The Impact of Moist Frontogenesis and Tropopause Undulation on the Intensity, Size and Structural Changes of Hurricane Sandy (2012)

Hurricane Sandy (2012) was one of the most destructive hurricanes causing more than $50 billion damage and a total of 147 casualties. It has a record-breaking storm size in the extended best track that began in 1988, with an averaged radius of the tropical-storm-forced wind of 660 km at 24 h prior to landfall. The storm underwent several intensity changes with continuous size expansion as it moved northward from the southwestern Caribbean Sea to landfall at the New Jersey shoreline. Although Sandy's track and landfall were well predicted 5 days in advance, little remain certain on the physical processes leading to the multiple intensity changes, and the continued growth of the storm size during its life cycle as well as the timing of its extratropical transition (ET). Furthermore, it still remains unclear about the roles of two upper-level troughs (i.e., a polar and a subtropical one) and low-level baroclinity in determining the re-intensification of the storm prior to landfall. In this study, we examines the relative roles of moist frontogenesis and tropopause undulation in determining the intensity, size and structural changes of Hurricane Sandy using a high-resolution cloud-resolving model. A 138-h simulation reproduces Sandy's four distinct development stages: (i) rapid intensification, (ii) weakening, (iii) steady maximum surface wind but with large continued sea-level pressure (SLP) falls, and (iv) re-intensification. Results show typical correlations between intensity changes, sea-surface temperature and vertical wind shear during the first two stages. The large SLP falls during the last two stages are mostly caused by Sandy's moving northward into lower-tropopause regions associated with an eastward-propagating midlatitude trough, where the associated lower-stratospheric warm air wraps into the storm. Meanwhile, three spiral frontogenetic zones and associated rainbands develop internally from the northwestern eyewall to the outer northeastern quadrant during the last three stages, respectively, when Sandy's southeasterly warm current converges with an easterly cold current associated with an east-Canadian high. Cyclonic inward advection of absolute angular momentum along each frontal rainband accounts for the continued expansion of the tropical-storm-forced wind and structural changes, and merging of the final two survived frontal rainbands generates a spiraling jet in Sandy's northwestern quadrant, leading to the re-intensification of the storm. Moreover, cyclonic advection of the east-Canadian colder air into Sandy's southern semicircle increases radial thermal contrasts and SLP gradients, helping maintain a (balanced) swirling jet in its southern sectors. We conclude that moist frontogenesis plays more important roles than the lower-stratospheric warmth in determining Sandy's size and structural changes as well as re-intensification with little impact of an approaching cold front.