DYNAMIC ASPECTS OF THE OCTOBER STORM, 1987 - A STUDY OF A SUCCESSFUL FINE-MESH SIMULATION

A diagnostic study of a successful numerical simulation of the October storm 1987 is described. This good, non-operational forecast was obtained using the Meteorological Office's fine-mesh model run from a revised analysis at 00 GMT on 15 October 1987. The Meteorological Office's mesoscale model was also run using initial conditions at 12 GMT 15 October obtained by interpolating the above fine-mesh forecast fields to examine the benefits of very high horizontal resolution (15 km grid length). Diagnostic maps and cross-sections derived are used to address two scientific issues: (i) the role of weak moist slantwise stability (or, alternatively, positive slantwise convective potential energy) (ii) the dynamical significance of the striking cloud structure (the cloud head) seen in the satellite imagery early in the storm's life. It appears that the baroclinic zone, in which the storm develops, is highly receptive to forcing by an upper trough and the warm boundary-layer air just ahead of the main surface front is characterized by substantial slantwise convective available energy. The cloud head is identified with a region of intense slantwise ascent in the warm air just above the main frontal zone. Its great meridional extent is associated with the upper tropospheric outflow and horizontal spreading of air which has recently undergone rapid slantwise ascent. The low-level temperature field exhibits, throughout the development period, a remarkable shape-preserving thermal ridge of considerable intensity. The sensitivity of the forecast to the removal of surface energy fluxes and latent heat of condensation are investigated separately. It is found that on the timescale of the simulation, surface energy fluxes have negligible impact on the explosive development. On the other hand, the contribution of latent-heat release to the development appears to dominate the dry baroclinic instability process.