A Polar Low Pair over the Norwegian Sea

Abstract

During the Lofotes cyclone experiment (LOFZY 2005), two polar lows developed one behind the other inside a cold-air outbreak from the north in the lee of Spitsbergen on 7 March 2005. Buoys, ship, and aircraft measurements as well as satellite imagery are applied to analyze the polar low bulk properties, the horizontal and vertical structure, and the mass, moisture, and heat budget. The lifetime of the system until landfall at northern Norway was 12 h. The generation occurred under the left exit region of an upper-level jet with 70 m s?1. Both polar lows had a radius of 100?130 km and extended to a height of about 2.5 km. The propagation speeds were within 14?17 m s?1 and correspond to the vertically averaged wind velocity of the lowest 2.5 km. In the polar low centers the pressure was about 2?3 hPa lower and the air was 1?2 K warmer and drier than in the surroundings. Aircraft measurements in the second of the two polar lows show an embedded frontlike precipitation band north of the center. Here, the highest low-level winds with 25 m s?1 and the largest fluxes of sensible and latent heat with 290 and 520 W m?2, respectively, were measured (areal averages amounted to 115 and 190 W m?2). Aircraft data show mass convergence in the subcloud layer (0?900 m) and divergence in the cloud layer (900?2500 m). Moisture supply by evaporation from the sea surface was about twice as large as that by convergence in the subcloud layer. The condensation rate in the cloud layer nearly equaled the rate of evaporation at the sea surface. Almost all condensed cloud water was converted to precipitation water. Only half of the precipitation at the cloud base reached the sea surface.