Mountain wave-induced rotors in the lee of the Hex River Mountains

Abstract

Prefrontal mountain waves frequently occur in the lee of the mountains of the Western Cape province in South Africa. On 22 July 2010, four weather balloon soundings were conducted in this area on the upwind (one) and lee sides (three) of the Hex River Mountains. The purpose of these soundings was to detect rotors that occurred below trapped lee mountain waves, which can pose a significant danger to general aviation aircraft. A portable sounding system was employed to obtain atmospheric data of wind, temperature, pressure and humidity at 1-s intervals. These values were used to determine a Scorer parameter profile and Froude number for the upwind side of the mountain, which indicated the possibility for trapped lee wave development. Lee side rotors were analyzed using the reversed flow, updraft and downdraft speeds, along-ridge horizontal vorticity, potential temperature, wind speed, wind direction and Richardson number as determined from the sounding data. The vertical extent and approximate position of updraft areas of the rotors could be determined using areas of vertical wind shear, Richardson numbers below 0.25 and positions of local maxima in along-ridge horizontal vorticity. Upwind flanks of rotors were analyzed using along-ridge horizontal vorticity maxima and updraft areas. The top of one rotor was penetrated by a balloon.

Keywords:

• soundings
• Scorer parameter
• Froude number
• reversed flow
• along-ridge horizontal vorticity

Notes

^1. Current address: South African Weather Service, Private Bag 5991, Walmes 6065, South Africa.

Additional information

Notes on contributors

D. van der Mescht

1 1. Current address: South African Weather Service, Private Bag 5991, Walmes 6065, South Africa.