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Weather Radar
The
WSR-88D (NEXRAD) equipment at the Facility is a PUP associated with the
actual radar at Mobile, AL (latitude 30°40'44"
north and longitude 088°14'24"
west).The height of the Mobile radar
is 289' MSL.
a.NEXRAD
helps identify sea breeze fronts before activity is visible on satellite
images.To identify sea breeze fronts
use either low elevations from Base Reflectivity or Composite Reflectivity
products.The front will normally
begin to appear northwest through northeast from the center of the radar
in the non-precipitable range (below 18 dBz) of return as a thin line of
radar return in the range of 1-14 dBz.
b.Severe
weather detection with the NEXRAD is slightly different in the warm, moist,
unstable air masses of the southeast region than it is in a region where
the freezing level is generally lower.This
makes the hail indicators on the NEXRAD fairly useless in the local region.One
method in helping to detect for possible severe weather was researched
at the Tampa, Florida radar site.It
involves using a Vertically Integrated Liquid (VIL) Value of the Day (VOD)
determined by the following formula:1500 ¸
(úT500ê+÷T400ê),
where temperatures are in degrees Celsius.The
formula for both analyzed and forecast temperatures is used at each level,
with the average value from each equation used to determine the VIL Value
of the Day.
For
example:
500
temp = -11.7 °
C
400
temp = -24.3 °
C
forecasted500
temp = -13.9 °
C
forecasted400
temp = -26.1 °
C
1500 ¸
(11.7+24.3) =(don't forget to use
absolute values)
1500 ¸
36 = 41.67 »
42,
and1500¸
(13.9+26.1) =(these are the forecasted
values)
1500 ¸
40 = 37.5 »
38.
In the example
above, the two values are averaged [(42+38) ¸
2 = 40] to give you the VIL VOD of 40.
If
there is a greater than 15 unit change in the VIL in one volume scan which
then exceeds the VOD, investigate the storms further.If
there is a greater than 20 unit change in the VIL in 2-3 volume scans which
then exceeds the VOD, investigate the storms further.
When
a VIL VOD is reached, it holds more significance if there is more than
one pixel of high value.If there
are four or more pixels just below the VIL VOD, investigate storms further.
When
looking at reflectivity, a 50 dBz return or greater above 30,000' is a
good indicator of possible severe weather.
When
using the VIL product with tropical storms and hurricanes, maximum values
are normally limited to the mid 20s, with the average values between 5
and 10.This helps in detecting
embedded thunderstorm activity within the tropical storms.VILs
ranging from approximately 15 to 25 are associated with the embedded activity.
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