Radar Basics

Dual Doppler Analyse

Dual Doppler measurements combine the data obtained simultaneously from two (or more) adjacent radars to exploit additional information in determining the horizontal or, optionally, three-dimensional wind field.

A single radar can measure only the radial speed relative to the radar through the Doppler frequency. The wind speed over the ground can only be calculated under the assumption of a uniform occurrence of the wind from the values of a complete rotation of the radar antenna by using the zeros in the Doppler frequency measurement to determine the wind direction. This measurement becomes more accurate and detailed when two adjacent radars provide data on the same reflecting object. From these two measured Doppler frequencies, the exact direction of motion and speed over the ground can be calculated.

A proper measurement geometry exists when both radars are about 30 to 60 km apart. They should measure the reflecting object from different aspect angles. The most accurate values result from measurements with a side angle difference of 90°. All these values are located on a circle with the diameter equal to the distance between the two radars, on the circumference of which both radars are located. The difference between the measured side angles should be preferably wider than 60° but smaller than 120° because or else the Doppler frequency difference is too small, and thus the measurement becomes inaccurate.

For a horizontal wind field, the two vector components U (east-west) and U (north-south), may be calculated as follows (all angles in degrees):

(1)

• ν_R1 = radial velocity 1st radar
• ν_R2 = radial velocity 2nd radar
• β₁ = bearing angle 1st radar
• β₂ = bearing angle 2nd radar
• θ₁ = elevation angle 1st radar
• θ₂ = elevation angle 2nd radar

Figure 2: Example of a horizontal wind field.

Figure 2: Example of a horizontal wind field.

From these two vector components, a small vector can be drawn for each range cell within the usable geometric region (grayed out in Figure 1) that expresses wind direction and, with its length, wind speed. There are also representations in which the direction vectors are all the same size and the velocities are color-coded.