Meteorological Precursors to the Explosive Intensification of the QE II Storm

Abstract

The objective of this research is to define the meteorological conditions prior to the explosive development of the QE II storm. By using conventional data and detailed McIDAS satellite imagery we document the genesis of this storm along a preexisting line of active surface Frontogenesis, 12 h before the onset of its extraordinarily rapid 24-b central pressure fall of nearly 60 mb. This particular surface cyclone, having formed on the western edge of a convective complex, is shown to be a lower-tropospheric warm-core phenomenon at the time of its birth. During the first 12 h of existence, the cyclone deepened 7 mb and its surface relative vorticity increased to 17 ? 10?5 s?1. The cyclone had intensified sufficiently, after only 6 h, to have developed the characteristic pattern of strong cold and warm frontogenesis regions. During the 24-h period of explosive intensification, a strong midtropospheric trough interacted with the already well-developed surface cyclone. This period corresponds to the cyclone?s transformation into a larger and deeper system. The particular midtropospheric trough is traced on its southeastward path from the Canadian Northwest Territories until it interacts with the QE II storm during its explosive intensification. This upper-tropospheric trough is also found to be associated with another distinct and intensifying surface cyclone, whose identity is maintained until after the initiation of the QE II storm?s explosive intensification. We demonstrate that this particular surface cyclone has a deep, cold-core structure during the initial 12 h of the separate and shallow QE II storm. This documentation of a separate, independent origin and development for each of the surface and upper-tropospheric cyclonic disturbances involved in this explosive cyclone intensification motivates us to suggest a two-stage process of cyclone development that may be unique to the explosively developing cyclone. The first stage involves the genesis and development of the surface cyclone. For this particular case, the surface cyclogenesis occurs as a shallow frontal wave that develops independently of an upper-tropospheric trough. This frontal wave develops strong winds of 18 m s?1, extending to 300 km south of its center, and a sufficient amount of cyclonic vorticity (17 ? 10?1 s?1 in this case) to dramatically enhance the surface response to the approach of the upper-tropospheric trough. The interaction of the upper-tropospheric trough and the strong surface cyclone constitutes the onset of stage two of the development process that corresponds to the cyclone's explosive intensification period. This research suggests that not all cyclogenesis can be regarded as a classical type ?B? development in which the surface cyclone forms in response to an approaching upper-tropospheric trough. Rather, we suggest that a surface cyclone?s explosive intensification may typically involve the interaction of separate surface and upper-tropospheric cyclonic disturbances, each of whose development may be substantial enough, before their interaction, to warrant their individual examination.