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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 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

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

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 Wei-Jen Chang
and
Rangarao V. Madala

Abstract

No abstract available.

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Jainn-Jong Shi
,
Simon Wei-Jen Chang
, and
Sethu Raman

Abstract

The structure and dynamics of the outflow layer of tropical cyclones are studied using a three-dimensional numerical model. Weak and strong tropical cyclones are produced by the numerical model when starting from idealized initial vortices embedded in mean hurricane soundings. The quasi-steady state outflow layers of both the weak and strong tropical cyclones have similar characteristics 1) the circulations are mainly anticyclonic (except for a small region of cyclonic flow near the center) and highly asymmetric about the center, 2) the outflow layer is dominated by a narrow but elongated outflow jet, which contributes up to 50% of the angular momentum transport and 3) the air particles in the outflow jet mostly originate from the lower level, following “in-up-and-out” trajectories.

We found that there are secondary circulations around the outflow jet, very much like those associated with midlatitude westerly jet streaks. In the jet entrance region, the secondary circulation is thermally direct. That is, the ascending motion is located on the anticyclonic shear side of the jet, and the descending motion on the cyclonic shear side. There is a radially outward (perpendicular to the jet) flow above the jet and inflow below it. In the jet exit region, the secondary circulation is weaker and reversed in its direction (thermally indirect). The secondary circulations leave pronounced signatures on the relative humidity, potential vorticity, and tropopause height fields. The secondary circulation is more intense in the stronger tropical cyclone (with a stronger outflow jet) than in the weaker tropical cyclone.

The sensitivities to upper-tropospheric forcing of the outflow are tested in numerical experiments with prescribed forcings. It is found that the simulated tropical cyclone intensifies when its upper levels within a radius of approximately 500 km are accelerated and forced to be more divergent. Convection plays a key role in transforming the upper level divergence into low level convergence. In another experiment, additional regions of convection are initiated in the ascending branches of the circum-jet secondary circulations away from the inner region when the outflow jet between the radii of 500 and 1000 km is accelerated. These regions of convection become competitive with the inner core convection and eventually weaken the tropical cyclone. In both experiments, cumulus convection is the major link between the upper-level forcing and tropical cyclone's response.

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