ALBERT

Description: During 3–4 March 2008, the Norwegian IPY-THORPEX field campaign successfully carried out three flight missions that observed the full life cycle of a polar low over the Norwegian Sea. Here the three-dimensional structure of the polar low has been investigated using dropsonde data from the three flights. The polar low developed in a cold air outbreak, with temperature differences between the sea surface and 500 hPa of about 45–50°C. Cross-sections show that the horizontal gradients of potential temperature weakened as the polar low matured, suggesting that baroclinic energy conversion took place. Dropsonde data of potential temperature and relative humidity show evidence of a tropopause fold, which is possibly a manifestation of upper-level forcing. This is corroborated by potential vorticity inversion, which shows a dominant role of upper-level forcing throughout the polar low's lifetime. During the cyclogenesis stage the polar low circulation was confined below 700 hPa, with a northerly low-level jet of 26 m s −1 . In the mature stage, its circulation reached up to the tropopause (∼450 hPa), with maximum wind speed between 700 and 900 hPa of about 26–28 m s −1 . At this stage the polar low warm core was about 3 K warmer than surrounding air masses. The deep moist towers at the eye-like structure of the polar low extended up to the tropopause with relative humidity values above 70%, indicating a possibly important role for condensational heating in the development. Estimates of surface fluxes of sensible and latent heat using temperature and moisture from the dropsonde data show latent heat fluxes west of the polar low increasing from 175 to 300 W m −2 as the low matured, while the sensible heat fluxes rose from 200 to 280 W m −2 , suggesting a gradually increasing contribution of surface fluxes to the energetics of the polar low with time. Copyright © 2011 Royal Meteorological Society