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Last modified
7/28/2009 2:40:56 PM
Creation date
4/24/2008 2:56:27 PM
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Template:
Weather Modification
Title
Snow Accumulation Algorithm for the WSR-80D Radar: Supplemental Report
Date
11/1/1999
Weather Modification - Doc Type
Report
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<br />A program was written to extract S accumulations from 24-hour files generated by the SAA for pixels <br />directly over reported surface measurements of snow and/or snow depth. For simplicity, S was converted <br />to an SAA-derived SD using a snow density of 0.10. All data from 8 November 1998 through <br />27 February 1999 were examined. Table 3 indicates the number of observations. <br /> <br />Table 3.-Left: The numbers of simultaneous daily radar and surface measurements of SO for the various radars from November 1998 <br />through February 1999 for three intensity thresholds. Right: The SAAlSO ratios and square of the correlation coefficient (R2) <br />for ratios vs range for SO,2: 1.0 inch <br /> <br /> Data pairs Average SAAlSD ratios, p =snow density <br />KBIS KABR KMVX KMPX KOLH Thresholds KBIS KABR KMVX KMPX KOLH All <br />288 528 235 892 621 aIlS,SO 1.33 1.04 1.48 1.25 1.60 1.24 ratio p= 1/1 0 <br />8 42 17 55 11 SO~ 1.0" 0.95 0.74 1.06 0.90 1.14 0.88 ratio p= 1/14 <br />2 19 5 22 3 SD~3.0" .110 0.41 0.45 0.12 0.62 0.14 R2 <br /> <br />Ratios of surface measurements to 24-hour SAA accumulations for S and SD were made and plotted <br />against range from the radar. The S graphs for the individual radars had too few data points to be <br />meaningful. Plotting together all points for all radars resulted in graphs too noisy for interpretation. <br />Figures 6 a-e show the SAAlSD (derived/observed) ratios for the five radars along with a reference line <br />for perfect agreement and no range distortion. The points are classified according to accumulation <br />thresholds. The x points are for S of at least 0.10 inch or SD of at least 1.0 inch. The boxes are for SD <br />of at least 3.0 inches. There is much scatter in the five graphs, as expected for presumably unsheltered <br />locations and various timing offsets. Isolating the heaviest storms did not decrease the scatter. <br /> <br />Least squares fits (not shown) of range to the logarithm of the SAAlSD ratio confirmed nearly pure <br />scatter and only a slight trend for a decreasing SAAlSD ratio with range. The square of the correlation <br />coefficient (R2) is given as the bottom line at the right of table 3, with only 1.4 percent of the scatter <br />explained by range when all data are combined. The average of the logarithm of the ratio is converted <br />back to the ratio at the right side of table 3 for two assumed snow densities, 1/10 (as plotted in figures 6a- <br />e) and 1/14 (closer to actual). For the lesser density, the average ratios are close to unity, indicating an <br />alpha that is nearly correct. <br /> <br />If the range correction was in significant error, then the data points should noticeably diverge from the <br />horizontal reference line near 230 lan. There seems to be no systematic bias with range for these data <br />sets. The data noise is much greater than any range effect. Therefore, there is no justification at present <br />to change the range correction that was derived from vertical gradient data. <br /> <br />6. BRIGHT-BAND WARNING <br /> <br />The vertical profile of Ze can reveal bright band effects. However, at far range the effects get blended <br />with dry snow and light rain because of the larger vertical extent of the radar beam with range. Virga <br />can sometimes look like a weak bright band effect, producing a maximum in reflectivity near cloud <br />base. Correct identification of bright band effects needs confirmation from an outside data source. <br />Rawinsondes can provide the altitude of the melting level but they are often widely separated in distance <br />from the radars; the NEXRAD network is much denser than the rawinsonde network. Furthermore, <br /> <br />10 <br />
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