Severe thunderstorm outflow: 26 June 2009

From CSU-CHILL

Overview

A dissipating thunderstorm generated locally-severe outflow winds on 26 June 2009. Strong outflow winds were found at a distance of ~10 km ahead of the main precipitation echo.

Introduction

During the mature and dissipating stages of a thunderstorm, downward-flowing air currents reach the ground beneath the storm. Divergent horizontal winds are produced as the downdrafts interact with the surface and fan out from the base of the storm. As time passes, the low-level outflow typically expands away from main precipitation area (and its associated high reflectivity core). The CSU-CHILL radar observed an example of this evolution during the afternoon hours of 26 June 2009.

Reflectivity RHI scans

PPI scans were primarily done on 26 June 2009; RHI scans were occasionally added to the scan sequence. The reflectivity data from two such RHI scans through the storm of interest are shown below. The azimuth angles differ by only one degree; the time difference between the scans is ~7 minutes. The reflectivity field tends to lose intensity and subside towards the surface over this time period. The echo summit height also decreases; these are typical characteristics of a thunderstorm entering the dissipating life cycle stage.


Click play to begin animation

Delay: ms

Linear
Rock

Radial velocity RHI scans

The corresponding radial velocity patterns confirm the existence of approaching (negative sign) wind speeds of ~25 mps near the surface at a range of 45 km. By the second image time, the leading edge of this outflow airmass has crossed the 40 km range mark.


Click play to begin animation

Delay: ms

Linear
Rock

Combined reflectivity and radial velocity PPI scan

The following plot contains the reflectivity and radial velocity data from a 0.5 degree elevation angle PPI sweep made just after the second RHI scan shown above. Areas with inbound radial velocitiy magnitudes exceeding 20 and 25 mps are depicted by solid contour lines. An SPC spotter report confirmed the existence of high wind speeds at 2011 UTC. The maximum radial velocities had expanded to a location some 10 km east of the reflectivity core at the time of the PPI scan (2017 UTC). An eastward-protrusion of the reflectivity contours was also evident in the vicinity of the strong outflow. This low-level echo configuration is similar to that described in Fujita and Byers (MWR, Feb. 1977, p129 - 146.)