Tuesday, 6 August 2013
Holladay-Halsey (DoubleTree by Hilton Portland)
Strong temperature inversions frequently cap moisture within the marine atmospheric boundary layer. Since atmospheric refractivity is directly dependent upon the layered structure of the atmosphere these temperature and moisture gradients represent an important forecast problem for the Navy. Since these gradients are often removed during the averaging process, understanding their mean structure over time requires conditional sampling. In this study high-resolution (3-km horizontal spacing) forecasts and observations at Oakland, California were analyzed for inversions, as defined by either constant or increasing temperature with height. Predicted and observed inversions were sorted by height, depth, and intensity for the months of May through August through a four-year period from 2009-2012. The distribution of Inversion intensity, as defined by the positive deviation from an isothermal lapse rate, was found to evolve through the warm season. In May the distribution follows a quasi-exponential curve with maximum occurrence frequencies near deltaT=0° C. By July a distinct secondary maximum develops for intensities near deltaT=10° C. The bimodal structure was most apparent in the morning soundings, with flatter responses in the evening. The forecast intensity distributions represented the observations fairly well within the first 12-hours. However, by 24-hours the bimodal structure was replaced by a relatively flat distribution. These changes were not immediately apparent in the mean forecast vertical temperature profiles, which varied by about 2° C through the forecast lead times.
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