Simon W. Chang

Numerical Simulations of the Ocean's Nonlinear, Baroclinic Response to Translating hurricanes

Abstract 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 oceans 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 storms 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 Geislers linear solutions. However, vertical mixing produces significant differences in the depth of the thermocline behind the storm.

The Interaction between Hurricane Opal (1995) and a Warm Core Ring in the Gulf of Mexico

Abstract Hurricane Opal (1995) experienced a rapid, unexpected intensification in the Gulf of Mexico that coincided with its encounter with a warm core ring (WCR). The relative positions of Opal and the WCR and the timing of the intensification indicate strong air–sea interactions between the tropical cyclone and the ocean. To study the mutual response of Opal and the Gulf of Mexico, a coupled model is used consisting of a nonhydrostatic atmospheric component of the Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS), and the hydrostatic Geophysical Fluid Dynamics Laboratory’s Modular Ocean Model version 2 (MOM 2). The coupling between the ocean and the atmosphere components of the model are accomplished by conservation of heat, salt, momentum, as well as the sensible and latent heat fluxes at the air–sea interface. The atmospheric model has two nests with spatial resolutions of 0.6° and 0.2°. The ocean model has a uniform resolution of 0.2°. The oceanic model domain ...

Response of the Hurricane Boundary Layer to Changes of Sea Surface Temperature in a Numerical Model

Abstract An axisymmetric, multilayer hurricane model is used to investigate the hurricanes 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 modifi...

A Satellite Observational and Numerical Study of Precipitation Characteristics in Western North Atlantic Tropical Cyclones

Abstract Special Sensor Microwave/Imager (SSM/I) observations were used to examine the spatial and temporal changes of the precipitation characteristics of tropical cyclones. SSM/I observations were also combined with the results of a tropical cyclone numerical model to examine the role of inner-core diabatic heating in subsequent intensity changes of tropical cyclones. Included in the SSM/I observations were rainfall characteristics of 18 named western North Atlantic tropical cyclones between 1987 and 1989. The SSM/I rain-rate algorithm that employed the 85-GHz channel provided an analysis of the rain-rate distribution in greater detail. However, the SSM/I algorithm underestimated the rain rates when compared to in situ techniques but appeared to be comparable to the rain rates obtained from other satellite-borne passive microwave radiometers. The analysis of SSM/I observations found that more intense systems had higher rain rates, more latent heat release, and a greater contribution from heavier rain to...

A Multi-Level Model of the Planetary Boundary Layer Suitable for Use with Mesoscale Dynamic Models

Abstract In this paper a simple model of the planetary boundary layer (PBL) is proposed. The surface layer is modeled according to established similarity theory. Above the surface layer a prognostic equation for the mixing length is introduced. The time-dependent mixing length is a function of the PBL characteristics, including the height of the capping inversion, the local friction velocity and the surface heat flux. In a preliminary experiment, the behavior of the PBL is compared with observations from the Great Plains Experiment.

The Mutual Response of the Tropical Cyclone and the Ocean

Abstract 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 oceans 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.

Free Surface Effects on the Near-Inertial Ocean Current Response to a Hurricane

Abstract During the passage of hurricane Frederic in 1979, four ocean current meter arrays in water depths of 100–950 m detected both a baroclinic and a depth-independent response in the near-inertial frequency band. Although the oceanic response was predominately baroclinic, the hurricane excited a depth-independent component of 5–11 cm s−1. The origin and role of the depth-independent component of velocity is investigated using a linear analytical model and numerical simulations from a 17-level primitive equation model with a free surface. Both models are forced with an idealized wind stress pattern based on the observed storm parameters in hurricane Frederic. In an analytical model, the Greens function (K0) is convolved with the wind stress curl to predict a sea surface depression of approximately 20 cm from the equilibrium position. The near-inertial velocities are simulated by convolving the slope of the sea surface depression with a second Greens function. The barotropic current velocities rotate ...

A Numerical Study of the Interactions between Two Tropical Cyclones

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 Fujiwharas 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 deve...

Tropical Cyclone-Upper Atmospheric Interaction as Inferred from Satellite Total Ozone Observations

Abstract The Nimbus-7 Total Ozone Mapping spectrometer (TOMS) was used to map the distribution of total Ozone in and around western Atlantic tropical cyclones from 1979 to 1982. It was found that the TOMS-observed total Ozone distribution within the subtropics during the tropical cyclone seasonal correlated well with the tropopause topoghraphy, similar to earlier middle-latitudinal observations. This relationship made it possible to use TOMS to monitor the propagation of upper-tropospheric subtropical transient waves and the mutual adjustment between the tropical cyclone and the upper-tropospheric waves during their interaction. These total ozone patterns reflected the three-dimensional upper-tropospheric transport processes that were conducive for storm intensification and weakening. It was also found from satellite observations and numerical model simulations that modification of the environmental distribution of total ozone by the tropical cyclones was primarily caused by the secondary circulation asso...

Impact of assimilating SSM/I rainfall rates on numerical prediction of winter cyclones

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...