.
Interpreting Doppler Radar Velocities
straight line winds and rotation
We will now investigate how Doppler
velocity patterns vary in a small area
of the display where the flow field is uniform, rotating, and flowing
outward from a point. These patterns
would be similar to
what an operator would see with uniform winds,
when a mesocyclone associated with a severe
thunderstorm was present, or when a downburst
was present.
We will view a small area located north of the
radar as seen in the figure below.
Image by:
Brown & Wood
This diagram shows the
location and size of the 27 x 27 nautical mile displaying window used
for simulated Doppler velocity patterns of
convective storm features. The
window is 65 nautical miles due north of the radar located at the center
of the overall display region.
First, let's look at what a uniform flow field would look like in
the small box. The
radial velocity
pattern on the right is what the radar
would see if the wind speed was uniform at 50 knots and the wind was blowing
from the west towards the east. Note the zero line goes down the center of
the box because the winds are perpendicular to the beam along that line.
Next we look at a wind field which is rotating about a point at the center
of the box. The wind speeds are 40 knots, 2.5 miles from the circulation
center, and decrease outward to zero at the edge of the box. This crudely
represents the mesocyclone circulation associated with a severe thunderstorm.
The Doppler
radial velocity display again
shows the zero line down the middle of the box since winds along this line
are perpendicular to the radar beam.
However, to the left and right of the line, a sharp velocity couplet appears
with strong outbound velocities just to the right and strong inbound velocities
just to the left. This is the classic signature of rotation with strong
inbound and outbound velocities flanking the beam axis.
Straight line winds often originate in circulations called
downbursts, in
which cool descending air strikes the ground at high speed and spreads out
in all directions. The leading edge of this cold outflow is called a
gust front. The image above shows the
Doppler
radial velocities
associated with an idealized
downburst.
This Doppler velocity pattern corresponds to an axisymmetric divergent
flow where the center of the downburst is in the center of the
box 65 nautical miles north of the radar.
Note the strong velocity couplet, with the zero line perpendicular to the
beam axis and strong inbound and outbound velocities along the beam axis.
directional shear
tornado signatures