Abstract
During March 2013 a series of polar lows originated in a high‐vorticity (>10−3 s−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−1 over 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.
Highlights
Myriads of mesoscale vortices emerge over the Norwegian Sea during the extended winter period (November–April) (Kolstad, 2011; Rojo et al, 2015)
Journal of the Royal Meteorological Society published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society
The so-called marine cold-air outbreak (MCAO) index, which is proportional to the potential temperature difference between surface skin temperature and 700 hPa height (Bracegirdle and Kolstad, 2010), on 26 March was at its highest of the whole ACCACIA campaign period, implying that the observed polar lows (PLs) developed during the strong advection of an Arctic air mass over the relatively warm (a)
Summary
Myriads of mesoscale vortices emerge over the Norwegian Sea during the extended winter period (November–April) (Kolstad, 2011; Rojo et al, 2015). The chronic lack of in situ observations at high latitudes makes it challenging to forecast these small-scale and explosive weather phenomena accurately, even for modern numerical weather prediction (NWP) systems (Kristiansen et al, 2011) This shortcoming is especially true at the early stages, when incipient perturbation PLs can be embedded in convergence zones with large horizontal wind shear. Wagner et al (2011) used two lidars, including a Doppler wind lidar, to provide high-resolution cross-sections of water-vapour mixing ratio, backscatter ratio, and horizontal wind speeds through the inner part of a PL These observations provide a very thorough description of a PL ‘eye’ and were essential to compare against the authors’ axisymmetric and NWP models.
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