Weather radars send out electromagnetic waves similar to wireless computer networks and mobile phones. The signals are sent out as short pulses which may be reflected by objects in their path, in part reflecting back to the radar. Weather radars are designed to detect rain drops, hail or snow and from the intensity of the returned signal we can estimate how heavy the rainfall is and where it is located.
Doppler radars not only perform this function, but they are also able to measure the speed of movement of the reflecting particle as well. Generally as precipitation falls it moves with the wind, so the Doppler radar can measure this movement and provide wind information as well as rainfall intensity. This gives more information and a better understanding of current weather to the radar data user.
Doppler radars derive their name due to the method with which they measure the velocities. The Doppler Effect was proposed by Christian Andreas Doppler in 1842, and states that the frequency of a wave, perceived by an observer, varies with the motion of the source of the wave relative to the observer. This effect is most commonly noticed with sound waves, an example of which is the apparent lowering in pitch of an ambulance siren as the vehicle moves past. The Doppler Effect is also used in devices to measure the speed of cars and even the speed of a ball in sport.
Doppler radars measure the phase of each pulse of energy as it returns to the radar, and any changes in phase can be related to motion of the reflecting particles. This is a similar effect to the change in frequency, hence the name Doppler radar.
The wind velocity can be separated into two components known as the radial and transverse components, and these are displayed in Figure 1. The radar is only able to sense the motion directly along the radial, either towards or away from the radar, because the transverse component has no effect on the phase of the returning electromagnetic wave. Hence the total wind speed is not measured, only the portion that is directed towards or away from the radar. This is an important concept to understand when interpreting Doppler wind images.
Figure 1. The radial and transverse components of velocity.
In Figure 2, the wind direction is from left to right of page, and is represented by the black lines. The component of the total velocities that is towards or away from the radar is shown by the blue and red lines, and these are the radial velocities. At the locations marked as A and B, the total wind velocity is entirely down the radial, so the radar will display the total velocity. The radial component decreases away from these points, to such an extent that at position C the total wind is entirely transverse and the radar would display zero wind speed.
Figure 2: Radial components of total wind as seen from Doppler radar.
The maximum velocity that can be correctly displayed by a Doppler radar is known as the Nyquist velocity, and this is dependent on the wavelength and frequency of pulses emitted by the radar. Common Nyquist velocities for Bureau of Meteorology Doppler radars are 95km/hr and 140km/hr.
|Radar||Maximum Velocity (Nyquist Velocity)|
|Adelaide (Buckland Park)||141 km/hr|
|Brisbane (Mt Stapylton)||188 km/hr|
|Darwin (Berrimah)||96 km/hr|
|Sydney (Terrey Hills)||94 km/hr|
|Wollongong (Appin)||140 km/hr|
So what happens if the radial velocity is larger than the Nyquist velocity? In this situation we commonly see velocity aliasing, where the displayed velocity "wraps around" to the other end of the palette.
The electromagnetic waves that weather radars emit travel at the speed of light. Therefore, by measuring the time it takes for a single pulse to return, the distance of the reflecting particle from the radar can be calculated. Doppler weather radars emit between 500 and 1000 pulses every second, but the pulses are so short that the radar still spends 99.8% of its time "listening" for returning pulses. At 1000 pulses per second, the maximum distance that a pulse can travel out to a reflecting particle and then return to the radar before the next pulse is emitted is 150 kilometres. This is the maximum Doppler range and no data is displayed outside this range. The maximum Doppler range for each Bureau of Meteorology radar is listed in the below table.
|Radar||Maximum Doppler Range|
|Adelaide (Buckland Park)||200 km|
|Brisbane (Mt Stapylton)||150 km|
|Darwin (Berrimah)||150 km|
|Sydney (Terrey Hills)||300 km|
|Wollongong (Appin)||200 km|