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Last modified
7/28/2009 2:32:29 PM
Creation date
1/8/2008 11:54:38 AM
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Weather Modification
Sponsor Name
USBR Technical Serivce Center, River Systems & Meteorology Group
Project Name
Snow Accumulation Algorithm for the WSR-88D Radar, Version 1
Title
Snow Accumulation Algorithm for the WSR-88D Radar, Version 1
Prepared For
USBR
Prepared By
Arlin B. Super and Edmond W. Holroyd
Date
6/1/1996
State
CO
Weather Modification - Doc Type
Scientific Study
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<br />replace outliers by the simple average of its eight neighbors. The Snow Algorithm <br />does the conversion back and forth only for those neighbors in order to replace only <br />the outlier itself; as in the PPS version, the neighbors are not affected. <br /> <br />(c) The new hybrid scan file is used to keep the bottom of the beam close to, but at least <br />500 feet above, the terrain. <br /> <br />(d) The tilt test is not done because snow may be falling from shallow clouds illuminated <br />only by the 0.50 tilt beam at far ranges. <br /> <br />(e) The biscan maximization for the two lowest tilts is not done. Instead, the lowest <br />acceptable tilt is used as a partial protection against virga. <br /> <br />2) Rate: <br /> <br />The PPS uses only a Z-R relation for rain. The constants for the relation are stored in an <br />adaptable parameters file. This Snow Algorithm has found different constants for snow that <br />depend on geographic (climate) region. The observed range dependency for snow has not yet <br />been addressed. <br /> <br />8) Accumulation: <br /> <br />The PPS sums rainfall for '10 by 2-km sample volumes and also for 1/4 LFM (limited fine <br />mesh) grid boxes. Accumulations are made by I-hour, 3-hour, and storm total intervals. This <br />version of the Snow Algorithm sums snowfall in polar coordinate arrays (230 km x 3600) at <br />the basic resolution of 10 by 1 km. They may be resampled into any other grid style. The <br />accumulation grids presently used by the PPS are entirely acceptable for snow. The three <br />time periods for accumulations are the same as in the PPS version. The Snow Algorithm <br />does not yet contain the continuity and quality checks done by the PPS version, but no <br />changes are anticipate~. <br /> <br />4) Adjustment: <br /> <br />The PPS algorithm contaiI)s coding to compare radar-derived accumulations with those from <br />precipitation gages. No such adjustments are yet included in the Snow Algorithm. <br /> <br />5) Products: <br /> <br />The PPS algorithm produces 2- by 2-km grids of rainfall totals at I-hour, 3-hour, and storm <br />total intervals. The accumulations from the Snow Algorithm can be transformed to the same <br />grids that are presently in use. <br /> <br />10.2.2 The V AD Algorithm <br /> <br />The V AD algorithm is described in section 3.11 in part C of Federal Meteorological Handbook <br />No. 11 (1991) and in chapter 6 of part B of the Handbook. Upper winds are needed at 1000- <br />foot increments above sea level. A range, default 30 km, is assigned for data gathering <br />purposes. For each 1000-foot level, the range at which each tilt's beam crosses that level is <br />calculated. The tilt yielding the closest range to the 30 km is then used. All valid velocity <br />measurements at that tilt and range are fitted to a sine wave as a function of azimuth. The <br /> <br />39 <br /> <br />
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