An increasing moisture source and gravity waves in
the velocity field can also be detected. Gravity waves
should coincide with the alternating increases and
decreases in reflectivity. This may also be apparent in
the Echo Tops product if the minimum reflectivity
threshold of 18 dBZe is met.
In the Clear Air mode, cloud detection maybe best
using VCP-31 due to a better signal-to-noise ratio. If
the minimum reflectivity threshold of 18 dBZ is met,
the Echo Tops product can provide an indication of the
top of a cloud layer.
Layer Composite Reflectivity
products may show returns at given layers, indicative of
clouds. These products should be supported with the
use of the Reflectivity product to determine the
existence and extent of the cloud layers. Stratus clouds
and fog will usually not be detected by the radar due to
small cloud particle size.
The Velocity Azimuth Display Wind Profile product
may be useful in determining moisture advection for
development of cloud layers, depending on distance
from the radar.
For further discussion of preconvective, convective,
multicell and supercell development as well as severe
storm identification, refer to part D of the FMH-11.
THE STRUCTURE OF LARGE-SCALE
PRECIPITATING WEATHER SYSTEMS
The character of precipitation is largely controlled
by vertical air motions. Radar observations of the aerial
extent, and lifetime of precipitation systems are
evidence of the physical processes at work in the
atmosphere. Depending on the dominant mechanism
responsible for the vertical air motion, precipitation is
usually classified into one of these two types:
1. Stratiform Widespread, continuous
precipitation produced by large-scale ascent due to
frontal or topographical lifting or large-scale horizontal
air convergence caused by other means.
2. Convective Localized, rapidly changing,
showery precipitation produced by cumulus-scale
convection in unstable air.
The distinction between stratiform and convective
precipitation is not always clear in practice. Widespread
precipitation, for example, is often accompanied by
fine-scale structures, or embedded convective elements.
In fact, precipitation systems generally are composed of
a wide spectrum of scales and intensities. Nevertheless,
it is usually possible to classify precipitation patterns by
their dominant scale.
Stratiform Rain or Snow
Stratiform precipitation is most often produced in
nimbostratus clouds or dissipating cumulus clouds.
Upward air motions are weak and the vertical structure
of the reflectivity pattern is closely related to the
precipitation patterns by their dominant scale.
The presence of stratiform precipitation facilitates
wind measurements with the velocity azimuth display
(VAD) to much higher altitudes than possible in clear
air (part B of the FMH-11). The wind profiles observed
during precipitation may be useful in determining the
nature of fronts. A layer of warm air advection (veering
of wind with height) is evident from the S pattern near
the surface (at close slant ranges). Cold air advection
(backing with height) is present at greater altitudes (far
slant ranges). Generally, the reflectivity pattern depicts
stratiform precipitation without prominent small-scale
On some occasions, mesoscale precipitation bands
form within the stratiform precipitation area. The band
is just ahead of and parallel to the wind shift line marking
the location of the cold front. Other bands have different
orientations with respect to cold fronts. Bands also
occur in the vicinity of warm fronts and in precipitation
areas away from frontal locations, particularly in the
warm sector of a large-scale system.
Mesoscale Convective Systems
Some large-scale precipitating systems begin as
combinations of a number of convective elements. The
resulting system, although still feeding on unstable air,
takes on a character much different from typical
Regions of strong
convection and heavy showers can become randomly
distributed within a larger area of developing stratiform
precipitation, or the strong convection can be limited to
the large systems leading edge with the rest of the
system primarily composed of stratiform rain. When
organized in a linear fashion, the convective cells are
typically distributed along a band about 20 km (11 nmi)
wide and hundreds of kilometers long. The bands are
usually related to low-level convergence and wind
shear, but the Earths topography also affects the
structure of the rain areas. The characteristics of
precipitation bands have been categorized, but it is not
completely understood why precipitation has the strong