WMOD00447

• Title: Interagency Agreement Funds Usage Period Report - February 2001
• Date: 2/1/2001
• Weather Modification - Doc Type: Report

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Ron ge - ~ m

Figure 3. A linear relation is assumed for the
vertical gradient of S for the first five tilts at
the 35 km range.

Figure 4. A plot of the correction factor with
ran!~e, based on the vertical gradient of S.

Table 1 presents data used in a range correction derivation based on a vertical gradient of
apparent S. Listed in the first two columns are the angh~s 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 dewee beam
intersects those same altitudes at the ranges indicated in the third column. 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 0.56 at the
4.3 degree tilt. Between the two end points, the relation is assumed to be linear and is plotted in
figure 3. 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 4 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 norttlem 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.

The SAA work was reported at a GCI P meeting at the University of Maryland (17-18 May 1999)
and at OPE Workshops in Boulder, Colorado (14 April 1 H99), and Reno, Nevada (11 June 1999).

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