Deriving Surface Values of Refractivity from an Operational C-band Precipitation Radar Network

We describe the refractivity technique which is being implemented within the UK operational rain radar network and should provide additional data to improve nowcasting and short term prediction of local rainfall. The technique relies on measuring the radar return from clutter; if clutter is truly stationary any change in the phase of the return must be due to a change in the speed of the wave, in other words a change in the refractivity of the air. The technique is currently quasi-operational and being tested on the Cobbacombe (Devon) and Hameldon Hill (Lancashire) radars and should soon be rolled out over the complete network. The radar performs a low level PPI scan every five minutes. In addition to the standard radar observables, the transmitted frequency, the absolute phase and a quality index for the clutter at each gate, which is based on the variability of the I and Q samples for the 22 transmitted pulses, are also recorded. By looking at the changes in the phase over an hour, we can derive a map of the hourly refractivity changes on a spatial scale of 2-4km. Comparisons with refractivity derived from observations at synoptic stations are very encouraging. One particularly important aspect we are addressing is to attach an error to the refractivity changes derived from the radar so that they can be used for data assimilation.