tendency to become organized into the characteristic
scales and patterns that are observed.
Precipitation areas can be grouped into categories
of size and lifetime. Observations show that synoptic
areas that are larger than 29,000 nmi2 have lifetimes of
one day or longer; large mesoscale areas that range from
2,900 to 29,000 nmi2 last several hours; small mesoscale
areas that cover 54 to 216 nmi2 last about an hour; and
elements that are about 5.4 nmi2 in size usually last no
longer than half an hour. Although the smallest
elements have the highest rain rates, the major
contribution to the total rainfall over large areas comes
from the small and large mesoscale features.
The Mesoscale Convective System (MCS) includes
all precipitation systems 11 to 270 nmi wide that contain
Examples in middle-latitudes are
large isolated thunderstorms, squall lines, Mesoscale
Convective Complexes (MCCs), and rainbands. The
aerial extent of these systems is generally too large to
be covered by a single radar. Examinations of
composite maps from a network of radars is required to
capture the full extent of most MCSs.
Refer to part B of the FMH-11 for further discussion
of mesoscale convective systems.
THE BRIGHT BAND
In stratiform precipitation where lower portions of
the echo are at above freezing temperatures, a thin layer
of relatively high reflectivity (bright band) is often
observed just below the level of 0°C. As snowflakes
descend into this layer, they begin to melt and stick
together. The radar reflectivity of the large wet
snowflakes is higher, principally because of their large
size and because the dielectric constant of water exceeds
that of ice by a factor of five. Descending further below
the bright band, the snowflakes become more compact,
break up, and become raindrops. The raindrops fall
faster than snowflakes so their concentration in space is
diminished. This decrease in size and number density
of hydrometers accounts for the lower reflectivity just
below the bright band.
The last section of this chapter will deal with the
Next Generation Weather Radar (NEXRAD)
WEATHER SURVEILLANCE RADAR
LEARNING OBJECTIVES: Recognize criteria
for the three groups of alert thresholds.
Understand data access procedures for the
WSR-88D. Be familiar with the various user
functions, as well as archiving procedures.
The following discussion will deal with weather
radar alert areas and thresholds, the editing and sending
of data, and the various user functions of the WSR-88D.
ALERT AREAS AND THRESHOLDS
Radar product generators (RPGs) can automatically
issue alerts upon detection of user-specified
meteorological phenomena. Automated alerts relieve
the operator ffom constantly monitoring the Principle
User Processor (PUP) for significant meteorological
phenomena and allow automatic generation of paired
products when a particular phenomena occurs. The
forecaster determines various thresholds for
meteorological phenomena. The PUP operator then
selects which phenomena and associated threshold
values to use, based upon local watch/warning criteria.
Alert thresholds allow several individual PUPs to
select agency unique alert threshold criteria.
threshold criteria are broken down into three groups
preset at the Unit Control Positions (UCPs):
1. Grid Group- Alert areas based on geographical
points or grid boxes that occur within a user-defined
alert area. The first grid box within a defined alert area
that meets or exceeds a phenomena threshold triggers
2. Volume Group These are alerts occurring
within a user-defined area requiring completion of a
volume scan. Detection is based on meteorological
phenomena meeting or exceeding an assigned threshold
or algorithm output. If there is more than one storm
exceeding a category threshold in the same alert area,
the alert triggers on the stronger storm.
3. Forecast Group Alert categories with this
group are storm based only and are triggered when
phenomena is located in or forecast to enter an active
alert area. This group is storm oriented (based on the