4
NEXRAD

THE NEXRAD NETWORK1

Appendix B summarizes the technical characteristics of the NEXRAD system. Crum and Alberty (1993) and Serafin and Wilson (2000) provide additional background on the system characteristics. Coverage over the eastern two-thirds of the country is essentially complete, although significant limitations exist in coverage near the surface (NRC, 1995). The NEXRAD coverage sometimes is portrayed in terms of the coverage at 3.05 km (10,000 ft) above the level of the radar sites. The 3.05-km (10,000-ft) criterion was based on language in the Weather Service Modernization Act (P.L. 102-567; Sec. 702(4)), and the above-site-level extension appears to have been an interpretation of that language. The extent of near-surface coverage was a consideration in the NEXRAD site selection process (Leone et al., 1989), but the 3.05-km (10,000-ft) above-site-level representation does not convey a true picture of the low-level coverage. There is considerable variation in above ground level (AGL) or above mean sea level (MSL) coverage where sites were established in mountainous terrain. Thus, there are some substantial gaps in western regions (see, for example, Figure 4.4), and the combination of high-altitude sites and mountainous terrain presents difficult problems in several areas (Westrick et al., 1999; Maddox et al., 2002).

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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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California 4 NEXRAD THE NEXRAD NETWORK1 Appendix B summarizes the technical characteristics of the NEXRAD system. Crum and Alberty (1993) and Serafin and Wilson (2000) provide additional background on the system characteristics. Coverage over the eastern two-thirds of the country is essentially complete, although significant limitations exist in coverage near the surface (NRC, 1995). The NEXRAD coverage sometimes is portrayed in terms of the coverage at 3.05 km (10,000 ft) above the level of the radar sites. The 3.05-km (10,000-ft) criterion was based on language in the Weather Service Modernization Act (P.L. 102-567; Sec. 702(4)), and the above-site-level extension appears to have been an interpretation of that language. The extent of near-surface coverage was a consideration in the NEXRAD site selection process (Leone et al., 1989), but the 3.05-km (10,000-ft) above-site-level representation does not convey a true picture of the low-level coverage. There is considerable variation in above ground level (AGL) or above mean sea level (MSL) coverage where sites were established in mountainous terrain. Thus, there are some substantial gaps in western regions (see, for example, Figure 4.4), and the combination of high-altitude sites and mountainous terrain presents difficult problems in several areas (Westrick et al., 1999; Maddox et al., 2002). 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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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California referred to as reflectivity and designated by Z; the Doppler (radial) velocity, designated by v or vr; and the width of the Doppler spectrum, designated by σv. 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. A value for each quantity is available for every resolution cell of the radar, as defined by its antenna beamwidth and the sampling rate along the beam axis, although the latter is constrained presently to no less than half the pulse duration. Displays of these quantities, together with other products (Radar Operations Center, 2002) and results of the algorithms discussed below, are developed from the base data in a radar product generator (RPG) unit. The products include estimates of precipitation accumulations for 1-hour and 3-hour periods as well as a storm-total product. In addition, a series of computer algorithms operates on the base data—and some also incorporate auxiliary information such as temperature profiles—examining the echo patterns and their continuity in space and time in order to identify significant weather features such as mesoscylones, tornado vortex signatures, or the presence of hail. Outputs of these algorithms are displayed as icons superimposed on the basic radar displays or in auxiliary tables. The number and the variety of potential algorithms continue to increase as scientific knowledge about the relationship between echo characteristics and storm properties improves and available computational resources increase. Data Display, Dissemination, and Archiving A principal user processor (PUP) associated with each NEXRAD installation, and numerous additional remote PUPs, provided the initial data display capability. The PUP was essentially a mainframe minicomputer, with a monitor, that operated programs to generate displays from a rather limited set of possibilities. As computer technology has advanced, open-systems architecture has been implemented to replace both the RPG and the PUP units. Thus, the current open RPG (ORPG) generates and displays the various products as well as relays the relevant data for display on other systems. Although the NEXRAD system of radars is of major value as a stand-alone weather-observing network, additional value is obtained through the integration of NEXRAD data with other weather observations (e.g., wind profilers, satellites, the National Lightning Detection Network, surface measurements, other radar systems) and associated analyses. As described earlier, this synthesis is carried out in the National Weather Service (NWS),

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California the Advanced Weather Interactive Processing System (AWIPS), and dissemination of NEXRAD data within the NWS is handled through the AWIPS system. Equivalent systems support the Federal Aviation Administration (FAA) and Department of Defense (DoD) users of NEXRAD data. Dissemination to outside users, formerly handled by vendors, is now accomplished through the Base Data Distribution System (BDDS), with distribution over the Internet. AWIPS and similar systems are expected to mature dramatically and grow in use in the future as the science of meteorology and the technology of information processing, display, and dissemination continue to advance and merge with societal needs for improved weather information and forecasting. Archiving of the NEXRAD base data—referred to as Level II data—was previously accomplished by magnetic tape recording at the sites, with the tapes being shipped to the National Climatic Data Center (NCDC) (Crum et al., 1993). Experience showed that only a little more than half of the national dataset reached the archive in retrievable form. Results of the Collaborative Radar Acquisition Field Test (CRAFT; Droegemeier et al., 2002) established the capability to transmit NEXRAD data over the Internet to NCDC and increase the fraction of retrievable data. A separate archive of a set of the derived products—referred to as Level III data—provides basic data for such things as research, training, and legal inquiries. Users and Uses of the Data and Products The principal NEXRAD user agencies are the Department of Commerce (DOC), DoD, and Department of Transportation (DOT). The primary mission organizations within these agencies are the NWS, the Air Force Weather Agency (AFWA), the Naval Meteorology and Oceanography Command (NMOC), and the FAA. As discussed in Chapter 3, the NWS is responsible for the detection of hazardous weather and for warning the public about these hazards in a timely, accurate, and effective way. 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. The AFWA provides worldwide meteorological and airspace environmental services to the Air Force, Army, certain other DoD organizations, and intelligence agencies. NMOC supports the U.S. Navy, U.S. Marine Corps, and certain other DoD organizations. The primary missions of these

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California DoD agencies are to provide timely information on severe weather for the protection of DoD personnel and property; to provide weather-related information in support of decision-making processes at specific locations; and to support military aviation. The FAA’s responsibility requires it to gather information on the location, intensity, and development of hazardous weather conditions as well as to provide this information to pilots and air traffic controllers and managers. The group of users has expanded dramatically to include, among others, a very large atmospheric sciences and hydrometeorological research community in universities and research laboratories throughout the world; other federal, state, and local government organizations and private sector providers; and distributors and users of weather and climate information gleaned from meteorological radar measurements and associated products (see NRC, 2003). The latter include data that are either taken or derived directly from radar measurements, as well as information derived through intelligent integration of radar data with other measurements and analyses of weather events. Limitations of the System A variety of limitations impede the ability of the NEXRAD system to meet the needs of all of its varied users. Some limitations, such as the divergence of the radar beam with increasing range, are inherent to any radar system. Others, such as the inability to acquire data in small elevation steps during shallow winter precipitation episodes, can be overcome by rather straightforward hardware or software modifications, the latter of which will be facilitated by the greater flexibility afforded by the open-systems architecture. The ongoing program of research and development should provide at least partial solutions to some of the other problems. Serafin and Wilson (2000) provide a good summary of the recognized deficiencies of the NEXRAD system. 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) conditions, and the occasional impact of bird echoes on the Doppler velocity data. The problem of range-velocity folding, common to all pulse-Doppler radars, has proved to be quite serious in the NEXRAD system. Studies of several techniques now under way should yield means of mitigating the range-velocity folding problem. Spatial coverage limitations are imposed in the first instance by the curvature of Earth. This limitation constrains the available coverage to mini-

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California mum altitudes that increase with distance from the radar site. The problem is exacerbated by any obstacles in the radar environment that constitute a radar horizon extending above 0.0° elevation angle. A further constraint currently imposed on the NEXRAD system limits the minimum elevation angle to no lower than 0.5°, adding to this difficulty. This problem is of special concern for radars at high-altitude sites in mountainous areas, such as the Sulphur Mountain NEXRAD. Similar difficulties arise in areas subject to intense precipitation from shallow cloud systems, such as places in the lee of the Great Lakes affected by lake-effect snowstorms. NEXRAD scans also are restricted to some maximum elevation angle (currently 20.0°), mainly to provide an acceptable scan update rate (see below), but the result is a “cone of silence” data gap above each radar site. 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. Data voids resulting from overshoot, beam blockage, the cone of silence, and vertical gaps are determined by the geometry of the radar installation and the scan strategy. Overshoot leads to a data void caused by the elevation of the lowest beam above the surface. Beam blockage and the cone of silence prevent the acquisition of data from affected regions of the atmosphere. Gaps in vertical coverage result from the usual scan strategies that have fairly coarse vertical beam spacing at high elevation angles to achieve more rapid volume scans. The trade-off here is between accepting longer times for the volume scan, accepting larger vertical gaps (i.e., fewer tilts), or enlarging the cone of silence by limiting the elevation of the highest tilt. Some products are more tolerant of vertical gaps than others. Early termination of volume scans by NEXRAD operators seeking more rapid updates of low-level base data occasionally introduces additional voids in the high-level data. In regions of low echo strength due to the scarcity of reflectors, the signal can become so weak that the wind velocity cannot be resolved. The resulting velocity data void can affect the products involving the velocity data. Some compensation is possible by changing the waveform or the scan strategy, as in the present clear-air mode volume coverage pattern (see the following section for a discussion on volume coverage patterns). A final deficiency concerns NEXRAD precipitation estimates (J. A. Smith et al., 1996; Anagnostou et al., 1998), which are important to a variety of

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California applications. There are fundamental problems in converting the measured reflectivity to precipitation rates, as discussed in depth later in this chapter. NEXRAD SCAN STRATEGIES Surveillance of the atmospheric volume surrounding a NEXRAD site is provided through one of several available scan routines known as volume coverage patterns (VCPs). The VCPs summarized in Table 4.1 are commonly used. 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. Although these update cycles may be limiting during rapidly evolving convective weather, such update rates are adequate to capture relevant precipitation features for flash flooding (e.g., J. A. Smith et al., 1996, 2000). The elevation steps between scans at the low end of the VCPs are usually 1°, or a bit less (i.e., essentially equivalent to the radar beamwidth, which usually is defined as the angular distance between points on either side of the beam axis where the power density is half the value along the axis). This corresponds roughly to the Rayleigh resolution criterion2 for distinguishing a pair of adjacent point sources (or targets, in the radar domain). However, it does not provide the most complete depiction of storm structures that are distributed in the vertical. As noted by Brown et al. (2002), smaller elevation increments at the lowest angles would provide additional useful information, especially for echoes at long range that may extend only up into the first few elevation steps of the current VCPs. The forthcoming open RDA-RPG combination is capable of implementing a more flexible variety of VCPs, which will include ones with minimum elevation steps of order 0.5° as suggested by Brown et al. to accommodate this concern. One such VCP is to be implemented in the NEXRAD network in 2004. The smaller elevation steps will be useful in estimating precipitation in many instances, such as when the beam is blocked at the lowest tilt angle. 2   The Rayleigh resolution criterion for the resolving power of an antenna is that two points can just be resolved (i.e., distinguished as two points rather than one) when their angular separation, θ, is given by θ = 1.22*λ/D, where λ is the wavelength of the radiation and D is the diameter of the receiving antenna (Kidder and Vonder Haar, 1995).

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California TABLE 4.1 NEXRAD Volume Coverage Patternsa Scan Strategy Number of 360° Azimuthal Scans Number of Unique Elevation Steps Elevation Range of Azimuthal Scans Time to Complete (min) Clear air (short pulse) 7 5 0.5° to 4.5° 10 Clear air (long pulse) 8 5 0.5° to 4.5° 10 Precipitation 11 9 0.5° to 19.5° 6 Severe weather 16 14 0.5° to 19.5° 5 aThe azimuthal scan at the two lowest elevation angles (three for clear air long pulse) is repeated to permit one scan in a low-PRF (pulse-repetition frequency) surveillance mode (to map the reflectivity field) and another in a high-PRF Doppler mode (to measure radial velocities). At higher elevations, these functions are done during the same azimuthal scan. 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. Although the low-level coverage permits estimates of precipitation at the height of the beam, such data voids can interfere with attempts to use the vertical profile of reflectivity as an aid in projecting those estimates down to the surface. Problems with low-level coverage from high-altitude radar sites in mountainous terrain were noted in the National Research Council NEXRAD coverage report (NRC, 1995) and have been discussed further by Westrick et al. (1999), NRC (2002), and Maddox et al. (2002). There is no easy compromise to achieve both low-level and wide-area radar coverage in mountainous terrain, but the current constraint to a minimum elevation angle of 0.5° clearly exacerbates the problem. It has already been noted that the use of lower, even negative, minimum elevation angles would mitigate the problem for NEXRAD sites in Montana (Brown et al., 2002) and Utah (Wood et al., 2003b). The utility of this approach in other situations depends on the degree of beam blocking that might result in the directions of importance. The NWS has abandoned the use of the hybrid scan rainfall estimation scheme (Fulton et al. 1998) in western mountainous areas. Only the lowest nonblocked elevation angle is now used. Thus, a modification of the scan scheme to use negative elevation angles may not pose a huge algorithmic problem. PRECIPITATION ESTIMATION Estimation of rainfall using radar data has been a well-researched topic (Box 4.1) and has received extensive attention in the literature. The main

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California BOX 4.1 Radar Basis for Characterizing Precipitation The scattering of electromagnetic waves by precipitation particles and their propagation through precipitation media form the basis of radar-based characterization of precipitation. Figure 4.1 below shows the geometry of the scattering volume within the precipitation medium for ground radars. 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. At the S-band frequencies (i.e., 2.7–3.0 GHz) at which NEXRAD operates, the volumetric radar cross section, η, is given by, (1) where the dimensionless factor is the complex dielectric constant of the precipitation particle, and (1) assumes that all of the particles are composed of the same material (e.g., water). N(D) is the particle size distribution and D is the equivolume diameter of a particle. FIGURE 4.1 Geometry of ground-based radar, where the shaded area shows the resolution volume. SOURCE: V. Chandrasekar, Colorado State University.

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California The reflectivity factor Z is defined as (2) The received power is determined by the backscattering properties of the hydrometeors in the resolution volume as well as by the forward-scattering properties of the particles over the propagation path from the radar to the resolution volume. From the received voltages, the reflectivity factor, Z, and other radar measurements can be inferred. The most commonly used measurement for precipitation estimation is the reflectivity factor. In practice, it has to be measured at a specific polarization state; for NEXRAD, a horizontal polarization state is used. The measurements of precipitation described above can be elaborated on further for rain. 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. The raindrop size distribution (DSD) describes the probability density distribution function of raindrops and can be expressed as (3) where N(D) is the number of raindrops per unit volume per unit size interval (D to D + ΔD), nc is the total drop number concentration, and fD(D) is the probability density function of raindrop size. The Z-R algorithm relating the reflectivity factor, Z, to rainfall rate, R, is the most widely used rainfall estimation technique and is the basis of current NEXRAD rainfall estimates. Initially it was used as an engineering solution to the problem of estimation of rainfall from radar. The commonly used Z-R relationships are of the form (4) In the literature, the coefficients a and b both have been varied to account for such things as variability in the type of rainfall with climatic regions and season. In reality, the relationship of rainfall rate to the concentration and size of raindrops can vary from storm to storm and even between different locations and times within a storm. The radar beam geometry and its relationship to that of the storm can exacerbate these difficulties. Thus, it is not uncommon for raw radar-estimated precipitation rates and accumulations to be in error by 50 percent or more. For example, in cases of flooding from warm rain convective storms in Virginia and Texas, Baeck and Smith (1998) found the radar estimates to be in error by a factor of 3 or more.

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California However over time, various researchers have tried to provide a physical basis for the Z-R relation (see Steiner et al., 2004, for a review). One such approach is the concept of normalized drop size distributions (Sekhon and Srivastava, 1971; Willis, 1984; Bringi and Chandrasekar, 2001; Testud et al., 2001). Using the normalized form of the DSD, Z and R can be related as (5) where b is approximately 1.5 and Nw is the normalizing constant defined as the intercept of an equivalent exponential size distribution with the same water content. Equation (5) indicates that most of the fluctuations in Z-R relations can be ascribed to variability in Nw. The normalized DSD principle fixes b and accounts for all the variability in rainfall through changing Nw. Zawadzki (1984) has argued that the measurement procedure can dominate the process of deriving a ground rainfall rate from reflectivity data. Bringi et al. (2003) have studied the variability of Nw over different climatic regimes and conclude that there are significant systematic variations in Nw that could account for changes in the Z-R relations. Nevertheless, it is clear that attention to all aspects of the retrieval of rainfall rate from the radar is necessary, and the relative impact of physical processes and practical measurement issues will depend on the prevailing situation. advantage of using radar to estimate precipitation is that measurements can be made over large areas, with fairly high temporal and spatial resolution. In addition, radars can provide rapid updates of the three-dimensional structure of precipitation. Because of these advantages, radar measurements of precipitation have enjoyed widespread use for meteorological applications. Radar rainfall estimation can be classified broadly into (1) physically based and (2) statistical-engineering-based methods. Physically based rainfall algorithms, as defined here, rely on physical models of the rain medium independent of ground observations, whereas statistical-engineering methods rely on modifications to the algorithm based on the volumetric structure of the radar echoes or the information from related gauge observations. Physically based approaches attempt to solve the inverse electromagnetic problem of obtaining resolution-volume-averaged precipitation estimates from radar backscatter and forward-scatter measurements; they

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California essentially require a polarimetric capability that will become available on NEXRAD in the future (see Chapter 8). Engineering solutions, on the other hand, seek the best possible estimate of rainfall on the ground, recognizing that radar measurements are made aloft by using some feedback mechanism such as gauge data. 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. Neural networks in this context refer to computing systems that train themselves from observations to yield ground rainfall estimates from remote sensing observations. A class of hybrid procedures also is evolving that combine the advantages of physically based and statistical-engineering solutions. The potential applications of quantitative radar precipitation measurements are very broad, including hydrology, agriculture, forestry, studies of the water cycle and water resource management, nowcasting floods, and data assimilation and validation in numerical modeling. Reviewing all of the research findings and issues related to radar rainfall estimation is beyond the scope of this report; we refer the interested reader to discussions by Wilson and Brandes (1979), Austin (1987), Bringi and Chandrasekar (2001), and Krajewski and Smith (2002). Here the focus is on the sources of uncertainty affecting radar rainfall estimates in complex terrain and coastal areas (e.g., Young et al., 1999). One problem with characterizing such remote estimation of rainfall is the lack of systematic strategies and approaches to quantify and characterize the uncertainty structure of the radar rainfall products. Radar rainfall errors can be conceptualized as both systematic (i.e., causing bias) and random, and their statistical determination using ground-based systems requires appropriate approaches that take into account the uncertainty of the reference estimate (Krajewski and Smith, 2002; Habib et al., 2004). In particular, consideration of space-time scale and sample-size effects is important. 3   Nonparametric relationships refer to algorithms that cannot be expressed in a simple parametric equation, such as Z = aRb, where R is the rain rate, Z is the reflectivity factor (see Box 4.1), and a and b are constants of the algorithm.

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California from anomalous propagation. Voids created by censoring the contaminated data can be filled by horizontal interpolation (if the void is only a small area) or by vertical extrapolation of noncontaminated data taken at higher elevations. 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. Next in importance is the removal of echoes contaminated by the bright-band effect. These regions of extremely high reflectivity can be particularly pernicious, even leading to flood warnings in light rain. The height of the 0°C isotherm can be obtained from soundings, model outputs, and more directly, from the vertical structure of radar data themselves. Using all these sources of information is ideal; during frontal passages, the height of the bright band may change rapidly and radar information is crucial. With the 1.0° beamwidth of NEXRAD, the signature of the bright band can be detected clearly up to ranges of 50–70 km, depending on the thickness and intensity of the bright band. Several strategies can be used to overcome bright-band contamination; the simplest is to substitute information from below the bright-band region (if possible) or use the reflectivity of snow above the region and extrapolate the measurement to ground. Zones of partial beam blocking must be identified for each antenna elevation and a compensation factor applied to the data. This is particularly critical in complex terrain; Chapter 7 provides illustrative examples for the Sulphur Mountain radar. A question can arise as to whether it is better to use non-blocked data from higher-elevation beams or to compensate for the blocked fraction of the lowest beam. Once the decontamination and correction of data are completed, extrapolation to the ground must be done. The broadening of the radar beam with range, the minimum beam elevation angle, and Earth’s curvature all serve to increase the distance between the radar scattering volume aloft and the surface. As this distance increases with range, it often is referred to as the range effect. Here the removal of the bias associated with the vertical profile of reflectivity (VPR) is necessary. This profile can be estimated from observations at short ranges, where the scans reach down to the surface or near the surface. However, the VPR can be highly variable in time and space, so that the VPR determined at short ranges may be quite different from that at far ranges; a conservative approach is preferable. At short ranges, where voids were created by removal of ground-contaminated data, the nearby VPR should be sufficient for an effective extrapolation. The Z-R relationships also can be obtained by a direct radar-gauge comparison. This can be done in various ways. One commonly used proce-

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California dure is via regression of synchronous radar and gauge measurements. Alternative approaches have been suggested for developing observation-based mapping from radar observations to rainfall on the ground. One such procedure is the neural network technique, which provides a mechanism to build a nonparametric relation between surface rainfall and the vertical profile of reflectivity aloft (Liu et al., 2001). THE EVOLVING NEXRAD SYSTEM The NEXRAD system configuration is not static, but rather continues to evolve through an ongoing NEXRAD Product Improvement (NPI) Program. Stated objectives of this program (Saffle et al., 2001) are to ensure the capability to implement advances in science and technology to improve forecasts, watches, and warnings; minimize system maintenance costs; and support relatively easy upgrades in technology so that a large-scale NEXRAD replacement program may be indefinitely postponed. The NPI Program provides a means for introducing continuing improvements in science and technology into the NEXRAD system on an ongoing basis. The NPI Program works to develop and introduce system improvements in an orderly and seamless manner. Thus, the NEXRAD system a decade hence will be substantially improved over that of today. The current NPI Program emphasizes two major thrusts. One is to replace the data acquisition and processing systems in the original NEXRAD with open-system hardware and software. This development increases the overall capability of the NEXRAD system for data acquisition, processing, display, dissemination, and archiving; facilitates implementation of new algorithms for processing radar data; and reduces costs for system operation and maintenance. Field deployment of the ORPG component, which executes the NEXRAD algorithms and produces the image products, was completed in FY 2002. The ORPG improvements provide a capability for (1) data quality improvements, such as AP clutter detection and suppression and identification of nonprecipitation echoes; (2) 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. Introduction of the second component, the open RDA (ORDA) unit, is in progress. The ORDA improvements will include (1) availability of a modern Doppler spectral processing platform,

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California including digital receivers, for improved data fidelity; (2) a provision for range-velocity ambiguity mitigation techniques using phase coding and dual-pulse repetition interval waveforms; (3) the capability for polarimetric sensing and processing to more accurately measure hydrometeor properties, such as drop size distributions and precipitation phase; and (4) custom VCPs to allow site-specific volume scans adapted to local weather needs. These improvements are expected to be operational in the next few years. The third major component of the original NEXRAD, the PUP display unit, has been replaced with open-architecture systems in different ways by the three major NEXRAD user agencies. The second major thrust of the NPI Program is directed toward introduction of a polarimetric capability for NEXRAD. Such a capability could provide improved precipitation measurements as well as new capabilities for identifying hydrometeor types (e.g., recognizing the presence of hail, discriminating between rain and snow regions) and enhanced ability to screen out artifact echoes such as those caused by ground clutter or birds. The polarimetric capability is scheduled for implementation in the next few years. The NPI Program is not limited to modifications of NEXRAD itself. An enhanced software environment, termed Common Operations and Development Environment (CODE), is being provided to facilitate use of the open-architecture system capabilities and linkage of the NEXRAD data to other agency weather data systems such as AWIPS. Data from other radar systems also can augment the NEXRAD dataset, and plans and procedures are being developed to incorporate data from appropriate FAA and other radar systems. These include the FAA’s Terminal Doppler Weather Radar (TDWR) and short- and long-range surveillance radars (Air Route Surveillance Radar [ARSR] systems and Airport Surveillance Radar [ASR]) (see Chapter 8), as well as atmospheric wind profilers (Rich, 1992) and radar systems operated by television stations and other private entities. This development will enhance the available coverage and also the backup capabilities in case of a NEXRAD outage. The national coverage, improved accuracy, and rainfall estimation capabilities of the NEXRAD system have advanced the practice of hydrologic forecasting and water resource management. If dual-polarization capabilities are incorporated into the current system as planned, further improvements in precipitation measurements will occur, particularly in the quantification of high-intensity rainfall rates and the characterization of snowfall. However, closer radar spacing would be needed for representative near-surface coverage throughout the continental United States. The latter implies the

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California need for an affordable means of dealing with the inherent inability of any widely spaced network to provide comprehensive near-surface surveillance over large portions of the country. SITING OF THE NEXRADS In preparation for installation of the national network of NEXRADs, surveys were performed nationwide by SRI International, Inc., and Metcalf & Eddy, Inc., to evaluate potential radar sites. A three-step process was employed by the survey team to site the NEXRADs. First, an Initial Site Assessment was done, which entailed an examination of a potential site by the analyses of maps and other data without visiting the site. Then a Preliminary Site Survey was conducted by visiting the site to assess the potential radar coverage and the ability of the site to fulfill the requirements of the three principal users (i.e., FAA, NWS, DoD). A report was prepared for the DOC as a result of the preliminary siting. 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. General Siting Considerations To select the best sites for the NEXRADs, the DOC outlined four key factors that were important in the selection of sites, including (1) optimization of radar location for an unobstructed view in the prevailing direction of approaching hazardous weather; (2) minimization of ground clutter over the areas of greatest interest; (3) minimal impact on the environment and electromagnetic interference; and (4) no degradation of existing radar coverage over coastal regions. More detailed information on siting weather radars is provided by Leone et al. (1989), Doviak and Zrnic (1985), and the NEXRAD Siting Handbook (NEXRAD Joint System Program Office, 1983). Site selection was based on providing maximum radar coverage in the priority areas required by the NWS, FAA, and DoD over highly populated regions while keeping installation and operational costs at a minimum. Both the NWS and the FAA require radar coverage over large areas, the former needing coverage over high-population areas to detect severe weather,

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California particularly at low altitudes, and the latter needing coverage of both airport terminal areas and airways en route. The NWS priority regions of coverage were based on climatological records of the frequency of occurrence and prevalent locations of severe weather in the area of interest. Outside the priority coverage areas, NEXRAD Doppler radars can detect most types of hazardous weather out to a range of approximately 148 km. The DoD requires radar coverage within 65 km of its highest-priority military and civilian facilities. Weather radar sites in existence prior to the installation of NEXRADs were given first consideration as potential sites, but they did not always meet the requirements outlined by the DOC; this was the case for the Sulphur Mountain radar (see below). An important consideration in the siting of a NEXRAD was its contribution to the national network of NEXRAD coverage. Overlapping coverage of NEXRADs can be crucial for the early detection of severe weather from directions not well observed by the closest NEXRAD. An additional criterion for the network of NEXRADs was the ability to provide coverage of FAA airways from 1.83 km (6000 ft) AGL to 21.34 km (70,000 ft) MSL over the continental United States east of the Rocky Mountains and over the San Francisco-San Diego Corridor (to the Sierra Nevada Mountains) and 3.05 km (10,000 ft) AGL to 21.34 km (70,000 ft) MSL elsewhere. NEXRAD Site Surveys for Los Angeles, California (Sulphur Mountain Site) Initial, preliminary, and in-depth site surveys were performed for the Los Angeles, California, area during 1986–1987. A NWS Weather Surveillance Radar-1974 C-band (WSR-74C) radar was located in the Los Angeles basin at the time. Due to the severe ground clutter return from the surrounding 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), were known to be quite severe. Given these limitations in radar performance and the NWS requirement of priority radar coverage down to 0.61 km (2000 ft) AGL to detect severe weather in the prevailing approach zones (Figure 4.2), an elevated site was considered critical to minimize the effects of ground clutter and AP. Several candidate sites were examined including Sulphur Mountain, Saddle Peak, and Castro Peak. In response to a request by the NWS Office of Hydrology, these sites were visited more than once to address their suitability in providing the best radar coverage possible in a region of com-

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California 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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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California plex terrain. Saddle Peak was eliminated from consideration due to the existence of several communication towers that were already in place, the performance of which would have been negatively affected by the installation of NEXRAD. 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. This site is located 78 km northwest of downtown Los Angeles, 22 km north of the NWS Los Angeles-Oxnard WFO, and 19 km northeast of downtown Ventura and the town of Ojai (see Chapter 6). The site is at the top of Sulphur Mountain at an elevation of 0.83 km (2726 ft) MSL (Figure 4.3). A series of ridges blocks radar coverage to the north, but the site survey indicated that the radar would be well positioned to provide good coverage over Los Angeles and surrounding communities (Figure 4.4) out to the 148-km range and over potential flooding regions at the base of the San Gabriel Mountains (Leone and Johnson, 1986). Using selected clutter models, the survey team examined degradation of radar coverage due to ground clutter for the Sulphur Mountain site. It was anticipated that the radar would be able to reliably detect weather beyond the 3.7-km to 9.3-km range. The overall intensity of ground clutter return was expected to be much lower than that observed at the WSR-74C radar site. Adjustments in the pulse-repetition frequency (PRF) used by the radar were expected to improve weather coverage in a particular area of interest by minimizing the overlap of long-range—that is, greater than 148-km—radar echoes with ground clutter areas. Because the minimum elevation angle of the main beam is 0.5°, the current institutional limitation, the center of the radar antenna would have to be located at a height at least level with the average height of the nearby hills in the directions of interest. The necessary height of the radar tower was determined to be approximately 24 m (81 ft), thus a standard 20-m tower was proposed (Burns et al., 1987). At an elevation of 0.5°, the height of the center of the radar beam at a range of 148-km will be approximately 2.57 km (8432 ft) above the site level. Figure 4.5 provides an illustration of this concept graphically, but at a range of approximately 120 km from the radar, where the height of the center of the beam at 0.5° elevation above site level is 1.91 km (6266 ft) (i.e., the difference between the 2.74-km [8990-ft] height of the beam above MSL at 120 km range and the 0.83-km [2726-ft] height of the Sulphur Mountain site). Thus, the elevation of the

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California FIGURE 4.3 Topographical map showing the location of the Sulphur Mountain radar site at an elevation of 0.83 km (2726 ft). The site elevation was initially estimated at 2720 ft, as shown in this figure, but it was later updated to 2726 ft. SOURCE: SRI International, Inc.

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California 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) and for the immediate surrounding radars, including Vandenberg Air Force Base (AFB), Edwards AFB, and San Diego. Note that the March AFB radar has since been replaced with the Santa Ana radar. SOURCE: SRI International, Inc.

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California FIGURE 4.5 Cartoon depicting the Sulphur Mountain radar, at an elevation of 0.83 km (2726 ft) and with a 20-m tower, transmitting at the current minimum beam elevation of 0.5°. The cartoon shows how the distance between the beam scan and Earth increases with distance from the radar due to Earth’s curvature and the rising of the beam. The 3.05 km (10,000-ft) ASL (above site level) swath also is shown. SOURCE: Modified by V. Chandrasekar from SRI International, Inc.

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Flash Flood Forecasting Over Complex Terrain: With an Assessment of the Sulphur Mountain NEXRAD in Southern California proposed Sulphur Mountain site was deemed sufficient to provide radar coverage above the strong inversion layer that often covers the Los Angeles area. After completion of the preliminary and in-depth site surveys, construction at the site began on November 15, 1993. 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. A complete chronology of events leading up to the siting of the Sulphur Mountain NEXRAD can be found in Appendix C.