THE STRUCTURE AND EVOLUTION OF AN EXPLOSIVE CYCLONE NEAR ICELAND

In the first study of a series of studies of intense cyclones in the a ctive region around Iceland, the explosive synoptic-scale ''Greenhouse Low'' that hit Iceland on 3 February 1991 is studied. We have investi gated this cyclone mainly through numerical simulations with an NWP mo del (HIRLAM), but also by careful reanalysis of surface observations a nd satellite pictures. The cyclone formed in an unusually baroclinic a ir mass just north of the Azores in the early hours of 2 February 1991 . A baroclinic wave formed and developed, aided by latent heat release . During this period, we have found evidence of symmetric instability in a deep layer. Between 18 UTC 2 February and 06 UTC 3 February, the central pressure fell by more than 30 hPa, as the cyclone deepening co ntinued. Over the next 6 h, a further deepening of 15 hPa occurred, as an upper level potential vorticity anomaly caught up with and reinfor ced the cyclone. At this time, hurricane-force winds were observed at numerous sites in the southern and western parts of Iceland, with mean winds up to 57 m s(-1). As the cyclone continued its northward moveme nt, a pressure rise of 30.4 hPa in 3 h was recorded at one station. Re runs with the HIRLAM model, using ECMWF analyses from 12 UTC 2 Februar y as initial conditions, captured all the main features of the observe d development, helping us to draw the above picture. Corresponding run s from 00 and 06 UTC had considerable errors in both positioning and d eepening of the storm. The sensitivity of the simulations to the remov al of surface energy fluxes and latent heat of condensation was invest igated separately. It was found that over the time span of the simulat ion, surface energy fluxes had negligible impact on the explosive deve lopment. The contribution of latent heat release to the development ap pears to be about half that of dry baroclinic instability. These findi ngs are discussed in view of related studies in the literature.