Near‐surface strong winds in a marine extratropical cyclone: acceleration of the winds and the importance of surface fluxes

Abstract

A rapidly developing extratropical cyclone named Tini brought strong winds to Ireland and the United Kingdom on 12 February 2014. A mesoscale-model simulation is used to analyze the development of the strong winds through the terms in the horizontal momentum equation. The maximum of near-surface wind speed equatorward of the cyclone was composed of two different airstreams that underwent different paths to acceleration. First, horizontally moving air in the cold conveyor belt was accelerated by the along-flow pressure gradient force but was decelerated by friction. Second, descending air accelerated into the eastern end of the maximum of near-surface wind speed and was associated, in part, with a sting jet, caused by the increasing along-flow horizontal pressure gradient force at lower levels. When this descending air entered the boundary layer, it too was decelerated by surface friction. Surface fluxes of heat and moisture were necessary to destabilize and deepen the boundary layer, allowing mixing of the strongest winds from the free troposphere down to the surface. A simulation with the surface fluxes turned off during cyclogenesis showed a more stable boundary layer around the bent-back front, which inhibited the strongest winds from reaching the surface. The descent of the sting-jet air was associated with a maximum in quasigeostrophic omega, which consisted of both synoptic-scale and mesoscale descent, the latter associated with frontolysis occurring at the end of the bent-back front. Thus, the near-surface wind maximum was created by the synoptic-scale and mesoscale dynamics, whereas localized moist processes were negligible.

Invigoration of convection by an overrunning diabatically modified cloud‐top layer

“…In other research (Browning et al , ) a similar role for evaporative cooling in destabilizing the boundary layer was identified in a rapidly deepening cyclone at the stage of frontal fracture. Destabilization of the boundary layer can be due to the evaporative cooling mechanism as in these studies or to surface fluxes if the underlying surface is warm enough (Slater et al , ). Further research is required to establish the relative importance of these mechanisms but, one way or another, a destabilized boundary layer is an important ingredient of situations when extratropical storms produce damaging surface winds.…”

The role of mesoscale instabilities in the sting‐jet dynamics of windstorm Tini

“…As recognised by Browning (), gradient wind balance is sufficient to account for a large part of the strength of the winds in the frontal‐fracture region. Subsequent studies (Schultz and Sienkiewicz, ; Coronel et al ; Slater et al ) have demonstrated association between SJ‐type airstreams and frontolysis, divergent Q vectors, and/or quasi‐geostrophic omega. This body of work has established that downwards advection of strong winds by the frontolytical secondary circulation (implicitly found in the frontal‐fracture region of a cyclone) followed by acceleration by the low‐level pressure gradient can give rise to strong winds.…”

“…The last subsection contains a comparison with a coarser model simulation; this comparison is included to reconcile our results, which show a clear contribution of mesoscale instability release to the generation and descent of the SJ in windstorm Tini , with the results of other studies that have concluded that this mechanism does not contribute. In particular, Slater et al () also show results from a simulation of windstorm Tini , but attribute the SJ‐like feature to large‐scale dynamics. Since their simulation used a resolution that is too coarse to resolve mesoscale instability release, this part of our study addresses the role that model resolution has on the diagnosed contributions of mechanisms to SJs.…”

What is a sting jet?

“…Browning and Field, ), but definitive evidence of the existence of the sting jet requires trajectories and/or other diagnostics (e.g. Smart and Browning, ; Slater et al ., ; ).…”

“…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 ., ).…”