WMOD00562

• Title: Snow Accumulation Algorithm for the WSR-80D Radar: Second Annual Report
• Date: 6/1/1997
• Weather Modification - Doc Type: Report

Table 4. -Summary of R values between indicated gage hourly SWE accumulations and the range bins directly
overhead (0.0 m S.I) and the range bins calculated by the reverse trajectory method (see text) for the four indicated
assumed fall speeds for larger snowflakes.

I. R values for Cleveland using Ze = 330 S2.0 and minimum hourly gage accumulations of 0.005 inch. No hourly
radar SWE minimum was used. N is the number of hourly data pairs available.

Gage Range Assumed Snowflake Fall Speed (m S.I)
No. (km) N
0.0 0.5 1.0 1.5 2.0
1 36 301 0.61 0.39 0.57 0.58 0.57
2 61 398 0.71 0.61 0.68 0.69 0.70
1&2 699 0.66 0.47 0.62 0.63 0.64
3 87 344 0.63 0.40 0.58 0.61 0.63
4 115 368 0.70 0.25 0.53 0.65 0.66
5 146 388 0.55 0.07 0.18 0.30 0.38
n. Same as above except for Denver using Z, = 150 S2.0.
Gage Range Assumed Snowflake Fall Speed (m S.I)
No. (km) N
0.0 0.5 1.0 1.5 2.0
1 25 106 0.74 0.76 0.77 0.78 0.75
2 24 100 0.71 0.73 0.68 0.66 0.68
3 49 151 0.71 0.68 0.68 0.68 0.68
7 25 101 0.60 0.55 0.55 0.58 0.58
1,2,3 357 0.72 0.72 0.70 0.70 0.70

The Cleveland calculations show the advection scheme actually reduces the degree of association between
radar estimates and gage observations, in opposition to its designed purpose. The best performance of
the advection scheme is with 2.0 m S.I fall speed. This value is unrealistically fast for most snowflake
types, especially for the area's lake effect storms known to frequently have low density snowfall. Use of
the fastest speed simply results in use of a range bin fairly close to the nearest neighbor except at far
range.

Use of the advection scheme in the Cleveland area is clearly counterproductive. Two major reasons for
its failure are speculated to be:

1. Use of a single fall speed is a gross oversimplification. At any given instant, the larger snowflakes
may be falling over a significant range of speeds in different portions of the radar's area of coverage.
But no measurements are available concerning what these speeds actually are or how they vary in
space and time.

2. V AD winds calculated near the radar may be unrepresentative of actual winds at farther ranges.

Examination of the Denver R values in table 4 reveals a different situation than for Cleveland. The
greatest R-value is for the nearest neighbor range bin for two of the four gages. But gages 1 and 2 have
greatest degrees of association with fall speeds of 1.5 and 0.5 m S.I, respectively. Unlike Cleveland, the
Denver R values do not vary much with fall speed for any site. However, three of the sites are located

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