Diagnostic Study of Explosive Cyclogenesis during FGGE

Abstract

Two rapidly developing extratropical maritime cyclones are studied using the European Centre for Medium-range Weather Forecasting (ECMWF) level III-b analyses from the First GARP Global Experiment (FGGE). The first cyclone forms over the western North Pacific Ocean along an intense frontal zone south of Japan, while the second develops in a polar air mass over the North Atlantic Ocean. Quasi-Lagrangian diagnostic techniques in isobaric coordinates are used in a synoptic investigation and mass and vorticity budget diagnostic evaluation of storm development. Reliable diagnostics are obtained from the ECMMWF analyses over these ocean areas using 0000 and 1200 UTC data. Although the cyclones develop under almost unperturbed upper level flow resembling Petterssen type A development, rapid deepening in both cases occurs when an approaching upper tropospheric jet with appreciable shear vorticity advection becomes favorably superposed over the surface low. Stability decreases in the low troposphere during the development period aiding in the rapid development of an intense mass-circulation and low tropospheric vorticity production by the divergence term. These results suggest upper-level forcing plays a greater role in the initiation of explosive oceanic development than the Petterssen description suggests. This study also shows FGGE SOP-1 data and analyses can support diagnostic studies of oceanic cyclogenesis.