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On the determination of precipitation efficiency
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1981
[PDF-66.08 MB]
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Description:Assuming conservation of energy, an air column may be followed across an obstacle such as a mountain range, and also a convergence zone. The efficiency of this zone for releasing water or ice toward the ground may be computed from the ratio of the difference in specific humidity at the inflow and outflow end of the computation boundary to the difference in specific humidity at the inflow and at the greatest height, or lowest pressure, attained by the column. This method was applied to storms crossing the Wasatch Mountains of southern Utah for January to March 1981. Efficiency near 100 percent was determined for more than half of the sample of 18 cases. Further, nearly 60 percent of the imported moisture was left over the mountains in these efficient cases, a high value. The two controls of the total precipitate, moisture difference between inflow and outflow, and strength of total mass flow, contributed equally to the fallout. Synoptically, most situations involved short wave troughs in the westerlies, as was expected. In post-experiment analysis, it was found that most of the trajectories followed across the mountain area were on the cyclonic shear side of jet streams; this was not anticipated. The computational technique is illustrated with two examples, one with northwest and the other with southwest upper flow.
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Rights Information:CC0 Public Domain
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