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Ludwig Umscheid and Peter R. Bannon

Abstract

Three finite-difference global grids [the original Kurihara (OK), a modified Kurihara (MK) and latitude- longitude (LL)] are tested by comparing numerical solutions with a barotrpic free-surface model to a high-resolution control run and by comparing forecasts with a general circulation model to observations.

With the free surface model, 30-day integrations are made for three different resolutions of each grid and with three initial conditions two mathematical patterns and one 500 mb observed field. The LL grid performed well on the mathematical patterns, especially the case with zonal wavenumber 4. The numerical solutions with the high resolution MK grid also performed satisfactorily for both mathematical patterns. The OK grid did not perform as well, particularly on the case with zonal wavenumber 1. For the observed case, the LL grid in general had lower rms errors although the solutions did not depend as strongly on the three different grid types as the solutions for the mathematical patterns.

For the three-dimensional cases, the GFDL nine-level model was used for 14-day forecasts for observed conditions in March and 3-day forecasts in November. Forecast sensitivity to the different grids is low for short range. The MK grid had the lowest 500 mb rms errors for the duration of both forecasts. Both the MK and LL grids were free of the problem of anomalously high geopotential heights over the North Pole that occurred with the OK grid.

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Mankin Mak and Peter R. Bannon

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Analytic solutions are obtained for a prototype semigeostrophic frontal model in which cumulus heating is parameterized by applying the conventional wave-CISK scheme and the scheme of Mak in the geostrophic coordinate. Such heating schemes give rise to a vertically tilted heating distribution as suggested by observational evidence of clouds in frontal zones. The wave-CISK scheme produces a larger influence than the Mak scheme and preferentially excites smaller-scale components of the solution. Both schemes generate a significant frontogenetic development at higher levels over the front, although their impact on the temperature near the surface is smaller. The various facets of the model solutions for moist frontogenesis are compared with those for dry frontogenesis. The main differences are the significant enhancement by the heating of frontogenesis aloft and the nongeostrophic component of the circulation. Specifically, the magnitude of the ascending motion over the surface cold front can be readily increased several fold, and its length scale reduced to be comparable to that of the front itself. The propagation of the cold front into the warm sector is also slightly enhanced as a result of the heating.

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Timothy Sotack and Peter R. Bannon

Abstract

Lamb’s hydrostatic adjustment problem for the linear response of an infinite, isothermal atmosphere to an instantaneous heating of infinite horizontal extent is generalized to include the effects of heating of finite duration. Three different time sequences of the heating are considered: a top hat, a sine, and a sine-squared heating. The transient solution indicates that heating of finite duration generates broader but weaker acoustic wave fronts. However, it is shown that the final equilibrium is the same regardless of the heating sequence provided the net heating is the same.

A Lagrangian formulation provides a simple interpretation of the adjustment. The heating generates an entropy anomaly that is initially realized completely as a pressure excess with no density perturbation. In the final state the entropy anomaly is realized as a density deficit with no pressure perturbation. Energetically the heating generates both available potential energy and available elastic energy. The former remains in the heated layer while the latter is carried off by the acoustic waves.

The wave energy generation is compared for the various heating sequences. In the instantaneous case, 28.6% of the total energy generation is carried off by waves. This fraction is the ratio of the ideal gas constant R to the specific heat at constant pressure c p. For the heatings of finite duration considered, the amount of wave energy decreases monotonically as the heating duration increases and as the heating thickness decreases. The wave energy generation approaches zero when (i) the duration of the heating is comparable to or larger than the acoustic cutoff period, /N A ∼ 300 s, and (ii) the thickness of the heated layer approaches zero. The maximum wave energy occurs for a thick layer of heating of small duration and is the same as that for the instantaneous case.

The effect of a lower boundary is also considered.

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Mankin Mak and Peter R. Bannon

Abstract

Abstract not available.

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Adam R. Edson and Peter R. Bannon

Abstract

A nonlinear, numerical model of a dry, compressible atmosphere is used to simulate the hydrostatic and geostrophic adjustment to a localized prescribed injection of momentum applied over 5 min. with a size characteristic of an isolated, deep, cumulus cloud. This theoretical study is relevant to the initialization of updrafts in compressible numerical weather prediction models. The four different forcings studied are vertical, divergent horizontal, and nondivergent horizontal momentum forcings, and a prescribed transverse circulation. These forcings are applied to an isothermal atmosphere, a nonisothermal atmosphere, and an atmosphere with a nonisothermal troposphere capped by an isothermal stratosphere. These scenarios are studied by analyzing the resulting perturbation fields and the energetics of the system. Potential vorticity is used to determine the possibility of steady atmospheric states. The energetics of the system are examined to observe the creation and propagation of atmospheric waves. Both traditional and available energetics are used to determine the presence and strength of these waves. Traditional energetics consist of kinetic, internal, and potential energies while available energetics consist of kinetic, available potential, and available elastic energies. The efficiencies are similar for these different energetics, though they represent different phenomena. The traditional energetics show a strong dependence on the presence of a Lamb wave, whereas in the available energetics the Lamb wave has little or no effect.

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Jonathan W. Smith and Peter R. Bannon

Abstract

The response to an instantaneous diabatic warming and the resulting hydrostatic and geostrophic adjustment in compressible and anelastic models is examined. The comparison of the models includes examining the initial conditions, time evolution, potential vorticity, and both the traditional and available energetics. Between the two models, the buoyancy flow fields and potential vorticity perturbations are qualitatively and quantitatively similar. Traditional and available energetics can both be accurately conserved within the models. There are some short-lived (e.g., several minutes) differences in the model solutions as the compressible model undergoes an acoustic adjustment that contains vertically propagating acoustic waves and horizontally propagating Lamb waves. The acoustic waves are effectively eliminated in an upper-level numerical sponge layer using Rayleigh damping. Moreover, the relative computational efficiency and accuracy of the two models are assessed.

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Peter R. Bannon and Thomas L. Salem Jr.

Abstract

The Ekman-Taylor boundary layer model is solved for the case of a linear variation of the geosptophic wind with height. The two-layer model couples a Monin–Obukhov similarity layer to an Ekman layer with a vertically constant eddy diffusivity. The presence of the thermal wind contributes both an along-isotherm and a cross-isotherm component to the boundary layer flow. The along-isotherm flow is supergeostrophic and results from the net downward transport of geostrophic momentum by the eddies. The cross-isotherm flow is toward the warm air and results from the Coriolis deflection of the geostrophic momentum-rich air aloft that has been mixed downward. The effect of the baroclinity (i.e., the thermal wind shear) on the wind field is conveniently summarized geometrically.

The model predicts that the surface vorticity increases in regions of cyclonic thermal vorticity (i.e., the vorticity of the thermal wind). However, anticyclonic thermal vorticity produces convergence of the low-level warmward flow and rising motion. Thus, a warm core cyclone experiences increased boundary layer convergence.

The effects of horizontal gradients in the turbulent momentum mixing on the surface vorticity, convergence, and rising motion are ascertained. For example, there is convergence of the Ekman mass transport and an upward contribution to the boundary layer pumping for mixing gradients directed downstream or to the right of the surface geostrophic wind and directed upstream or to the left of the surface thermal wind. The mixing gradients appear most sensitive to variations in the surface stability (i.e., the air - surface temperature difference).

A case study estimates the influence of these processes on the surface vorticity in a frontal zone. The surface vorticity is shown to be displaced behind (i.e., coldward of) its geostrophic location, in agreement with observations.

An appendix provides justification for the generalized Prandtl boundary layer approximation that, to lowest order, the pressure and thermal fields (and their vertical variations) in the boundary layer are those associated with the large-scale interior flow.

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Jeffrey M. Chagnon and Peter R. Bannon

Abstract

This study compares the response to injections of mass, heat, and momentum during hydrostatic and geostrophic adjustment in a compressible atmosphere. The sensitivity of the adjustment to these different injection types is examined at varying spatial and temporal scales through analysis of the transient evolution of the fields as well as the partitioning of total energy between acoustic waves, buoyancy waves, Lamb waves, and the steady state.

The effect of a cumulus cloud on its larger-scale environment may be represented as a vertical mass source/sink and a localized warming. To examine how the response to such injections may differ, injections of mass and heat that generate identical potential vorticity (PV) distributions and, hence, identical steady states, are compared. When the duration of the injection is very short (e.g., a minute or less), the injection of mass generates a very large acoustic wave response relative to the PV-equivalent injection of heat. However, the buoyancy wave response to these two injection types is quite similar.

The responses to injections of divergent momentum in the vertical and horizontal directions are also compared. It is shown that neither divergent momentum injection generates any PV and, thus, there is no steady-state response to these injections. The waves excited by these injections generally propagate their energy in the direction of the injection. Consequently, an injection of vertical momentum is an efficient generator of vertically propagating, horizontally trapped, high-frequency buoyancy waves. Such waves have a short time scale and are therefore very sensitive to the injection duration. Analogously, an injection of divergent horizontal momentum is an efficient generator of horizontally propagating, vertically trapped low-frequency buoyancy waves that are relatively insensitive to the injection duration. Because of this difference in the response to horizontal and vertical injections of momentum, the response to the injection of an isolated updraft differs depending on whether a compensating horizontal inflow/outflow is also specified. This additional specification of inflow/outflow helps filter acoustic waves and encourages a stronger updraft that is not removed as rapidly by the buoyancy waves. This finding is relevant to the initialization of updrafts in compressible numerical weather prediction models.

Injection of nondivergent momentum generates waves in the regions of convergence/divergence produced by the deflection of the current by Coriolis forces. The energy partitioning for such an injection is sensitive to the width and depth of the current relative to the Rossby radius of deformation, but the response is insensitive to the duration of injection for time scales shorter than several hours.

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Joseph A. Zehnder and Peter R. Bannon

Abstract

The interaction Of developing two-dimensional cold and warm frontal systems with a mesoscale mountain ridge is examined. The flow of the rotating model atmosphere is assumed to be inviscid, adiabatic, and Boussinesq. The geostrophic momentum approximation is made. An imposed horizontal deformation field forces the frontogenesis. The nonlinear model equations are solved numerically in physical space using terrain-following Coordinates which incorporate the fully nonlinear, lower boundary condition. Comparison of the model results with and without topography enables assessment of the impact of the mountain on the frontogenesis.

A scale analysis indicates that the most general problem of frontal interaction with an infinite mountain ridge encompasses a seven-dimensional parameter space. The scale analysis provides justification for die two-dimensional geostrophic momentum approximation and defines an inverse Richardson number as a measure of the importance of the ageostrophic advection. Sensitivity of the model to variations in the orographic and frontal Richardson numbers and the ratio of the orographic and frontal length scales is examined.

Flow over the mountain ridge results in retardation of the surface cold front on the upstream side, while rapid advection of the front across the mountain top yields an advancement of the frontal position downstream. The combination of acceleration and deceleration produces a net 100 km advancement of the front far downstream compared with the front-only case. The front is significantly weakened on the upslope side, but reappears stronger in the lee. Aloft, the upper-level front advances a similar distance in the presence of the mountain and undergoes a slight weakening. The general character of the interaction is relatively independent of the initial frontal strength.

Superposition of a warm front with the orographic disturbance results in strengthening on the upstream side and weakening in the lee. The retardation and advancement are similar to those of the cold front.

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Peter R. Bannon and Joseph A. Zehnder

Abstract

The linear problem of rotating, stratified, adiabatic, hydrostatic, Boussinesq airflow over a mountain ridge is solved analytically for the case where the spatially uniform, normally incident airflow is the sum of a steady and sinusoidally varying component. The mountain generates a response at the fundamental frequency of the wind and all higher harmonics.

During flow acceleration, the evanescent (vertically decaying) modes deepen and broaden the high-low pressure asymmetry across the ridge and increase the mountain drag. In contrast, the evanescent modes for steady airflow product only a symmetric mountain anticyclone that generates no drag. The influence of the acceleration is more pronounced for mesoscale and synoptic-scale ridges (i.e., ridges whose Rossby number is order unity or smaller) and when the fundamental period is near the inertial period.

The transience also amplifies the magnitude of the maximum wave drag over its value predicted from steady airflow theory using the instantaneous wind speed. The total acceleration reaction due to both evanescent and wave modes can be larger than this steady airflow drag.

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