Flash Flood Forecasting Over Complex Terrain With an Assessment of the Sulphur Mountain NEXRAD in Southern California (2005) / Chapter Skim
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4 NEXRAD
4 NEXRAD
Pages 35-58
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From page 35... ...
. Primary Data and Derived Products The NEXRAD is a pulse-Doppler system that measures three primary characteristics of the radar echoes: the radar reflectivity factor, commonly 1 Adapted from NRC (2002)
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These base data variables, derived in the radar data acquisition (RDA) unit, express the zeroth, first, and second moments, respectively, of the Doppler spectrum of the echoes.
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The NWS also provides essential weather information in support of the nation's river and flood prediction program, as well as in support of civilian aviation, agriculture, forestry, and marine operations. The national information database and infrastructure formed by NWS data and products can be used by other government agencies, the private sector, the public, and the global community.
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Those that affect the primary variables directly include contamination by ground clutter, both that in the normal radar environment and that arising during anomalous propagation (AP) con ditions, and the occasional impact of bird echoes on the Doppler velocity data.
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Regions devoid of data pose a difficult problem for NEXRAD algorithms. The primary reasons for data voids are beam overshoot, beam blockage due to obstructions, the cone of silence near the radar, and gaps in vertical coverage arising when large elevation steps are used between azimuth traverses in the scan strategy, along with regions of low echo strength, data masking due to data corruption, and planned and unplanned outages.
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The clear-air patterns cover the lowest layers of the atmosphere in 10 minutes and provide such things as wind profiles and indications of sea breeze fronts or storm outflow boundaries that could trigger convective activity. The "precipitation" and "severe weather" patterns cover the full depth of storm activity in 5 to 6 minutes and provide more frequent updates on evolving storms.
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From page 41... ...
SOURCE: Adapted from Crum et al., 1993. Elevation steps greater than the radar beamwidth at the high elevation angles in those VCPs, plus the cone of silence over the radar site itself, lead to some data voids, particularly for echoes near the radar.
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From page 42... ...
The power received at the radar from precipitation is composed of the backscattered power contribution from all of the particles in the radar resolution volume. Therefore, it is useful to work with radar cross sections per unit volume that can be related to microphysical properties of precipitation.
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The distributions of raindrop sizes and shapes form the building blocks for deriving physically based rain rate algorithms. Although practical considerations may be just as important, the physical approach provides guidance in developing algorithms for rainfall estimation.
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Radar rainfall estimation can be classified broadly into (1) physically based and (2)
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Although both physically based techniques and engineering solutions have their role in precipitation measurements, only the engineering approach is presently applicable with NEXRAD. Engineering techniques that focus primarily on accurate estimation of rainfall on the ground range from simple techniques, such as tuning the algorithm coefficients with season or with radar range, to more sophisticated approaches, such as the derivation of nonparametric relationships3 between the reflectivity factor and the rainfall rate or the use of neural networks.
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to avoid 1 1 1 1 unrealistically high values that may result from contamination of the radar data by hail. The radar-based estimates of rainfall rate then are accumulated into hourly totals that are compared to contemporaneous gauge rainfall amounts at the locations of reporting gauges.
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Practical Issues of Estimating Precipitation Using the Reflectivity Factor As described in Box 4.1, the measured radar reflectivity factor, Z, and the rain rate, R, are linked physically by the raindrop size distribution, DSD. If radar can measure Z sufficiently close to the ground so that the precipitation intensity does not change over this height, the only uncertainty in the transformation from Z to R arises from the DSD variability.
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or by vertical extrapolation of noncontaminated data taken at higher eleva tions. The latter can be done in the same manner as for data from far range where the lowest elevation angles necessarily sample precipitation at an appreciable height above ground.
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new polarimetric-based products such as improved precipitation estimation and hydrometeor particle identification; and (3) new products that may be assimilated directly into operational numerical models.
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the capability for polarimetric sensing and processing to more accurately measure hydrometeor proper ties, such as drop size distributions and precipitation phase; and (4) custom VCPs to allow site-specific volume scans adapted to local weather needs.
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Finally, based on a favorable preliminary site survey, an In-Depth Site Survey was performed, which included a second site visit and another report for the DOC containing updated information and a detailed analysis of the site and radar coverage. Estimated site preparation costs, anticipated environmental and historical impacts caused by site construction and future radar operations at the site, and measurements of electromagnetic frequencies in the area were included in the report.
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From page 52... ...
Due to the severe ground clutter return from the surround ing metropolitan area, the WSR-74C site was not considered a favorable location for the installation of NEXRAD. Furthermore, AP effects, resulting from interaction of the radar beam with the coastal marine inversion layer that frequently persists in Los Angeles and surrounding metropolitan areas (see Chapter 5)
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NEXRAD 53 FIGURE 4.2 The Sulphur Mountain radar site shown with respect to the prevailing area from which precipitating systems approach and to the cities of Los Angeles, Ventura, and Oxnard. SOURCE: SRI International, Inc.
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Castro Peak was abandoned as a potential site because its elevation was not high enough to keep the main radar beam above the climatological range of variability of inversion heights, and thus the integrity and utility of the radar data would be diminished by the frequency of AP effects. The Sulphur Mountain site did not present these problems, and eventually it was selected by the survey team as the best site to meet the priority radar coverage requirements.
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NEXRAD 55 FIGURE 4.3 Topographical map showing the location of the Sulphur Mountain radar site at an elevation of 0.83 km (2726 ft)
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56 FLASH FLOOD FORECASTING OVER COMPLEX TERRAIN FIGURE 4.4 Radar coverage 1.22 km (4000 ft) above site level for the Sulphur Mountain radar (i.e., covering the Los Angeles area)
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The 3.05 km (10,000-ft) ASL (above site level)
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Work included clearing of the site and placement of forms for the concrete foundation. The construction crew returned to the site on November 28, 1993, to remove the forms, and the Sulphur Mountain radar was erected on December 16, 1993.
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