The Chilbolton Group of STFC Rutherford Appleton Laboratory is involved in meteorological research. Funding agencies include the NERC, ESA, EU and the Met. Office.
One of the familiar features of a modern weather presentation is a picture from the UK radar network, operated by the Meteorological Office. Researchers use the facilities at Chilbolton to develop advanced techniques for future meteorological radar systems. For example, the Met. Office has used high resolution information about the vertical structure of rain systems to improve the predictions of rainrates from the present national network.
Many of the research projects are made possible by the high resolution of the Chilbolton radar system. This high resolution is a result of the huge 25 metre antenna used by the main 3 GHz radar.
Another main strength of the system is its polarisation capability. The polarisation characteristics of the Chilbolton radar allow researchers to study the characteristics of hydrometeors.
"Polarisation Diversity Radar Measurements of Precipitation" or "Raindrops are not drop-shaped"
Hydrometeors (rain, snow, ice crystals, graupel, hail) are usually non-spherical, as has been measured in wind tunnel experiments. In fact, the amount of deformation is related to the dropsize and this can be detected using differential polarisation.
Although one might expect raindrops to be drop-shaped, they are not. This is because they are falling at their terminal velocity, which squashes the drops so they are wider than they are thick, as shown in the figure below. The bigger the drops are, the more smartie shaped or oblate they become. The shape of the raindrops affects how they reflect vertically and horizontally polarised radar beams. Small round (0.5-1mm) raindrops reflect back both types of beams with the same amplitude. Larger, more oblate raindrops reflect the two polarisations at different amplitudes. Therefore, the difference in amplitude between vertically and horizontally polarised beams reflected by raindrops gives researchers information about the size of the raindrops in clouds and rain.
The Chilbolton radar can transmit vertically and horizontally polarised beams alternately. The system records the energy reflected back by the different beams. The difference between the two allows researchers to find out the dropsize of hydrometeors overhead.
Removal of ambiguity in drop size distribution
In the early 80s there was great excitement at the removal of uncertainty in the Drop Size Distribution so that better rainrates could be derived, better than just using a Z-R relationship. In fact, the errors in R often come from other uncertainties.
Perhaps a more significant result of polarisation measurements is the ability to perform hydrometeor identification, to differentiate liquid water from ice using their different dielectric properties and to identify various form of ice (snow, hail, crystals).
Parameters available from the Radar
- Reflectivity in the horizontally polarised channel: dBZH (mm6m-3)
- Ratio of reflectivity in horizontally and vertically polarised channels (Differential Reflectivity): ZDR
- Ratio of cross-polar to co-polar reflectivity (Linear Depolarisation Ratio): LDR
- Doppler Radial Velocity: v (ms-1)
- Difference in phase between horizontally and vertically polarised channels: FDP(degrees)
These measurements are typically available in 300 range gates, each of 300 m length, at 0.25 s intervals.
The main Chilbolton radar has full Doppler capability. This make it possible for researchers to determine radial air velocities in precipitation, which in turn gives information about the wind field and allows work in dynamical meteorology.
The Doppler Principle
The Doppler principle is the effect which makes the tone of a fire engine change as it passes an observer, switching from an apparently high pitched, high frequency tone to a lower pitched, lower frequency tone. In reality, the fire engine is continuously emitting sound waves at a constant tone or frequency. However, if it is moving towards the observer the wave fronts arrive closer together which is equivalent to a higher frequency, heard as a higher tone. As the fire engine starts to move away from the observer, the wave fronts arrive less frequently and the tone sounds as it if is of a lower frequency.
Radar waves can experience the same Doppler effect. Radio waves are scattered off a target, which in this case is rain. The rain moves with the wind and is therefore a 'tracer' for the wind's velocity. Therefore, when the wind is moving towards the radar, the radar's radio wave frequency is increased; when the wind is moving away, it is decreased. This effect can be detected by the Chilbolton radar. The size of this Doppler shift is proportional to the velocity of the radial wind in the direction the antenna is pointing.