Working principle of Doppler weather radar

The Doppler effect was named in memory of Austrian physicist and mathematician Christian John Doppler, who first proposed this theory in 1842. Doppler believes that the wavelength of object radiation changes because of the relative motion between the light source and the observer. In front of the moving wave source, the wave is compressed, the wavelength becomes shorter and the frequency becomes higher (blue shift). Behind the moving wave source, the opposite effect is produced. The longer the wavelength, the lower the frequency (red shift). The higher the speed of the wave source, the greater the effect. According to the red/blue shift degree of light wave, the speed of wave source moving along the observation direction can be calculated. The displacement of the spectral line of the star shows the speed of the star moving along the observation direction. Unless the speed of the wave source is very close to the speed of light, the degree of Doppler frequency shift is generally very small. All wave phenomena (including light waves) have Doppler effect.

Weather radar intermittently emits electromagnetic waves (called pulsed electromagnetic waves) into the air. Electromagnetic waves travel in a nearly straight path in the atmosphere, and the speed is close to that of light waves. On the propagation path, if a meteorological target is encountered, the pulse electromagnetic wave is scattered by the meteorological target, and the electromagnetic wave scattered back to the radar (called echo signal, also called backscattering) shows the spatial position of the meteorological target on the screen.

In radar detection, the spatial position of meteorological target is expressed by the linear distance r (also called oblique distance) from radar antenna to target, elevation angle and azimuth angle of radar antenna. The oblique distance r can be determined according to the propagation speed c of electromagnetic waves in the atmosphere and the time interval between the detection pulse and the echo signal. The propagation speed of electromagnetic wave in the atmosphere is slightly lower than that in vacuum, but it has little effect on the slope accuracy, so it is approximately represented by C.