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Simon Wei-Jen Chang

Abstract

The impact of satellite-sensed winds on the intensity forecasts of tropical cyclones is evaluated by a simulation study with an axisymmetric numerical model. The parameterized physics in the forecast model are deliberately made different from those in the model that generates the observation. Model-generated “observations” are assimilated into forecasts by 12 h dynamic initialization.

A series of 24 h forecasts with and without assimilation of satellite-sensed winds are conducted and compared with the observations. Results indicate that assimilation with marine surface (or low-level) wind alone does not improve intensity forecasts appreciably, that a strong relaxation coefficient in the initialization scheme causes model rejection of the assimilation, and that an attenuating relaxation coefficient is recommended. However, when wind observations at the outflow level are included in the assimilation, forecasts improve substantially. The best forecasts are achieved when observations over the entire lower troposphere are assimilated.

Additional experiments indicate the errors in the satellite observations contaminate the forecast. But the assimilation of inflow and outflow winds still improve the intensity forecast if the satellite observation errors are less than or about the same magnitude of those in the initial wind field.

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Simon Wei-Jen Chang

Abstract

Numerical simulations with a primitive equation model which includes parameterized physics are conducted to study the effects of an island mountain range on translating tropical cyclones. The idealized topography with a 200 m peak is introduced over a 12 h growth period. The initial state contains a nonlinearly balanced vortex embedded in a uniform, unsheared, tropical easterly flow.

Many orographic effects are produced similar to those observed for typhoons passing over mountain ranges. The storm tends to translate at about twice the speed of the basic flow near the mountain, while its intensity is reduced. Air flows mostly around the mountain range instead of over it, forming a ridge on the windside and a trough on the leeside slopes. The tropical cyclone's passage induces a mean cyclonic circulation around the mountain with strongest amplitudes at low levels. As a result, the model tropical cyclone makes a cyclonic curvature in its path around the north end of the island mountain.

Further numerical experiments suggest that cumulus heating which maintains the tropical cyclone forces the cyclonic circulation around the mountain. In the experiment with an unforced, quasi-barotropic vortex we found that the lower level circulation is blocked by the mountain range. As the original low-level center fails to pass the mountain range, a secondary low-level circulation center forms in the induced lee trough. The secondary low-level center develops as the upper level center comes into phase.

A vorticity budget is performed for the 700 mb airflow prior to landfall and confirms the importance of diabatic processes in producing the observed orographic effects. Diabatic processes generate convergence to maintain the vorticity of the tropical cyclone. The horizontal advection of positive vorticity in conjunction with the leeside vortex stretching, results in the mean positive vorticity around the mountain.

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Simon Wei-Jen Chang

Abstract

An efficient, multilayer model for predicting the diurnal variations in the thermal and momentum fields in the planetary boundary layer (PBL) is proposed for incorporating into mesoscale or large-scale dynamical models. The ground temperature is given by a soil slab heated (or cooled) by net radiation and sensible heat from the atmospheric surface layer and a ground thermal reservoir. The surface heat flux can be generated by two mechanisms: 1) the convective mixing depending on the temperature difference between the ground and the screen level and 2) the mechanical mixing depending on the wind stress. Following Blackadar (1976), a prediction equation is employed for the screen-level temperature. In the PBL, the heat and momentum exchanges are computed by a Richardson number adjustment scheme. Heat and momentum exchanges occur mainly due to thermal instability under convectively unstable conditions and due to shear instability under convectively stable conditions. A case study shows good agreement between model results and observation. Additional experiments are performed to test the scheme under calm and stronger wind situations. Since no explicit diffusion coefficient is needed in the adjustment scheme, the model time step is not restricted by computational stability requirements of the diffusion term. This PBL parameterization scheme is therefore very appealing for use in numerical models that use large time steps yet have good vertical resolutions in the PBL.

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Simon W. Chang

Abstract

No abstract available.

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Simon W-J. Chang

Abstract

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 Wei-jen Chang

Abstract

A planetary boundary-layer (PBL) parameterization based on the generalized similarity theory (GST) was tested in tropical cyclone models. This parameterization, with only one layer, is desired in modeling tropical cyclones for computational speed. The momentum, sensible heat and moisture fluxes are mutually dependent in this parameterization through nondimensional gradient equations. The internal structure of the PBL is determined implicitly through universal functions.

In comparison with a complex, one-dimensional, multilayer PBL model, the GST parameterization yields accurate moisture fluxes, but slightly overestimates the momentum flux and underestimates the sensible heal flux. The GST parameterization produces very realistic dynamics, energetics and thermal structure in an axisymmetric tropical cyclone model. This GST parameterization, although unable to treat the diffusion across the PBL inversion, is judged superior to drag coefficient parameterization and is a good alternative to the more expensive, multilayer parameterization.

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Simon W. Chang

Abstract

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 Wei-Jen Chang

Abstract

The interactions between atmospheric vortex pairs are simulated and studied with a nondivergent barotropic model and a three-dimensional tropical cyclone model.

Numerical experiments with nondivergent barotropic vortex pairs show that the relative movements of the vortices are sensitive to the separation distance and the characteristics of the swirling wind of the vortex. No mutual attraction is found in any of the nondivergent barotropic vortex pairs tested.

Results from the three-dimensional tropical cyclone model show that on a constant ƒ-plane with no mean wind, the movements of the two interacting tropical cyclones consist of a mutual cyclonic rotation, attraction and eventual merging, in agreement with Fujiwhara's description. The displacement of one interacting storm in the mutual rotation is proportional to the combined strength of the binary system, but inversely proportional to the size of the storm and to the square of the separation distance. The rate of merging is related to the development of a mean secondary circulation on the radial–vertical plane, and is quite independent of the strength of the two tropical cyclones.

The latitudinal variation of the Coriolis parameter adds a northwest beta drift to the trajectories. Depending on their relative strength and location, the beta drift either speeds up the merging process or separates the two interacting tropical cyclones.

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Simon W. Chang and Teddy R. Holt

Abstract

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

Abstract

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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