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Simon W. Chang

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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-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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Richard A. Anthes and Simon W. Chang

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An axisymmetric, multilayer hurricane model is used to investigate the hurricane's response to sudden changes of sea surface temperature (SST). The model contains a parameterization of the planetary boundary layer (PBL) which includes matched formulations for the surface layer and the mixed layer. The heat, moisture and momentum fluxes are mutually dependent through Monin-Obukhov similarity theory.

The height of the model hurricane PEL is 400–500 m, below which the potential temperature and specific humidity are nearly invariant with height. The flow in the hurricane PBL is characterized by subgradient tangential velocities and nearly uniform cross-isobaric flow angles. The sensible heating from the ocean is insignificant, but the evaporation is large. The magnitudes of the equivalent drag coefficients are approximately one-third those of the exchange coefficients for heat and moisture.

As the SST is suddenly decreased (increased), the steady-state model hurricane experiences two stages of modification. The first stage consists of adjustments of the hurricane PBL featuring a weakened (enhanced) dynamic and thermodynamic coupling of the storm with the ocean. No important changes of intensity occur during this stage, which lasts several hours. The decrease (increase) of kinetic energy dissipation offsets part of the decrease (increase) of kinetic energy generation. The second stage is characterized by a steady modification of storm intensity. The fluctuations of intensity in these experiments are less pronounced than those shown by a similar model with a conventional bulk parameterization of the hurricane PBL.

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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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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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Simon W. Chang and Teddy R. Holt

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A series of observing system simulation experiments (OSSE) and real data assimilation experiments were conducted to assess the impact of assimilating Special Sensor Microwave/Imager (SSM/I)-estimated rainfall rates on limited-area model predictions of intense winter cyclones.

For the OSSE, the slow-moving, fronto- and cyclogenesis along the cast coast of United States during the second intensive observation period (IOP 2) of the Genesis of Atlantic Lows Experiment (GALE) (26-28 January 1986) was selected as the test case. The perfect “observed” rainfall rates were obtained by an integration of a version of the Naval Research Laboratory (NRL) limited-area model, whereas the “forecast” was generated by a degraded version of the NRL model. A number of OSSEs were conducted in which the “observed” rainfall rates were assimilated into the “forecast” model. Rainfall rates of various data frequencies, different vertical beating profiles, various assimilation windows, and prescribed systematic errors were assimilated to test the sensitivity of the impact. It was found that assimilation of rainfall rates, in general, improves the forecast in terms of sea level pressure S1 scores when either the “observed” or model-determined vertical beating profiles were used. The improvement was insensitive to the error in rainfall magnitude estimates but was sensitive to errors in geographic locations of the precipitation. More frequent observations (additional sensors in orbits) had positive but gradually diminishing benefits.

Real SSM/I-measured rainfall rates were assimilated for the rapid-moving, intense marine cyclone of IOP 4 of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) (4–5 January 1989), which started from an initial offshore disturbance with a minimum pressure of 998 mb at 0000 UTC 4 January and developed into a very intense storm of 937 mb 24 h later. The NRL model simulated a well-behaved but less intense cyclogenesis episode based on the RAFS (Regional Analysis and Forecast System) initial analysis, reaching a minimum sea level pressure of 952 mb at 24 h. The first SSM/I aboard a DMSP (Defense Meteorological Satellite Program) satellite flew over the marine cyclone at 0000, 0930, and 2200 UTC 4 January and measured rainfall rates over portions of the warm and cold fronts associated with the cyclone. The SSM/I rainfall rates at 0000 and 0930 UTC were assimilated into the model as latent heating functions in ±3-h windows with model-determined vertical profiles. Two different methods were used to define the latent heating rates for the model in the assimilation experiments: 1) the model heating rates were defined by the maximum of the model computed and the SSM/I measured, and 2) the model beating rates were replaced by the SSM/I-measured rainfall rates within the SSM/I swath. Results of the assimilation experiments indicated that the assimilation in general leads to better intensity forecasts. The best forecast with assimilation predicted a 24-h minimum surface pressure of 943 mb, cutting the forecast error of the “no sat” forecast by 50%. This most efficient assimilation was carried out with assimilations of two-time SSM/I observations using the swath method. Further analysis indicated that the assimilation also resulted in better track and structure forecasts.

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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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Norman A. Phillips and Simon W. Chang

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The principles of variational analysis are reviewed in a symbolic manner, with emphasis on the error introduced by a failure to use an exact constraint. A technique to approximate a nonlinear exact constraint is suggested, with the object of avoiding error magnification in regions of good data, in the process of analyzing slow mode amplitudes for nonlinear mode initialization. The technique amounts to subtractingall fast modes from the data fields that form the input to the variational analysis. The analysis procedure is then focused on only the analysis of slow mode fields. These general considerations are demonstrated by computations with the vortex model of Tribbia, and show how nonlinear mode techniques can improve initial analyses in a more significant way than the mere elimination of noise. A review of the relative merits and weaknesses of optimum interpolation and variational analysis suggests a logical way to use both techniques in an operational analysis system.

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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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