High-Resolution Full-Scale Observations of Thunderstorm Outflow Winds

Both impinging jet models and thermodynamic-allowing simulations have proved very successful in understanding the vertical structure of thunderstorm outflow, yet very few high resolution observational datasets exist for comparison. The primary goal of this research is to provide high-resolution observations of thunderstorm outflows. With these observations, many of the poorly understood aspects of thunderstorm outflow winds, such as wind profile evolution and the effects of outflow structure on the distribution of higher momentum, can be investigated and compared to high resolution numerical models.

Data collection began in the spring of 2011 as thunderstorm events deemed capable of producing severe wind were targeted locally and throughout the Great Plains. Both TTUKa mobile Doppler radars and a fleet of 24 rapidly deployable surface observation stations called StickNet were employed to collect the datasets. A novel radar scanning strategy was employed in which coordinated Range-Height Indicator (RHI) scans were used to obtain dual-Doppler wind speed and direction profiles ahead of, along, and behind thunderstorm outflow boundaries. In addition to dual-Doppler wind profiles, the coordinated RHIs also provided information on the vertical structure of the outflow. Coordinated single elevation Plan Position Indicator (PPI) scans were also periodically included into the scanning strategy to provide the basis for horizontal dual-Doppler synthesis. The StickNet fleet (which measures wind speed, direction and other atmospheric variables at 2.25 meters) was deployed to measure the near surface winds for comparison with the TTUKa radar data and to allow for the computation of turbulence parameters. Initial analysis of the collected data echo the findings from previous model and observational studies in that thunderstorm outflow structure and the resulting wind profiles vary vastly through space, time, and event. Several classic features also appear in the RHIs at the leading edge of selected outflow cases including large horizontal rotors and outflow nose generation and collapse.

Data were also collected in several outflow events at Reese Technology Center in close proximity to a 200 meter instrumented tower. Data from the tower served to validate wind profiles computed from the coordinated RHIs during the outflow events. In general, the magnitudes of the dual-Doppler wind speeds were slightly higher than the wind speeds recorded by the tower. It was also noted that turbulence statistics computed with the radar data were lower than those computed with the tower data. Smaller scales of turbulence appear to be "lost" within the volumetric average of each radar bin (15 m in range with a 0.49° half power beamwidth). This effect was most pronounced near the surface where terrain induced turbulence is most prevalent.