WMOD00459

• Title: Snow Accumulation Algorithm for the WSR-88D Radar, Version 1
• Date: 6/1/1996
• Weather Modification - Doc Type: Report

for Algorithm development. Reclamation scientists did acquire the existing November
through February Denver WFO hourly snowfall data, as well as daily snowfall amounts from
all cooperative observer gages within range of the Denver radar.

Besides the problem of sparsity of Denver data during the dry winter of 1994-95, it was
discovered that the standard mode of operation that winter was not to use the Clutter Bypass
Map. Rather, because of frequent ground-based inversions and superrefraction, maximum
suppression was routinely applied to the entire radar area of coverage. Communication with
OSF specialists has revealed that suppression application might have commenced with radial
wind speeds (i.e., toward or away from the radar) as great as 10 knots. Suppression is
increased as the radial wind speed decreases (Chrisman et aI., 1994). Radial winds less than
10 knots are not uncommon over large portions of the lowest tilt radar beam (0.50 elevation
angle) during many Denver-area winter storm periods. Consequently, meteorological returns
were likely often suppressed even in regions without ground clutter. With the uncertainty
of how much suppression was applied at a given place and time and the scarcity of surface
snowfall observations it is difficult to see how much use can be made of 1994-95 winter data
from Denver for establishing a Ze-S relationship. However, some attempts were made.

All hourly precipitation data from the Denver WFO were examined for the three snowstorms
of the 1994-95 winter with associated Level II data. The only hourly values of record are
Trace, 0.01, 0.02 and 0.06 (1 value only) inches, so the available range is very limited.
Comparisons ofthese data withZe values directly above the WFO gage revealed large scatter.
The scatter was possibly caused in part by over-application of clutter suppression and
partially by the lack of range in the snowfall observations.

Twenty-four-hour precipitation totals from all area cooperative observer gages were also
examined and compared to Ze values dirEdlyoverhead. This comparison resulted in even
larger scatter than that observed with thl~ Denver WFO hourly data. At that point, it was
decided that resources would be better spent working with the upcoming 1995-96 winter
observations than dealing further with the limited and uncertain measurements from the
prior winter.

3. Observations from the 1995.96 Winter

3.1 Snowfall Measurement ConsideraUons

Accurate snowfall measurement is difficult because wind effects can cause significant to
severe gage undercatch as demonstrated by several authors over many decades. For example,
Goodison (1978) reported that gage under<:atch by an un shielded Belfort Universal gage can
exceed 50 percent with wind speeds as low as 2.5 m s'l, and 75 percent with a 7-m S-l wind.
Furthermore, Goodison showed that even .a Belfort gage equipped with an Alter wind shield
can exceed 50-percent undercatch with a 5 m S-l wind speed. Goodison and others have
shown that the degree of undercatch is laven greater for some gage types, including the
Fisher-Porter, because of their shape.

Another problem is that many existing recording gages in the national network are of the
Fisher-Porter type with resolution of only 0.10-inch water equivalent. This resolution is an
order of magnitude less than the 0.01 inl~h (or less) provided by a Belfort gage. In most
regions of the U.S. which commonly experience snowfall, only a small fraction of all snowfall
hours have a melted water equivalent of 0.10 inch or more. The infrequent occurrence of

3