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Timothy A. Bullock and John R. Gyakum

Abstract

The phenomenon of explosive cyclogenesis is studied from the perspective of the synoptic-scale framework within which various intensities of maximum 24-h pressure falls are occurring. This study is accomplished with a construction of composite groups of cyclones that have experienced similar maximum intensification rates within a specified 5° latitude-longitude geographical domain over the Kuroshio Current in the western North Pacific Ocean. An examination of diagnostics computed from the composite fields of geopotential height and temperature reveals several trends. As the degree of intensification increases, the downstream surface ridge and attendant warm, moist inflow become more prominent, the cyclonic vorticity of the initial surface circulation is greater, the downstream frontogenesis is stronger and occurs through a deeper layer of the troposphere, and the location and strength of the vertical-motion forcing become more favorable for development. As a consequence of these results, it is concluded that synoptic-scale forcing mechanisms extending over a large domain, in a composite sense, play a role in determining the amount of intensification experienced by a cyclone. These mechanisms supporting cyclogenesis include not only dynamic support in the form of midtropospheric thermal and vorticity advection but also by deep tropospheric frontogenetic processes occurring both upstream and downstream of the surface low.

Since these mechanisms are well resolved by contemporary numerical models and routinely available data, the aforementioned trends might be used operationally to evaluate the potential for cyclone intensification.

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John R. Gyakum, Paul J. Roebber, and Timothy A. Bullock

Abstract

We examine the idea that antecedent vorticity development, defined as the surface vorticity spinup in the period prior to a cyclone's maximum intensification, is an important dynamical conditioning process for explosive cyclogenesis. Previous suggestions from case study research that subsequent intensification may be proportional to the intensity of the preexisting circulation are supported through the systematic study of a large sample of weakly and explosively developing cyclones in the North Pacific and North Atlantic basins.

Additional support for this concept is found with an examination of composite weakly and strongly developing cyclones at the onset of their most rapid intensification period. At this onset, the strongly developing cyclone composite has substantially stronger surface circulation and vorticity than is found in a composite of the weak cases. Ensembles of successive forecasts of an explosive cyclogenesis case during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) suggest similar dynamical behavior, in that small errors in the surface intensity subsequently amplify into larger errors only 12 h later under predominantly similar upper-level conditions.

The temporal evolution of large-scale geostrophic vorticity for 62 cases of cyclogenesis shows that stretching in the presence of relative vorticity is present throughout the life cycle of both the weakly and rapidly developing cases. An examination of 794 cyclones in the North Pacific basin reveals a general trend of increased maximum development as the antecedent deepening increases. Explosively developing cyclones are preferentially characterized by at least 12 h of antecedent development.

We investigate the relationship between the amplitude of the 500-mb quasigeostrophic-ascent forcing and maximum surface cyclone intensification and find a significant positive correlation, as previous studies have shown. However, computations with model-based surface convergence suggest that the response to the upper-level forcing is conditioned by the low-level antecedent vorticity development. Furthermore, variations in successive numerical weather prediction model forecasts of maximum cyclone intensification are well correlated with variations in the initial surface vorticity as well as variations in the 500-mb forcing.

This study suggests that explosive development is typically characterized by a nonlinear interaction between two cyclonic disturbances in the lower and upper troposphere. These disturbances, in some cases, may have formed independently of one another. Thus, the correct simulation of the full life cycle of these cyclones, including the antecedent phase, may be crucial for accurate numerical forecasts.

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