Mean flow and turbulence characteristics of a nocturnal downburst recorded on a 213-m tall meteorological tower

Abstract

This study presents rare measurements and analysis of a nocturnal thunderstorm downburst on the 213-m tall Cabauw tower in the Netherlands. The event occurred between 0200 and 0300 UTC 12 March 2008 and was measured using four ultrasonic 10-Hz anemometers positioned at 3, 60, 100, and 180 m above ground level. One-second gusts in the outflow exceeded 30 m s21 at 60 m and above. This wind event was accompanied by an abrupt change of wind direction from southwest to west. While the shift in wind direction corresponded with the change of upwind surface roughness, the time series of turbulence intensity and other turbulence characteristics were not affected. The statistical properties of this event were compared against the largest European database of thunderstorm winds measured in the Mediterranean. The study also demonstrated that primary and secondary vortex structures-secondary vortex being rarely observed in actual downbursts-developed at the forward edge of the cold outflow. The estimated diameter of the downdraft was 1200 m at 70 m above ground level. The measured velocity profiles and friction velocity were compared against theoretical predictions of the Monin-Obukhov similarity theory (MOST). MOST without stratification adjustment overestimated measured friction velocity twofold. Alternative values for surface roughness during the outflow were derived based on the measured friction velocity and MOST-based fit of measured velocity profiles. Ceilometer and radar measurements were supplementary data in this analysis.