Forward and Reverse Shear Environments during Polar Low Genesis over the Northeast Atlantic

Abstract

The synoptic and subsynoptic environments associated with polar low genesis are examined. Ambient pre–polar low environments are classified as forward or reverse shear conditions based on the angle between the thermal and mean wind. Forward shear environments are associated with a synoptic-scale ridge over Scandinavia, featuring a zonally oriented baroclinic zone extending throughout the troposphere with a wind speed maximum at the tropopause. Similar to typical midlatitude cyclogenesis, concurrent wavelike development occurs both in the lower and upper troposphere along the baroclinic zone and the mean propagation direction is eastward, parallel to isolines of sea surface temperature. Reverse shear environments exhibit a distinctly different structure and are characterized by a trough over Scandinavia, associated with a synoptic-scale, occluded cyclone. The genesis area exhibits strong cold air advection on its right-hand side and polar low development occurs on the warm side of an intense low-level jet. The environment resembles the characteristics conducive to secondary development associated with frontal instability. Polar lows developing in this configuration propagate mainly southward, perpendicular to isolines of sea surface temperature. The two genesis environments exhibit similar temperature differences between the sea surface and atmosphere near the surface, yet the magnitude of the surface fluxes is approximately double during reverse shear conditions due to stronger low-level winds. The ratio between surface sensible and latent heat fluxes is close to unity for both shear environments.

Polar low variability and future projections for the Nordic and Barents Seas

“…Terpstra et al . () and Michel et al . () cautioned that the SST– T 500 criterion may favour selection of so‐called reverse‐shear polar lows (over forward‐shear), but they also noted that the most intense polar lows were of this type.…”

“…This could be more of importance in the Barents Sea, where a larger fraction of the polar lows are forward‐shear than in the Nordic Seas (Terpstra et al . ). As with any statistical analysis based on thresholds obtained in historical climate, it is not certain that the SST– T 500 threshold used for polar low detection may hold under future climate change.…”

An objective global climatology of polar lows based on reanalysis data

“…The STARS dataset version 2 provides a list of 185 PL tracks over the Nordic Seas from January 2001 to March 2011 (Noer et al , ). The PLs are subjectively identified by forecasters at the Norwegian Meteorological Institute by inspection of satellite infrared data, scatterometer winds and the operational weather forecasting model HIRLAM 4 (Rojo et al , ; Terpstra et al , ), for evaluation of PL occurrence in different datasets (e.g. (Laffineur et al , )(Smirnova and Golubkin, )), and the evaluation of objective detection methods (e.g (Zappa et al , )).…”

“…A threshold of 43 K for this parameter would exclude 30.9% of the PLs and therefore appears to be too high. Also, Terpstra et al s*() noted that this threshold excludes a considerable number of PLs in the North Atlantic.…”

“…(Papritz et al , )). Commonly, a threshold of SST – T 500 > 43 K is used, although Terpstra et al s*() and Smirnova and Golubkin () argue that this threshold excludes a considerable number of PL cases. Bracegirdle and Gray () investigated different temperature parameters, and found the difference between the wet‐bulb potential temperature at 700 hPa and the SST to be the most effective of their considered parameters to separate PLs from other cyclones.…”