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  • Author or Editor: MICHAEL L. KAPLAN x
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Jennifer M. Cram and Michael L. Kaplan

Abstract

A variational method has been developed to assimilate VAS temperature and moisture gradient information into a mesoscale model. A series of experiments were conducted to test the sensitivity of both adiabatic and diabatic versions of the model to VAS data assimilations for the 20–21 July 1981 case.

The VAS data for this case are compared to the rawinsonde data and VAS moisture imagery. The retrieved VAS temperature fields captured the asynoptic development of strong mesoscale temperature gradients although the VAS relative humidity fields were generally too smooth.

The synoptic-scale effects of the assimilation of VAS data were negligible. The greatest impact was on the mesoscale forecasts of the patterns of convective instability. The assimilation of the strong VAS temperature gradients resulted in the short-term forecast of greater convective instabilities across Oklahoma, where observed convection subsequently developed. The additional assimilation of relative humidity gradients did not significantly change the patterns of the forecast instabilities. Increasing the number of successive assimilations improved the subsequent forecasts of convective instability.

For this case, the greatest improvements from assimilation resulted from the resolution of the strong mesoscale temperature gradients by the asynoptic VAS data. The assimilation of this structure into the model resulted in forecasts of convective instability and precipitation more closely resembling the patterns of the observed convection.

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John W. Zack and Michael L. Kaplan

Abstract

A series of mesoscale numerical simulations of the AVE-SESAME I case (10 April 1979) were performed in order to analyze the dynamical processes that result in the production of an environment favorable for the development of severe local convective storms. The investigation focused on the relative contributions of quasi-adiabatic inertial and isallobaric adjustments attributable to the geometry of the tropospheric flow and the fluxes of heat, moisture and momentum from the surface of the earth.

The model simulations support many of the conclusions deduced by Kocin et al. in their analyses of the observations taken during the field experiment. The quasi-adiabatic simulations support the existence of a coupled upper-tropospheric and lower-tropospheric jet streak system. However, the dynamical coupling is more complex than the straight line jet streak model utilized by Uccellini and Johnson. The departures are attributable to two sources. First, there is a time-varying curvature in the exit region due to the propagation of a meso-αscale trough through the area while a longer wave trough remains relatively stationary. Second, the exit region experiences significant changes in the mass field due to the presence of differential horizontal thermal advection. These two effects produce significant alterations to the classical exit region patterns of vertical motion and man divergence. In addition, them processes phase with a pattern of significant horizontal variations in the fluxes of heat, moisture and momentum in the planetary boundary layer. The combination of these processes result in the amplification of the low-level pressure tendencies and an increase in the strength of the low-level jet streak.

The combination of mass-momentum adjustments associated with the jet streak system and low-level flux gradients results in the creation of significant amounts of buoyant energy and the vertical motion necessary for its release. The simulation experiments suggest that the 6 h increase in buoyant energy over the areas that subsequently experience convection is approximately half the result of the quasi-adiabatic processes and half the result of the surface fluxes of heat and moisture.

This study has three major contributions. First, it indicates the possible importance of the phasing of deep tropospheric mass-momentum adjustments with differential surface fluxes of heat and momentum. Second, it extends the understanding of jet-streak exit region dynamics to the case of cyclonically curved flow in the presence of differential horizontal thermal advection. Third, it reveals the rapidity with which circulation patterns associated with a jet streak exit region can change.

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JAMES E. JIUSTO and MICHAEL L. KAPLAN

Abstract

Three yr of winter lake-storm data were analyzed to determine snowfall distribution patterns downwind of Lake Erie and Lake Ontario. The total amount of snowfall and the area of ground cover in each of 23 lake-effect storms were determined for both lakes. Total snowfall mass was highly dependent on time of year; November and early December storms were two to five times more productive than January storms. A considerable variation in snow density (snowfall depth to melt water ratio) could be attributed mainly to differences in snow crystal type.

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Sen Chiao, Yuh-Lang Lin, and Michael L. Kaplan

Abstract

This paper investigates the local circulation associated with a heavy orographic rainfall event during 19–21 September 1999 [Mesoscale Alpine Programme Intensive Observing Period 2B (MAP IOP-2B)]. This event was simulated with a 5-km horizontal grid spacing using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The MM5 simulation reproduced the basic features such as the timing and location of the deep trough and the associated precipitation evolution, though the total amount of precipitation is slightly higher than that measured by rain gauges (∼30% in 24 h). The near-surface flow was dominated by an easterly jet originally from the Adriatic Sea and a southerly jet from the Gulf of Genoa. A significant westward turning occurred when the southerly flow approached the south side of the Alps. This deflection was caused by boundary layer friction and rotation, as well as mountain blocking effects. Flow was generally from the south above the surface. Precipitation was mainly concentrated on the windward slopes, especially near the Lago Maggiore region. Sensitivity experiments have been conducted to investigate the effects of upstream orography, the western flank of the Alps, boundary layer friction, and horizontal resolution. The results indicate that precipitation distribution and amount over the southern upslope region of the Alps were not directly related to either the coastal Apennine Mountains or the west flank of the Alps. The boundary layer friction reduces the total amount and alters the distribution of rainfall by weakening the wind near the surface. The 1.67-km horizontal grid spacing simulation indicates that heavy rainfall tended to be concentrated in the vicinity of individual mountain peaks. The total amount of rainfall was overpredicted along the windward slopes because of the strong upward motion that occurred on the upslope of the barrier. The results indicate the importance of dynamical forcing associated with upslope-induced and near-surface horizontal velocity convergence-induced vertical motion, which increases rapidly as horizontal resolution increases.

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

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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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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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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Michael L. Kaplan, John W. Zack, Vince C. Wong, and Glen D. Coats

Abstract

Mesoscale model simulations with and without diurnal planetary boundary layer heat flux are compared to a detailed surface analysis for a case of an isolated tornadic convective complex development. The case study, 3-4 June 1980, is of particular interest because of the development of several destructive tornadic storms within the Grand Island, Nebraska metropolitan area during a period of relatively weak synoptic scale forcing. This type of case presents an opportunity for the mesoscale numerical simulation of the subtle interactions between an upper tropospheric jet stream and surface diabatic heating. Model simulations runwith and without diurnal surface sensible heating show marked differences in processes both within and above the planetary boundary layer (PBL). The results of the simulations indicate that the evolution of the subsynoptic scale low pressure system and its accompanying low level jet streak, areas of moisture convergence, and regions of convective instability are influenced by the interaction of the deep surface-heated PBL with a weak synoptic scale jet streak. The model simulations show that the distribution and evolution of tropospheric velocity divergence cannot be realistically decoupled from the thickness changes caused by PBL heating in this case of relatively weak dynamic forcing. Modifications in the simulated velocity divergence and low level warm advection caused by PBL heating led to a more realistic pattern of pressure falls, low level jet formation, and a significant reduction of the lifted index values near the region of observed convection. Comparisons with observations, however, also indicate that the modeling system still requires: I) enhanced soil moisture information in the data base utilized for its PBL parameterization to achieve the proper amplitude and distribution of surface sensible heat flux and 2) the proper parameterization of convective scale processes such as latent heating to completely capture the evolution of the subsynoptic scale low pressure system into a mesoscale low pressure system. The most significant implication of these modeling results is that previous dynamical models of upper and lower tropospheric coupling during the pre-stormenvironment should include consideration of the effects of diurnal surface sensible heating upon a pre-existing jet streak.

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