The structure and evolution of an explosive cyclone near Iceland

Abstract

In the first study of a series of studies of intense cyclones in the active region around Iceland, the explosive synoptic-scale “Greenhouse Low” that hit Iceland on 3 February 1991 is studied. We have investigated this cyclone mainly through numerical simulations with an NWP model (HIRLAM), but also by careful reanalysis of surface observations and satellite pictures. The cyclone formed in an unusually baroclinic air 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 continued. Over the next 6 h, a further deepening of 15 hPa occurred, as an upper level potential vorticity anomaly caught up with and reinforced 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 movement, a pressure rise of 30.4 hPa in 3 h was recorded at one station. Reruns with the HIRLAM model, using ECMWF analyses from 12 UTC 2 February as initial conditions, captured all the main features of the observed development, helping us to draw the above picture. Corresponding runs from 00 and 06 UTC had considerable errors in both positioning and deepening of the storm. The sensitivity of the simulations to the removal of surface energy fluxes and latent heat of condensation was investigated separately. It was found that over the time span of the simulation, surface energy fluxes had negligible impact on the explosive development. The contribution of latent heat release to the development appears to be about half that of dry baroclinic instability. These findings are discussed in view of related studies in the literature.