Near-Surface Flow Characteristics of a Thunderstorm Outflow Event

Thunderstorms are the primary source for significant wind events and therefore control the design wind speed for the vast majority of regions worldwide. However, thunderstorm winds are not represented in wind tunnel experiments or current building codes and design techniques. A lack of understanding and documentation of the near surface (200 m and below) wind flow characteristics and associated wind loads on structures has led to this absence. One type of wind event, particularly of interest, is the thunderstorm outflow. Strong winds aloft are transported to the surface where it diverges laterally, producing strong to severe winds near the ground. The magnitude of the winds generated by the outflow, its distribution with height, and the flow characteristics deviate significantly from a typically assumed synoptic boundary layer wind. The non-stationary, turbulent, and short-lived nature of an outflow, as well as the continuous modification of the wind profile and other factors, makes it extremely difficult to simulate and model. With a better understanding of severe wind events, such as a thunderstorm outflow, structural design techniques and codes can be improved.

The primary objective of this research is to investigate the near surface flow characteristics, wind profile, and meteorological features associated with a severe West Texas thunderstorm outflow event by utilizing data from the Texas Tech University Wind Science & Engineering (WISE) Research Center's multi-level 200 m tower coupled with data from the National Weather Service WSR-88D Doppler Radar. Analysis of the outflow incorporates wind speed and direction time histories acquired from multiple tower heights, turbulence statistics (i.e., turbulence intensity, gust factor, etc.), thermodynamic data (i.e., temperature, relative humidity, pressure, etc.), and radar observations.

A terrain-induced thunderstorm that had developed on 12 July 2006 produced two outflows that were sampled by the 200 m tower. Based on radar reflectivity, the first outflow had no well-defined fine-line. The only indication of this outflow was the radial spread of scatterers that traveled ahead of the main storm. The second outflow did produce a fine-line on radar, however, only a portion of the boundary was sampled by the tower as the storm began to dissipate and move southeast of the platform. Results indicated the strongest wind speeds occurred ~30 min. after the passage of the first outflow boundary, but well before the second boundary passed. In addition, periodic oscillations were observed in the wind speed records throughout the depth of the tower. Thermodynamic and turbulence data also showed unique characteristics. Lastly, the mean wind speed profile for the event showed a typical low-law profile from 116.4 m downward. However, the wind speed at the top instrument height, 158.2 m, was slightly lower than the 116.4 m level, indicating a deviation from the typically assumed profile.