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Last modified
7/28/2009 2:35:41 PM
Creation date
3/11/2008 2:43:45 PM
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Template:
Weather Modification
Title
Investigations of Winter Storms Over the Wasatch Plateau During the 1991 Utah/NOAA Field Program
Prepared For
Utah Department of Natural Resources Division of Water Resources
Prepared By
Arlen W. Huggins, Melanie A. Wetzel, Patricia A. Walsh
Date
10/1/1992
State
UT
Weather Modification - Doc Type
Report
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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />radiative transfer software of Schroeder and Westwater (1991) applied to the entire set of Mt. <br />Pleasant soundings. In order to simulate radiative transfer through the depth of the atmosphere <br />above the top of the Wasatch Plateau, the soundings were processed from an initial height of <br />2698 m MSL, the height of the radiometers positioned at the DOT site as in Fig. 2.1. The <br />coefficients computed in this manner are given in Table 2.1, together with coefficients from <br />various other wintertime projects which have employed radiometers. The coefficients appear <br />in the computations of water vapor and liquid water as follows: <br /> <br />Yap = cv1 + (tau20)(cv2) + (tau30)(cv3) <br />Liq = ell + (tau20)(c12) + (tau30)(cl3), <br /> <br />(1) <br />(2) <br /> <br />where tau20 is the absorption at 20.6 GHz, tau30 is absorption at 31.65 GHz, and ev1, cv2, etc. <br />are the statistical retrieval coefficients. <br />The absorption at each frequency contributes to the computation of both vapor and liquid, <br />with the relative size of the second and third coefficients determining the amount of influence <br />imposed by each absorption value. For vapor, since cv3 is always relatively large, the influence <br />of tau30 is great when liquid is plentiful. For the computation of liquid the effect of altitude on <br />the second coefficient (c12) is obvious in Table 2.1. As the site altitude increases c12 becomes <br />less negative. At the highest sites, the 1987 and 1991 Utah/NOAA sites, c12 becomes negligible <br />when compared to cl3. This is apparently due to the fact that water vapor is much more <br />concentrated in the lowest levels of the atmosphere, so emissions at 20.6 GHz decrease rapidly <br />with height and the influence on liquid measurements also decreases. The implication is that for <br />high altitude sites (in winter months) the computation of liquid depth is approximately <br />independent of measurements in the 20.6 GHz channel. This is demonstrated in Fig. 2.2 where <br />brightness temperature at 31.65 GHz (proportional to tau30) is plotted against the computed <br />liquid depth for a period on 2 March. The lack of scatter about the nearly straight line results <br />because variations normally caused by 20.6 GHz measurements are insignificant due to the small <br />size of el2. For this reason the problems encountered in the 20.6 GHz channel in the DR! <br />radiometer during the 1991 season had little effect on liquid water measurements. <br />The USBR radiometer data were processed at the USBR office in Denver using the same <br />retrieval coefficients used in processing DR! radiometer data. The DR! received the USBR data <br />as separate fIles for each day of the 1991 field program. The correction to produce a zero offset <br />in liquid was applied to each day and some bad data, caused by a wet reflector or component <br />failure, were removed from the data set. For presentation with other data sets, and for <br /> <br />2-4 <br />
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