Structure of a shear-line polar low

Abstract

During March 2013 a series of polar lows originated in a high-vorticity (>10−3s−1) shear zone that was associated with a prolonged marine cold-air outbreak over the Norwegian Sea. A detailed analysis of one shear-line polar low at the leading edge of the outbreak is presented using comprehensive observations from a well-instrumented aircraft, dropsondes, scatterometer and CloudSat data, and numerical modelling output from a convection-resolving configuration of the Met Office Unified Model. The maximum low-level wind gradient across the shear line was 25 m s−1over 50 km. High winds to the north and west were within the cold air mass and were associated with large surface turbulent heat fluxes and convective clouds. Low wind speeds to the south and east of the shear line were associated with low heat fluxes and a clear ‘eye’ in the polar low. Shear-line meso-gamma-scale instabilities merging into the polar low appeared important to its structure and development. The model captured the shear line and the polar low structure very well–in particular the strength of the horizontal shear and the mesoscale thermodynamic fields. The spatial structure of convective cloud bands around the polar low was simulated reasonably well, but the model significantly underestimated the liquid water content and height of the cloud layers compared to the observations. Shear-line polar lows are relatively common, however this case is arguably the first to be examined with a wide range of in situ and remote observations allied with numerical model output.