Session 17B.3 The Super Tuesday outbreak: forecast sensitivities to single-moment microphysics schemes

Thursday, 30 October 2008: 5:00 PM
South Ballroom (Hilton DeSoto)
Andrew L. Molthan, University of Alabama Huntsville, Huntsville, AL; and J. L. Case, S. R. Dembek, G. J. Jedlovec, and W. M. Lapenta

Presentation PDF (2.3 MB)

The “Super Tuesday” Outbreak of February 5-6, 2008 gained notoriety for the numerous severe weather reports that occurred within several states holding presidential primaries. Other impacts also included widespread, heavy precipitation throughout the Midwest and significant snowfall accumulation across the Great Plains. Therefore, this multi-faceted event provides a good case study to examine the microphysical properties of simulated phenomena and sensitivities within a numerical weather prediction (NWP) model.

As operational numerical weather prediction is performed at increasingly finer spatial resolution, convective precipitation traditionally represented by sub-grid scale parameterization schemes is now being calculated explicitly by single- or multi-moment, bulk water microphysics routines. These schemes are also being used to acquire representative profiles of latent heat release and other cloud characteristics. As with any parameterization, their reliability must be measured through performance metrics. Furthermore, the use of these schemes within an operational forecast system requires an understanding of limitations or estimate of biases so that forecasters can be aware of potential errors.

In order to progress toward these goals, three forecasts are generated using the Weather Research and Forecasting (WRF) model, simulating the event within a domain and parameterization selection designed to mirror experimental real-time forecasts generated by the National Severe Storms Laboratory. Various single-moment, six-class bulk water microphysics schemes are examined: the WRF scheme (WSM6), the Goddard 3-ice scheme with graupel (GSFC6G) and the Goddard 3-ice scheme with hail (GSFC6H). Hourly precipitation rates are compared to the NCEP Stage IV gridded analyses during the 36-hour model integration. In addition, differences between observed and modeled cloud reflectivity distributions are noted when comparing forecast warm frontal convection to observations made by the WSR-88D network. Differences among microphysics schemes are related to key microphysical assumptions, and sensitivities of reflectivity profiles to these assumptions are investigated.

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