better approximates this proportion, though still underestimates it. This indicates that the modeled storms in the 1-km simulation that are too small to be resolved on the 4-km grid tend to be left-movers. Generally, the 1-km simulation is shown to be the best representation of the observed storm motion in terms of both velocity magnitude and direction.

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