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Ying-Hwa Kuo, M. A. Shapiro, and Evelyn G. Donall

Abstract

This study addresses the relative contributions of adiabatic baroclinic and diabatic processes and their interaction in the evolution of a rapidly intensifying marine cyclone. Two numerical experiments were performed using a limited-area mesoscale model. The adiabatic simulation showed that the surface cyclone was associated with the quasi-geostrophic vertical motion forcing of a midtropospheric short wave for a period greater than 12 hours, suggesting the presence of deep baroclinic forcing during the evolution of the storm. The full-physics simulation produced major cyclogenesis with a central pressure of 967 mb and a deepening of 37 mb in 24 h. The model simulated the development of comma-shaped cloud patterns, which compared favorably with satellite observations of the storm. Further analysis showed that the rapid cyclogenesis was strongly related to moist frontogenesis at the warm front. During rapid storm intensification, the heavy precipitation, the generation of vorticity, strong surface frontogenesis, and large surface pressure falls all took place in the vicinity of the warm front.

Quasi-geostrophic vertical velocity diagnosis of the full-physics simulation suggested a strong interaction between baroclinic and diabatic processes in the course of rapid development. The latent heat release significantly modified the frontal structure of the storm to reinforce its adiabatic secondary circulation. As a result, the adiabatic component of the vertical motion in the full-physics simulation was three times larger than that in the adiabatic simulation. Moreover, the upward and downward vertical motion induced by latent heat release was in phase with the secondary circulation associated with the adiabatic frontogenesis. The enhanced frontal circulation provided strong low-level moisture convergence to stimulate further frontal precipitation, establishing a positive feedback. Because of the large amount of latent heating associated with the warm frontal precipitation, diabatic heating was the dominant forcing mechanism for the vertical motion of the simulated storm during its rapid intensification. These results clearly indicate that it is not appropriate to treat the contribution of latent heat release (or other physical processes) to rapid development as a linear addition to the adiabatic dynamics, as has been done in most model sensitivity experiments. Rather, extratropical cyclogenesis should be viewed in the context of moist baroclinic instability with nonlinear interactions between the baroclinic dynamics and diabatic processes.

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Ying-Hwa Kuo, Evelyn G. Donall, and Melvyn A. Shapiro

Abstract

A series of observing system simulation experiments was conducted to investigate the feasibility of shortrange numerical weather prediction using a network of profilers. A mesoscale model was used to generate datasets which mimic observations from a network of profilers and from an array of rawinsondes. The sensitivity of the model forecast to the characteristic measurement errors of a number of hypothetical profiler networks was tested.

Our results demonstrate that profiler wind observations would have a positive impact on short-range numerical weather prediction with a simple static initialization. We also found that forecasts based on retrieved temperatures (calculated from profiler wind data) are significantly better than those based on direct radiometric temperature measurements (using climatology as the first guess for radiometric retrieval). However, the temperature fields from either radiometric measurements or from thermodynamic retrieval need further improvement-before they can be as accurate as the radiosonde temperature observations for model initialization.

Various hypothetical networks, each having a regular array of stations at a separation of 360 km, provided the initial conditions for short-range numerical forecasts. These predictions can be ranked by performance in the following order. (1) profiler wind with radiosonde temperature and moisture; (2) mixed profiler and rawinsonde wind with rawinsonde temperature and moisture; (3) rawinsonde wind; temperature and moisture; (4) profiler wind and moisture with retrieved temperature., and (5) profiles wind, temperature and moisture.

It was found that, with a domain of 4320 × 2880 km centered at 40°N and a grid spacing of 40 km, accuracy in both the wind field and the temperature field is needed to define the initial state of the model properly. Even within the mesoscale range, the wind field and the temperature field adjust to each other during the course of the model integration. This is because temperature and wind errors associated with observing systems are often projected onto several different vertical modes at a wide range of horizontal scales, both larger and smaller than the Rossby radius of deformation, thus forcing the mutual adjustment of wind and mass fields.

These conclusions are considered tentative because only one synoptic situation was tested with a simple static initialization procedure. Further modeling studies should utilize a four-dimensional data assimilation technique to take advantage of the high temporal resolution of the profiler observations. Also, the experimental procedure should be repeated for more synoptic events to obtain statistically significant results.

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Paul J. Neiman, M. A. Shapiro, Evelyn G. Donall, and Carl W. Kreitzberg

Abstract

On 25–27 January 1988, the National Oceanic and Atmospheric Administration's Wave Propagation Laboratory, Drexel University, and the Office of Naval Research carried out a combined pre-ERICA research aircraft investigation of a major marine cyclone moving northeastward over the Canadian Maritime Provinces. Flight-level and dropwindsonde observations documented the diabatic modification of the cyclone's warm sector marine boundary layer (MBL) as it moved out over cold underlying water. These observations and results from the Blackadar one-dimensional boundary layer model both show that heat fluxes were directed downward from the warm sector MBL into the cold ocean. Vertical gradients of these downward heat fluxes diabatically cooled the lower portion of the warm sector MBL and generated large static stability within the entire layer. The increase in stable stratification allowed large vertical wind shear to exist within this layer and strong wind speeds to exist at its top. The increase in static stability within the warm sector MBL acted to concentrate isentropic potential vorticity in this layer, but these changes also weakened the horizontal gradients of temperature, moisture, and wind velocity within the adjacent warm- and cold-frontal zones at the surface.

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Nelson L. Seaman, Francis L. Ludwig, Evelyn G. Donall, Thomas T. Warner, and Chandrakant M. Bhumralkar

Abstract

The Pennsylvania State University (PSU)/National Center for Atmospheric Research (NCAR) mesoscale model was modified and used to simulate the evolution of meteorological conditions in the vicinity of St. Louis, Missouri, from near sunrise to noon on 25 July 1975. Observations obtained during the METROMEX (Metropolitan Meteorological Experiment) and RAPS (Regional Air Pollution Study) field programs were available for comparison with modeled conditions. The PSU/NCAR model used a nested grid with two-way interaction between the coarse mesh (7.5 km) and the fine mesh (2.5 km), where the fine domain covered the city and its immediate suburban and rural surroundings. Realistic three-dimensionally variable initial and lateral boundary conditions were obtained from the observations so that the numerical experiments could be used for quantitative evaluation of certain urban effects. After simulation of the actual conditions (control experiment), the importance of a number of processes on the urban planetary boundary layer (PBL) structure were investigated. The PBL effects were isolated by using realistic surface parameters as well as those based on the preurban conditions and an expanded urban environment. Sensitivities to surface evaporative fluxes, radiative processes, and different surface roughness associated with urbanization were examined. The control simulations of the temperature, boundary layer depth, specific humidity and wind fields exhibited essentially the same behavior as observed in the urban PBL throughout the morning forecast period. Unlike many other documented cases that displayed strong urban-induced low-level convergence, the confluence on this morning was relatively weak, with the center of the heat island displaced (in both the simulation and the observations) downwind (south) of the city. A relative minimum in windspeed was associated with the center of the displaced heat island. The sensitivity experiments clearly demonstrated and maintenance of the urban PBL perturbation.

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