Formation of Wind Gusts in an Extratropical Cyclone in Light of Doppler Lidar Observations and Large-Eddy Simulations

Abstract

Damaging gusts in windstorms are represented by crude subgrid-scale parameterizations in today’s weather and climate models. This limitation motivated the Wind and Storms Experiment (WASTEX) in winter 2016–17 in the Upper Rhine Valley over southwestern Germany. Gusts recorded at an instrumented tower during the passage of extratropical cyclone “Thomas” on 23 February 2017 are investigated based on measurements of radial wind with ≈70-m along-beam spacing from a fast-scanning Doppler lidar and realistic large-eddy simulations with grid spacings down to 78 m using the Icosahedral Nonhydrostatic model. Four wind peaks occur due to the storm onset, the cold front, a precipitation line, and isolated showers. The first peak is related to a sudden drop in dewpoint and results from the downward mixing of a low-level jet and a dry layer within the warm sector characterized by extremely high temperatures for the season. While operational convection-permitting forecasts poorly predict the storm onset overall, a successful ensemble member highlights the role of upstream orography. Lidar observations reveal the presence of long-lasting wind structures that result from a combination of convection- and shear-driven instability. Large-eddy simulations contain structures elongated in the wind direction that are qualitatively similar but too coarse compared to the observed ones. Their size is found to exceed the effective model resolution by one order of magnitude due to their elongation. These results emphasize the need for subkilometer-scale measuring and modeling systems to improve the representation of gusts in windstorms.

Large Roll Vortices Exhibited by Post-Tropical Cyclone Sandy during Landfall

“…Diamonds in Figure 7 show that rolls in the WRF simulation consistently possessed large horizontal scales (which ranged from 5 to 8 km), although they were somewhat smaller than the observed rolls. As noted in Section 2.2 and by Pantillon et al [19], the simulation's resolution of 500 m may not suffice to fully resolve rolls of this size, so the wavelengths measured from the WRF simulation must be used with caution. Moreover, because we were not able to pursue any simulation sensitivity analyses of potentially important effects on roll characteristics, such as surface roughness and PBL parameterization, we cannot rule out that the close resemblance of roll signatures manifested by WRF with those observed by radar was coincidental.…”

“…Furthermore, rolls with different characteristics and driving mechanisms have since been identified and classified [8,9], with multiple types of rolls also identified and classified for hurricanes [10][11][12][13][14][15]. Large eddy simulations have been used to characterize rolls at finer granularity in recent years [16][17][18], and most recently, rolls have even been observed to be associated with high wind bands in extratropical cyclones [19]. Theoretical underpinning for roll formation has been developed by Mourad and Brown [20] and specifically for hurricane environments by Foster [21].…”

Evaluating the dependence structure of compound precipitation and wind speed extremes

“…Most heavy precipitation events in the investigation domain in winter are associated with extratropical cyclones. Within extratropical cyclones, wind speed maxima and precipitation maxima are often linked to fronts and conveyor belts (Parton et al, 2010;Catto and Pfahl, 2013;Pfahl et al, 2014;Pantillon et al, 2020) and this may results in co-located extremes.…”

Investigation of the Weather Conditions During the Collapse of the Morandi Bridge in Genoa on 14 August 2018 Using Field Observations and WRF Model

“…The lidar scanned four different elevation angles (ϕ) between 2.5 • and 10 • above the horizontal plane with a 2.5 • increment. These results are particularly interesting because Figure 13 is among the few sets of published lidar velocity measurements of a thunderstorm gust front [41]. Unfortunately, the heavy rain that occurred behind the gust front affected the lidar measurements by reducing the range of data acquisition from about 5 km at 09:15 UTC to 1-2 km at 09:30 UTC.…”