Figure 2-35.—Line Echo Wave Pattern (LEWP). Uneven acceleration causes the line to crest at position (1) and bulge at the point
of acceleration, position (2).
beneath the purple shaded areas. And, as with all radar
products, successful coverage depends on the
availability of scatterers.
Base Velocity (VEL) Product
Doppler radar measures only the radial component
of motion (directly toward or away from the antenna).
The Base Velocity product (VEL) presents the mean
radial velocity data. While only two-dimensional data
is received, a pseudo 3-D analysis is displayed.
Weather features are extracted in both the horizontal
and vertical, producing valuable information. As with
Base Velocity, a separate product is available for each
elevation angle in the current volume coverage pattern
(VCP).
The Base Velocity (VEL) product can be used to
examine internal storm motions, as well as overall
atmospheric flow. In the horizontal, this includes
convergence, divergence, rotation, and wind shifts. In
the vertical, this product helps locate low and mid-
level jet streams and inversion layers. Base Velocity is
also useful in locating aviation hazards, such as
turbulence and low-level wind shear (LLWS). Any
range folded areas, as on base reflectivity, are
displayed in purple.
All velocity data from this product must be
referenced to a "zero-Doppler" velocity line. The zero-
Doppler velocity line depicts points along the radar
beam where no Doppler motion is being detected.
Remember from our discussion of velocity that this
does not mean that no motion is occurring. It simply
implies that motion in this region is not being detected
by the radar.
Doppler velocities are color-coded while wind
directions are not. Cooler colors, such as blues and
greens, depict velocities moving toward the antenna
(negative values). Conversely, warm colors (red,
orange, yellow) are outbound (positive values). Gray
colors indicate where the wind component is
perpendicular to the radial direction from the radar
(zero velocity). Determining the wind direction from
Base Velocity data can be difficult and requires
practice.
Look at figure 2-36. View (A) shows a vertical
profile of uniform wind speed that veers with height.
View (B) depicts the same general wind field as it
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