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

Abstract

his study 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) reintensification. 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 northward movement into lower-tropopause regions associated with an eastward-propagating midlatitude trough, where the associated lower-stratospheric warm air wraps into the storm and its surrounding areas. The steady maximum surface wind occurs because of the widespread SLP falls with weak gradients lacking significant inward advection of absolute angular momentum (AAM). Meanwhile, three spiral frontogenetic zones and associated rainbands develop internally in the outer northeastern quadrant during the last three stages when Sandy?s southeasterly warm current converges with an easterly cold current associated with an eastern Canadian high. Cyclonic inward advection of AAM along each frontal rainband accounts for the continued expansion of the tropical storm?force wind and structural changes, while deep convection in the eyewall and merging of the final two surviving frontogenetic zones generate a spiraling jet in Sandy?s northwestern quadrant, leading to its reintensification prior to landfall. The authors conclude that a series of moist frontogenesis plays a more important role than the lower-stratospheric warmth in determining Sandy?s size, intensity, and structural changes.