Polar low le Cygne:: Satellite observations and numerical simulations

A polar low (PL) which occurred in October 1993 over the Norwegian Sea is investigated front an observational and a numerical point of view. This PL has several unique features: it developed early in the season, it lasted for about 3 days. and its trajectory was such that it passed over weather stations so that 'conventional' observations of the low are available. The conditions of the formation, development and decay of the PL are investigated using a double approach: satellite data front several instruments are used together to document the mesoscale structure of the low, and two versions of a limited-area model are run to investigate the dynamics of the low. Numerical model fields are compared to quantities derived from TIROS-N Operational vertical Sounder, the Special Sensor Microwave/Imager, and satellite radar altimeter data. In spite of a better spatial resolution of the models, humidity and surface wind speeds are less organized in the simulations than in satellite retrievals. The number of vertical levels. especially for the lowest layers of the atmosphere, appears to be an essential component for a good simulation of the trajectory of the low. There is, however, good overall agreement between modelled and satellite-derived fields, and the good quality of the simulations allows inferences to be made regarding the essential physical and dynamical processes taking place during the formation and development of the PL. We find that the PL was the result of favourable flow conditions Lit the surface in the form of a shallow arctic front established south of the ice edge, together with all Upper-level potential-vorticity anomaly setting the stage for a positive interaction. Later on. the strong surface sensible- and latent-heat fluxes contributed to the extensive vertical development. This Study demonstrates the usefulness of the approach adopted here. which relies not only on simulations but also on observations to act a very complete description of such disturbances.