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Last modified
7/28/2009 2:40:51 PM
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4/24/2008 2:55:43 PM
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Weather Modification
Title
Snow Accumulation Algorithm for the WSR-80D Radar: Final Report
Date
7/1/1998
Weather Modification - Doc Type
Report
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<br />are quite limited. It has been necessary to rely on surface observations provided by other agencies and <br />most of these in mountainous terrain are proving to be far below the lowest unblocked radar beam. <br />Therefore, the rugged terrain is presenting special challenges to determining relationships between Ze and <br />S. A supplemental report is due in October 1999 which will include the results from those additional <br />data sets, and will discuss further SAA work by Reclamation scientists between now and then. <br /> <br />3.5 Ground Clutter <br /> <br />An important consideration in selecting surface precipitation sites for comparison with radar is whether <br />ground clutter potentially existed at particular surface locations. Whether ground clutter suppression was <br />applied or not, cluttered ground locations can significantly modify resulting Ze values. Super and <br />Holroyd (1996) briefly discussed the WSR-88D clutter suppression scheme as did Fulton (1998). More <br />details of this fairly complicated scheme are given by Chrisman et al. (1994). The scheme may be totally <br />automated (with different operator selected settings) or operators may manually select one or more <br />regions to suppress and decide on the amount of suppression in each. Often a combination of automated <br />suppression, using "clutter bypass maps" and "notch widths," and manually selected suppression are <br />going on simultaneously. The problem with dc~aling with potentially suppressed areas is that no <br />automatic record is made of what suppression scheme(s) was being invoked at any given time. Individual <br />NWS FOs mayor may not keep written records of suppression being applied, but even where records <br />exist it would not be practical to discern the effect on the Level II reflectivity data since suppression is <br />applied on a pulse by pulse basis. It is important to realize that clutter suppression is applied at the radar <br />prior to final production and transmission of Level II data. Therefore, all WSR-88D algorithms can be <br />influenced by whatever clutter suppression scheme is being applied. <br /> <br />In view of the uncertainties of clutter suppression and its effects on Ze measurements, the authors <br />attempted to avoid gage sites where clutter suppression was likely. This generally resulted in gage sites <br />not being located closer than about 25 kIn from the radar because ground clutter is usually common in <br />this near-radar region. Attempting to avoid ground clutter was done in most cases by examination of <br />clutter bypass maps provided by the NWS FOs. which maintained particular WSR-88Ds. Such maps, <br />which are recommended to be generated at seasonal intervals, indicate where clutter was detected and <br />where it was not under conditions approximating standard refraction. Of course, manual suppression <br />could still be imposed on normally uncluttered regions without the authors' knowledge. But this situation <br />is unlikely during snowfall periods when anomalous propagation does not appear to be a significant <br />problem. <br /> <br />When clutter bypass maps were not readily available, the RDRHGT program displays developed by <br />Barker (1994) were examined. The terrain elevation files used with this very useful display program <br />have only 1 kIn horizontal resolution. However, agreement has been reasonable with other information <br />such as large scale maps and occultation files, used for correction of partial blockage, which the OSF has <br />had developed from 0.1 kIn horizontal resolution terrain elevation files for each radar. <br /> <br />Based on various analyses performed with the radar and gage/snow board data, and on examination of <br />PPI (plan-position indicator) plots of many stOlm total accumulations for the various radars, it is not <br />believed that ground clutter, or its suppression, posed a serious problem for any of the surface <br />measurement sites used in this study. A few exceptions were found where clutter affected Ze data over <br />Albany snow board locations. Measurements from those sites were removed from further consideration. <br /> <br />7 <br />
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