A climatology of North Atlantic polar lows and their phase space derived from thermal wind and thermal asymmetry

  • Christopher Fairless

Student thesis: Phd


Polar lows are gale-producing, mesoscale (200 - 2000 km) maritime storms that form poleward of the main storm tracks. They are associated with rapid intensification, high winds, and intense precipitation. Their small scale makes them notoriously difficult to forecast and they can damage shipping, marine infrastructure and coastal communities. They get their energy from a combination of baroclinicity, latent heat release and surface fluxes, and the relative importance of these mechanisms determines a polar low's thermal structure and properties.The Arctic System Reanalysis is the first reanalysis suitable for the direct, automated detection of polar lows. In this thesis it is used to create a climatology for the years 2000-2010. Relative vorticity maxima are tracked at 850 hPa, and polar lows are defined as maritime systems in unstable polar air masses (a SST - 500 hPa temperature difference exceeding 43 °C) with winds exceeding 15 m s -1 within 150 km of the low centre and with a SLP minimum at least once along the track.An annual mean of 289 polar lows are found in the North Atlantic, which is higher than previous automated climatologies, which apply additional intensity criteria during the identification process. Polar lows occur from the storm track to the ice edge, and have frequency maxima in the Norwegian Sea and between the Greenland tip and Iceland, where cold-air outbreaks and regions of shallow baroclinicity are most frequent.The climatology is used to populate a cyclone phase space (Hart, 2003), representing polar lows' life cycles by a measure of baroclinicity and the cold-core/warm-core structure in two layers. Three modes of polar low development are identified. Forward-shear cold-core polar lows (24.7% of lows) form according to conventional models of baroclinic development, are most frequent near the storm track and the ice edge, and seldom achieve notable vorticities. Forward-shear polar lows in which a warm-core structure develops and deepens within a deep cold-core environment, forced in the early stages by an upper-level trough (38.4% of lows) are common throughout the domain. Polar lows in reverse-shear environments (34.1% of lows) also have a warm-core structure, and are most frequent east of the Greenland tip and in the Nordic Seas, and are not found near the storm track. Deep warm core systems have the highest surface wind speeds and precipitation rates. A subset of these are the symmetric, warm-core polar lows, also known as 'Arctic hurricanes'. They can develop from warm-core systems in forward- or reverse-shear environments and have low or moderate wind shear, strong surface fluxes, high winds and intense convective precipitation.
Date of Award1 Aug 2016
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorGeraint Vaughan (Supervisor) & David Schultz (Supervisor)


  • cyclones
  • baroclinicity
  • Arctic System Reanalysis
  • cyclone tracking
  • Arctic
  • polar low
  • climatology

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