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Robert E. Tuleya

Abstract

This study investigates two cases of the FGGE III-B tropical cyclone genesis study of Tuleya (1988) in more detail. These two cases occurred within a week of one another in the tropical North Atlantic in August 1979. One disturbance developed into Hurricane David, the other did not develop past the depression stage. At one point in their evolution the disturbances had quite similar values of low-level vorticity. In the developing case of Hurricane David, the disturbance propagated along in a low-level wave trough with an accompanying high wind maximum. In the nondeveloping case the initial disturbance was also embedded in a wave trough with an associated wind maximum. This low-level wave propagated westward leaving the depression in its wake. The different environmental flow was responsible for the different behavior. Synoptic and budget analyses revealed significant differences in disturbance structure and vorticity and equivalent potential temperature tendencies at the time of approximate equal strength of the two disturbances. The evolution of these two disturbances was quite robust even to reasonable increases to the initial relative humidity.

Supplementary experiments of the developing case were performed by altering the sea surface temperature and surface evaporation. It was found that the difference in storm evolution was minor in a case when climatological mean values of sea surface temperatures were specified. The climatological mean values were ∼0.5 K lower than the August 1979 mean used in the control simulation. In addition, an experiment without evaporation led to a propagating easterly wave with little development. Furthermore, when the evaporation was specified to a climatological constant value, there was intensification into a weak tropical storm with a rather peculiar structure. Apparently, at least in this case, processes other than evaporation-wind feedback led to moderate storm intensification.

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Robert E. Tuleya

Abstract

This study utilizes the First GARP Global Experiment's (FGGE) analyzed dataset and a relatively fine scale regional model in combination to investigate the feasibility of numerically simulating tropical disturbances during the FGGE year, 1979. Four different cases were investigated including a cyclone. TC-17, in the Indian Ocean, a developing hurricane, David, and a nondeveloping wave in the Atlantic, and a multi-storm case, Tip and Roger, in the Pacific.

The results were promising when using ECMWF FGGE data in that simulations of genesis or nongenesis were achieved in the three developing cases and in the one nondeveloping case. The accuracy of the intensification rates varied from case to case. For example, in the simulation of TC-17, the maximum low level winds were simulated to be ∼45 m s−1 while observations indicated winds of only 22 m s−1. However, in the case of David, the maximum winds increased at a slower rate than observed, while in the case of Tip the slow intensification rate was correctly simulated. An interesting result was the high correlation between model precipitation patterns in the simulations and observed satellite cloud photos. These results indicate that the environment in which an incipient disturbance is embedded plays a major role in the genesis process. An additional striking result was the wide variability of storm development and structure from case to case. Tropical storm David was simulated to be a relatively small scale storm whereas Tip was simulated to be a storm with an enormous area of gale force winds. The model simulations also produced different distributions of the low level wind maximum relative to the moving storm with banding of a number of meteorological fields, including precipitation and vorticity. The formation of storms was related to the presence of an incipient disturbance possessing cyclonic low level vorticity, and ample high relative humidity together with a strong coupling between the disturbance phase speed and the upper level flow field. Most cases including the nondeveloping wave contained upper level anticyclonic conditions. All cases included a weak warm, upper level anomaly including the nondeveloping wave case. Also, it was found that environmental upward motion is not always an accurate indicator of genesis.

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Robert E. Tuleya

Abstract

Hurricane models have rarely been used to investigate the observational fact that tropical disturbances seldom form, develop, or intensify over land. Furthermore, rather ad hoc assumptions have been made when modeling landfall. The general consensus is that energy supplied primarily through surface fluxes is necessary for tropical cyclone development and maintenance. In the past, rather a priori assumptions have been made such as the elimination of surface sensible and latent heat fluxes over land or the reduction of surface land temperature. By incorporating an improved version of the Geophysical Fluid Dynamics Laboratory (GFDL) tropical cyclone model with diurnal radiation and a bulk subsurface layer with explicit prediction of land temperature, a series of experiments was performed to test the sensitivity of surface boundary conditions to tropical cyclone development and decay at landfall.

A triply nested version of the GFDL model was used in an idealized setting in which a tropical disturbance, taken from the incipient stage of Gloria (1985), was superposed on a uniform easterly flow of 5 m s−1. A control case was performed for ocean conditions of fixed 302-K SST in which the initial disturbance of about 998 hPa developed to a quasi-steady state of 955 hPa after one day of integration. Using identical atmospheric conditions, a series of experiments was performed in which the underlying land surface was specified with different values of thermal property, roughness, and wetness. By systematically changing the thermal property (i.e., heat capacity and conductivity) of the subsurface from values typical of a mixed-layer ocean to those of land, a progressively weaker tropical system was observed. It was found that the initial disturbance over land failed to intensify below 985 hPa, even when evaporation was specified at the potential rate. The reduction of evaporation over land, caused primarily by the reduction of surface land temperature near the storm core, was responsible for the inability of the tropical disturbance to develop to any large extent. Under land conditions, the known positive feedback between storm surface winds and surface evaporation was severely disrupted.

In sensitivity experiments analogous to the all-land cases, a series of landfall simulations were performed in which land conditions were specified for a region of the domain so that a strong mature tropical cyclone similar to the ocean control case encountered land. Again as in the all-land case, the demise of the landfalling storm takes place due to the suppression of the potential evaporation and the associated reduction of surface temperatures beneath the landfalling cyclone. Even when evaporation was prescribed at the potential rate, a realistic rapid filling (36 hPa in 12 h) ensued despite the idealized nature of the simulations. Although not critical for decay, it was found that surface roughness and reduced relative wetness do enhance decay at landfall.

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Robert E. Tuleya and Yoshio Kurihara

Abstract

The role of the environmental wind in tropical storm genesis is studied using a numerical simulationmodel. The model used is an 11-level, primitive equation model covering a channel domain of 25° span withopen lateral boundaries at 5.5 and 30.5°N. A number of experiments were integrated for 96 h in which theinitial zonal mean flow was specified differently. The superposed initial wave disturbances were identicalin all experiments.

The dynamic coupling between the upper-level winds and the low-level movement of the disturbance wasfound to be an important factor in explaining the role of the environmental wind in storm genesis. Anotherimportant factor is the impact of the low-level winds on the latent energy supply. This supply is affectedby the relative inflow into a disturbance and by the transfer of momentum from aloft into the boundarylayer in a large area surrounding the disturbance.

According to the model results, the storm genesis potential is definitely biased toward easterly verticalshear (easterlies increasing with height) of the environmental flow rather than westerly shear when themean surface flow is easterly, i.e., 5 m s. The initial perturbation developed into a vigorous tropicalstorm when an easterly vertical shear of 15 m s was specified between 150 and 850 mb. In an experimentwith a specified westerly vertical wind shear of 15 m s, the perturbation failed to develop beyond a weaktropical depression. In a third experiment with no vertical wind shear but with anticyclonic shear aloft, atropical storm also developed. In analyzing the structure of the disturbances at the early wave stage it wasfound that the vertical shear modulated the vertical velocity and rainfall patterns relative to the troughaxis.

In studies involving the horizontal wind shear of the basic flow, it was found that cyclonic shear at lowlevels and, to a lesser extent, anticyclonic shear at upper levels are conducive for storm genesis. Theexperimental results also indicate a significant change of structure of the disturbance between uniformwesterly and easterly flows. Under uniform westerly environmental flow, the initial perturbation developedmore and its low-level structure became more characteristic of mid-latitude cyclones.

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Robert E. Tuleya and Yoshio Kurihara

Abstract

In a three-dimensional numerical model of a tropical disturbance, a spectrum of development stages, from a weakening wave to a mature tropical storm, was obtained with a 5 K range (298 to 303 K) sea surface temperature (SST). However, the apparently large SST sensitivity of the model was found to be modulated by other factors including the large-scale environmental temperature and humidity. Through the use of this model, problems concerning a critical value of SST necessary for storm development were discussed.

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Yoshio Kurihara and Robert E. Tuleya

Abstract

A three-dimensional, 11-level, primitive equation model has been constructed for a simulation study of tropical cyclones. The model has four levels in the boundary layer and its 70×70 variable grid mesh encloses a 4000-km square domain with a 20-km resolution near the center. Details of the model, including the parameterization scheme for the subgrid-scale diffusion and convection processes, are described.

A weak vortex in the conditionally unstable tropical atmosphere is given as the initial state for a numerical integration from which a tropical cyclone develops in the model. During the integration period of one week, the sea surface temperature is fixed at 302K.

The central surface pressure drops to about 940 mb, while a warm moist core is established. The azimuthal component of mean horizontal wind is maximum at about 60 km from the center at all levels. A strong in-flow is observed in the boundary layer. At upper levels, a secondary radial-vertical circulation develops in and around the region of negative mean absolute vorticity. In the same region, the azimuthal perturbation of horizontal wind is pronounced. At the mature stage, the domain within 500 km radius is supplied with kinetic energy for asymmetric flow by both barotropic and baroclinic processes. At 60 km radius, the temperature perturbation field is maintained by condensation-convection heating at upper levels and by adiabatic temperature change due to vertical motion at lower levels. An area having an eye-like feature is found off the pressure center.

Structure of spiral bands in the outer region is extensively analyzed. The phase relationship among the pressure, horizontal motion, vertical motion, temperature and moisture fields is discussed. The spiral band behaves like an internal gravity wave. Once the band is formed in an area surrounding the center, it propagates outward apparently without appreciable further supply of energy, as far as the present case is concerned.

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Robert E. Tuleya and Yoshio Kurihara

Abstract

A GFDL tropical cyclone model was applied to simulate storm landfall. The numerical model is a three-dimensional, primitive equation model and has 11 vertical levels with four in the planetary boundary layer. The horizontal grid spacing is variable with finest resolution being 20 km near the center. This model was used successfully in the past to investigate the development of tropical cyclones over the ocean.

In the present experiments, a simple situation is assumed where a mature tropical cyclone drifts onto flat land. In such a case, the landfall can he simulated by changing the position of the coastline in the computational domain rather than by moving the storm. As the coastline moves with a specified speed, the surface boundary conditions are altered at the shore from those for the ocean to those for the land by increasing the surface roughness length and also by suppressing the evaporation.

Despite the simplicity and idealization of the experiments, the cyclone's filing rates are quite reasonable and a decay sequence is obtained. Notable asymmetries in the wind, moisture and precipitation fields exist relative to the coastline at the time of landfall. Roughness-induced, quasi-steady convergence and divergence zones am observed where onshore and offshore winds encounter the coastline. Spiral hands propagate and exist over the land area. A comparison of the energy and angular momentum budgets between ocean and land surface boundary conditions indicates a simultaneous broadening and weakening of the storm system in the decay process. The latent energy release through condensational processes is initially augmented over land by greater moisture convergence in the planetary boundary layer which counteracts the lack of evaporation from the land surface.

Supplementary experiments indicate that the suppression of evaporation is the most important factor in the decay of a storm upon Landfall. When the evaporation is suppressed, the storm eventually weakens whether the surface roughness is increased or not. An increased surface roughness, which causes increased inflow in the boundary layer, has little immediate negative impact on the storm intensity. Indeed, if the supply of latent energy is sufficient, a storm can deepen when encountering an increase in surface roughness. The decay rate in a later period well after landfall is influenced by the rate with which the water vapor of the storm system is depleted in the earlier period immediately after landfall.

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Yoshio Kurihara and Robert E. Tuleya

Abstract

The genesis of a tropical storm is studied using a numerical simulation model. The model used is an 11-level primitive equation model covering a channel domain of 25° span with open lateral boundaries at latitudes 5.5 and 30.5°N. The initial basic flow field is based on the mean condition at 80°W during Phase III of GATE. The superposed wave disturbance is initially confined in the lower troposphere. The time integration of the model is carried out to 96 h, during which a tropical storm develops accompanied by an upper level anticyclone.

The genetic sequence of the disturbance system, from a shallow easterly wave into a tropical depression and further into a tropical storm, is described. The minimum surface pressure of the system deepens from 1008.4 to 1002.6 mb at 96 h. The maximum surface wind at 96 h is above 17 m s−1. The relative vorticity at 950 mb intensifies from 43 × 10−6 s−1 at the initial time to 237 × 10−6 s−1 at 96 h. The surface convergence increases from 24 × 10−6 10−6 s−1 to 71 × 10−6 s−1. The processes involved in the above transformation are extensively discussed. Attention is given to the change in the area of rainfall and cloud from a zonal pattern to a cluster-type, the deepening of the cloud within the system, the appearance of horizontal tilt of the trough axis and the time variation of its vertical tilt, the evolution of the vertical motion field, the thickening of the convergence layer around the depression center, the formation of a warm core at 335 mb and its downward extension, the appearance of a cold core at a higher level, etc. The intensification of the vortex and the growth of a warm core are analyzed by examining the budgets of vorticity and heat at the tropical depression stage. The vorticity increase at low levels is due to stretching of the vortex. Relative horizontal advection causes a decrease of vorticity in some outer areas. At upper levels, the upward protrusion of positive vorticity from below and relative horizontal advection cause a positive tendency. Both the effect due to horizontal divergence and the twisting up of a horizontal vortex make negative contributions. The net effect at upper levels is to produce a compact positive vorticity area within a large region of negative vorticity. Upper level warming is largely due to the excess of the condensation-convection heating over the cooling effect associated with the upward motion. The appearance of an upper level disturbance in the present model is caused entirely by the forcing from below. Supplemental experiments confirm that, although the diabatic heating effect of radiation plays an important role, the heating due to the condensation of water vapor is essential for the formation of a tropical storm in the present case.

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Yoshio Kurihara and Robert E. Tuleya

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Yoshio Kurihara and Robert E. Tuleya

Abstract

A scheme of dynamic initialization of a primitive equation model is proposed with an emphasis on the dynamic adjustment in the boundary layer. The pre-initialintion analysis is important since the restorative method is used in the subsequent dynamic initialization. The first phase of dynamic initialization is designed to establish a reasonable boundary layer structure. For this purpose, a time integration of the primitive equations is performed under a strong constraint such that all meteorological fields except momentum in the boundary layer are frozen. Use of an implicit form for the vertical diffusion term is recommended. The second phase is formulated to reduce the high-frequency noise in the final initialized field. Cyclic integration with a selective damping scheme is carried out under a restorative constraint.

The proposed scheme is applied to a case of simple zonal flow and the evolution of the boundary layer flow is shown. The scheme is also tested for a cam of mature tropical cyclone. Starting from the wind data in the free atmosphere only, the initial condition of the model is derived. Subsequent time integration of the model compares favorably with the integration in a control experiment.

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