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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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John Manobianco, John W. Zack, and Gregory E. Taylor

This paper describes the capabilities and operational utility of a version of the Mesoscale Atmospheric Simulation System (MASS) that has been developed to support operational weather forecasting at the Kennedy Space Center (KSC) and Cape Canaveral Air Station (CCAS). The implementation of local, mesoscale modeling systems at KSC/CCAS is designed to provide detailed short-range (< 24 h) forecasts of winds, clouds, and hazardous weather such as thunderstorms. Short-range forecasting is a challenge for daily operations, and manned and unmanned launches since KSC/CCAS is located in central Florida where the weather during the warm season is dominated by mesoscale circulations like the sea breeze.

For this application, MASS has been modified to run on a Stardent 3000 workstation. Workstation-based, real-time numerical modeling requires a compromise between the requirement to run the system fast enough so that the output can be used before expiration balanced against the desire to improve the simulations by increasing resolution and using more detailed physical parameterizations. It is now feasible to run high-resolution mesoscale models such as MASS on local workstations to provide timely forecasts at a fraction of the cost required to run these models on mainframe supercomputers.

MASS has been running in the Applied Meteorology Unit (AMU) at KSC/CCAS since January 1994 for the purpose of system evaluation. In March 1995, the AMU began sending real-time MASS output to the forecasters and meteorologists at CCAS, Spaceflight Meteorology Group (Johnson Space Center, Houston, Texas), and the National Weather Service (Melbourne, Florida). However, MASS is not yet an operational system. The final decision whether to transition MASS for operational use will depend on a combination of forecaster feedback, the AMU's final evaluation results, and the life-cycle costs of the operational system.

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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, 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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Michael L. Kaplan, John W. Zack, Vince C. Wong, and James J. Tuccillo

Abstract

The development of a comprehensive mesoscale atmospheric simulation system (MASS) is described in detail. The modeling system is designed for both research and real-time forecast applications. The 14-level numerical model, which has a 48 km grid mesh, can be run over most of North America and the adjacent oceanic regions. The model employs sixth-order accurate numerics, generalized similarity theory boundary-layer physics, a sophisticated cumulus parameterization scheme, and state of the art analysis and initialization techniques. Examples of model output on the synoptic and subsynoptic scales are presented for the AVE-SESAME I field experiment on 10–11 April 1979. The model output is subjectively compared to the observational analysis and the LFM II output on the synoptic scale. Subsynoptic model output is compared to analyses generated from the AVE-SESAME I data set.

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David P. Bacon, Nash’at N. Ahmad, Zafer Boybeyi, Thomas J. Dunn, Mary S. Hall, Pius C. S. Lee, R. Ananthakrishna Sarma, Mark D. Turner, Kenneth T. Waight III, Steve H. Young, and John W. Zack

Abstract

The Operational Multiscale Environment Model with Grid Adaptivity (OMEGA) and its embedded Atmospheric Dispersion Model is a new atmospheric simulation system for real-time hazard prediction, conceived out of a need to advance the state of the art in numerical weather prediction in order to improve the capability to predict the transport and diffusion of hazardous releases. OMEGA is based upon an unstructured grid that makes possible a continuously varying horizontal grid resolution ranging from 100 km down to 1 km and a vertical resolution from a few tens of meters in the boundary layer to 1 km in the free atmosphere. OMEGA is also naturally scale spanning because its unstructured grid permits the addition of grid elements at any point in space and time. In particular, unstructured grid cells in the horizontal dimension can increase local resolution to better capture topography or the important physical features of the atmospheric circulation and cloud dynamics. This means that OMEGA can readily adapt its grid to stationary surface or terrain features, or to dynamic features in the evolving weather pattern. While adaptive numerical techniques have yet to be extensively applied in atmospheric models, the OMEGA model is the first model to exploit the adaptive nature of an unstructured gridding technique for atmospheric simulation and hence real-time hazard prediction. The purpose of this paper is to provide a detailed description of the OMEGA model, the OMEGA system, and a detailed comparison of OMEGA forecast results with data.

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