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Harold D. Orville

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Harold D. Orville

Abstract

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Harold D. Orville
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Harold D. Orville

Abstract

No abstract available.

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Harold D. Orville
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Harold D. Orville

Abstract

The initiation of cumulus clouds over mountainous terrain is investigated photogrammetrically. Stereo pairs taken at one- and two-minute intervals are analyzed for four days of cumulus initiation over the Santa Catalina Mountains, northeast of Tucson, Arizona. Charts of growth rates, cloud position, and tracings of clouds over the mountain ridges are presented. The environmental conditions, represented by the Tucson radiosonde and rawin soundings, are related to the growth characteristics. Two days with easterly components in the wind and two with westerly components are analyzed. The growth characteristics can be vastly different, depending upon the water vapor content of the air and the ambient winds. There is no obvious effect of the lapse rate on the initial growth rates.

The clouds form over the principal mountain ridges with their base topography in general agreement with the ridge topography on three of the four days. The ambient winds determine the position of the clouds with respect to the ridge line. The days with fairly strong winds show evidence of cloud formation in suspected lee waves. The growths in the waves are more vigorous and extend to greater heights.

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Harold D. Orville

Abstract

The initiation of cumulus clouds over mountainous terrain is investigated by means of a numerical model. Two-dimensional motion is simulated over a mountain and valley. Changes at the mountain surface of both temperature and water vapor initiate the motion. The equations are similar to Ogura's (1963) but include an extra buoyancy term due to water vapor.

Five cases have been numerically integrated. Cases 1 and 4 are included to demonstrate the dynamic effect of water vapor by comparison with a previously integrated “dry model.” Case 1, which allows evaporation at the mountain surface, causes the upslope motion to develop at a 20 per cent faster rate than the dry case. Case 4, which allows no evaporation at the surface, augments the motion over that of the dry case by approximately 10 per cent. A comparison of the results with Braham and Draginis' (1960) observation of potential temperature and water vapor over the Santa Catalinas shows some similarities but indicates that the numerical model has eddy mixing effects that are too small.

Case 2 is included to model cloud initiation on a typical Tucson summer day with run in the mountains. The initial environmental stability is greater than in Case 1 (2.8C km−1 potential temperature change compared to 1.0C km−1 for Case 1), but the water vapor content in Case 2 is greater. The effect is to slow the development of the slope winds and the development of the cloud. Cloud initiation occurs after approximately two hours from the assumed initial equilibrium conditions. The cloud development extends over 30 min. The position of the stream function center with respect to the cloud outline is crucial to the shape and evolution of the cloud. A second stream function center rising beneath the first initiates a second growth surge in the cloud.

Case 3 is included to show the effect of decreasing the mountain slope from 45° to approximately 26°. The mountain ridge is 300 m lower than in the other cases. Environmental conditions are the same as those in Case 1. The motion develops slightly slower, a cloud forming 100 m lower and 9 min later (at 72 min) than in Case 1.

Case 5 has the same initial conditions as Case 2 but has an eddy mixing coefficient of 40 m sec−1, ten times greater than that in the other cases. The larger diffusion coefficients result in a broader upslope flow and a later cloud initiation than in Case 2.

The results are compared with photogrammetric data presented by Orville (1965).

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Harold D. Orville

Abstract

Equations developed by Ogura (1962) for a two dimensional axially symmetric thermal initiation problem are adjusted to two rectilinear dimensions and used in a numerical study of mountain upslope winds. The winds are initiated by an assumed potential temperature change at the surface of a mountain 1 km high with a 45° slope. A level plain 2 km long extends from the base of the mountain. Two cases are considered, one in a neutral environment, the second in a slightly stable environment to approximate conditions over the Santa Catalina Mountains near Tucson, Arizona. A neutral environment leads to formation of a bubble which moves up and away from the slope. As the bubble rises above the ridge line, some similarities with previous initial-bubble numerical studies are seen. A stable environment slows the development of the upslope winds and causes columnar shaped convection over the mountain slope. The results of the two cases are compared with each other, with previous numerical studies on isolated thermal bubbles and with a few observational studies on upslope winds. Pressure deviations for the model are computed and appear reasonable. Many of the features of the upslope wind are reproduced in the model.

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Harold D. Orville
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