Cloud Banding and Winds in Intense European Cyclones: Results from the DIAMET Project

Abstract

The Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project aims to improve forecasts of high-impact weather in extratropical cyclones through field measurements, high-resolution numerical modeling, and improved design of ensemble forecasting and data assimilation systems. This article introduces DIAMET and presents some of the first results. Four field campaigns were conducted by the project, one of which, in late 2011, coincided with an exceptionally stormy period marked by an unusually strong, zonal North Atlantic jet stream and a succession of severe windstorms in northwest Europe. As a result, December 2011 had the highest monthly North Atlantic Oscillation index (2.52) of any December in the last 60 years. Detailed observations of several of these storms were gathered using the U.K.’s BAe 146 research aircraft and extensive ground-based measurements. As an example of the results obtained during the campaign, observations are presented of Extratropical Cyclone Friedhelm on 8 December 2011, when surface winds with gusts exceeding 30 m s–1 crossed central Scotland, leading to widespread disruption to transportation and electricity supply. Friedhelm deepened 44 hPa in 24 h and developed a pronounced bent-back front wrapping around the storm center. The strongest winds at 850 hPa and the surface occurred in the southern quadrant of the storm, and detailed measurements showed these to be most intense in clear air between bands of showers. High-resolution ensemble forecasts from the Met Office showed similar features, with the strongest winds aligned in linear swaths between the bands, suggesting that there is potential for improved skill in forecasts of damaging winds.

Subsynoptic‐scale features associated with extreme surface gusts in UK extratropical cyclone events

“…During the 2008–2014 period, there were at least three known SJ‐producing ETCs identified in the literature, Friedhelm on 8 December 2011 [ Baker et al ., ; Martínez‐Alvarado et al , ; Vaughan et al , ], Ulli on 3 January 2012 [ Smart and Browning , ], and Christian on 28 October 2013 [ Browning et al ., ]. SJs were also allocated as the cause of 0.1%DMGSs during each of these events during our study.…”

Cyclones, windstorms and the IMILAST project

“…We would however place this case in the CJ category, given that the CJ was present beneath, at the surface. Meanwhile dropsonde observations near to windstorm Friedhelm, in December 2012, as discussed in Baker et al (2013) and Vaughan et al (2014), seem to have an elevated SJ structure, probably with a CJ beneath. This interpretation also finds some support in model trajectory analysis by Martinez-Alvarado et al (2014).…”

“…Observational data for Friedhelm in Vaughan et al (2014) highlight what can happen in the over-land case if there is significant inland propagation of vigorous marine-based convection. Should this occur (perhaps via the support of broadscale forced ascent) then vertical transfer of momentum associated with the convection (and indeed with evaporation) can accordingly help elevate inland gusts at the surface even when insolation is not really a factor.…”

Diabatic Heating and Cooling Rates Derived from In Situ Microphysics Measurements: A Case Study of a Wintertime U.K. Cold Front

“…The BAe 146 is a state-of-the-art airborne laboratory, fitted with a comprehensive set of meteorological probes and microphysics instrumentation to provide measurements of both liquid and ice particles across a range of sizes [see Vaughan et al (2014) for the DIAMET payload]. FAAM is jointly run by the U.K. Natural Environment Research Council (NERC) and the Met Office.…”

“…The Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project (Vaughan et al 2014) conducted four U.K.-based field campaigns between September 2011 and August 2012, which provided a comprehensive set of both in situ and ground-based radar measurements in relation to the microphysics and dynamics of a variety of frontal systems. These measurements are used in combination with numerical models to improve our understanding of the way mesoscale structures develop and give rise to severe weather in extratropical cyclones, and to help improve the prediction of such features in weather forecast models.…”