Moist Frontogenesis and Low-Stratospheric Warmth during the Extratropcial Transition of Hurricane Sandy (2012)

Prof. Lance Bosart has made many important contributions during the past five decades to our understanding of tropical and extratropical storms, and mesoscale convective systems (MCSs) as well as to our daily weather analysis and forecasting processes. Although I did not receive direct graduate education from him and never collaborated with him, I have benefited so much from his work throughout my professional career, especially during my very earlier career stage. Specifically, my Ph. D. dissertation on the numerical modeling of the Johnstown flood of July 1977 (Zhang 1985) would not be successful without his objectively analyzed data at 1°´1° latitude-longtitude resolution over a 1000 km ´ 1000 km domain that resulted from his highly cited observational study of the case (i.e., Bosart and Sanders 1981). Because both the squall line and MCC, which accounted for the generation of the Johnstown flood, occurred in the warm section far ahead of a large-scale cold front, the then PSU/NCAR MM3 model could not initiate the two MCSs at the right location and timing without his carefully analyzed data (Zhang and Fritsch 1986). Instead, the model produced areas of deep convection far from the observed. Of more importance to my career growth is that his analysis of the subsequent oceanic cyclogenesis, after the MCSs moved off shore following the Johnstown flood, motivated me and my student to perform 90-h simulations of the oceanic cyclogenesis events showing many interesting features associated with the thermodynamical transformation and the generation of low-level cyclonic vorticity during tropical cyclogenesis (Zhang and Bao 1996a,b). A successful study of this case opened my mind for subsequent extension of my research interests from midlatitude MCSs to tropical cyclones. During the presentation, I will show more how Prof. Bosart’s work influenced my professional growth.

In addition, I will present some recent work, through supervision of one of my Ph. D. students, on the impact of moist frontogenesis and low-stratospheric warmth on the extratropical transition of Hurricane Sandy (2012), which has also been one of the research strengths of Prof. Lance Bosart. This work was completed by analyzing a 138-h WRF simulation of the case, first showing that the model 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 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, respectively, when Sandy’s southeasterly warm current converges with an easterly cold current associated with an east-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 survived frontal rainbands generate a spiraling jet in Sandy’s northwestern quadrant, leading to its re-intensification prior to landfall. We conclude that a series of moist frontogenesis plays more important roles than the lower-stratospheric warmth in determining Sandy’s size and structural changes as well as re-intensification.

Bosart, L. R., and F. Sanders, 1981: The Johnstown flood of July 1977: A long-lived convective system, J. Atmos. Sci., 38, 1616-1462.

Zhang, D.-L., 1985: Nested-grid simulation of the meso-b-scale structures and evolution of the Johnstown flood of July 1977. The Penn State University, 270pp.

Zhang, D.-L., and J.M. Fritsch, 1986: A case study of the sensitivity of numerical simulation of mesoscale convective systems to varying initial conditions. Mon. Wea. Rev., 114, 2418-2431.

Zhang, D.-L. and N. Bao, 1996: Oceanic cyclogenesis as induced by a mesoscale convective system moving offshore. Part I: A 90-h real-data simulation. Mon. Wea. Rev., 124, 1449-1469.

Zhang, D.-L. and N. Bao, 1996: Oceanic cyclogenesis as induced by a mesoscale convective system moving offshore. Part II: Genesis and thermodynamic transformation. Mon. Wea. Rev., 124, 2206-2225.