The multiscale structure and evolution of an Oklahoma winter-precipitation event

Those living in Norman in 1994 might remember the rather complex winter-precipitation event that occurred on 8-9 March in Oklahoma and neighboring states. This event created hazardous road conditions throughout Oklahoma, resulting in hundreds of traffic accidents and two fatalities. Snow accumulations greater than 30 cm (12 in) were measured within a ~50 km-wide corridor in northern Oklahoma. South of the heaviest snowfall - in Norman, for example - convective cells produced mixed-phase precipitation and significant cloud-to-ground lightning.

The structure and role of such storm-scale features in the overall winter precipitation has received little attention in the literature. This motivated Jeff Trapp, Dave Schultz, Alexander Ryzhkov and Ron Holle to analyze a unique dataset collected on this day using the Cimarron polarimetric radar; the Twin Lakes, Oklahoma, WSR-88D (KTLX) complemented the Cimarron radar, allowing in some instances dual-Doppler retrieval of 3-D wind fields. Conventional surface and upper-air observations also were analyzed to provide a synoptic-scale and mesoscale context for the storm-scale analysis and to determine the larger-scale forcing of the heavy snow that accumulated in a narrow band.

We found a broad region of generally stratiform snowfall coincided with lifting provided through frontogenesis forcing aloft, and persisted in the northern half of Oklahoma during our six hour period of study. Within this stratiform region, periods of heavier snow - at times accompanied by in-cloud lightning - were associated with embedded elevated convective elements that likely aided the production of the large accumulation of snow in a narrow band. More intense convective cells formed south of the stratiform precipitation, yet still well north of the surface cold front, in air with little or no surface-based conditional instability. One cell was a prolific lightning producer and particularly long-lived, owing to its rotational dynamics (Figure 2); in other words, it had a midlevel mesocyclonic vortex couplet! A transition zone existed between the areas of stratiform snowfall and convective rainfall and was accompanied by a unique signature in the polarization radar data. There are more details of this study in the March 2001 issue of Monthly Weather Review, and online at http://ams.allenpress.com.By Jeff Trapp