Flash Flood Forecasting Over Complex Terrain With an Assessment of the Sulphur Mountain NEXRAD in Southern California (2005) / Chapter Skim
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8 Potential Improvements in Flash Flood Warnings
8 Potential Improvements in Flash Flood Warnings
Pages 117-145
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From page 117... ...
This opens a wide range of opportunities for intelligence acquisition technology and the engineering of regional expert systems to capitalize on the human intelligence at local Weather Forecast Offices (WFOs) and to make the most of artificial intelligence applications to process, classify, interpret, and synthesize inhomogeneous data.
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From page 118... ...
. NEXRAD AND OTHER RADAR SOURCES Potential improvements to the radar hardware and operation are build ing on a flexible radar scan strategy and upgrades to polarimetric capability, which will assist in expanded volume coverage, data quality control, and precipitation estimation.
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From page 119... ...
The impact of a changed scan strategy on those products and algorithms imposes additional technical hurdles and software requirements; thorough planning and testing will be necessary before implementation of any new scan strategy. Recommendation: The National Weather Service should improve nationwide NEXRAD coverage of low-level precipitation and wind, especially for elevated radar sites in complex terrain, through the adoption of a modified scan strategy that will allow scanning at lower elevation angles.
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From page 120... ...
Because of increased level of random errors for phase measurements at lower rain rates, hybrid methods that combine rainfall estimation algorithms based on radar reflectivity, differential reflectivity, and specific differential phase are developed that blend these techniques for the
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. Besides contributing to improved rainfall estimation, dual-polarization measurements are very effective for data quality control, for example, detecting anomalous propagation and ground clutter contamination.
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Coverage of low-level precipitation may thus have to be achieved either by changing the radar scan strategy or by deployment of additional radars at lower altitudes. Estimation Uncertainty Characterization of the uncertainty of radar-based rainfall estimates should be as important as the estimates themselves.
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Radars deployed by the FAA include the Terminal Doppler Weather Radar (TDWR) , the Airport Surveillance Radar (ASR)
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(b) Airport Surveillance Radar The FAA uses ASRs to monitor and track en route aircraft as well as the traffic near major airports.
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The ARSR-4 collects reflectivity data, but is limited by a minimum reflectivity level of 30 dBZ and has a problem with properly representing maximum reflectivity within a storm. The FAA and the NWS are currently considering upgrading these radars with a new Doppler weather processor to provide wind velocity information and improved reflectivity data.
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From page 126... ...
The web site http://www.weatherexpress.com/wmradar.htm contains a list and links for local TV station weather radars in the United States. In addition to using radar data in their daily broadcasts, several television networks also provide reflectivity-based, precipitation imagery on their web sites.
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From page 127... ...
in their broadcasts, along with providing routinely updated, full 360° radar information in near real time on its web site. Data Access and Mosaic Radar Capabilities Following the installation of NEXRAD across the nation, commercial vendors such as WSI and Kavouras (now Meteorlogix, LLC)
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From page 128... ...
Access to data from TDWR, ASRs, and research and local TV station radars, especially in regions of complex terrain such as Southern California, can help alleviate gaps in radar coverage resulting from beam blockage by high terrain and problems associated with overshooting beams. Figure 8.3, which does not reflect all of the blockage zones, provides an example of how this may be possible by using an existing ASR-8 located at Santa Barbara to provide radar coverage (blue range rings)
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POTENTIAL IMPROVEMENTS IN FLASH FLOOD WARNINGS 129 FIGURE 8.2 Mosaic of radar reflectivity data produced from a combination of NWS and DoD NEXRADs and one research radar in Oklahoma, Kansas, and Texas. SOURCE: Rita Roberts, National Center for Atmospheric Research.
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represent radar coverage from the Sulphur Mountain NEXRAD; the blue rings show coverage from an ASR-8 located in Santa Barbara; and the red rings show coverage from an ASR-9 located at Los Angeles International Airport. SOURCE: Rita Roberts, National Center for Atmospheric Research.
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From page 131... ...
Through this collaborative effort, all of the above facilities will have real-time access to the upgraded WSR-74C data and surrounding NEXRAD datasets. Recommendation: To extend radar coverage, all available regional real-time weather radar data should be made accessible to the NWS WFOs, including Federal Aviation Administration (FAA)
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SOURCE: Rita Roberts, National Center for Atmospheric Research.
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real-time radar information may alleviate some of the problems and thus improve the useful radar coverage available to forecasters. Nonetheless, there may still be smaller areas hidden by terrain that would require permanent or temporary (e.g., areas burned by wildfires)
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134 FLASH FLOOD FORECASTING OVER COMPLEX TERRAIN FIGURE 8.6 Schematic of a network of smaller radars that operate as a distributed, collaborative adaptive system.
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To maximize their use, these systems should be networked together, data formats standardized, and metadata established. MODELING, DATA ASSIMILATION, AND DECISION SUPPORT SYSTEMS National numerical weather prediction (NWP)
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There also is evidence that the explicit version of the model can predict finer distinctions in the precipita tion structure, with features such as bow-echoes, lines of supercell storms, and the convective and stratiform regions of squall lines, than possible with some of the existing operational models. Plans are under way to run and assess the performance of the WRF model during the 20042005 winter season (Weisman, personal communication)
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From page 137... ...
One such system that has been run operationally at the Washington, D.C.-Baltimore, Maryland, WFO is the four-dimensional Variational Doppler Radar Analysis System (VDRAS; Sun and Crook, 2001) which assimilates single-Doppler radar data into a numerical cloud model.
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From page 138... ...
simulated peak streamflows exceeded that achieved by the NWS Northwest River Forecast Center (RFC) results, although the results were highly sensitive to data density and quality.2 Prediction Uncertainty Numerically predicted meteorological parameter fields, such as tem perature, wind, and precipitation, exhibit uncertainty that depends on the model physics, initial conditions, and forecast lead time (e.g., Cortinas and Stensrud, 1995; Harris et al., 2001; Germann and Zawadzki, 2004; Walser et al., 2004)
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From page 139... ...
Inevitably, capacity-building and ensuring the continuity of forecasting expertise constitute a great challenge for the NWS. This opens a wide range of opportunities for intelligence acquisition technology and the engineering of regional expert decision support systems to capitalize on the human intelligence at local WFOs and to make the most of artificial intelligence applications to process, classify, interpret, and synthesize inhomogeneous data.
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From page 140... ...
This system assists local forecasters by monitoring precipitation accumulations at the hydrologic catchment scale to interpret a hydrologic threat within the context of an evolving meteorological situation to provide short-term, high-resolution flash flooding guidance. A prerequisite for utilization of FFMP is the topographic mapping of all the basins and subbasins covered by a WFO, including their hydrologic characteristics such as a basin's outline and average slope, soil characteristics and conditions, foliage coverage, and channel hydraulic measures.
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The high-resolution information and zoom-in capability and the automated bookkeeping of the FFMP system enable forecasters to stay abreast of potentially hazardous weather situations more easily, issue much more specific warnings, and do so with improved skills. Thus, FFMP bears the potential to greatly enhance flash flood monitoring, prediction, and warning.
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From page 142... ...
already have such topographic and hydrologic information for some of their water sheds; WFO collaboration with such agencies may somewhat lessen this burdensome task and facilitate the implementation of FFMP. Future developments of FFMP may include incorporation of numeri cally predicted precipitation to increase the lead time in issuing flash flood warnings, which would be most valuable for regions of complex terrain.
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From page 143... ...
The potential for complete and partial radar beam blockage can be evaluated in the context of hydrologic basins for which coverage is sought. Basin size, average slope, orientation with respect to the movement of dominant weather patterns, characteristics of soil, land cover and land use, and channel hydraulic aspects determine the amount of rain that is likely to cause flash flooding and where it may occur.
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the VDRAS cloud model to obtain vertical motions in the boundary layer; and (4) national numerical model output to delineate regions of atmospheric instability (Saxen et al.
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From page 145... ...
The polarimetric modification would improve the data quality and quantitative precipitation measurement capabilities of NEXRAD. Real-time data assimilation systems that incorporate observations into high-resolution mesoscale numerical models provide rapidly updated wind and precipitation forecasts.
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