Quasi-Lagrangian Diagnostics Applied to an Extratropical Explosive Cyclogenesis in the North Pacific

A case of explosive extratropical cyclogenesis in the North Pacific is analyzed employing quasi-Lagrangian diagnostic techniques in isobaric coordinates. The First GARP Global Experiment (FGGE) Level III-b data set from the European Center For Medium-range Weather Forecasting (ECMWF) is used in a synoptic investigation and mass and vorticity budget diagnostic evaluation of storm development. Mid-tropospheric positive vorticity advection (PVA) acts in concert with low-level thermal advection to enhance surface layer organization during the initial periods. Explosive development occurs under upper-level zonal flow when the surface system, developing along the low-level baroclinic zone, moves under the front left quadrant of a strong jet streak. Stability in the lower troposphere decreases steadily as an intense two-layer mass circulation is established. The divergence term provides the largest contribution to the vorticity tendency during explosive development due to substantial surface layer convergence and upper-level divergence. (Author)