VOLUME COVERAGE PATTERNS
Smart scanning strategies are extremely important
in optimizing the WSR-88D’s detection capability.
Unlike older units, the WSR-88D has several different
operating modes that can function automatically and
continuously. The antenna is also capable of scanning
(rotating) at several different elevation angles.
The WSR-88D has a beamwidth of 1° and thus
creates 360 "beams" or radials of information per
elevation angle. The area covered by one complete
360° rotation at one elevation angle is called an
elevation scan. The area covered by the radar beam as
the antenna rotates through several elevation scans (0°
through 20°) is known as a volume scan.
The RDA can invoke computer-driven
instructions known as Volume Coverage Patterns
(VCPs). VCPs determine how thoroughly the
atmosphere is to be scanned. They dictate the number
of areas to scan, the specific elevation angles to
examine, and the time allotted for this task. The
scanning process is repeated indefinitely, or until a
change in VCP is made. Some products may be
displayed after just one elevation scan. Thus, each
"slice" of the atmosphere is immediately available for
examination. Other products are not available until all
elevation angles have been scanned for that particular
VCP. These products are called volumetric products
because the entire volume of radar coverage must be
scanned before they can be produced.
Two operational modes exist with the WSR-88D:
mode A (precipitation mode) and mode B (clear air
mode). Each mode contains two VCPs designed
specifically for that type of environment. Each VCP
possesses certain capabilities, as well as limitations.
Mode A (precipitation mode)
Mode "A" is used during periods of precipitation
and convective activity. This mode consists of two
short pulse (1.57 microsecond) modes, VCP 11 and
VCP 21.
VCP 11.—This pattern scans 14 elevations in 5
minutes. It contains fewer data gaps than other VCPs.
This increases radar accuracy, making VCP 11 a
logical choice for examining nearby storms. VCP 11
provides the most complete coverage of all VCPs, and
it is normally used when significant echoes are present
or severe weather is occurring or anticipated. Weather
events are more accurately depicted on final products.
Unfortunately, VCP 11 requires a high degree of data
processing, significantly increasing RPG workload.
VCP 21.—This scans nine elevations in 6 minutes.
It provides adequate coverage for non-severe events
(continuous rain, drizzle). Increased data gaps (less
scans) effect performance close to the RDA. However,
adequate coverage is provided for distant storms.
Mode B (clear-air mode)
"Clear-air" does NOT imply cloud-free skies, but
rather the absence of precipitation from radar. In other
words, the criteria for the WSR-88D to use the
precipitation mode (VCP’s 11 and 21) has not been met
over the coverage area of the radar. Mode "B" contains
VCPs 31 and 32; both complete five elevations in 10
minutes, both scan slower than mode A, and both
contain larger data gaps than VCP 11 or 21. However,
in precipitation-free environments, these VCPs are
extremely helpful in detecting the early formation of
convective precipitation. They are also helpful in
detecting air mass discontinuities and monitoring
precipitation onset.
VCP 31.—This pattern contains the longest pulse
(4.7 microseconds) and the lowest PRF of all VCPs
(recall, a longer pulse contains more power and
increase sensitivity). VCP 31 scans slowly, to allow
sufficient return of energy from clear-air scatterers.
The greatest drawback to using VCP 31 is its reduced
Doppler capability. While winds and motions are still
observed, stronger winds are more likely to aliase
when scanning with VCP 31.
VCP 32.—This pattern uses relatively shorter
pulses and a higher PRF than VCP 31. It remains
sensitive enough to observe most scatterers while
increasing Doppler thresholds in clear-air mode. Of
course, the weakest features may be lost.
In clear-air mode, the decision to use VCP 31 or 32
(long pulse vs short pulse) is dictated by the
meteorological situation and is not always an easy
choice to make. For example, on windy days where
ample scatterers are present (clouds, moisture, etc.),
VCP 32 may be the best choice. Its shorter pulse makes
it less likely to aliase velocities. When the atmosphere
contains few scatterers, such as on cold, dry days,
longer pulses may be required (VCP 31).
The radar makes some decisions by itself. The
WSR-88D is designed to operate continuously in "B"
mode, but switches automatically to "A" mode
whenever precipitation is detected. Precipitation is
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