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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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Jennifer M. Cram and Roger A. Pielke

Abstract

The geostrophic stream and potential functions of Sangster and the derived flat pressure field of Pielke and Cram are further compared. A simple numerical experiment with an idealized mountain-atmosphere system compares the two methods and their sensitivity to lateral boundary conditions. The flat pressure field of Pielke and Cram is essentially equivalent to the streamfunction of Sangster, although the two methods have different boundary condition formulations and resulting different sensitivities to these boundary conditions.

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Roger A. Pielke and Jennifer M. Cram

Abstract

A methodology to analyze a flat surface geostrophic wind and pressure is presented which eliminates the arbitrariness of the standard reduction of surface pressure to mean sea level. The procedure utilizes a surface geostrophic wind defined in terms of a terrain-following coordinate system to derive a flat ground surface pressure field which is consistent in concept (i.e., nondivergent except for the f variation with latitude) with the currently applied MSL analyses. With this approach, interpretation of synoptic weather patterns in areas of elevated complex terrain will be more accurate.

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Jennifer M. Cram, Roger A. Pielke, and William R. Cotton

Abstract

An observational and numerical study of the squall line that occurred on 17–18 June 1978 is presented. This squall line was initially triggered by the strong surface convergence along a cold front and stretched from Illinois to the Texas Panhandle. The squall line was aligned with the surface front during its initial development (at 0000 UTC 18 June 1978), but then propagated faster than the front, resulting in a separation of approximately 200 km by 0300 UTC and 300–400 km by 0600 UTC. The Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS) is used to model the squall-line development and propagation. Results are described from several experiments that tested the sensitivity to the use of the Kuo-type cumulus parameterization scheme and grid-scale microphysical processes. The simulations that included the cumulus parameterization scheme accurately modeled the initial development of the squall line and its subsequent movement away from the front.

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Jennifer M. Cram, Roger A. Pielke, and William R. Cotton

Abstract

A numerical study of the squall line that occurred on 17–18 June 1978 was described in Part I of this paper. The squall line was collocated with a surface front during its initial development (at 0000 UTC 18 June 1978), but then propagated faster than the front, resulting in a separation of approximately 200 km by 0300 UTC and 300–400 km by 0600 UTC. In this paper (Part II), the movement of the squall line in the model is shown to be due to the propagation of a deep tropospheric internal gravity wave in a wave–CISK-like (Conditional Instability of the Second Kind) process. The thermal and dynamic perturbations associated with the hypothesized wave are shown to be consistent with internal gravity wave theory, and the characteristics of the wave are compared to similar results from other wave-CISK studies. The current literature favors the mechanism of gust front convergence to explain squall-line propagation, although there are other modeling studies that show specific instances of squall-line propagation as being due to internal gravity waves. It is suggested that a spectrum of scales of forcing may exist and be responsible for squall-line propagation, but many models and observations may be able to detect only the gust-front-type processes. The 17–18 June 1978 squall line probably did not propagate solely as the result of any one mechanism, but instead as the product of several active mechanisms. The dominant mechanism in these modeling simulations was an internal gravity wave, and it seems reasonable that the gravity wave was at least one of the mechanisms responsible for the actual propagation of the 17–18 June 1978 squall line.

An unsuccessful attempt to model the squall line with a 5-km grid spacing and without a cumulus parameterization is also discussed. Briefly, the squall line did not develop properly on that scale and did not separate from the front.

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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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