A polar low observed over the Norwegian Sea on 3–4 March 2008: high-resolution numerical experiments



A polar low (PL) observed over the Norwegian Sea by the IPY-THORPEX research aircraft campaign during 3–4 March 2008 was studied by a series of fine-mesh (3 km) experiments using the state-of-the-art Weather Research and Forecasting (WRF) model. The full-physics experiment simulated the PL intensity, baroclinic nature, surface wind speed and track rather well compared to dropsonde observations and satellite images. Two types of sensitivity experiments were designed to analyse the physical properties of the PL. First, physical processes such as condensational heating and sensible and/or latent heat fluxes were switched off throughout the whole simulation. In the second type, these processes were turned off at later times, which minimized the modification of the polar low environment caused by the absence of one or all of them over a long time period, making it suitable to study the direct effect of the physical processes on the PL itself. These two types of sensitivity experiments suggested the following: low-level baroclinic energy conversion was of primary importance for the PL development, while other physical processes had a minor direct impact on the PL intensity. The surface latent heat fluxes, and to a lesser extent sensible heat fluxes, played an indirect role in the sense that they set up and supported the baroclinic environment vital for the PL development. Condensational heating, however, was essential to set up the initial baroclinic environment favourable for the PL intensification, while it had a modest indirect effect thereafter. The experiments indicated that in the late mature stage, after baroclinicity vanished, surface energy fluxes (sensible and/or latent heat) might have contributed to the PL energetics. How the variability in the simulated moist convection, the thermal structure of the PL core, and surface energy fluxes in each experiment affected the PL was difficult to answer. Copyright © 2012 Royal Meteorological Society