Profiling the Local Tornadic Environment

Several years ago, the National Research Council convened a committee to help define affordable and effective solutions to the need for a national integrated mesoscale observing system (a “network of networks”) near the Earth's surface. Of particular importance was the need to obtain vertical profiles of moisture and wind using both passive and active remote sensing techniques.

The Windsor, Colorado EF3 tornado event of 22 May 2008 was a highly unusual event in a number of respects, and thus a good candidate for study of the utility of ground-based remote sensing of the local severe thunderstorm environment. By generating a profile every few minutes, a microwave radiometer at Boulder, Colorado (50 km from Windsor) and a 404-MHz wind profiler at Platteville (located where the tornado initially touched down) were able to detect rapidly changing conditions immediately prior to the explosive development of the Windsor storm. The radiometric profiler observed a rapid increase in the CAPE values to 3500 J/kg in the 2h preceding touchdown of the tornado associated with ~8K increase in equivalent potential temperature θe below 700 hPa (1.3 km AGL) above an elevated, but relatively constant value in the preceding 6h that morning. In addition, the radiometer Neural Network information indicated decreases of~3K in θe above 600 hPa during the morning. Potential instability below 800 hPa doubled just prior to tornado touchdown in association with pronounced heating and moistening. Vertical wind shear in the 0–4 km AGL layer (some missing data above 4 km prevented determination of the normal 0–6 km shear) increased dramatically from 5 to 30 m s-1 in the 10h prior to storm development, with most of that increase occurring in the last 3 h.

These rapid changes in the local storm environment were consistent with analyses performed on a 1.7-km resolution grid with the Local Analysis and Prediction System (LAPS). A narrow ribbon of high equivalent potential temperature air with large CAPE values was diagnosed in the LAPS analyses directed precisely at Windsor in association with strong low-level southeasterly flow. This study confirms that continuous sampling of the near-storm environment by ground-based remote sensing systems can alert forecasters to rapidly changing conditions that can help pinpoint severe storm initiation.