WMOD00425

• Title: Demonstration of Improved Operational Water Resources Management Through Use of Better Snow Water Equivalent Information
• Date: 9/1/2002
• Weather Modification - Doc Type: Report

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location) from nine Minnesota snoWSlonns, from 1996-97 data collected ,It J5-km range. This
relationship was Iransfonncd to he a function of range rather than elevation. It was realized.
however. that a VPR correction would be superior to a range correction in certain circumstances.
i.e.. whenever the hybrid scan specifics that z.: (thercf{)fC S) estimates be gathered from tilts thai
are stepped upward to avoid terrain blockage. This upward step worsens the partial beam filling
problem described in section 5, diminishing z.: and S compared to the desired values at the
ground (depicted by figure 3). Such sleps may occur close to the radar. where the minimal Of
l.CTO range correction will not compensate for this diminishment.

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Sectorized Hybrid Scan
For radials with blockage

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~
--~
~~~~.c:-:

20%

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T

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T

...-~-
_.-.-----.---
-~--~~
-"'A.. "

70%

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Figure 3.-Examples 01 hybrid scan and its possible effect on S. Top
shows small amount (20%) 01 blockage of lowest beam so that dala is
still gathered beyond the blockage (checkerboard beam). Bottom
shows large amount (70%) of blockage of the lowest beam so that
dala are no longer collected from thaI beam (black beam) but rather
the next beam above. Yellow outline and Xs connOle shallow cloud
and snow particles. respectively.

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Such circumstances arc not prescnt for most radars in the north-cemral U.S. (figure 2), because
these areas arc relatively nat with little significant blockage of the lowest (0.50) hearn. Three
radars, however. KTFX. KBLX, and KUDX. have more than 60% blockage of this lill in some
sectors, according to the radar height (RDRHGT) program (Barker, 199~). Therefore, the
artificial reduction in S by blockage and the hybrid scan step is a serious prohlem at these sites.

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The mean range correction of a that has been applied for the north-central Slates in the past is
shown by the curved line in figure I. As evident from Ihat figure. a profiles are site-specific with
range and. therefore, height. Therefore, VPRs are also site.specific. Nevertheless. even a crude
seasonal eSlimate can significantly improve precipilation estimales at midrange to far range (Joss
and Waldvogel. 1989; Vignal et al.. 20eX)). Our new VPR correction is a single profile hased on
many additional measurements from several different geographic regions.

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