WMOD00564 - Laserfiche WebLink

Home Browse Search

4. The SAA underestimated snow water equivalent. The Ze -S relation used was not optimum for this storm. Ze=50 S.O would have been better and would have tripled the snowfall. Using default adaptable parameters (Ze=150 S2.0), optimized for best overall performance, results in poor SAA products in some cases. If the SAA had been run on-site, the adaptable parameters could have been changed, depending on the storm type. 5. The default snow density (1/14) routinely used was too great for this storm. SD is calculated by dividing S by the snow density. S was already underestimated. Therefore the default density resulted in an even greater underestimation of snow depth. The snow density is an adaptable parameter that could also have been changed if the SAA were running on-site. This case demonstrates that "one size" does not fit all. Although the SAA is producing daily and storm totals of S within about 0.20 inches of quality surface measurements according to the Level II data tests, the problem is that Reclamation is not changing a few adaptable parameters as changing conditions dictate. Vertical profiles of reflectivity, described in Sec- tion 7.3, for the 2 days and two radars are shown in figure 1. Profiles were generated with the coding presented in appendix A for the purpose of understanding the virga problem. The left two columns are for January 2 and 3 for KMPX, and the right two columns are for KABR. Each column contains 24 graphs, one per hour, with time progressing downwards. The graph for the hour ending at 12 UTC (near sunrise) is at the bottom. For each graph, the vertical axis is altitude above the radar from 0 to 5 km. The horizontal axis is cumulative percent of range bins at each altitude, generated from all available tilts ofNIDS data. Far ranges cannot contribute to the bottom of each graph because of the beam height. The coloring, as shown by the key, indicates the magnitude of reflectivity. The upper left graphs, for KMPX, show an apparent virga pattern because the storm is at far ranges where the lowest beam of the radar cannot see the lower altitudes. In the middle of the left column it is seen that the snow, when it is close enough to be seen at ~,. ~""'" ~""" ..."'''' ~"" , ~"" """'" '~.."""" :::::::: lL-.J:'::::::. '~.. : :lo, ' , ' . ' , ' . ' o ' , .. I I I " " I ( "., "" Ii. l I . " I , " _ 0 5 HI 1520 25 31) 35 .:lOdS! . ~ ,I ':J ' "''',,'' 'l-""" '51"'''' ""40'00'80', : ~: 1 ""'"",eo,,,, , - ~- ~ .........'~........Il......\......... "",~""",.' Figure 1.-Vertical profiles of reflectivity during a shallow snowstorm in arctic air. 4