Effects of Surface Energy Fluxes during the Early Development and Rapid Intensification Stages of Seven Explosive Cyclones in the Western Atlantic

Abstract

A series of eight numerical experiments were conducted on seven cases of explosive marine cyclogenesis, using the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. The main objective was to elucidate the role of surface energy fluxes both during and preceding rapid deepening. Results from the 24-h experiments initialized at the commencement of the period of most rapid deepening showed that the fluxes occurring during this stage had essentially no effect on the deepening rate. However, substantial effects of the fluxes were found in 48-h experiments initialized early in the development. When the fluxes were withheld from the 48-h simulations, the predicted cyclones were weakened on average by 7.1 mb at 24 hours and 13.5 mb at 48 hours. Evidently the fluxes occurring during the 24 hours preceding rapid deepening did affect the storm development throughout its lifetime. Additional 48-h experiments confirmed the relatively small impact of the concurrent fluxes during the rapid development stage, as found in the 24-h experiments. Detailed analysis was performed on a storm that was representative of the sample as a whole. In the early development stage, when the cyclone was located over the warm ocean in the vicinity of the GulfStream, strong energy fluxes occurred near the low center. The low-level heating and moistening caused by the fluxes contributed to the development of a coastal front and significantly reduced the atmospheric stability in and near the frontal region. The frontogenetical action and stability reduction resulted in greatly increased precipitation amounts near the storm center. The incipient cyclone was 11 mb deeper in a 24-h full-physics simulation than in a simulation without fluxes. During the later stage of the development, the cyclone crossed the GulfStream and moved over colder waters. The surface energy fluxes in the vicinity of the storm then weakened and even reversed direction, while large upward fluxes persisted well to the rear of the cyclone. This pattern of low-level heating and cooling was not conducive to storm intensification. However, the latent beat released in precipitation of moisture supplied by earlier fluxes did contribute to further deepening. To fully explain the deepening rates at different stages of development, one needs to consider both the effects of the fluxes occurring at a given stage and the delayed effects of earlier fluxes.