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Revision 6 Feb/2009
KMD 550/850 FIS Addendum
10. Echoes from migrating birds, bats, and insects will be displayed as
circular patterns of level 1 reflectivity centered near NEXRAD sites.
11. Significant variation in humidity with altitude can cause reflections
from the Earth's surface. These reflections are also displayed as large
circular or oval areas of uniform low intensity.
12. Differences will be noted between the base reflectivity (VDL) and
composite (XM) products. The base reflectivity product displays the
lower extent of cloud masses, so the extent of water droplets appears to
be less. The composite product provides more airborne water reflection
seen at higher angles as viewed from the NEXRAD ground sites, so
higher level features of the convective activity will be given. For example,
the "blowout" portion of thunderstorms (the anvil portion) containing high
altitude ice particles is often depicted as light precipitation and might be
confused for lower altitude light precipitation. Either base or composite
product provide accurate, but somewhat time delayed depiction of high
levels of precipitation for fully developed or late stage thunderstorms.
NEXRAD LIMITATIONS
The following are limitations on the use of NEXRAD data:
1.
NEXRAD does not provide sufficient information to determine cloud
layers, precipitation characteristics (frozen vs. liquid, hail vs. rain, etc),
nor does it provide information on turbulence, only the amount of
reflected energy from the precipitation.
2.
The displayed NEXRAD product does not provide sufficient detail to
infer future weather trends. Due to delays involved in product creation
and transmittal, the pilot should always review the age bar to determine
information currency.
3.
Due to NEXRAD site
location limitations, terrestrial
blockages and outages,
NEXRAD coverage gaps
exist. Coverage gaps are dis-
played as a cross hatched
pattern as shown in Figure 2.
Lack of reflectivity in a cov-
erage gap area should not be
construed as a lack of precip-
itation. Coverage gaps are
portrayed indentically on
either VDL or XM.
4.
The resolution of NEXRAD VDL data is 4 kilometers (km).Thus,
when zoomed in on the display, each square block is 4 km on a side.
The intensity level reflected by the square will be the highest level sam-
pled within the 4 km area.
Value Added Service Weather Products
Figure 2
Coverage Gap
