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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6. SAA REFINEMENT

6.1 Adjustments to the Hybrid Scan and Occultation Files

The radar's hybrid scan strategy (O'Bannon, 1997) is primarily based on a minimum 500-foot
clearance of the terrain by the beam bottom. This scheme, which uses a terrain elevation file,
was used initially. Since it was discovered that there were still echo anomalies evident after
applying this scheme, a new approach was taken. Numerous cases of either widespread
precipitation or clear air echoes were composited for the separate tilts for all radar sites to reveal
these echo anomalies, which were caused by the beam striking surface targets and producing
persistent echo enhancement (ground clutter echoes) or persistent echo reduction (preprocessing
suppression from zero Doppler shift). The hybrid scan files were then constructed to specify the
lowest tilt that avoided such anomalies. Case studies were run with the new hybrid scan files,
and the files were hand-edited to remove further localized errors caused by echo anomalies.
These procedures, though laborious, allowed for "manual" adjustments of reflectivity in the
anomalous areas and the best QPE available. We call the result our empirical hybrid scan.

The widespread precipitation cases revealed radial streaking, signifying inadequacies in the
occultation file. By using azimuthal profiles of SWE, estimates were made for the dB additions
to particular radials beyond some appropriate range at which blockages began. The occultation
file was manually adjusted, and the cases were rerun until anomalies were minimized. Unlike the
original occultation files that attempted no correction if more than 4 dB additions were needed,
the new changes maintained a 4-dB correction to 230 km if a vertical blockage greater than
60 percent was apparent. We preferred an insufficient correction of first tilt reflectivities to
having the hybrid scan file stepping up to the second tilt, which overshot the tops of far range
echoes. In a few mountainous areas where the first tilt was totally blocked, the hybrid scan files
were adjusted to rise above the obstruction for the rest of the farther ranges. These changes were
implemented before the 2000-2001 winter season.

6.2 Vertical Profile of Reflectivity Correction

This correction and others in the remainder of section 6 were first described in Hunter et aI.,
2001. The importance of some type of VPR correction is reinforced by the literature. Joss and
Waldvogel (1990) assert that VPR measurement is ". . .the main problem in using radar for
precipitation measurements and hydrology in operational applications." This is affirmed by
several researchers, including Koistinen (1991), Galli and Joss (1991), Andrieu and Creutin
(1991), and Smith (1990). Joss and Waldvogel (1990) reinforce the importance of
VPR correction by asserting that it should be done before any other adjustment, such as gauge
data. Hunter (1996) recapitulated these findings regarding the operational WSR-88D. The NWS
embarked on a VPR correction for precipitation in general in the late 1990s, as described by
Seo et al. (2000).

As described in section 4.2, Reclamation had developed a simple second-order polynomial range
correction. This correction was actually formulated from a mean of several VPRs (more
precisely vertical profiles of snowfall rate S, since that is calculated from Ze at each vertical

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