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Kuo-Hsu Su, Ping-Tung Shaw, and Leonard J. Pietrafesa

Abstract

Distinct offshore and upward phase propagation at periods of 12 and 24 days was previously observed in the horizontal flow field on the outer shelf in the southern Mid-Atlantic Bight during the Shelf Edge Exchange Processes Experiment in spring 1988. A linear, forced-wave model is invoked to explain the observed phase propagation. It is found that the observed phase propagation can exist when currents are in resonance with a vorticity source on the continental slope. It is essential that the thermohaline field be characterized by an upper-ocean stratification an the slope for resonance to occur. The phase difference near resonance is weakly dependent on the bottom frictional parameter and the alongshore length scale of forcing. Resonance is due to onshore propagation of topographic waves. It is suggested that low-frequency oscillations on the outer shelf in the Mid-Atlantic Bight in winter an associated with sources on the upper slope.

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Li-Huan Hsu, Shih-Hao Su, Robert G. Fovell, and Hung-Chi Kuo

Abstract

Typhoons with “deflection tracks” (DTs) within a 200-km distance of the mountainous island of Taiwan are examined. We analyze 84 landfalling typhoons that compose 49 DT cases turning to the left-hand side, including 18 with very large deflection angles (DA > 20°) and another 7 having looped tracks (LTs). Most of the large DA and LT cases are “northern landfall” type, reaching Taiwan’s east coast poleward of 24°N and originally possessing relatively slow translation speeds (~4 m s−1). Their average translation speeds, however, increase by 50% in the 3 h prior to landfall. The WRF Model is used to simulate DT cases, and potential vorticity (PV) tendency diagnosis is used to interpret the contributions of the horizontal advection (HA), vertical advection (VA), and diabatic heating (DH) terms. The northern landfall tropical cyclones (TCs) possess significant cross-mountain flow to the south of the storm near the coast, resulting in vorticity stretching (the VA effect) and subsidence warming. The subsidence suppresses storm convection and produces heating asymmetries (the DH effect) that can induce significant southwestward deflections. The cross-mountain VA and DH effects are weaker for the “southern landfall” storms. The results explain well the observed increase of translation speed prior to landfall in DT cases and show that the HA effect, in general, does not contribute to the track deflection. Our results highlight the impact of topography on TC track by the vorticity stretching effect and by asymmetric diabatic heating.

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Robert G. Fovell, Yizhe Peggy Bu, Kristen L. Corbosiero, Wen-wen Tung, Yang Cao, Hung-Chi Kuo, Li-huan Hsu, and Hui Su

Abstract

The authors survey a series of modeling studies that have examined the influences that cloud microphysical processes can have on tropical cyclone (TC) motion, the strength and breadth of the wind field, inner-core diabatic heating asymmetries, outer-core convective activity, and the characteristics of the TC anvil cloud. These characteristics are sensitive to the microphysical parameterization (MP) in large part owing to the cloud-radiative forcing (CRF), the interaction of hydrometeors with radiation. The most influential component of CRF is that due to absorption and emission of longwave radiation in the anvil, which via gentle lifting directly encourages the more extensive convective activity that then leads to a radial expansion of the TC wind field. On a curved Earth, the magnitude of the outer winds helps determine the speed and direction of TC motion via the beta drift. CRF also influences TC motion by determining how convective asymmetries develop in the TC inner core. Further improvements in TC forecasting may require improved understanding and representation of cloud-radiative processes in operational models, and more comprehensive comparisons with observations are clearly needed.

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