WMOD00283

• Sponsor Name: USBR, Technical Service Center, River Systems 7 Meteorology Group
• Project Name: Use of NEXRAD WSR-88D Radar Snow Accumulations Algorith in the GCIP LSA-NC
• Title: Use of NEXRAD WSR-88D Radar Snow Accumulations Algorith in the GCIP LSA-NC
• Prepared For: USBR Technical Service Center
• Prepared By: Arlin B. Super
• Date: 1/1/1998
• State: MN
• Weather Modification - Doc Type: Report

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lowest tilt angles. Only those volume scans with complete data from the two lowest
tilts were used as input to the SAA.

A number of volume scans either had no Level III reflectivity information at all or
were missing either the lowest or second lowest scans, both needed by the SAA. It
is not known why Level III data were sometimes missing in the data set received
from DCAR. Although mostly Level III observ~tions were missing, occasionally
Level III files existed without corresponding Level II data.

All volume scans without both types available were "flagged" to be ignored during
SAA runs. Thus, the Level II and Level III comparisons to be presented used
exactly the same data except that the latter had only 5.0-dBZ resolution. All KMPX
scans during the 1996-97 winter were made in precipitation scanning mode (YCP
21) to accommodate real-time field testing of the SAA.

The weakest interval for Level III data in that scanning mode ranges from
undetectable to 4.5 dBZ. This interval was not used in the SAA runs because of the
uncertainty as to the actual dBZ value. The weakest interval used was from 5.0
through 9.5 dBZ. A value of 5.0 dBZ corresponds to 0.005 inch h-1 (trace) for the Ze-
SWE relation used; that is Ze = 200 SW~'o, discussed in section 2. Reflectivities
less than 5.0 dBZ were ignored in both Level II and Level III runs because such
values were indeterminate with Level III data. Corresponding low radar-estimated
SWE rates would not contribute significantly to storm total accumulations.

SAA operation required assign.m.ent of a discrete dBZ value to each 5.0-dBZ-wide
Level III interval which contained up to 10 Level II values. The simplest approach
would be to assume that the midpoint of each Level III interval is the value most
representative of the interval. Because values of 5.0,5.5, --- 9.0, and 9.5 are
contained in the "5 to 10" dBZ interval, the midpoint would be 7.25 dBZ, and other
midpoints would be at increments of 5.0 dBZ above that value. However, use of
midpoints for greater intervals biased the data toward larger than actual (Level II)
values as explained below.

Frequency distributions ofKMPX Level II values were calculated over each of the
six weighing gages for the entire 1996-97 winter. The distributions showed, for
example, a greater frequency of values of25.0 and 25.5 in the "25 to 30" dBZ
interval than values of 29.0 and 29.5 dBZ. Level II values have an approximately
Gaussian frequency distribution that "tails off' toward both the smaller and greater
values. The weak reflectivity "tail" was usually below 5.0 dBZ and, consequently,
had no influence on the presented results. Distributions were fairly "flat" from 5.0
to 15.0 dBZ. Frequencies rapidly decreased for values greater than about 20 dBZ
near the radar and about 15 dBZ at farther ranges. The decrease might be expected
since the probability of greater dBZ values will decrease with range because
sampling is higher in the clouds due to earth's curvature and beam spreading.

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