The role of evaporating showers in the transfer of sting‐jet momentum to the surface

Abstract

The most damaging winds in a severe extratropical cyclone often occur just ahead of the evaporating ends of cloud filaments emanating from the so‐called cloud head. These winds are associated with low‐level jets (LLJs), sometimes occurring just above the boundary layer. The question then arises as to how the high momentum is transferred to the surface. An opportunity to address this question arose when the severe ‘St Jude's Day’ windstorm travelled across southern England on 28 October 2013. We have carried out a mesoanalysis of a network of 1 min resolution automatic weather stations and high‐resolution Doppler radar scans from the sensitive S‐band Chilbolton Advanced Meteorological Radar (CAMRa), along with satellite and radar network imagery and numerical weather prediction products. We show that, although the damaging winds occurred in a relatively dry region of the cyclone, there was evidence within the LLJ of abundant precipitation residues from shallow convective clouds that were evaporating in a localized region of descent. We find that pockets of high momentum were transported towards the surface by the few remaining actively precipitating convective clouds within the LLJ and also by precipitation‐free convection in the boundary layer that was able to entrain evaporatively cooled air from the LLJ. The boundary‐layer convection was organized in along‐wind rolls separated by 500 to about 3000 m, the spacing varying according to the vertical extent of the convection. The spacing was greatest where the strongest winds penetrated to the surface. A run with a medium‐resolution version of the Weather Research and Forecasting (WRF) model was able to reproduce the properties of the observed LLJ. It confirmed the LLJ to be a sting jet, which descended over the leading edge of a weaker cold‐conveyor‐belt jet.

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

“…Browning [] identified one of the causes of the most extreme surface winds to be associated with a mesoscale feature at the tip of the cloud head as the “sting at the end of the tail” (shortened to sting jet; SJ) (Table and Figure b). This proved to be the case for the 16 October 1987 storm [ Clark et al ., ], for Windstorm Jeanette in October 2002 [ Parton et al ., ], and Christian in October 2013 [ Browning et al ., ], amongst others. However Baker [] found that the strongest surface winds during Windstorm Gudrun of January 2005 were associated with the cold conveyer belt (CCB) as it wrapped around the low‐pressure center and acted in the same direction as the motion of the cyclone.…”

What is a sting jet?

“…At first sight, it might be tempting to assume that the importance of descent is in bringing down high momentum from aloft, as suggested by Neiman and Shapiro (). As explained below, this assumption might be relevant in transporting momentum through the boundary layer (e.g Vaughan et al, ; Browning et al ., ), but, in the free troposphere, the air that becomes the sting jet starts off with low momentum (Clark et al ., ; Slater et al ., ; ). The question therefore remains as to the cause of the acceleration of the sting‐jet air as it descends within the free troposphere.…”

“…Some researchers have found evidence to support the importance of these localised moist processes to the sting‐jet formation and intensity (e.g. Clark et al ., ; Gray et al ., ; Martínez‐Alvarado et al ., 2014b; Browning et al ., ), but others have described cases in which the importance of moist processes was questionable (e.g. Schultz and Sienkiewicz, ; Baker, ; Baker et al ., ; Smart and Browning, ; Slater et al ., ; ; Coronel et al ., ).…”

“…Evaporative cooling is believed to be important in sting jets sometimes not so much in accounting for the acceleration of the sting jet as in reducing the static stability. This evaporative cooling facilitates the transport of high momentum associated with an already developed sting jet in the free troposphere down through the boundary layer to the surface (Browning et al ., ). Another way to weaken the static stability is through surface fluxes of heat and moisture (e.g.…”

Development of localized damaging wind gusts associated with a frontal wave and mesoscale vortex across south Wales on 18 May 2015

“…Inspection of quarter‐hourly radar images indicates that the line convection evolved from one of the smaller intense rainfall elements located just north of Swansea at 0842 UTC (Figure (c)) and then persisted for several hours as it continued eastwards. The location of the line convection along the inner edge of the leaf cloud and near the tip of the back‐bent front is consistent with Browning's () conceptual model of the frontal structure in cyclones of the Shapiro–Keyser type (Shapiro and Keyser, ) and with other recent cases of cyclones with prominent back‐bent fronts, including the damaging St Jude's Day storm of 28 October 2013 (Browning et al ., ).…”