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Steven E. Koch, Fuqing Zhang, Michael L. Kaplan, Yuh-Lang Lin, Ronald Weglarz, and C. Michael Trexler

Abstract

Mesoscale model simulations have been performed of the second episode of gravity waves observed in great detail in previous studies on 11–12 July 1981 during the Cooperative Convective Precipitation Experiment. The dominant wave simulated by the model was mechanically forced by the strong updraft associated with a mountain–plains solenoid (MPS). As this updraft impinged upon a stratified shear layer above the deep, well-mixed boundary layer that developed due to strong sensible heating over the Absaroka Mountains, the gravity wave was created. This wave rapidly weakened as it propagated eastward. However, explosive convection developed directly over the remnant gravity wave as an eastward-propagating density current produced by a rainband generated within the MPS leeside convergence zone merged with a westward-propagating density current in eastern Montana. The greatly strengthened cool pool resulting from this new convection then generated a bore wave that appeared to be continuous with the movement of the incipient gravity wave as it propagated across Montana and the Dakotas.

The nonlinear balance equation and Rossby number were computed to explore the role of geostrophic adjustment in the forecast gravity wave generation, as suggested in previous studies of this wave event. These fields did indicate flow imbalance, but this was merely the manifestation of the MPS-forced gravity wave. Thus, the imbalance indicator fields provided no lead time for predicting wave occurrence.

Several sensitivity tests were performed to study the role of diabatic processes and topography in the initiation of the flow imbalance and the propagating gravity waves. When diabatic effects owing to precipitation were prevented, a strong gravity wave still was generated in the upper troposphere within the region of imbalance over the mountains. However, it did not have a significant impact because moist convection was necessary to maintain wave energy in the absence of an efficient wave duct. No gravity waves were present in either a simulation that disallowed surface sensible heating, or the “flat terrain” simulation, because the requisite MPS forcing could not occur.

This study highlights difficulties encountered in attempting to model the generation of observed gravity waves over complex terrain in the presence of strong diabatic effects. The complex interactions that occurred between the sensible heating over complex terrain, the incipient gravity wave, and convection highlight the need for much more detailed observations between wave generation regions over mountains and the plains downstream of such regions.

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Steven Businger, Michael E. Adams, Steven E. Koch, and Michael L. Kaplan
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Steven Businger, Michael E. Adams, Steven E. Koch, and Michael L. Kaplan

Abstract

Mesoscale height and temperature fields can be extracted from the observed wind field by making use of the full divergence equation. Mass changes associated with irrotational ageostrophic motions are retained for a nearly complete description of the height field. Above the boundary layer, in the absence of friction, the divergence equation includes terms composed of the components of the wind and a Laplacian of the geopotential height field. Once the mass field is determined, the thermal structure is obtained through application of the hypsometric equation.

In this paper an error analysis of this divergence method is undertaken to estimate the potential magnitude of errors associated with random errors in the wind data. Previous applications of the divergence method have been refined in the following ways. (i) The domain over which the method is applied is expanded to encompass the entire STORM-FEST domain. (ii) Wind data from 23 profiler and 38 rawinsonde sites are combined in the analysis. (iii) Observed profiler and rawinsonde data are interpolated to grid points through a modified objective analysis, and (iv) the variation in elevation of the profiler sites is taken into account.

The results of the application of the divergence method to the combined wind data from profiler and rawinsonde sites show good agreement between the retrieved heights and temperatures and the observed values at rawinsonde sites. Standard deviations of the difference between the retrieved and observed data lie well within the precision of the rawinsonde instruments. The difference field shows features whose magnitude is significantly larger than the errors predicted by the error analysis, and these features are systematic rather than random in nature, suggesting that the retrieved fields are able to resolve mesoscale signatures not fully captured by the rawinsonde data alone.

The divergence method is also applied solely to the profiler data to demonstrate the potential of the divergence method to provide mass and thermal fields on a routine basis at synoptic times when operational rawinsonde data are not available. A comparison of the heights derived from the profiler winds with those independently measured by rawinsondes indicates that valuable information on the evolution of atmospheric height and temperature fields can be retrieved between conventional rawinsonde release times through application of the divergence method. The implications of the results for applications of the method in weather analysis and in numerical weather prediction are discussed.

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Michael L. Kaplan, Yuh-Lang Lin, David W. Hamilton, and Robert A. Rozumalski

Abstract

Meso-beta-scale numerical model simulations and observational data are synthesized in an effort to develop a multistage paradigm for use in forecasting tornadic convection in the southeastern United States. The case study to be utilized as an example of the multistage sequence of events is the Palm Sunday 1994 outbreak, which culminates with the development of an unbalanced mesoscale jet streak or jetlet that focuses a given region for significant values of low-level vertical wind shear, low-level confluence and vertical vorticity, midtropospheric cooling, and storm-relative helicity. The five-stage paradigm includes 1) the existence of a jet exit region accompanying a deep balanced thermally indirect circulation south of the outbreak and a return branch ageostrophic low-level southerly jet, both typically accompanying the subtropical jet stream and the leading edge of hot continental air; 2) the existence of a jet entrance region accompanying a deep balanced thermally direct circulation north of the outbreak and a return branch ageostrophic low-level northerly jet, both typically accompanying the polar jet stream and the leading edge of rain-cooled air; 3) the geostrophic adjustment of the wind in the southern jet to the emerging/intensifying mass field perturbation, that is, intensification of the cross-stream mesoscale pressure gradient force, caused by the juxtaposition of the rain-cooled air southeast of the polar front and hot air accompanying the continental front where evaporational cooling as well as surface heating merge resulting in unbalanced jetlet formation; 4) the low-level mass adjustment underneath the new mesoscale midtropospheric unbalanced jetlet induces a return branch low-level unbalanced jetlet as well as vertical motion patterns oriented along the stream; and 5) adiabatic cooling ahead of the unbalanced jetlet and sinking behind it accompanying the along-stream vertical circulation increases the intensity of downstream destabilization and upstream downward momentum fluxes, producing a favorable environment for severe convection.

This new synoptic/dynamical overview assigns added importance to the subtropical jet, defines the continental air front, and also defines the unbalanced jetlet for use in predicting the presevere storm environment.

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Jennifer M. Cram, Michael L. Kaplan, Craig A. Mattocks, and John W. Zack

Abstract

Conventional synoptic rawinsonde data do not have a fine enough temporal or spatial resolution to accurately resolve mesoscale features. Profiling networks are one potential source of these data although they provide only wind information. A methodology following Fankhauser and Kuo and Anthes is used to retrieve height and temperature analyses from actual profiler wind data using the full divergence equation. Simulation experiments were fist completed to test the feasibility of using the available profiler network spacing to define mesoscale atmospheric structure and to test the boundary conditions used in the retrieval process. Real profiler and rawinsonde data were then used to retrieve height analyses. The real-data results are compared to independent microbarograph surface pressure data and radiometer height data. Retrieved heights on 13 April 1986 from the four-node Colorado profiler network revealed the presence of a mesoscale trough that was not resolvable by the standard rawinsonde network, but was corroborated by PROFS mesonet data and Denver radiometer data. This study differs from previous work in that actual profiler data were used in the height retrievals, and the retrieved heights were verified against independent asynoptic data.

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Michael L. Kaplan, Steven E. Koch, Yuh-Lang Lin, Ronald P. Weglarz, and Robert A. Rozumalski

Abstract

Mesoscale model simulations are performed in order to provide insight into the complex role of jet streak adjustments in establishing an environment favorable to the generation of gravity waves on 11–12 July 1981. This wave event was observed in unprecedented detail downstream of the Rocky Mountains in Montana during the Cooperative Convective Precipitation Experiment. The high-resolution model simulations employ a variety of terrain treatments in the absence of the complicating effects of precipitation physics in order to examine the complex interactions between orography and adiabatic geostrophic adjustment processes.

Results indicate that prior to gravity wave formation, a four-stage geostrophic adjustment process modified the structure of the mid- to upper-tropospheric jet streak by creating secondary mesoscale jet streaks (jetlets) to the southeast of the polar jet streak in proximity to the gravity wave generation region (WGR). During stage I, a strong rightward-directed ageostrophic flow in the right exit region of the polar jet streak (J1) developed over west-central Montana. This thermally indirect transverse secondary circulation resulted from inertial-advective adjustments wherein momentum was transported downstream and to the right of J1 as air parcels decelerated through the exit region.

During stage II, a highly unbalanced jetlet (J2) formed just northwest of the WGR in response to the inertial-advective forcing accompanying the ageostrophic circulation associated with J1. The mass field adjusted to this ageostrophic wind field. An adiabatic cooling and warming dipole resulting from this thermally indirect secondary circulation was the cause for frontogenesis and a rightward shift in the midtropospheric pressure gradients. Since this secondary circulation associated with J2 occurred above a dramatic vertical variation in the thermal wind, the vertical transport of potentially colder air from below was larger ahead of and to the right of J1, thus shifting the new jetlet (J2) well away from J1 as the mass field adjusted to the new wind field.

Stage III was established when the new mass field, which developed in association with J2 during stage II, set up a dynamically unbalanced circulation oriented primarily across the stream, and directly over the WGR. This new leftward-directed ageostrophic cross-stream flow (A) formed between jetlet J2 and the original exit region of the polar jet streak J1.

Finally, a midlevel mesoscale jetlet (J3) is simulated to have developed in stage IV over the WGR in response to the integrated mass flux divergence associated with both the stage II and III adjustment processes. This lower-level return branch circulation to jetlet J2 was further enhanced by velocity divergence accompanying the localized cross-stream ageostrophic wind maximum (A), which develops during stage III. The entire multistage geostrophic adjustment process required about 12 h to complete over a region encompassing approximately 400 km × 400 km.

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V. Mohan Karyampudi, Michael L. Kaplan, Steven E. Koch, and Robert J. Zamora

Abstract

In this first of a two paper series, a sequence of dynamical processes involving the evolution of a mesoscale Ice cyclone and its subsequent interaction with a mesoscale tropopause fold downstream of the Rocky Mountains is investigated. These scale-interactive phenomena, which resulted from the jet streak interaction with the topography, were examined in detail using the observational data obtained from the Program for Regional Observing and Forecasting Services' mesonetwork and wind profilers, as well as conventional surface and rawin-sonde data and Total Ozone Mapping Spectrometer satellite data over the Colorado region for the severe weather event that occurred during 13–14 April 1986.

Large-scale analysis indicated that as a baroclinic low pressure system approached the Rockies with its attendant upper-level jet streak, a typical prestorm environment over western Kansas formed in the early morning hours of 13 April. Hourly mesonet data analysis revealed the formation and eastward progression of a mesoscale Ice cyclone with a trailing wind-shift line identified as an internal bore initiated by a cold front (i.e., a prefrontal bore) in Part II. Analysis of winds and divergence including diagnostically derived temperature and height fields from Colorado wind profilers indicated that as the jet streak momentum propagated into a Acre stable region in the midtroposphere created by low-level adiabatic warming and midlevel cooling on the leeside of the Rockies, unbalanced flow conditions resulted at scales less than the Rossby radius of deformation. AS a consequence of geostrophic adjustment processes, mesoscale tropopause folding and upper-level frontogenesis occurred over the profiler network. Unbalanced upper-level frontogenesis resulted from the tilting of the isentropes by along-stream ageostrophic indirect circulations comprised of horizontal vertical velocity gradients across the tropopause fold. As the mesoscale tropopause fold extruded downwards to midlevels in association with the descending secondary upper-level jet streak forced by the geostrophic adjustment process, Ice cyclogenesis occurred due to the phasing of the upper-level front with the low-level Ice cyclone.

Synthesis of the mesonetwork and profiler observations suggest that high momentum in the midtroposphere associated with the descending branch of the jet stream just ahead of the prefrontal bore but behind the dryline. This surge of southwesterly momentum at the surface, largely responsible for blowing dust, was mostly ageostrophic and contributed to an increase in surface vorticity and moisture convergence as well as frontogenesis around the lee cyclone. A mesoscale conceptual model is proposed in order to explain the dynamical sequence of events involving lee cyclogenesis, dust stroms, and a tropopause fold that led to the severe weather environment over the Great Plains. In the companion paper (Part II), observational evidence of an internal bore occurring ahead of a cold front and comparisons with simple numerical model results are presented in order to understand the initiation and propagation of the prefrontal bore and its influence in triggering a squall line father downstream.

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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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David W. Hamilton, Yuh-Lang Lin, Ronald P. Weglarz, and Michael L. Kaplan

Abstract

The three-dimensional responses of simple stably stratified barotropic and baroclinic flows to prescribed diabatic forcing are investigated using a dry, hydrostatic, primitive equation numerical model (the North Carolina State University Geophysical Fluid Dynamics Model). A time-dependent diabatic forcing is utilized to isolate the effects of latent heat release in a midlatitude convective system. Examination of the mass-momentum adjustments to the diabatic forcing is performed with a focus on the development of an isolated midlevel wind maximum. The results of both cases suggest the formation of a midlevel wind maximum in the form of a perturbation meso-β-scale cyclone, which later propagates downstream as the heating is decreased. The scale of the perturbation cyclone remains at a sub-Rossby radius of deformation length scale. Therefore, the mass perturbations adjust to the wind perturbations as the mesocyclone propagates downstream. Transverse vertical circulations, which favor ascent on the right flank of the wind maximum, appear to be attributed to compensatory gravity wave motions, initially triggered by the thermal forcing, which laterally disperses as the heating is reduced.

The simple model simulations are used to explain more complex results from a previous mesoscale modeling study (the Mesoscale Atmospheric Simulation System, MASS), in which it was hypothesized that an upstream mesoscale convective complex triggered a midlevel jetlet through geostrophic adjustment of the wind to the latent heat source. The MASS simulated jetlet attained a transverse vertical circulation that favored ascent on the right flank of the midlevel jetlet. The jetlet and accompanying transverse vertical circulations later propagated downstream aiding in the formation of the 27–28 March 1994 tornadic environment in Alabama and Georgia.

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Tracy M. Backes, Michael L. Kaplan, Rina Schumer, and John F. Mejia

Abstract

This study presents the climatology of the vertical structure of water vapor flux above the Sierra Nevada during significant cool season (November–April) precipitation events. Atmospheric river (AR) and non-AR events are analyzed to better understand the effect of this structure on precipitation patterns. Daily measurements of cool season precipitation at seven weather stations around the Tahoe basin from 1974 to 2012 and NCEP/CPC gridded daily precipitation analysis along the Sierra crest for the period 1948–2012 are examined. NCEP–NCAR reanalysis and soundings from Oakland are used to look at upper atmospheric conditions, including the presence of vapor transport by low- and midlevel jets on storm days as well as upstream static stability in relation to significant precipitation events. Key findings are as follows: 1) ARs play a disproportionately large role in generating Tahoe basin precipitation during the cool season; 2) strong midlevel vapor transport needs to occur in tandem with low-level transport to achieve the most extreme 2-day precipitation in the Tahoe basin; 3) when low- to midlevel vapor transport is present on days with a defined AR, the local maximum in 2-day precipitation intensity decreases with distance from the Sierra crest, and on non-AR days, the relative increase in 2-day precipitation intensity due to low- and midlevel vapor transport does not vary based on distance from the Sierra crest; 4) AR and non-AR moisture fluxes are significantly modified by upstream static stability; and 5) understanding the impacts of ARs and their lower- and midlevel moisture flux structure are crucial components of the hydrometeorology in this region.

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