WMOD00564

• Title: Snow Accumulation Algorithm for the WSR-80D Radar: Supplemental Report
• Date: 11/1/1999
• Weather Modification - Doc Type: Report

3IlOO I
2500
2000
i
i 1500
w
j
1000
500

-,

~ ~ ~ M ~
F_cII_SWE_

i
----.J
Figure 4.-A linear relation is assumed for the vertical
gradient of S for the first five tilts at the 35 km range.

2.9
2.8
2.7
2.6
;; 2.5
..J 2.4
~ 2.3
LL 2.2
2.1
c
o 2.0
-' 1.9
o 1.8
ill 1.7
L
L 1.6
j 1.5
1.4 --1
1.3
1.2
1.1
1.0

2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0

o 20 40 60 80 100120140160180200220

Ro n 9 e - ~ m

Figure 5.-A plot of the correction factor with range,
based on the vertical gradient of S.

9

Table 2 presents data used in a range correction derivation based on a vertical gradient of apparent S.
Listed in the first two columns are the angles for the first five tilts of the radar antenna and the altitudes
above the radar of the beam centers at a range of 35 km. The 0.5 degree beam intersects those same
altitudes at the ranges indicated in the third coJlumn. A linear vertical gradient in S was defined, based on
nine Minnesota storms. The ratio of the S aloft to that in the 0.5 degree beam at 35 km is listed in the
fourth column, based on a median value of 056 at the 4.3 degree tilt. Between the two end points, the
relation is assumed to be linear and is plotted in figure 4. The inverse of that ratio, in the fifth column, is
the correction factor needed to convert S aloft to S near the surface (approximate elevation of the radar),
based on the vertical gradient. Figure 5 shows a plot of that correction factor against the ranges of
column 3. The parabolic fit to the five points gives the relation,

F = 1.04607 - 0.0029590*R + 0.0000506*R2

for correction factor F and range R (km). That relation is being used for all radars of the northern plains
in our real-time calculations of S from NIDS data. It produces a correction factor of 3.0 at 230 km range.
However, by that extreme range, the lowest radar beam is usually overshooting the clouds and no
correction is possible.

The range correction parameters were determined from quality precipitation and snow depth data from
sheltered instruments near the KMPX radar. For the 1998-1999 season, we had to rely on cooperative
observers and other volunteers to report their snow observations. None of the sites were inspected for
quality, including determining if the sites were sheltered from winds. Therefore, the snow depths from
these surface sites may be greatly distorted by drifting and scouring. The timings of the observations
were generally irregular, not at some standard number like 12:00 UTC. These must suffice because there
are no other data.