Mesoscale analysis by numerical modeling coupled with satellite-based sounding
Date
1988-11
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Abstract
This dissertation deals with the development of a system for time-continuous mesoscale analysis and its use in studying the mesoscale distribution of summertime convective cloud development in the Northeastern Colorado region. There were two basic components of the system — a version of the CSU Regional Atmospheric Modeling System (RAMS) and an algorithm for retrieving temperatures and water vapor concentrations from VISSR Atmospheric Sounder (VAS) data. The system was designed to avoid some of the problems that researchers have encountered when satellite-retrieved parameters have been input to models. The primary distinguishing feature of the new method is that there is an intimate coupling of the retrieval and modeling processes. Water vapor concentrations and ground surface temperatures were the foci of the analyses. In preparation for analysis experiments we tested the sensitivity of a two-dimensional version of the model to various controls on the behavior of water vapor concentrations and surface temperatures. For water vapor mixing ratios, variations that might be caused by analysis errors had very little impact on the dynamics of circulations in the pre-convective stage. In contrast, ground surface temperature variations were shown to have a large impact on circulations, so analysis errors are very relevant to pre-convective dynamics. The first comparisons of the coupled analysis method with other, related, methods was by means of two-dimensional simulations. Analyses in which surface temperatures were derived from satellite-retrievals were compared with the alternative of relying on energy balance computations. The energy balance computations were so sensitive to soil characteristics, which were simulated as unknown, that the satellite retrieval method gave better results even with cloud contamination. In water vapor analysis comparisons no single method was superior in every respect, but the coupled method performed relatively well. Vertical gradients and horizontal gradients were well represented, and the method was relatively insensitive to a common problem in pre-convective analysis — contamination of satellite data by increasing amounts of small convective clouds. Analysis methods were further compared in a three-dimensional case study for 21 August 1983. The horizontal and time variations of satellite-retrieved surface temperatures closely corresponded to the conventional shelter temperature observations, but had much greater detail. In contrast, the energy balance-based temperatures tended to increase too quickly during the morning and lacked some of the observed gradients. According to the retrievals, there can be very large mesoscale gradients in temperatures at the ground surface even on the relatively flat plains. In the case study water vapor analyses there were substantial differences among the results of the several methods that were intercompared. The study demonstrated that, when the first set of satellite data is less reliable than the later sets, some of the contamination lingers throughout the time-continuous coupled analysis results. However, the coupled method generally appeared to be the most valuable of the methods considered in this study because it exploited the major strengths of the numerical model and the satellite data while making it relatively easy to recognize any impacts of their weaknesses. The results of this dissertation support the hypothesis that both ground surface temperatures and terrain variations can play important roles in pre-convective water vapor kinematics through their influences on vertical and horizontal winds. The development of convective clouds corresponded largely, but not exclusively, with convergence and deepening of low-level water vapor. The analysis system proved to be valuable for forecasting through the close correspondence between derived stability indices and later convective development. The new method is a step in the expanding capability of meteorologists to combine tools and sources of data for understanding and forecasting mesoscale phenomena.
Description
November 1988.
Principal investigators: Thomas H. Vonder Haar, James F.W. Purdom.
Principal investigators: Thomas H. Vonder Haar, James F.W. Purdom.
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Subject
Convection (Meteorology) -- Mathematical models
Convective clouds -- Mathematical models
Atmospheric temperature
Water vapor, Atmospheric