Introduction to the WSR-88D (NEXRAD) for Ornithological Research1
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Introduction to the WSR-88D (NEXRAD) for Ornithological Research1 Robert H. Diehl2,3 and Ronald P. Larkin4 ________________________________________ Abstract The system of Doppler weather surveillance radars gists, entomologists, and others. This system of radars known as WSR-88D or more popularly as NEXRAD differs from its predecessor in several important ways: helped transform radar ornithology in the United States it is Doppler capable and considerably more sensitive; into a field that today attracts considerable attention it is digital and established and increasingly sophisti- from scientists and laypersons alike. As interest in or- cated data distribution channels allow ready access to nithological applications of WSR-88D grows, so does data for a large user community; radars within the sys- the need to provide perspective on its use. In this paper, tem are uniform by design and widely distributed. we introduce WSR-88D and consider issues associated with interpretation of WSR-88D imagery and quantifi- With these improvements, the system boasted enor- cation of data. Because particular interest surrounds mous potential to advance our understanding of beha- radar’s potential to associate bird numbers with speci- viors and patterns of animal distribution and movement fic habitats or land use features, we address some of the across many spatial and temporal scales. As striking challenges in making such associations focusing speci- radar imagery of bird migrations began to appear in fically on migrating landbirds. Readers interested in scientific presentations and the popular media, many applying WSR-88D should gain appreciation of the biologists, governmental and non-governmental organi- technical challenges as well as the biological potential zations, and members of the general public realized the of this radar system for ornithological research. potential of WSR-88D for avian conservation, manage- ment, and education. However, imagery from WSR- 88D failed to convey the complexities of treating these data quantitatively while avoiding sources of bias. In this paper, we present a brief introduction to meth- Introduction odological issues concerning use of the WSR-88D for New technology spurs scientific progress. Bird migra- ornithological research. Although many concepts pres- tion research benefited greatly from the introduction of ented here apply to other radars, this paper is not in- radar as a biological tool (Lack and Varley 1945). tended as an exhaustive review of applied radar meth- Radar revealed the structure and intensity of nocturnal ods in ornithology (see also Eastwood 1967; Vaughn bird migration (e.g. Nisbet 1963, Eastwood 1967, Gau- 1985; Bruderer 1997a, b). Rather, our goal is to pro- threaux 1971) beyond the pioneering moon watching vide perspective and raise awareness regarding some studies which first showed nocturnal migration on a applications of WSR-88D in studying migrating birds. continental scale (Lowery 1951, Lowery and Newman Patterns of radar echo have numerous biological and 1966). Nisbet (1963) and Gauthreaux (1970) developed non-biological causes that must in part be understood novel approaches to quantifying bird densities from the to interpret and apply this technology properly. We analog displays of WSR-57 (Weather Surveillance introduce basic WSR-88D operation and its base Radar, design year 1957), the United States’ first gen- products (reflectivity, velocity, and spectrum width) eration of weather surveillance radar. Years later, com- and consider range bias, resolution, identity of echo missioning of the WSR-88D (Weather Surveillance sources, aspect effects, clutter, and refraction in prop- Radar, 1988 Doppler) or NEXRAD (NEXt generation erly interpreting and quantifying radar echoes from weather RADar) in the mid-1990’s represented a sub- WSR-88D. These are interacting factors. For example stantial leap in radar technology available to ornitholo- we present clutter and refraction as separate sections, __________ but when atmospheric conditions permit, refraction may dramatically increase the influence of clutter. Finally, 1A version of this paper was presented at the Third Interna- tional Partners in Flight Conference, March 20-24, 2002, we consider the association between radar echoes from Asilomar Conference Grounds, California. migrating birds just after takeoff with their source 2Department of Animal Biology, University of Illinois, Urbana, stopover habitats. Understanding and recognizing radar Illinois 61801, USA. artifacts such as velocity aliasing and second-trip echo, 3 Current address: Department of Biological Sciences, University however, are beyond the scope of this paper. Doviak of Southern Mississippi, Hattiesburg, Mississippi 39406, USA. and Zrnic (1993) and Rinehart (1997) consider these in E-mail: [email protected] 4Illinois Natural History Survey, 607 E. Peaabody Dr., some detail. For simplicity, we emphasize landbird Champaign, IL 61820, USA. migration, but the principles often apply more broadly. USDA Forest Service Gen. Tech. Rep. PSW-GTR-191. 2005 876 WSR-88D for Ornithological Research – Diehl and Larkin Figure 1֣ Locations of 154 radars comprising the current WSR-88D system. Radars in Guam and Puerto Rico are not shown. WSR-88D Products and Operation involving multiple radars, therefore, might require that data from each radar be treated individually. WSR-88D is a system of 154 S-band (10-cm wave- length) radars distributed across the continental United Each radar in the WSR-88D system switches between States, Alaska, Hawaii, Guam, and Puerto Rico (fig. 1). two modes of operation and characterizes echoes Because WSR-88D by design monitors weather through three base radar products–reflectivity, radial systems over large geographic areas, the system is velocity, and spectrum width. These modes differ pri- inherently well suited to studying the larger spatial marily in the number of sweeps in a volume scan, tem- scales of bird distributions and movements aloft poral resolution, and for Level III data, scaling and (Gauthreaux and Belser 2003, Gauthreaux et al. 2003). resolution of reflectivity. In clear-air mode, the radar’s The distribution of radars in the WSR-88D system does beam sweeps through five different elevations between not define the spatial extent that bird movements can ~0.5° and ~4.5° in ~1° increments to complete a vol- be studied by surveillance radar in North America. ume scan. Precipitation mode volume scans contain Both Canada (http://weatheroffice.ec.gc.ca/radar/index either 9 or 14 sweeps in varying increments ranging _e.html) and Mexico (http://smn.cna.gob.mx/radares/ from ~0.5° to ~19.5°. All figures depicting radar im- radares.html) maintain weather surveillance radar sys- agery in this paper show data from the 0.5° elevation tems that together increase the spatial scale of monitor- sweep. Stacking the sweeps from these volume scans, ing that might be possible if data from these different we can view vertical structure in bird density and systems were merged. movements (Diehl et al. 2003). However, earth’s cur- vature frequently limits the usefulness of such cross- Radars within the WSR-88D system are identical in sections to short ranges. design, simplifying system-wide understanding and application. However, operational parameters (see also Reflectivity ‘Resolution and Data Sources’) and other factors such as ground clutter rejection and radar siting (see below) Each image in figure 2 depicts echo strengths, or radar vary by radar (Crum and Alberty 1993). Analyses reflectivity factor (hereafter referred to as “reflectivi- ty”), as colors that vary with reflectivity. Reflectivity is USDA Forest Service Gen. Tech. Rep. PSW-GTR-191. 2005 877 WSR-88D for Ornithological Research – Diehl and Larkin usually presented in units of Z, a standard measure distributed throughout a volume of space known as a from radar meteorology which describes the amount of pulse volume. Complete description of this term is be- echo caused by distributed targets (such as rain, in- yond the scope of this paper; however the size of a sects, and birds) in the volume of space being pulse volume is determined by the width of the radar’s measured (see ‘Beam Geometry’ below). Because Z beam and the resolution in range along the beam. The varies greatly depending on the size and number of WSR-88D beam is conical in shape and nominally 1° targets, it is usually presented logarithmically as dBZ. wide. Along the beam, the amount of radio energy Under suitable circumstances, WSR-88D Z serves as a returning to the radar is recorded every kilometer for proportional substitute for bird density, although reflectivity, and every 250 m for velocity and spectrum understanding the relationship between Z and bird den- width. The pulse volume increases in size with distance sity remains an important and complex area of research from the radar and represents the smallest quantifiable (Gauthreaux and Belser 1998, Black and Donaldson measure of reflectivity or velocity. 1999, Gauthreaux and Belser 1999, Diehl et al. 2003). However, many applications of WSR-88D do not Figure 4 shows details of the beam geometry in reflect- require absolute estimates of bird numbers or density. ivity and velocity. In this two-dimensional view, each Comparing reflectivities may be perfectly adequate. rectangle represents a top down view of one pulse volume. Because the beam is conical, we know its Radial Velocity height matches its width. Notice range resolution in velocity is four times that in reflectivity