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Simon W. Chang

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Simon W-J. Chang

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An axisymmetric, multilayer, numerical tropical cyclone model with a well-resolved planetary boundary layer is used to test the response of local, instantaneous changes of sea surface temperature (SST). One experiment shows that the storm's intensity is steadily decreased as the SST in the inner 300 km is instantaneously cooled by 2°C. However, in the second experiment, in which the SST is cooled by 2°C outside the radius of 300 km, the storm shows no immediate and appreciable weakening. The intensity of the tropical cyclone in this case is maintained by enhanced evaporation in the inner 300 km and increased baroclinicity.

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Simon W. Chang

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An axisymmetric, hydrostatic ocean model containing a rigid bottom and a free surface is constructed to study the barotropic and baroclinic response in the upper and deep ocean to a wind stress corresponding to a stationary tropical cyclone. The numerical model covers a domain of 800 km and 1475 m in r- and z-directions, respectively, with a uniform radial resolution of 20 km and a stretched vertical resolution from 5 to 54 m. The vertical mixing is parameterized based on a local Richardson number and a mixing length.

The model ocean is spun up with the wind stress of Hurricane Eloise. A strong tangential circulation develops that extends to the ocean floor with a maximum speed of 1.2 m s−1 at the surface. The circulation on the r-z plane, which also extends to the ocean floor, oscillates with time with a maximum upwelling of 0.1 cm s−1 at the center. Surface height has a maximum depression of 57 cm. The deep overturning causes density changes deep in the ocean. A maximum temperature decrease of 3°C occurs in the mixed layer at the center; a maximum temperature increase of 0.45°C is found just below the thermocline at a radius of 200 km. The recovery of both the mass and momentum fields is very slow during the spindown. Inertial oscillations dominate in the spindown even in the deep ocean. Adjustments between the momentum and mass fields seem to converge to a state quite different from the prestorm state.

Direct comparison with observations is difficult because the model is only two-dimensional. Nevertheless, recent observations seem to suggest the existence of the barotropic response in the deep mean. The model suggests that the observed rapid response in the deep ocean is caused by the barotropic pressure gradient force, which arises from the storm-induced perturbation of the free surface.

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Simon W. Chang and Richard A. Anthes

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An asymmetric nonlinear ocean model is employed to investigate the oceanic response to moving hurricanes. A turbulent kinetic energy budget is used to parameterize the stress-induced vertical mixing. The results show that the ocean's response to a symmetric storm is stronger on the right of the storm track. Although the maximum speed of the induced current under the storm is not sensitive to the storm's translation speed, the speed does have a large influence on the temperature structure and the thermocline depth in the wake. Vertical motions associated with the inertia-gravity oscillations persist in the wake of the storm. A narrow ridge in the thermocline is left in the storm track for fast-moving storms. The results in many respects agree with Geisler's linear solutions. However, vertical mixing produces significant differences in the depth of the thermocline behind the storm.

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Lynn K. Shay and Simon W. Chang

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Free surface effects induced by an idealized hurricane based on observed air–sea variables in Hurricane Frederic are revisited to examine the barotropic and baroclinic response. Over five inertial periods comparisons between a one-layer and a 17-level model indicate a difference of 6–8 cm s−1 in the depth-averaged current and sea level oscillations of 4–5 cm. In a one-layer simulation, the surface slope geostrophically balances the depth-averaged current, whereas the 17-level model simulations indicate a near-inertially oscillating current of 7–8 cm s−1 found by removing the depth-averaged flow from the geostrophic currents induced by the surface slope. Surface undulations are driven by the depth-averaged nonlinear terms in the density equation, that is, [u ρx], [υρ y], and [w ρ z].

Based on fits of the 17 levels of demodulated horizontal velocities at 1.03f (f the Coriolis parameter) to the eigenfunctions, maximum amplitudes of the barotropic and first baroclinic modes are 7 and 58 cm s−1, respectively. The barotropic mode amplitude is consistent with the current found by removing the depth-averaged flow from the geostrophic current that contributes 2%–3% to the energy in the near-inertial wave pass band. Vertical velocity eigenfunctions at the surface indicate that the barotropic mode is at least 50 to 80 times larger than the baroclinic mode. Surface displacements by the barotropic mode have amplitudes of ±4 cm, explaining 90% to 95% of the height variations. The first baroclinic mode contributes about 0.2–0.4 cm to the free surface displacements. The weak barotropic near-inertial current provides a physical mechanism for the eventual breakup of the sea surface depression induced by the hurricane’s wind stress and surface Ekman divergence.

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Melinda S. Peng and Simon W. Chang

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Special Sensor Microwave/Imager (SSM/I) retrieved rainfall rates were assimilated into a limited-area numerical prediction model in an attempt to improve the initial analysis and forecast of a tropical cyclone. Typhoon Flo of 1990, which was observed in an intensive observation period of the Tropical Cyclone Motion Experiment-1990, was chosen for this study. The SSM/I retrieved rainfall rates within 888 km (8° latitude) of the storm center were incorporated into the initial fields by a reversed Kuo cumulus parameterization. In the procedure used here, the moisture field in the model is adjusted so that the model generates the SSM/I-observed rainfall rates. This scheme is applied through two different assimilation methods. The first method is based on a dynamic initialization in which the prediction model is integrated backward adiabatically to t = −6 h and then forward diabatically for 6 h to the initial time. During the diabatic forward integration, the SSM/I rainfall rates are incorporated using the reversed Kuo cumulus parameterization. The second method is a forward data assimilation integration starting from t = −12 h. From t = −6 h to t = 0, the SSM/I rainfall rates are incorporated, also using the reversed Kuo scheme. During this period, the momentum fields are relaxed to the initial (t = 0) analysis to reduce the initial position error generated during the preforecast integration. Five cases for which SSM/I overpasses were available were tested, including two cases before and three after Flo's recurvature. Forecasts at 48 h are compared with the actual storm track and intensifies estimated by the Joint Typhoon Warning Center. For the five cases tested, the assimilation of SSM/I retrieved rainfall rates reduced the average 48-h forecast distance error from 239 km in the control runs to 81 km in the assimilation experiments. It is postulated that the large positive impact was a consequence of the improved forecast intensity and speed of the typhoon when the SSM/I rain-rate data were assimilated.

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Teddy R. Holt and Simon W. Chang

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Sensitivity of coastal cyclogenesis to the effects of timing of diabatic processes is investigated using the Naval Research Laboratory mesoscale model. Numerical experiments were conducted to examine the sensitivity of the intensification and propagation of a coastal cyclone to changes in the timing of latent heat release due to cumulus convection, surface fluxes, and low-level baroclinicity.

The NMC Regional Analysis and Forecast System analysis of the GALE IOP 2 coastal cyclone was unable to resolve the initial subsynoptic-wale cyclogenesis. Hence, tracking and identification of a well-defined coastal cyclone was difficult operationally. However, the control model experiment having full physics, initialized with the NMC analyses, was able to properly simulate the development of the coastal cyclone. Results from the control experiment agree with the more accurate Fleet Numerical Oceanographic Center low-level analysis. The numerical experiments suggest the development of the surface cyclone was a result of proper superposition and interaction of the upper-level forcing and the low-level baroclinic zone.

Altering the timing of latent heat release due to cumulus convection in the control experiment indicates that for the initial 12 h of cyclogenesis, cumulus convection as determined by the modified Kuo scheme has little effect on the deepening of the surface system but strongly changes the alignment of the trough by retarding the eastward propagation. It is during the second 12 h of cyclogenesis that cumulus convection is crucial for rapid cyclogenesis. Imposing a zonal sea surface temperature, in addition to withholding cumulus heating, has the most impact once the system has reached the coast. The enhanced coastal baroclinicity due to the zonal SST distribution causes the surface cyclone to propagate closer to the coast and more slowly than the control experiment. Allowing no surface fluxes, in addition to no cumulus convection, cools and stabilizes the boundary layer and inhibits surface intensification. The strong coastal baroclinicity is weakened without surface fluxes and the cyclone remains well onshore.

An experiment to modify the phasing of the low-level baroclinic zone is conducted by imposing an additional linear increase in ground surface temperature to the typical diurnal heating cycle as well as eliminating ocean surface sensible beat flux for the initial 12 h of cyclogenesis. This results in a low-level temperature field that is out of phase with the typical diurnal surface evolution. The surface cyclone deepens much more rapidly [41 mb (24 h)−1] than the control experiment and remains more onshore with relatively little movement. In addition, potential vorticity analysis suggests that the upper levels for this experiment have much weaker protrusions of high potential vorticity into the lower troposphere compared to the control experiment.

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Simon W. Chang and Rangarao V. Madala

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A three-dimensional numerical model with a domain of 3000 km×3000 km and horizontal resolution of 60 km is used to study the influence of sea surface temperature (SST) on the behavior of tropical cyclones translating with mean flows in the Northern Hemisphere.

We find that tropical cyclones tend to move into regions of warmer SST when a gradient of SST is perpendicular to the mean ambient flow vector (MAFV). The model results also indicated that a region of warmer SST situated to the right side of the MAFV is more favorable for storm intensification than to the left side due to the asymmetries in air-sea energy exchanges associated with translating tropical cyclones. The model tropical cyclone intensifies and has greater rightward deflection in its path relative to the MAFV when translating into the region of warmer SST. The model tropical cyclone intensifies when its center travels along a warm strip, while it weakens along, but does not move away from, a cool strip.

The results suggest that the SST distribution not only affects the intensity and path of tropical cyclones frictionally, but also affects them thermally. The enhanced evaporation and convergence over the warm SST provide a favorable condition for the growth of the tropical cyclone, and lead to a gradual shift of the storm center toward the warm ocean.

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Simon W. Chang and Richard A. Anthes

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An axisymmetric hurricane model and an axisymmetric ocean model are integrated simultaneously for 24 h to investigate the mutual response of the two systems. The feedbacks between the hurricane and the ocean are negative. The weakening of the hurricane in response to the cooling of the ocean's surface by upwelling and mixing results in a lessened response of the ocean. The results suggest that appreciable weakening of a hurricane due to the cooling of the oceanic surface will not occur if it is translating at typical speed.

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Simon W. Chang and Harold D. Orville

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