Polar Lows – Moist Baroclinic Cyclones Developing in Four Different Vertical Wind Shear Environments Patrick Johannes Stoll1, Thomas Spengler2, Annick Terpstra2, and Rune Grand Graversen1,3 1Department of Physics and Technology, Arctic University of Norway, Tromsø, Norway 2Geophysical Institute, University of Bergen, and Bjerknes Centre for Climate Research, Bergen, Norway 3Norwegian Meteorological Institute, Norway Correspondence: Patrick Johannes Stoll (
[email protected]) Abstract. Polar lows are intense mesoscale cyclones that tensification of polar lows and propose instead that spirali- develop in polar marine air masses. Motivated by the large form clouds are associated with a warm seclusion process. variety of their proposed intensification mechanisms, cloud structure, and ambient sub-synoptic environment, we use 5 self-organising maps to classify polar lows. The method is applied to 370 polar lows in the North-East Atlantic, which 1 Introduction 35 were obtained by matching mesoscale cyclones from the ERA-5 reanalysis to polar lows registered in the STARS Polar lows (PLs) are intense mesoscale cyclones with a typ- dataset by the Norwegian Meteorological Institute. ERA-5 ical diameter of 200 - 500 km and a short lifetime of 6 - 36 h 10 reproduces most of the STARS polar lows. that develop in marine cold-air outbreaks during the extended We identify five different polar-low configurations which winter season (e.g Rasmussen and Turner, 2003; Yanase are characterised by the vertical wind shear vector, the et al., 2004; Claud et al., 2004; Renfrew, 2015; Rojo et al., 40 change of the horizontal-wind vector with height, relative to 2015). Despite numerical weather-prediction models being the propagation direction.