Roles of Upper-Level Processes in the Multi-Intensity Changes of Hurricane Sandy (2012)

Abstract:

The multi-intensity changes of Hurricane Sandy (2012) are examined by using a cloud resolving WRF model. An analysis of Sandy's life cycle shows four distinct stages: (1) rapid deepening, (2) weakening, (3) deepening with little intensification of rotational wind, and (4) re-intensification of vortex winds. Results from the model simulations indicate that Sandy's multi-intensity changes are closely related to (i) changes in the magnitude and direction of environmental vertical wind shear (VWS), (ii) upper-tropospheric warming associated with deep convection in the core region, (iii) lower-stratospheric warmth as the storm moves poleward into lower-tropopause regions, and (iv) the possible roles of inertial instability in the upper outflow regions as approaching a upper-level trough/jet stream.

Specifically, Sandy intensifies steadily since October 24 as it moves over warm SST surface from Caribbean Sea to Cuba Island. After the storm passes Cuba Island, its warm core begins to tilt under the influence of increasing VWS as it approaches to an upper-level subtropical jet stream, leading to the weakening of the storm. After October 27, Sandy deepens as it moves far away from the upper-level jet core VWS, though over a colder SST surface. By hydrostatic reasoning, we find that during the 3^rd stage Sandy's deepening results partly from the stacked upper-level warming in the core region but more from low stratospheric warmth as it moves to higher latitudes with lower tropopause height. Unlike the former scenario, this stratospheric warmth occurs over a meso-alpha-scale region encompassing the storm, thus causing widespread surface pressure falls. This explains why the rotational wind of Sandy shows little intensity changes while its central pressure keeps falling. During the final stage, organized deep convection in the core region increases upper-level tropospheric warming, leading to both the deepening of central pressure and re-intensification of rotational winds. It appears that the presence of inertial instability in the upper outflow channel may play some roles in the intensification of the storm. It is concluded that the intensity changes of Sandy are not only influenced by the tropospheric processes but also by the lower-stratospheric warmth, especially after moving into the mid-latitudes.