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 on 12 March 2008 between 02:00 and 03:00 UTC and was measured using four ultrasonic 10-Hz anemometers positioned at 3, 60, 100, and 180 m above ground level. 1-second gusts in the outflow exceeded 30 m s−1 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. 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.

An Analytical Solution to the Perturbation Analysis of the Interaction between Downburst Outflows and Atmospheric Boundary Layer Winds

“…For example, in describing the morphology of downburst outflows, Hjelmfelt (1988) showed that the outflow velocity of laboratory jets measured by Poreh et al (1967) fall within the range of variations of outflow velocity of numerous downbursts measured during the Joint Airport Weather Studies (JAWS) field campaign. This kinematic similarity is also supported by Wood et al (2001), Romanic et al (2020), andRomanic andHangan (2020). Wood et al (2001) used the functional form of the outflow velocity proposed by Verhoff (1963) for a wall jet and provided a self-similar solution to the velocity field of a downburst.…”

Experimental investigation of the near-surface flow dynamics in downburst-like impinging jets

“…However, the presence of SV is not commonly observed in real downbursts. A weak SV is observed by Sherman [65] in his study of a weak downburst close to Brisbane, Australia, as well as by Romanic [66] in his study of severe microburst recorded on a 213-m tall tower in the Netherlands. The presence of the SV, preceding the passage of the PV, was also reported by Canepa et al [19] in the downburst event recorded in Genoa on May 13, 2018 by means of a LiDAR vertical profiler.…”

Dynamic Response of Slender Vertical Structures Subjected to Thunderstorm Outflows

“…However, these studies were based on data collected by anemometers located in coastal areas, and thus have similar roughness lengths with which it is difficult to detect a significant dependence on the roughness. Other studies carried out on different sites have shown increasing trends for the vertical profile of the turbulence intensity, as well as greater values comparable to those observed during synoptic events [45][46][47]. Moreover, it was shown that the vertical profile of the turbulence intensity on the height changes during the thunderstorm, showing a decreasing trend during the ramp-up phase of the wind speed [32].…”