The following figure shows a plan view (looking down) of the dual Doppler geometry. The
horizontal wind is sampled by each radar which measures the radial component of the wind,
Vr1 and Vr2. These two radial velocity measurements at the same point, along with some
simple trigonometry, can be used to estimate the horizontal wind at the point, provided that
the crossing angle is not 0° or 180° (parallel beams).
Dual Doppler Geometry Plan View
Horizontal Wind Vector
Crossing
Angle
Baseline
Radar 2
Radar 1
Vr1
Vr2
Figure 58 Dual Doppler Geometry Plan View
This means that along the baseline (crossing angle 0°) or at far ranges (crossing angle 180°)
we cannot use the algorithm, since the two radars essentially measure the same radial wind.
While in theory the crossing angle could be anywhere between 0° and 180°, our radial
velocity measurements are not perfect so that in practice, a crossing angle of <20° leads to
unreliable wind estimates.
The NDOP product performs the dual Doppler algorithm to obtain estimates of the
horizontal wind field. NDOP makes the following assumptions:
• The vertical wind is assumed to be weak as compared to the horizontal wind.
• The input radial velocity measurements represent the radial velocity of the air, that is,
the velocity is unfolded and corrected for particle fallspeed.
The output of the NDOP product is a multi-level grid of horizontal wind vectors. These can
be displayed by themselves, or overlaid on other echoes.
More information
‣
NDOP Output Options (page 249)
IRIS Product and Display User Guide M211319EN-H
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