tional background on the system characteristics. Coverage over the eastern two-thirds of the country is essentially complete, though significant limitations exist in coverage near the surface. There are some gaps in western regions, and the combination of high-altitude sites and mountainous terrain presents difficult problems in several areas (Westrick et al., 1999).

Surveillance of the atmospheric volume surrounding a NEXRAD site is provided through one of several available volume coverage patterns (VCPs). The VCPs summarized in Table 2–1 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.

Primary Data and Derived Products

The NEXRAD is a pulse-Doppler system that measures three primary characteristics of the radar echoes: equivalent radar reflectivity factor, commonly referred to as reflectivity and designated by Zc; Doppler (radial) velocity, designated by v or vr; and the width of the Doppler spectrum, designated by sv. 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 (though the latter is constrained presently to no less than half the pulse duration).

These displays, together with products (summarized in Radar Operations Center, 2002) and results of the algorithms discussed below, are developed from

TABLE 2–1. WSR-88D 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 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 (Adapted from Crum et al., 1993)



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