Thermal Structure and Airflow in a Model Simulation of an Occluded Marine Cyclone

Abstract

A very fine mesh model simulation of the Ocean Ranger storm of February 1982 is used to study the thermal structure and airflow in an intense marine cyclone. In particular, the study investigates the structures of the occluded front and the secluded pool of warm air in the simulated cyclone and examines the formation of these structures with the help of a large number of air trajectories. It was found that in the model simulation the largest thermal gradient occurred along the occluded front, not along the warm or cold fronts, and that the intense gradient was a product of strong warm and occludofrontogenesis in the inflowing air. The occluded front was embedded within air that was earlier located within the baroclinic zone ahead of the low center, not air that was in proximity to the historical warm and cold fronts. The warm air in the seclusion likewise originated in the baroclinic zone ahead of the low. The seclusion at low levels resulted from a tongue of slower moving, relatively warm air being pinched off by colder, more rapidly moving air that circulated about the low center from front to rear. Major features of the simulated airflow were 1) a cloud-producing flow that rose from the downstream boundary layer and spread out in a fan-shaped pattern, forming the familiar book-shaped cloud signature of the mature cyclone, and 2) an upper-level dry airstream that advanced on the system from the west (upstream), producing the dry slot commonly seen in satellite imagery over and ahead of the surface occlusion. Air in the dry slot amended rapidly over the occluded front but remained cloud-free because of earlier upstream subsidence and drying. The seclusion of warm air at middle levels was a consequence of a tongue of warm, but not tropical, air being pinched off by colder air from both the front and rear of the system. Detailed feature of the simulated storm that differed from the classical occlusion model were 1) the sharpness of the thermal gradient along the inner part of the occluded front, 2) the presence of a secluded pocket of warm air near the tail end of the occluded front, 3) the formation of the occluded front by frontogenesis in the baroclinic air mass ahead of the low rather than by a joining of the initial warm and cold fronts, 4) the trans-formation of the low-level occluded front to a warm front aloft without the existence of an intervening cold front, and 5) the lack of mid- and upper-level cloud above the occluded-warm front along much of its length. The fundamental occlusion process described in the Norwegian model, however, was verified. Warm air of widely ranging temperature was squeezed aloft by the sinking and spreading of colder air that encircled the low center.