A potential vorticity signature for the cold sector of winter extratropical cyclones

Abstract

The cold sector of a midlatitude storm is characterized by distinctive features such as strong surface heat fluxes, shallow convection, convective precipitation and synoptic subsidence. In order to evaluate the contribution of processes occurring in the cold sector to the mean climate, an appropriate indicator is needed. This study describes the systematic presence of negative potential vorticity (PV) behind the cold front of extratropical storms in winter. The origin of this negative PV is analyzed using ERA-Interim data, and PV tendencies averaged over the depth of the boundary layer are evaluated. It is found that negative PV is generated by diabatic processes in the cold sector and by Ekman pumping at the low centre, whereas positive PV is generated by Ekman advection of potential temperature in the warm sector. We suggest here that negative PV at low levels can be used to identify the cold sector. A PV-based indicator is applied to estimate the respective contributions of the cold sector and the remainder of the storm to upward motion and large-scale and convective precipitation. We compare the PV-based indicator with other distinctive features that could be used as markers of the cold sector and find that potential vorticity is the best criterion when taken alone and the best when combined with any other.

Contribution of the cold sector of extratropical cyclones to mean state features over the Gulf Stream in winter

“…To identify the cold sector, we use an indicator combining potential vorticity (PV) at 975 hPa and surface sensible heat flux (Vannière et al , 2016). Such an indicator was tested in a composite of 57 extratropical cyclones in the Gulf Stream region, and it allowed a systematic and accurate detection of cold sector precipitation and subsidence (Vannière et al , 2016, provides a detailed discussion).…”

“…However, the large‐scale precipitation occurring in the cold sector must be interpreted with some caution. Indeed, as described by Vannière et al (2016), the cold sector indicator can capture some large‐scale precipitation from the WCB due to the vertically sloped nature of the cold front. The precipitation in splits into convective precipitation on the south side of the front and large‐scale precipitation north of it, corresponding to the entrance and the upper branch of the WCB respectively.…”

“…We propose instead to investigate the mechanism of this connection under different synoptic regimes, which we believe is a more physical approach. To do so, we will use recently developed tools (Vannière et al , 2016) to partition air–sea interactions depending on whether they occur in the cold sector of extratropical cyclones or in any other synoptic features. Following the American Meteorological Society (2016), we define the cold sector as the area within the circulation of an extratropical cyclone, behind the cold front, where cold air advection occurs.…”

“…The question thus arises as to what extent cold sector convective precipitation contributes to the mean climatological precipitation over the Gulf Stream. In a composite of 57 storms, Vannière et al (2016) showed that a significant part of precipitation, albeit weaker than in the warm conveyor belt (WCB), occurred in the cold sector. As 95 % of precipitation in the storm track region is produced by extratropical cyclones (Hawcroft et al , 2012), the cold sector could have a non‐negligible contribution to climatological precipitation in the western North Atlantic.…”

Diabatic heating and jet stream shifts: A case study of the 2010 negative North Atlantic Oscillation winter

“…Our results suggest that, in addition, the tropospheric latent heating is also fixed over the Gulf Stream during even the strongest shifts of the jet stream and hence makes a contribution to maintaining the baroclinicity and anchoring the upstream end of the storm track. Convection occurring in the cold sector of cyclones may also play a role by anchoring a band of precipitation over the Gulf Stream [Vannière et al, 2016]. This is a region of large moisture uptake by warm conveyor belts [Pfahl et al, 2014], so these are likely important factors.…”