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Regional-scale flows in complex terrain: an observational and numerical investigation




Bossert, James E., author

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An observational program has been conducted to obtain information concerning thermally driven flows in complex terrain on meso-(3 to meso-a scales (100 - 500 km). Data were collected from remote surface observing systems at exposed mountaintop locations throughout the state of Colorado, over the summers of 1984-1988. These field experiments have been called the Rocky Mountain Peaks Experiments (ROMPEX). The observations from ROMPEX have been supplemented with data from other remote surface networks, special soundings, upper-air observations, and radar and lightning strike information to provide an adequate description of the flows and weather of interest. The observations have shown the development of a recurrent "regional-scale" circulation system across the Colorado mountain barrier, operating on a diurnal time scale. The basic structure of the flow system consists of a daytime inflow phase toward the mountains along the Continental Divide, and a nocturnal outflow away from this high terrain. Long-term averages show this circulation system to be the dominant wind pattern at several high altitude stations, revealing its climatological significance. Attention has been focused upon the nocturnal phase of the circulation system along the western slope of the mountain barrier. Here, the winds are particularly strong and from a southeasterly direction, which is generally counter to the upper-level winds, and onset abruptly in early evening with steady flow thereafter. Soundings have shown this nocturnal current to be shallow and within a distinct stable air mass. Convective storms are found to enhance this southeasterly flow regime. Numerical simulations have been performed with the Colorado State University Regional Atmospheric Modelling System (CSU-RAMS) to provide further insight into the physical mechanisms forcing the observed regional-scale circulation system. The model simulations include both idealized two- and three-dimensional experiments, as well as a three-dimensional case study experiment using actual data for the initialization. The three-dimensional simulations use two-way interactive grid nesting and a realistic representation of topography over the region of interest. The idealized three-dimensional experiment showed that thermal forcing over realistic topography in conditions of negligible, or weak ambient flow, is capable of producing many of the flow features observed throughout the diurnal cycle. his experiment further showed how the deep mountain-plains solenoid along and above the Front Range crest evolves in late afternoon into a shallower density current, which then propagates westward over the mountains of the western slope. This unexpected flow phenomenon is the primary process responsible for the strong nocturnal southeasterly winds found in observations. Sensitivity experiments show that the particular terrain configuration through an east-west cross-section of the Colorado mountains is important to the generation of this unusual circulation. The strong thermal gradient produced by differential heating of the topography is the primary driving force in the density current evolution. Coriolis influence maintains the steady nocturnal south-southeast winds over the western slope. Additional experiments show that the diurnally evolving regional-scale circulation system over the Colorado Rocky Mountains is a robust feature which can occur over a range of ambient flow and stratification conditions. Soil moisture experiments reveal that wet soil along the eastern slope and dry along the western slope aids the development of the westward propagating density current. The diurnal evolution of the circulation system on the case study day was in fair agreement with many of the observed circulation features. This experiment also revealed that synoptic-scale forcing can influence the development of the regional-scale circulations in preferential regions along the eastern slope of the mountain barrier. As a result of the numerical experiments four phases of the thermally forced regional-scale diurnal circulation system have been identified. These consist of a daytime mountain boundary layer development phase, a late afternoon transitional phase, an evening propagating density current phase, and a latte night adjustment phase.


Fall 1990.
Also issued as author's dissertation (Ph.D.) -- Colorado State University, 1990.

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Atmospheric pressure -- Diurnal variations
Atmospheric circulation


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