Upper-level Forcing of Explosive Cyclogenesis Over the Ocean Based on Operationally Analyzed Fields

Abstract

Some of the recent research studies of explosive extratropical cyclogenesis have been based on detailed soundings or special reconstructions of the evolution in poststorm analysis. The focus of this cyclogenesis study is on the upper tropospheric and lower stratospheric features that may be determined from the operationally analyzed charts available to the maritime forecaster. To have the largest possible amount of over-ocean data, a sample of 23 explosive and nonexplosive deepeners from the western North Atlantic and western North Pacific regions during FGGE is examined. Potential vorticity and jet streak properties are derived from analyses prepared by the European Centre for Medium-Range Weather Forecasts. The vertical and horizontal resolution of the archived analyses are not adequate to depict the tropopause folding and downward transport of potential vorticity thought by some investigators to be important in explosive cyclogenesis. Thus, the emphasis is put on the relative maxima in potential vorticity that are present at 300 mb upstream of all cyclones. Storm tracks, with respect to these potential vorticity maxima, are counterclockwise, with the greatest sea level pressure decreases occurring when the storm is to the cast or southeast of the maximum, whereas pressure falls diminish when the cyclone is north of the maximum. Only five of the cases have a preexisting potential vorticity lobe that later becomes positioned in a favorable location relative to the surface feature to enhance cyclogenesis. In the remaining cases, the cyclone and the advection associated with the potential vorticity lobe propagate and develop concurrently. The presence of a jet maximum over the storm is a major factor in storm development with large pressure falls being directly related to higher 300-mb wind speeds. In 20 of the cases, the storm is in the left-ftont jet quadrant at some time during its development. A statistical analysis demonstrates that forecasting the actual values of 12-h pressure falls from the potential vorticity and wind fields is difficult. However, short-term forecasting of development within one of three intensity categories using a discriminant analysis technique may approach 90% accuracy for explosive cyclones.