Abstract

The Naval Research Laboratory’s limited-area numerical prediction system, a version of Navy Operational Regional Atmospheric Prediction System, was used to investigate the interaction between Hurricane Florence (1988) and its upper-tropospheric environment. The model was initialized with the National Meteorological Center (now the National Centers for Environmental Prediction)/Regional Analysis and Forecasting Systems 2.5° analysis at 0000 UTC 9 September 1988, enhanced by a set of Omega dropwindsonde data through a three-pass nested-grid objective analysis.

Diagnosis of the 200-mb level structure of the 12-h forecast valid for 1200 UTC 9 September 1988 showed that the outflow layer was highly asymmetric with an outflow jet originating at approximately 3° north of the storm. In agreement with the result of an idealized simulation (), there was a thermally direct, circum-jet secondary circulation in the jet entrance region and a thermally indirect one in a reversed direction in the jet exit region. In several previous studies, it was postulated that an approaching westerly jet had modulated the convection and intensity variations of Florence. In a variational numerical experiment in this study, the approaching westerly jet was flattened out by repeatedly setting the jet-level meridional wind component and zonal temperature perturbations to zero in the normal mode initialization procedure. Compared with the control experiment, the variational experiment showed that the sudden burst of Florence’s inner core convection was highly correlated with the approaching upper-tropospheric westerly jet. These experiments also suggested that the approaching upper-tropospheric westerly jet was crucial to the intensification of Florence’s inner core convection between 1000 and 1500 UTC 9 September, which occurred prior to the deepening of the minimum sea level pressure (from 997 to 987 mb) between 1200 UTC 9 September and 0000 UTC 10 September.

Many earlier studies have attempted an explanation for the effect on tropical cyclones of upper-tropospheric forcings from the eddy angular momentum approach. The result of this study provides an alternative but complementary mechanism of the interaction between an upper-level westerly trough and a tropical cyclone.

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 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.

Abstract

No abstract available.

Abstract

The influence of spatial variations of the oceanic mixed layer depth (OMLD) on tropical cyclones (TCs) is investigated using a coupled atmosphere–ocean model. The model consists of a version of the Naval Research Laboratory limited area weather prediction model coupled to a simple 2½-layer ocean model. Interactions between the TC and the ocean are represented by wind-induced turbulent mixing in the upper ocean and latent and sensible heat fluxes across the air–sea interface.

Four numerical experiments are conducted with different spatial variations of the unperturbed OMLD representing idealizations of broad-scale patterns observed in the North Atlantic and North Pacific Oceans during the tropical cyclone season. In each, the coupled model is integrated for 96 h with an atmospheric vortex initially of tropical storm intensity embedded in an easterly mean flow of 5 m s⁻¹ and located over an oceanic mixed layer that is locally 40 m deep. The numerical solutions reveal that the rate of intensification and final intensity of the TC are sensitive to the initial OMLD distribution, but that the tracks and the gross features of the wind and pressure patterns of the disturbances are not.

In every experiment, the sea surface temperature exhibits a maximum induced cooling to the right of the path of the disturbance, as found in previous studies, with magnitudes ranging from 1.6° to 4.1°C, depending on the initial distribution of the mixed layer depth. Consistent with earlier studies, storm-induced near-inertial oscillations of the mixed layer current are found in the wake of the storm.

In addition, numerical experiments are conducted to examine sensitivity of a coupled-model simulation to variations of horizontal resolution. Results indicate that the intensity and track of tropical cyclones are quantitatively sensitive to such changes.

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.

Abstract

Data from the Special Sensor Microwave/Imager (SSM/I) on board a Defense Meteorological Satellite Program (DMSP) spacecraft have been used to study the precipitation patterns associated with Hurricane Hugo (1989). Results indicate the intensification of Hugo was associated with increases in SSM/I-derived total latent heat release and increases in heavier rainfall rates near the storm center. This study also shows that SSM/I rainfall rates prior to the landfall of Hugo at Charleston, South Carolina, compared favorably with raingage observations. Additionally, data from the 85-GHz channel was used to monitor the extent of convection near the storm's center. As Hugo intensified, the areal coverage of deep convection increased. Furthermore, the 85-GHz brightness-temperature imagery was useful in determining the location of Hugo's low-level center. These results indicate the potential of using SSM/I data in the analysis and prediction of tropical cyclones in an operational environment.

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.

Abstract

A numerical study is conducted using the Naval Research Laboratory (NRL) limited-area model to study the evolution and structure of a rapidly intensifying marine cyclone observed during intensive observing period 4 (IOP 4; 4–5 January 1989) of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) over the North Atlantic Ocean.

The single grid version of the NRL model used in the study has 16 layers in the vertical with a horizontal resolution of 1/3° longitude and 1/4° latitude. The primitive equation, hydrostatic model in sigma coordinates includes parameterized physics of cumulus convection, radiation, and the planetary boundary layer. The National Meteorological Center (NMC) Regional Analysis Forecast System (RAFS) analysis is used to provide the initial and boundary conditions.

Starting from the 0000 UTC 4 January RAFS initialization, the control model simulates the ensuing cyclogenesis, deepening the initial disturbance from 998 to 952 mb in 24 h. While the simulated cyclone is about 15 mb weaker than that observed, the simulation reproduced many of the well-documented observed features of the IOP 4 cyclone, such as the remarkable comma-shaped precipitation pattern, bent-back warm front, warm-core seclusion, and secondary cold front. Control model results show that (i) the strongest temperature and water vapor gradients are aligned with the warm front and secondary cold front, not the primary cold front, (ii) the major precipitation and strongest vertical motion are along the warm front and its bent-back extension, (iii) the cyclonic circulation is displaced well to the southwest of the triple point, and (iv) the cellular convection occurs behind the secondary cold front accompanied by extreme surface sensible and latent beat transfer with a total maximum flux exceeding 3000 W m⁻² over the Gulf Stream approximately 100 km offshore of the Carolinas. A detailed analysis of model results is performed and is found to be in excellent agreement with available satellite and mesoscale observations.

Sensitivity experiments are also conducted to identify the importance of various dynamical and physical processes contributing to the rapid intensification. Results from sensitivity tests show that (i) the dynamic processes are more responsible for the rapid intensification and unique structure of the marine cyclone than the physical processes, (ii) both the sea surface heat transfer and the release of latent heat in clouds contribute positively to the cyclogenesis, (iii) physical processes combine to intensify the storm in a nonlinear fashion, and (iv) the formation of unique features associated with the IOP 4 storm such as the bent-back extension of the warm front, warm-core seclusion, and westward development of the low pressure center away from the triple point are not sensitive to physical processes.

Abstract

A numerical study of the East Coast snowstorm of 10–12 February 1983 has been conducted with the NRL mesoscale model. The three-dimensional, hydrostatic, primitive equation model has 91 × 51 horizontal grid points with a half degree resolution in a verification domain of 100°W to 60°W and 25°N to 45°N. There are ten layers in the vertical of equal σ(=p/p_s ) thickness. The model uses a split-explicit method for temporal integration and a second-order accurate spatial finite differencing. Model physics include precipitation on the resolvable scale and parameterized boundary layer and cumulus convection.

The NMC 2.5 degree hemispheric analyses are used as the basic dataset (called NMC analysis hereafter). Because significant details in the initial conditions contained in the original rawinsonde observations may have been smoothed out by the NMC analysis algorithm, the analyses are also altered to provide a closer fit to the rawinsonde data. Original rawinsonde data are used in this enhancement of the NMC analyses (called enhanced). Forecasts are made from both the NMC analysis and the enhanced analyses to determine whether the enhancement can improve the forecasts. The Barnes analysis scheme with parameters suitable for reducing the short wavelength noise on the model grid scale is used to enhance the NMC analyses with the original soundings. Three types of lateral boundary treatments—constant, tendency damping, and temporal relaxation boundary conditions—are tested and compared. Results from forecast experiments show that the boundary treatment has a great impact on the model performance. The constant boundary condition produces an unusable forecast after 12 h as judged by the S1 scores, while the relaxation boundary condition produces an excellent forecast. The enhancement of the initial conditions has a negligible effect on predictions when reasonable boundary updates are used for the snowstorm case. The enhanced dataset produces a slightly better but still useless forecast when constant boundary conditions are used.

Numerical experiments have also been conducted to test the sensitivity of the cyclogenesis to physical processes by suppressing one or more physical processes in the model. It is found that the evaporation from the ocean modulates the location and amount of precipitation. Without the evaporation, the-intensity of the cyclone remains the same but the center stays on the coast instead of staying off shore. The track of the snowstorm is such that the sensible beating from the ocean dampens the development of the cyclone by reducing the low-level baroclinicity. Without the sensible heating, the minimum surface pressure of the cyclone is 11 mb lower. The latent heating is found to be important for this case in which the maximum beating rate is 15°–20°C/day. When latent heating is suppressed, the cyclone translates at a much reduced rate and its central pressure is 10 mb higher after two days of simulation. These results from the sensitivity tests are of course case-dependent.

Abstract

Numerical simulations and diagnostics are performed for Typhoon Tip and Tropical Storm Faye, both of which occurred during 1979, the year of the First Global GARP (Global Atmosphere Research Program) Experiment (FGGE). The simulations are started from early in the life cycles of both disturbances, the former of which developed into a super typhoon, and the latter of which did not develop beyond the tropical storm stage. The numerical model employed was that of Madala et al. and is a modification of the one used in previous simulations by the authors. The primary modifications are the inclusion of a more sophisticated boundary layer parameterization, based on similarity theory, and the inclusion in the Kuo cumulus parameterization scheme of the nonmeasurable mesoscale latent heat release, as described by Krishnamurti et al. The initial conditions for both simulations were derived from the FGGE dataset of the European Centre for Medium-Range Weather Forecasts and from monthly mean sea surface temperatures provided by the National Meteorological Center (now the National Centers for Environmental Prediction). The initial intensities and the underlying sea surface temperatures were approximately the same for the two disturbances. In the simulations, Tip developed into an intense typhoon and Faye did not develop, as observed in the atmosphere, although the minimum surface pressures and maximum wind speeds attained do not agree quantitatively with the reported values.

The primary question the authors set out to answer is what special conditions exist at the early stages of the life cycles of tropical disturbances that allow one system to develop and another to fail to develop into a typhoon. The most significant difference found in the initial states of Tip and Faye was a large-scale eddy flux of angular momentum from the surroundings into the former and out of the latter, with maximum amplitudes located around 200 mb at radial distances from the vortex centers greater than 1000 km. These fluxes persisted for at least 24 h prior to the time the numerical simulations were started. While there were differences in the eddy heat fluxes as well, these were less significant. Diagnostic calculations reveal that the secondary radial circulation induced by the eddy fluxes of momentum and heat transported water vapor inward for Tip and outward for Faye, with the result that convection broke out at an early stage in the vortex center of Tip, but not in Faye. The convection intensified with time in Tip and subsequently became the dominant factor contributing to the moisture inflow and rapid vortex intensification.

The authors’ interpretation of the results of their numerical simulations and diagnostic calculations is that the secondary radial circulation induced by large-scale eddy fluxes of heat and momentum can serve either as a catalyst for typhoon formation or as a mechanism for inhibiting the further development of an incipient tropical disturbance, depending on the direction of the water vapor transport (into or out of the vortex core).