A polar low named Vera: the use of potential vorticity diagnostics to assess its development

Abstract

Abstract20 November 2008 the Norwegian Meteorological Institute issued an extreme weather warning for Trøndelag County. A storm was expected in the afternoon. The storm, a polar low (PL), was named ‘Vera’. Vera was the second of two polar lows that developed along a wedge of warm air at the rear of a synoptic‐scale low that moved northeastwards into northern Norway. The dynamical development may be explained by classic dynamical theory: low‐level warm air seclusion and shallow secondary circulation in a frontal zone that couples to transient upper level disturbances. The study focuses on the potential vorticity (PV) perspective of the PL development and we have modified the initial PV field to study the effect of each individual PV perturbation. Two PV and two low‐level temperature anomalies have been chosen. The PV anomalies retained their structure during the development while the temperature anomalies became less coherent and difficult to define by the end of the cyclogenesis period. By modifying the initial conditions in the upper troposphere over Greenland, it is shown that in this case the upper PV anomaly had the strongest effect on the development, and that this effect remained strong throughout the cyclogenesis until the polar low made landfall. The effect from the low level PV anomaly became large after onset of cyclogenesis, reflecting the positive feedback, although its impact on the development remained smaller than that of the upper PV anomaly. It is also shown that the topography of Greenland was important in determining the correct position of this polar low development. Copyright © 2011 Royal Meteorological Society