Geoscience Reference
In-Depth Information
2.1 Considerations and requirements for storm surveillance
Table 1 lists the radar parameters with which the surveillance mission is supported.
Discussions of the reasons behind choices in volume coverage and other radar attributes of
the WSR-88D network, with principal emphasis on Doppler measurements, follows.
Requirement
Values
Surveillance:
Range
Time
Volumetric coverage
460 km
< 5 min
hemispherical
SNR
> 0 dB, for Z= - 8 dBZ at r=50 km (exceeded by ~5 dB)
Angular resolution
1 o
Range sampling interval:
For reflectivity
For velocity
Δ r  1 km; 0 < r  230 km; Δ r  2 km; r  460 km
Δ r = 250 m
Estimate accuracy:
Reflectivity
Velocity
Spectrum width
1 dB; SNR>10 dB;  v = 4 m s -1
1 m s -1 ; SNR> 8 dB;  v = 4 m s -1
1 m s -1 ; SNR>10 dB;  v = 4 m s -1
Table 1. Requirements for weather radar observations.
2.1.1 Range
Surveillance range is limited to about 460 km because storms beyond this range are usually
below the horizon. Without beam blockage, the horizon's altitude at 460 km is 12.5 km; thus
only the tops of strong convective storms are intercepted. Quantitative measurements of
precipitation are required for storms at ranges less than 230 km. Nevertheless, in the region
beyond 230 km, storm cells can be identified and their tracks established. Even at the range
of about 230 km, the lowest altitude that the radar can observe under normal propagation
conditions is about 3 km. Extrapolation of rainfall measurements from this height to the
ground is subject to large errors, especially if the beam is above the melting layer and is
detecting scatter from snow or melting ice particles.
2.1.2 Time
Surveillance time is determined by the time of growth of hazardous phenomena as well as
the need for timely warnings. Five minutes for a repeat time is sufficient for detecting and
confirming features with lifetime of about 15 min or more. Typical mesocyclone life time is
90 minutes (Burgess et al., 1982). Ordinary storms last tens of minutes but microbursts from
these storms can produce dangerous shear in but a few minutes. Similarly tornadoes can
rapidly develop from mesocyclones. For such fast evolving hazards a revisit time of less
than a minute is desirable but not achievable if the whole three dimensional volume has to
be covered. The principal driver to decrease the surveillance time is prompt detection of the
tornadoes so that timely warning of their presence can be issued. Presently, the lead time for
tornado warnings (i.e., the time that a warning is issued to the time the tornado does
damage) is about 12 minutes (see Section 5).
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