Geoscience Reference
In-Depth Information
2.1.3 Volumetric coverage
The volume scan patterns currently available on the WSR-88D have maximum elevations up
to 20° and many are accomplished in about 5 minutes. Meteorologists have expressed a
desire to extend the coverage to higher elevations to reduce the cone of silence. It is fair to
state that the 30
o
elevation might be a practical upper limit for the WSR-88D. Top elevations
higher than 20
o
have not been justified by strong meteorological reasons.
2.1.4 Signal to noise ratio
The SNR listed in Table 1 provides the specified accuracy of velocity and spectrum width
measurements to the range of 230 km for both rain and snowfall rates of about 0.3 mm of
liquid water depth per hour. That is, at a range of 230 km the SNR is larger than 10 dB thus
the accuracy of Doppler measurements to shorter ranges is independent of noise and solely
a function of number of samples and Doppler spectrum width.
2.1.5 Spatial resolution
The angular resolution is principally determined by the need to resolve meteorological
phenomena such as tornados and mesocyclones to ranges of about 230 km, and the practical
limitations imposed by antenna size at wavelength of 0.1 m. Even though beamwidth of 1
o
provides relatively high resolution, the spatial resolution at 230 km is 4 km. Because the
beam of the WSR-88D is scanning azimuthally, the effective angular resolution in the
azimuthal direction is somewhat larger (Doviak & Zrnic, 2006, Section 7.8); typically, about
40% at the 3 RPM scan rates of the WSR-88D. This exceeds many mesocyclone diameters,
and thus these important weather phenomena, precursors of many tornadoes, can be
missed. Tornadoes have even smaller diameters and therefore can not be resolved at the 230
km range.
The range resolution is indirectly influenced by the angular resolution; there is marginal
gain in having range resolution finer than the angular one. For example better range
resolution can provide additional shear segments and therefore improve detection of
vortices at larger distance. The range resolution for reflectivity is coarser for two reasons: (1)
reflectivity is principally used to measure rainfall rates over watersheds which are much
larger than mesocyclones and (2) reflectivity samples at a resolution of 250 m are averaged
in range (Doviak & Zrnic, 2006, Section 6.3.2) to achieve the required accuracy of 1 dB.
2.1.6 Precision of measurements
The specified 1 dB precision of reflectivity measurements (Table 1) provides about a 15%
relative error of stratiform rain rate (Doviak & Zrnic, 2006, eq 8.22a). This has been accepted
by the meteorological community. The specified precisions of velocity and spectrum width
estimates are those derived from observations of mesocyclones with research radars. The 8
dB SNR is roughly that level beyond which the precision of velocity and spectrum width
estimates do not improve significantly (Doviak & Zrnic, 2006, Sections 6.4, 6.5). But, it is
possible that lower precisions can be tolerated and benefits can be derived therefrom. For
example, it has been proposed (Wood et al., 2001) that velocity estimates be made with less
samples (e.g., by a factor of two) in order to improve the azimuthal resolution. Although
