Showing papers in "Meteorology and Atmospheric Physics in 1992"

A comprehensive meteorological modeling system?RAMS

Abstract: This paper presents a range of applications of the Regional Atmospheric Modeling System (RAMS), a comprehensive mesoscale meterological modeling system. Applications discussed in this paper include large eddy simulations (LES) and simulations of thunderstorms, cumulus fields, mesoscale convective systems, mid-latitude cirrus clouds, winter storms, mechanically- and thermally-forced mesoscale systems, and mesoscale atmospheric disperision. A summary of current RAMS options is also presented. Improvements to RAMS currently underway include refinements to the cloud radiation, cloud microphysics, cumulus, and surface soil/vegetative parameterization schemes, the parallelization of the code, development of a more versatile visualization capability, and research into meso-α-scale cumulus parameterization.

The role of the convective “trigger function” in numerical forecasts of mesoscale convective systems

Abstract: Three-dimensional numerical model simulations of a mesoscale convective system are performed to evaluate the sensitivity of the simulations to differences in the convective trigger function The Penn State/NCAR mesoscale model with the Kain-Fritsch convective parameterization scheme is used as the modeling system for the study All simulations are performed on the June 10–11, 1985 squall line from the OK PRE-STORM field experiment Individual simulations differ only in their specification of the trigger function within the Kain-Fritsch scheme Comparison of results from 12 hour simulations indicates that the position, timing, and intensity of convective activity and mesoscale features vary substantially as a function of the trigger function formulation The results suggest that the convective trigger function is an integral part of the overall convective parameterization problem, and that great care must be exercised is designing realistic trigger function formulations, especially as model resolutions approach the scale of individual convective clouds

The second order adjoint analysis: Theory and applications

Abstract: The adjoint method application in variational data assimilation provides a way of obtaining the exact gradient of the cost functionj with respect to the control variables. Additional information may be obtained by using second order information. This paper presents a second order adjoint model (SOA) for a shallow-water equation model on a limited-area domain. One integration of such a model yields a value of the Hessian (the matrix of second partial derivatives, ∇2 J) multiplied by a vector or a column of the Hessian of the cost function with respect to the initial conditions. The SOA model was then used to conduct a sensitivity analysis of the cost function with respect to distributed observations and to study the evolution of the condition number (the ratio of the largest to smallest eigenvalues) of the Hessian during the course of the minimization. The condition number is strongly related to the convergence rate of the minimization. It is proved that the Hessian is positive definite during the process of the minimization, which in turn proves the uniqueness of the optimal solution for the test problem. Numerical results show that the sensitivity of the response increases with time and that the sensitivity to the geopotential field is larger by an order of magnitude than that to theu andv components of the velocity field. Experiments using data from an ECMWF analysis of the First Global Geophysical Experiment (FGGE) show that the cost functionJ is more sensitive to observations at points where meteorologically intensive events occur. Using the second order adjoint shows that most changes in the value of the condition number of the Hessian occur during the first few iterations of the minimization and are strongly correlated to major large-scale changes in the reconstructed initial conditions fields.

An explicit three-dimensional nonhydrostatic numerical simulation of a tropical cyclone

Abstract: A nonhydrostatic numerical simulation of a tropical cyclone is performed with explicit representation of cumulus on a meso-β scale grid and for a brief period on a meso-γ scale grid. Individual cumulus plumes are represented by a combination of explicit resolution and a 1.5 level closure predicting turbulent kinetic energy (TKE).

Fronts and orography

Abstract: Theoretical work on the impact of orography on cold fronts is reviewed and related to observations. This paper contains the following sections: 1. Introduction 2. Passive scalar fronts 3. Shallow water fronts 4. Fronts in deformation flows 5. Orographic jets 6. Cold-air damming 7. Fohn and fronts 8. Fronts in valleys 9. Cold surges 10. Lee cyclogenesis 11. Orographic rain and fronts 12. Pressure drag 13. Numerical simulations 14. Analysis

On the impact on forecast accuracy of the step-mountain (eta) vs. sigma coordinate

Abstract: Impact on forecast accuracy of the choice of the step-mountain (“eta”) vs. the traditional sigma coordinate is examined by using a model which can be run as either an eta or sigma system model via a simple switch in its code. This is done by (a) synoptic examination of the differences in a set of three forecasts, and (b) comparison of precipitation skill scores in a set of nine consecutive forecasts; each of these sets of forecasts was performed using one and then the other of the two systems. Both efforts indicate that greater accuracy is achnieved in the step-mountain coordinate forecasts. The three forecasts examined from the synoptic point of view are also compared against forecasts of another sigma system model. The two sigma models are found to have errors of an almost identical pattern (two cases) or to have errors which seemed to have the same basic cause.

Rain Rates Based on SSM/I, OLR and Raingauge Data Sets

Abstract: In this paper we present a methodology for evaluating rain rates from a mix of satellite and surface based observations. The component data sets include the outgoing longwave radiation (OLR), microwave radiometric data from the special sensor microwave imager (SSM/I) and surface raingauge data from the World Weather Watch. We have noted some deficiencies in the SSM/I algorithm-based rain rate over land areas; the OLR-based rain rates exhibit a larger lateral spread and lower intensities than the observed rainfall structures. The proposed combined method assimilates these three data sets to provide improved fields of global tropical rainfall.

An efficient non-hydrostatic forecast model

Abstract: A semi-implicit non-hydrostatic mesoscale forescasting model, which is used operationally in the United Kingdom Meteorological Office, has been reformulated using two time levels and semi-Lagrangian advection so as to permit longer timesteps. The boundaries are found to be an important influence on the model's stability and two methods of suppressing wave reflection are described with the aid of a one-dimensional experiment. The model's responses to two dimensional flow over a ridge and to three-dimensional flow over an isolated mountain are then demonstrated. The reformulated model is more efficient than the original provided a timestep at least twice as long can be used. It is also superior both in having minimal damping (except at the boundaries), and in the accuracy of the advection scheme.

Vortex initialization for typhoon track prediction

Abstract: A shallow water single level primitive equation model is ideally suited for studying the motion of a tropical cyclone. Three factors seem to be important in the initialization, i.e. the size, intensity and the initial speed and direction of motion of the storm. This study presents the results of sensitivity studies on the above parameters in the definition of a synthetic idealized vortex. The sensitivity studies include results of experimental forecasts for typhoons Betty of 1987 and Dan of 1989. The results of these studies show that the initial size, intensity and direction and speed of motion show considerable sensitivity to the predicted track. Finally a summary of the track forecast errors through 72 hours are presented for these storms.

A spectral fully compressible nonhydrostatic mesoscale model in hydrostatic sigma coordinates: Formulation and preliminary results

Abstract: The set of fully compressible nonhydrostatic equations governing a broad spectrum of atmospheric motion was transformed fromz coordinates to sigma coordinates under a hydrostatic base state. The hydrostatic base state may be either time-independent, such as a hydrostatic balance with-out motion or with motion such as a thermal wind balance, or time-dependent such as might be obtained from the result of integrating a hydrostatic model. The transformed set of equations can be used to predict and study all scales of at mospheric phenomena.

Turbulent dissipation of cold air lake in a basin

Abstract: Problems of turbulent dissipation of a cold air lake (CAL) and the inversion layer bordering CAL on the upper boundary are presented and studied with the compound model. In wintertime such cold air lakes can persist for days even if rather strong winds are blowing above them. The required conditions for CAL dissipation are removed processes of its formation or maintenance, as well as a sufficiently strong invasion of turbulence in the inversion layer from above down-wards. By this, the inversion layer at first becomes stronger and dissipation is stopped, until the increase of turbulent kinetic energy of the upper flow enables further dissipation. Such turbulent dissipation process is shown by the model for typical conditions and for different initial values of the relevent variables.

A 3-D modelsyystem for simulating thunderstorm microburst outflows

Abstract: A new three-dimensional numerical model system has been designed to study the complex near-surface flow features that arise from collisions between microburst outflow events and other sub-cloud phenomena with complex geometry. The model was designed specifically for implementation on massively parallel computers, and makes use of the high computation rates and large memory sizes of these machines to achieve spatial resolutions of 50 m or less in each dimension. Here we will report on one of the first model applications, a parameter study of colliding microburst outflows. Results from this study indicate that the collision zone between the two downdrafts can be a region of violent and complicated dynamics which can often lead to an elevated region of significant aircraft hazard.

A macrophysical model of the holocene intertropical convergence and jetstream positions and rainfall for the Saharan region

Abstract: Using a macrophysical model, it has been found possible to model the Late Pleistocene and Holocene history of glacial volume and mean temperature as a function of solar radiation seasonality and modulation of the solar radiation by volcanic aerosols (Bryson, 1988). From this model it has been possible to model the Indian monsoon with fair agreement with the field data (Bryson, 1989). The present paper represents an extension of this effort to the modeling of the position of the Intertropical Convergence in North Africa and the latitude of the atmospheric polar jetstream in the same longitudes.

Three-dimensional simulation of the Denver 11 July 1988 microburst-producing storm

Abstract: Near Denver on July 11 1988, a moderate-reflectivity thunderstorm produced a microburst of unusual intensity during the test operation of the Terminal Doppler Weather Radar (TDWR) system. This microburst-producing storm, unlike those previously investigated, contained a complex updraft structure and an intense microburst downstream from the main precipitation shaft. Of special significance is that several successive airliners inadvertently encountered the microburst during final approach to Stapleton Airport. This paper focuses on the numerical investigation of this storm-via simulation with the Terminal Area Simulation System (TASS). Evolution and structure of the storm, including hazard indices based onF-factor, are presented and compared with “observed” data, including that measured by Doppler radar and aircraft flight data recorders.

Propagation of fronts and frontogenesis versus frontolysis over orography

Abstract: Frontal propagation and its evolution over complex terrain features is not well understood. Moreover, one of the major sources of forecast error occurs over the earth's orography. This presentation reviews model studies of frontal interaction with smooth and relatively simple orographic profiles. Emphasis is placed on model predictions, of propagation speed and on the properties of the flow that favor either frontogenesis or frontolysis. The physical processes involved in the front-mountain interaction are isolated, with the aim of providing a basic physical understanding of the interaction process that may be used in the interpretation of more realistic but more complex flow models. Considered are bot homogeneous and density stratified fluid, both two- and three-dimensional orographic features, and both filtered (geostrophic and semigeostrophic) and unfiltered flow models. Potentially important areas of research that may add to knowledge of front-mountain interactions are briefly explored.

Numerical modeling studies of lee mesolows, mesovortices and mesocyclones with application to the formation of Taiwan mesolows

Abstract: Numerical experiments are performed for inviscid flow past an idealized topography to investigate the formation and development of lee mesolows, mesovortices and mesocyclones. For a nonrotating, low-Froude number flow over a bell-shaped moutain, a pair of mesovortices form on the lee slope move downstream and weaken at later times. The advection speed of the lee vortices is found to be about two-thirds of the basic wind velocity, which is due to the existence of a reversed pressure gradient just upstream of the vortices. The lee vortices do not concur with the upstream stagnation point in time, but rather form at a later time. It is found that a pair of lee vortices form for a flow withFr=0.66, but take a longer time to form than in lower-Froude number flows. Since the lee vortices are formed rather progressively, their formation may be explained by the baroclinically-induced vorticity tilting as the mountain waves become more and more nonlinear.

Cold surges along the front range of the Rocky mountains: Development of a classification scheme

Abstract: A classification scheme for cold surges occurring along the Front Range of the Rocky Mountains has been developed using NMC surface and upper air data. A separation is seen in the data between cases with “parent” anticyclones that moved from Canada south into the United States (Northern type) and those which moved from the Eastern Pacific Ocean east into the central US (Western type). The Northern surges are further broken down into three types, A, B and B′, when the degree of southward motion of the anticyclone and the leading edge of the cold air is considered. An anticyclone movement south of 40°N distinguishes type A, while anticyclone tracks remaining north of 40°N characterize types B and B′. The leading edge of the surge (the surge line) associated with the type A anticyclone travels south of 25δN, while the type B anticyclones produce two distinct surge line movements, one remaining north of 25°N and the other moving south of 25°N. It is not possible to break down the Western surges into distinct types. As a rule, the anticyclones associated with Western surges remain between 50°N and 30°N, with the surge line moving south to ∼18°N on average.

Fronts and the Alps: Findings from the Front Experiment 1987

Abstract: The “German Front Experiment 1987” was an experimental effort to determine the influence of the European Alps on cold fronts using data collected by aircraft, radar, surface and upper-air systems between October and December 1987. Measurements were concentrated in the Alpine foreland south of Munich and in the Inn valley in Austria. It was a cooperative effort by groups from Austria, Germany and Switzerland. This study briefly recapitulates the conduct of the experiment and the intense observation periods which were carried through. Then it is outlined to what extent the scientific objectives have been achieved. The findings of the experiments are grouped into the following topics: orographically induced frontogenesis, the interaction of foehn and front, orographically trapped gravity currents, flow into valleys and the modification of precipitation by the Alps.

Nonhydrostatic, mesobeta-scale, real-data simulations with the Penn State University/National Center for Atmospheric Research mesoscale model

Abstract: The Penn State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model is a widely used research tool that has been applied in a wide variety of real-data, mesoalpha-scale applications. Recently a nonhydrostatic version of this model has been developed by Dudhia (1993). It is the purpose of this paper to illustrate the capabilities of this modeling system by describing four examples of mesobeta-scale simulations: two of the cases involve maritime processes and two deal with continental weather events. All utilize fully three-dimensional sets of initial conditions that are based on real data, both standard data and from special measurements programs. One case employs the model in a data-assimilation configuration, wherein Newtonian relaxation terms are used in the equations to assimilate data from a variety of platforms. This example of nonhydrostatic four-dimensional data assimilation (FDDA) is performed for the purpose of generating a dynamically consistent four-dimensional data-set, however the same procedure can be used for model initialization. The first case, described in section 2, involves the simulation of a coastal front that forms offshore near the western edge of the Gulf Stream. In the second case, described in section 3, the model is used in the FDDA mode to define the mesobeta-scale windfield over the complex terrain of the region around Grand Canyon, Arizona. In sections 4 and 5 will be described the mesobeta-scale structure of cold fronts, one within a marine cyclone, and another near the Rocky Mountains.

Use of ECMWF Operational Analyses for Studies of the Tropical Cyclone Environment

Abstract: This study examined ECMWF operational analyses of the outflow layer of two tropical cyclones (Allen, 1980; Elena, 1985) during their passage across the Atlantic and Caribbean. Wind fields and related derived quantities were compared to those from objective analyses of specialized data sets. Errors in center position and storm motion from the ECMWF analyses were also evaluated.

Analysis and numerical modelling of a frontal passage associated with thunderstorm development over the Po valley and the Adriatic sea

Abstract: A case study is presented of the meteorological situation of 26 July 1987, when a moderate lee cyclone formed south of the Alps associated with a cold front moving from the northwest. A convective line developed over the Po valley and the northern Adriatic sea, slightly in advance of the position of the cold front which, over that area, was associated with a north-easterly low-level flow. The mesoscale situation over the Po valley before and after the frontal passage was complex, with different flow configurations and weather phenomena affecting the eastern and western portions of the region. The analysis of observations has been complemented by the results of numerical experiments run with a limited area, high resolution model, initialized using interpolated ECMWF analyses. Sensitivity experiments show the important roles played by the orography and by the condensation-evaporation processes in determining the mesoscale field environment favourable for the development of the organized convection.

Synoptic diagnosis of frontogenetic and cyclogenetic processes

Abstract: In the paper it is shown that theQ-vector defined by Hoskins et al. is a suitable aid for the diagnosis of frontogenetic and cyclogenetic processes. It allows a direct estimation of frontogenetic (or frontolytic) effects within the geostrophic wind field and of the macro-scale forcing of vertical motions. A partitioning of the forcing is possible into one part depending on divergences along the frontal zone and working in short baroclinic waves, and another part connected with frontogenesis (or frontolysis) and causing circulatory motions around the frontal zone. Thus the intercorrelation between both processes can be easily studied. With the described cases it is demonstrated that frontogenetic circulations often lead to the first stages of cyclogenesis but that a further forcing of ascent predominantly ahead of an upper trough is necessary for a deepening of the low. Also during the mature and occlusion stage of cyclogenesis and within the “downstream development” of baroclinic waves frontogenetic (or frontolytic) circulations play an important role in the regime of vertical motions.

Numerical Simulation of a Low-Precipitation Supercell Thunderstorm

Abstract: Numerical simulations of thunderstorms using initial conditions from an Oklahoma storm event on 26 April 1984 have been carried out using the Klemp-Wilhelmson cloud model Two thermodynamic and two horizontal wind profiles are mixed to create four initial storm environments The two initial thermodynamic profiles are designated as moderately and highly unstable Both hodographs have considerable vertical wind shear, with the extremely unstable profile having substantially stronger storm-relative curvature shear in the lowest few kilometers, as measured by the helicity of the environment Storms are initialized with two different temperature-perturbation warm bubbles A variety of storms, qualitatively covering the range observed on this day, are simulated With a strongly sheared, moderate instability environment, the two different temperature-perturbation simulations differ qualitatively, unlike the other pairs of simulations Of particular interest is the simulation carried out with the weaker temperature-perturbation It displays many of the characteristics of observed low-precipitation (LP) supercell storms Without artificially suppressing rainfall, this storm produces very little precipitation over the first 6000 s of its lifetime During this time, there is no strong downdraft at the lowest model level and vorticity at that level is much less than the values aloft We hypothesize that LP storms can be generated by smaller initial impulses than “classic” supercells in environments typically associated with a supercell storm, as speculated by Bluestein and Parks (1983), since they are more susceptible to turbulent entrainment in highly sheared environments

Dynamical aspects of frontal analysis

Abstract: Following the empirical rules of frontal analysis on synoptic scale weather maps, a criterion has been derived, which allows an objective determination of the frontal position and the frontal intensity. It can be shown that the extreme value lines of the crossfrontal second derivative of several atmospheric variables are relevant to resolve the structure of the ageostrophic secondary frontal circulation. This is being shown by the application of quasigeostrophic theory (Q-vector approach) as well as with the approach to ageostrophic motion by Phillips' approximation. With the aid of a case study the application of the method with regard to several atmospheric variables is shown. It is shown that the empirical rules may well be interpreted in terms of the dynamical behaviour of fronts.

A comparative study of the performance of various vertical discretization schemes

Abstract: Most finite-difference numerical weather prediction models employ vertical discretizations that are staggered, and are low-order (usually second-order) approximations for the important terms such as the derivation of the geopotential from the hydrostatic equation, and the calculation of the vertically integrated divergence. In a sigma-coordinate model the latter is used for computing both the surface pressure change and the vertical velocity. All of the above-mentioned variables can diminish the accuracy of the forecast if they are not calculated accurately, and can have an impact on related quantities such as precipitation. In this study various discretization schemes in the vertical are compared both in theory and in practice. Four different vertical grids are tested: one unstaggered and three staggered (including the widely-used “Lorenz” grid). The comparison is carried out by assessing the accuracy of the grids using vertical numerics that range from second-order up to sixth-order. The theoretical part of the study examines how faithfully each vertical grid reproduces the vertical modes of the governing equations linearized with a basic state atmosphere. The performance of the grids is evaluated for 2nd, 4th and 6th-order numerical schemes based on Lagrange polynomials, and for a 6th-ordercompact scheme. Our interpretation of the results of the theoretical study is as follows. The most important result is that the order of accuracy employed in the numerics seems to be more significant than the choice of vertical grid. There are differences between the grids at second-order, but these differences effectively vanish as the order of accuracy increases. The sixth-order schemes all produce very accurate results with the grids performing equally well, and with the compact scheme significantly outperforming the Lagrange scheme. A second major result is that for the number of levels typically used in current operational forecast models, second-order schemes (which are used almost universally) all appear to be relatively poor, for other than the lowest modes. The theoretical claims were confirmed in practice using a large number (100) of forecasts with the Australian Bureau of Meteorology Research Centre's operational model. By comparing “test” model forecasts using the four grids and the different orders of numerics with very high resolution “control” model forecasts, the results of the theoretical study seem to be corroborated.

High-resolution numerical modelling of convective precipitation over Northern Italy

Abstract: A number of forecast experiments were performed in order to assess the capability of reproducing, by means of a limited-area numerical model, the highly structured mesoscale circulations occurring in the Po Valley of Northern Italy during a north-westerly cold front passage across the Alpine chain, with particular attention to the modelling of the effects of organized convection. The case-study occurred during summer 1987 and the model used throughout was the 1989 version of the UB/NMC Limited Area Model (University of Belgrade, National Meteorological Centre of Washington). The model was integrated both with eta, η, and sigma, ϑ, as vertical coordinates and ECMWF initialized analyses were always used as initial conditions. ECMWF initialized analysis or operational forecast fields were also used for updating in time the lateral boundary conditions. Experiments show qualitative and quantitative agreement with observations, both in upper-air geopotential height fields, in MSLP and in cumulated precipitation. Several modelling issues were also investigated, e.g. sensitivity of the results to horizontal and vertical model resolution and to the influence of the lateral boundaries poitioning, finding large effects of the latter on quantitative precipitation fields. Difficulties in modelling very localized mesoscale phenomena, e.g. organized convective thunderstorms in the Po Valley and Alpine North Foehn in the Milan area, were generally encounted.

Rainfall over oceans : remote sensing from satellite microwave radiometers

Abstract: Microwave radiometer brightness temperature (Tb) measurements obtained from satellites over the oceans in dual polarization, at frequencies ranging from 6.6 to 85 GHz, reveal information about the rain and precipitation sized ice. These multifrequency measurements are composited from observations made by the Scanning Multichannel Microwave Radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I). TheTb measurements at 37 GHz, having a field of view (fov) of about 30 km, show relatively strong emissions due to rain, reaching values as large as 260 K over the tropical and mid-latitude rainbelts. Only marginal effects due to scattering by ice above the rain clouds are revealed. At frequencies below 37 GHz, where the fov is much larger than 30 km and the extinction is weaker,Tb is significantly smaller than 260 K. Additional information content about rain, at these low frequencies, is not appreciable. On the other hand, at 85 GHz (fov ≅15 km), where the extinction is very strong, the sea surface below the clouds is often masked and scattering due to ice above the rain clouds is vividly noticed. However, these high frequency measurements do not yield direct information about rain below the clouds.

Orographical Modification and Large Scale Forcing of a Cold Front

Abstract: The development of a cold front influenced by orography and large scale forcing is examined with a two-dimensional meso-scale model. The model is based on the primitive equations and uses the hydrostatic and anelastic approximations. Gradients of the basic flow and temperature field in the third dimension are taken into account during the simulations. Low diffusive numerical schemes and radiation boundary conditions reduce the numerical errors to an acceptable minimum for a two day simulation and avoid reflections at the upper and lateral boundaries. Frontogenetical forcing is included in the simulations by specifying either a vertically sheared or horizontally convergent basic zonal flow field. Model runs with an idealized cold front were carried out over flat terrain and in the presence of a bell shaped mountain ridge. The simulations show a weakening of the cold front on the windward side of the mountain ridge and a strong reintensification on the leeward side relative to the control runs without topography. Analysis of frontogenesis terms demonstrates the importance of convergence in the ageostrophic circulation and of along-front temperature advection for the development of the cold front. The strong intensification of the cold front on the leeward side of the mountain ridge can only partly be explained by superposition with the mountain induced wave. It is mainly caused by ageostrophic deformation forcing in the strong downward flow of this wave. The results also show that the cold front passage over the mountain ridge is not a continuous process. The formation of a new frontal structure on the leeward side of the mountain ridge, well separated from the primary one, is observed while the initial cold front still exists in the upslope region. Generally nonlinear interactions between the mountain wave and the cold front are the important mechanisms to explain these phenomena.

On the appearance of a cold front with a narrow rainband in the vicinity of the Alps

Abstract: First intensive observations of a narrow cold frontal rainband in southern Germany are described. The observations by Doppler radar and surface observations are in good agreement with others made in Great Britain and at the west coast of the United States. There are no principal differences between the observations close to the sea and those during the passage of the front across the Alpine foreland. The observed coexistence of short and elongated precipitation cells is explained by the local modification of convergence and vorticity. When approaching the Alps the precipitation patterns increased in size due to the upslope winds, and the pre-frontal low-level jet is reorganized completely in a manner such that the cross-frontal circulation was enhanced while the pre-frontal low-level jet disappeared.

Meso-beta scale numerical simulations of terrain drag-induced along-stream circulations Part I: Midtropospheric frontogenesis

Abstract: The problem of along-stream ageostrophic frontogenesis is studied by employing a numerical model at meso-alpha and meso-beta scales in simulations of the downstream circulations over the Front Range of the Rocky Mountains. Three-dimensional real data simulations at these two scales of motion are used to diagnose the transition from semigeostrophic cross-stream frontogenesis accompanying a propagating baroclinic upper-level jet streak to midtropospheric along-stream ageostrophic frontogenesis. This along-stream ageostrophic frontogenesis results from the perturbation of the jet streak by the Rocky Mountain range. The case study represents an example of internal wave dynamics which are forced by the drag of the Rocky Mountains on a strong jet streak in the presence of a low-level inversion.