A Case Study of Explosive Cyclogenesis in the Eastern Pacific

Abstract

An explosive cyclogenesis that occurred in the eastern Pacific on 13 November 1981 is described and discussed on the basis of surface and upper air analyses and satellite imagery. During one 12-h period the storm deepened by nearly 40 mb and subsequently attained a central pressure of 950 mb or less. Peak surface winds were in the 45?50 m s?1 range. The Limited Area Fine Mesh (LFM) model 24-h forecast initialized at 0000 GMT on the thirteenth completely failed to predict the storm development. The development took place within a strong baroclinic zone, as a shallow frontal wave traveled from the relatively stable environment of a long-wave ridge to the unstable environment of a long-wave trough. Winds at jet stream level exceeded 90 m s?1 prior to the explosive deepening. The lifted index in and ahead of the storm diminished and became negative prior to the rapid intensification. Measurements revealed that sensible and latent heat fluxes from the ocean contributed significantly to the reduced stability. Low-level advection of warm, moist air was perhaps an even greater cause of the reduction. Further evidence of the instability was afforded by satellite imagery, which showed outbreaks of deep convection at the leading edge of the warm frontal cloud band and along the upper part of the cold front. Very dry, ozone-rich air aloft penetrated the cyclone from the rear at the beginning of the rapid-deepening stage. The fully developed cyclone possessed a warm, hurricane-like core in the 1000?500 mb thickness. However, the spiral-shaped cloud pattern was typical of an extratropical occluded cyclone, and surface ship and buoy observations revealed an intertwining of warm and colder air within the core. A north-south cross section taken through the storm early in the deepening stage showed that the surface front was a shallow feature embedded within a zone of broad and deep baroclinity. A moist-neutral lapse rate prevailed within the frontal cloud band. The band was highly symmetrically unstable and would have remained symmetrically unstable had the lapse rate been considerably more stable. The rapid intensification is attributed to strong, deep baroclinic forcing in the presence of small effective static stability. The latter term, as used here, denotes the existence of weak or neutral static stability in the precipitating cloud mass combined with below normal dry static stability in the environment.