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Chaing Chen

Abstract

The purpose of this study is to examine the effect of wind shear on gravity currents in a neutral atmosphere by using a two-dimensional, nonhydrostatic primitive equation model. The depth of the gravity current is found to be directly related to the sign and the magnitude of the shear; a flow with a positive wind shear produces a gravity current of greater depth than that with a negative shear. As the positive wind shear (i.e., positive ∂u/∂z) increases, the gravity current becomes unstable. For sufficiently large positive shear, the gravity current displays a diffuse structure with two distinct gravity current heads. It is found that enhanced eddy mixing, triggered by the presence of a reversed (rear to front) flow in the prefrontal environment, is the source of this phenomenon. From a vorticity budget analysis, it is found that the rear-to-front flow is less efficient in “ventilating” or “removing” vorticity generated at the leading edge of the gravity current. Therefore, the accumulation of vorticity leads to the development of enhanced eddy circulations that subsequently destroy the gravity current head.

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Chaing Chen

Abstract

A nested grid, nonhydrostatic, elastic model using a terrain-following coordinate transformation is presented with a unique application of grid-nesting techniques to the time-splitting elastic model.

To minimize the wave reflection along the lateral and the upper boundaries of the nested fine-grid model, the conventional nested-grid scheme has been modified so that the fine-grid model has its own “independent” radiative boundary conditions as well as “dependent” nested boundary conditions. This modification is crucial to the success of nested grid simulations.

A simulation of the 10-m-high Witch of Agnesi Mountain provides the control to test this new model. The results show that the model produces the same solution as that derived from a simple linear analytic model.

The model used in the control case is then double-nested by a fine-grid model that has its primary domain zoomed into the area around the mountain. Using this configuration, one-way and two-way grid-nesting sensitivity experiments with various boundary conditions are performed. In order to examine the robustness of the newly developed nesting scheme, the model is further tested by conducting the triple-nesting case, the high-mountain case, and the downslope windstorm case.

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Chaing Chen and Craig Bishop

Abstract

No abstract available.

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Chaing Chen and William R. Cotton

Abstract

In order to simulate the stratocumulus-capped mixed layer, a one-dimensional stratocumulus model is developed. This model consists of five major points: 1) a one-dimensional (1D) option of the CSU Cloud/Mesoscale Model, 2) a partially diagnostic higher-order turbulence model, 3) an atmospheric radiation model, 4) a partial condensation parameterization, and 5) the drizzle process.

This model is tested against the observed structure of the marine stratocumulus layer reported by Brost et al. In this paper we also investigate the interactions among the following physical processes: atmospheric radiation, cloud microphysics, vertical wind shear, turbulent mixing, large-scale divergence, the sea surface temperature and the presence of high-level clouds above the capping inversion.

The model simulated fields were found to be in generally good agreement with observations, although the amount of cloud liquid water predicted was too large. This may have been a result of employing a wind profile that exhibits somewhat weaker shear than observed, since the sensitivity experiment with an unbalanced wind similar to that observed produced liquid water contents similar to the observed values.

It is also found that drizzle precipitation greatly alters the liquid water content of the cloud and the rate of radiative cooling. This then feeds back into the turbulence structure of the cloud.

For the case with large-scale subsidence and the presence of high-level clouds above the capping inversion, the effect of cloud top radiative cooling is found to become less important.

Longer time integrations (up to 6 hours) revealed a 15 to 20 min periodicity in cloud top entrainment. The length of the period of oscillation was regulated by the magnitude of shear and the presence of drizzle. Complete removal of shear and drizzle processes resulted in the elimination of sporadic entrainment.

Finally, sensitivity experiments were also conducted to examine the role of shortwave radiation. It is found that the influence of shortwave radiation on the cloud layer varies with the intensity of overlying large-scale subsidence and the moisture content of the airmass overlying the capping inversion.

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Chaing-Heins Chen and Harold D. Orville

Abstract

A two-dimensional time-dependent cloud model was used in this research to investigate the effects of mesoscale convergence on cloud convection. A Klemp and Wilhelmson type boundary condition was tested which allows inflow and outflow through the lateral boundary and also lets gravity waves pass out through the boundaries with minimal reflection. A relatively stable and easy method of superimposing a mesoscale convergence field was also introduced in this study. The main idea of this superposition is to decompose the velocity into mesoscale and cloud-scale velocities. The cloud-scale velocity is governed by the cloud convection, while the mesoscale velocity is governed by the mesoscale variable.

Two types of atmospheric soundings were run in this model. The first type is an unstable sounding and the second type is conditionally unstable with a low-level inversion. The results show that convergence weakens the temperature inversion and leads to strong convection in one case. Fewer, broader and more vigorous clouds were evident in another mesoscale convergence case.

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Chaing Chen, James W. Rottman, and Steven E. Koch

Abstract

A two-dimensional, nonhydrostatic, elastic numerical model has been used to study the generation of gravity waves for a stably stratified shear flow over an obstacle. When a low-level wind shear is included in the simulation, we find that the predictions for noticeable upstream effects based on Froude number for a uniform flow are no longer accurate. Upstream effects are encountered in the form of upstream propagating columnar disturbances and internal bores away from the obstacle. The limited parameter space studies conducted in this study suggest that the ratio of the shear depth to the obstacle height (d/H), the obstacle aspect ratio (H/L), and the Froude number (U/NH) are instrumental in determining the strength and the existence of these upstream disturbances. Thus, the present theoretical and empirical understanding of the importance of the Froude number for determining the nature of upstream effects should be modified substantially to include additional nondimensional parameters when shear is present.

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Chaing Chen, Craig H. Bishop, George S. Lai, and Wei-Kuo Tao

Abstract

A cold-frontal rainband, which occurred during the afternoon of 28 December 1988, is numerically simulated using the Penn State–NCAR three-dimensional MM5 modeling system. This case is characterized by a line of severe convection that has a gravity current–like structure along the leading edge of a strong surface cold front. The authors test whether this gravity current–like structure is associated with the cold-air outflow boundary generated by the evaporation of hydrometeors from the postfrontal precipitation system. It is found that the neglect of moist processes in the model did not significantly affect the gravity current–like structure of the front. PBL (planetary boundary layer) frictional processes, which produce cross-frontal low-level wind shear (u z), play an important role. The role of this cross-frontal low-level wind shear is to generate low-level convergence and to steepen the frontal slope. Thus, the observed intense NCFR (narrow cold-frontal rainband) is closely related to frictionally induced PBL processes.

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Yi Jin, Steven E. Koch, Yuh-Lang Lin, F. Martin Ralph, and Chaing Chen

Abstract

Numerical simulations of a gravity current in an environment characterized by complex stratification and vertical wind shear have been performed using a nonhydrostatic, two-dimensional, dry, primitive-equation model. Data from one of the most complete documentations to date of gravity waves associated with a gravity current, presented in an earlier study, are used both to prescribe the gravity current's environment and for validation of the simulated gravity current and its associated gravity waves. These comparisons indicate that the gravity current observed by a Doppler wind profiler and sodars was well simulated in terms of depth, density contrast, and propagation speed and that the model produced a variety of gravity waves similar in many ways to these observed.

Because uncertainties remained concerning the gravity wave generation mechanisms derived from the observations (e.g., wavelengths were not observed), the validated simulations are used to test these tentative hypotheses. The simulations confirm that trapped lee-type gravity waves formed in response to flow over the head of the gravity current and that Kelvin-Helmholtz (KH) waves were created because of shear atop the cold air within the gravity current. The 2.8-km wavelength of the simulated KH waves agrees with the 2- to 3-km wavelength inferred from the observations. However, the 6.4-km wavelength of the simulated lee-type waves is significantly shorter than the 12.5-km wavelength inferred from the observational data, even though wave periods (20-23 minutes) are nearly identical. Sensitivity tests indicate that the curvature in the wind profile associated with the low-level opposing inflow and an elevated isothermal layer worked together to support the development of the trapped lee-type waves. The model produces a deep vertically propagating wave above the gravity current head that was not present in the observations. As deduced in the earlier study, sensitivity tests indicate that the prefrontal, near-surface stable layer was too shallow to support the generation of a bore; that is, conditions were supercritical. Synthesis of detailed observations and numerical simulations of these mesoscale phenomena thus offers the broadest examination possible of the complex physical processes.

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V. Mohan Karyampudi, Steven E. Koch, Chaing Chen, James W. Rottman, and Michael L. Kaplan

Abstract

In this paper, Part II of a series, the evolution of a prefrontal bore on the leeside of the Rockies and its subsequent propagation and initiation of convection farther downstream over eastern Colorado and western Nebraska are investigated. The observational evidence for this sequence of events was obtained from combined analyses of high-resolution GOES satellite imagery and Program for Regional Observing and Forecasting Services mesonetwork data over the Colorado region for the severe weather event that occurred during 13–14 April 1986. A 2D nonhydrostatic numerical model is used to further understand the initiation of the bore and its ability to propagate farther downstream and trigger convection.

Analysis of satellite imagery and mesonet data indicated that an internal bore (ahead of a cold front), a moderate downslope windstorm, and a quasi-stationary hydraulic jump were generated within a few hours along the Iceslope as a Pacific cold front and its attendant upper-level jet streak advanced over the Rockies. The bore and the cold front then propagated eastward for several hours and interacted with a Ice cyclone, a dryline, and a warm front, initiating severe weather over Nebraska and Kansas. Wave-ducting analysis showed that favorable wave-trapping mechanisms such as a capping inversion above a neutral layer and wind curvature from a low-level jet, which appeared to he the most dominant ducting mechanism, existed across eastern Colorado and western Nebraska to maintain the bore strength. Numerical simulations of continuously stratified shear flow specified from upstream and downstream soundings suggested that the creation of a density current along the Ice slopes, a downstream inversion height lower than the upstream inversion height, and a strong curvature in the wind profile of the low-level jet are all needed to initiate and sustain the integrity of the propagating bore.

Based on the synthesis of observational analyses and 2D nonhydrostatic model simulations, a schematic illustration of the time evolution of the bore ahead of the Pacific cold front, the hydraulic jump associated with a mountain wave, and the arctic air intrusion from the north to the Ice of the Rockies are presented in the context of severe weather occurrence over western Nebraska and Kansas.

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Chaing Chen, Wei-Kuo Tao, Pay-Liam Lin, George S. Lai, S-F. Tseng, and Tai-Chi Chen Wang

Abstract

During the period of 21–25 June 1991, a mei-yu front, observed by the post–Taiwan Area Mesoscale Experiment, produced heavy precipitation along the western side of the Central Mountain Range of Taiwan. Several oceanic mesoscale convective systems were also generated in an area extending from Taiwan to Hong Kong. Numerical experiments using the Penn State–NCAR MM5 mesoscale model were used to understand the intensification of the low-level jet (LLJ). These processes include thermal wind adjustment and convective, inertial, and conditional symmetric instabilities.

Three particular circulations are important in the development of the mei-yu front. First, there is a northward branch of the circulation that develops across the upper-level jet and is mainly caused by the thermal wind adjustment as air parcels enter an upper-level jet streak. The upper-level divergence associated with this branch of the circulation triggers convection.

Second, the southward branch of the circulation, with its rising motion in the frontal region and equatorward sinking motion, is driven by frontal vertical deep convection. The return flow of this circulation at low levels can produce an LLJ through geostrophic adjustment. The intensification of the LLJ is sensitive to the presence of convection.

Third, there is a circulation that develops from low to middle levels that has a slantwise rising and sinking motion in the pre- and postfrontal regions, respectively. From an absolute momentum surface analysis, this slantwise circulation is maintained by conditionally symmetric instability located at low levels ahead of the front. The presence of both the LLJ and moisture is an essential ingredient in fostering this conditionally symmetric unstable environment.

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