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<br />. <br /> <br />The straight lines, labeled with the radar station identifier, are generally close fits to the array of <br />points (colored triangles) for that particular radar and its snow gage data. HOWeVE!r, the lines <br />certainly do not overlap; they are site specific. <br /> <br />The KCLE data are dominated by shallow lake effect storms, which the radar beam overshoots <br />at far ranges. <br /> <br />The KGJX and KBBX radars, at elevations differing I:>y about 10,000 feet, look at strongly oro- <br />graphic snowfalls which generate most of their precipitation growth in the layers close to the <br />ground. To avoid ground clutter, higher tilt radar data had to be used. However, that overshoots <br />the growth zone, resulting in the need for smaller alpha values to calculate the correct surface <br />precipitation. <br /> <br />All these straight lines present a mathematical problem. A range correction would be the ratio of <br />a standard alpha, such as 150, and the alpha values along the line. The corrections would go <br />infinite (inverse of zero) where the lines intersect the range axis and then negative at farther <br />ranges. Therefore, straight lines are inappropriate for range correction, even though the lines are <br />derived from real, precision surface measurements. For comparison, the NIDS version of the SAA <br />had a quadratic relationship for a range correction factor, as described below. That relation is <br />converted to an equivalent function of alpha varying with range and is shown as the thick curved <br />line in figure 2. Alpha is 150 until 36 km. Thereafter, alpha changes to a parabolic curve resulting <br />in a correction factor of about 3 (alpha of 50) at 230 km. The parabolic relationship avoids infinite <br />corrections but is a distortion of the actual relationship. <br /> <br />. Table 1.-Data used in the derivation of a range correction for the <br /> Minneapolis NEXRAD radar <br />Tilt Meters R, km Ratio Inv. <br /> --- <br />0.50 378 35.0 1.000 1.000 <br />1.45 958 73.4 0.890 1.124 <br />2.40 1,538 103.7 0.780 1.282 <br />3.35 2,117 129.5 0.670 1 .493 <br />4.30 2,696 152.4 0.560 1.786 <br /> <br />. <br /> <br />col 1 = NEXRAD antenna tilt angle - degrees - VCP21 <br />col 2= center of beam above radar at 35 km range (meters) <br />col 3= range, km to 0.5 deg beam center for altitude of col 2 <br />col 4= ratio of S to 35 km, 0.5 deg. tilt value; linear to 0.56 at 4.3 deg. <br />col 5= correction factor (reciprocal of col 4) <br /> <br />Range Correction from a Seasonal Averaged Vertical Profile of S <br /> <br />The range correction scheme previously developed for nine Minnesota snow storms of the 1996- <br />97 winter, described by Super (1998), was revisited. All the radar data were used in new <br />calculations with a somewhat different la-S relationship discussed by Super and Holroyd (1998). <br />The results were quite similar, with the median vertical profile of le suggesting about a 20 percent <br />decrease in la per kilometer of height above the ground. Alternatively, a seasonal average or <br />median gradient can be derived for S rather than Za. <br /> <br />5 <br />