1. The use of dual-frequency microwave links for the detection of sleet Background to the research A NERC-funded research project centred on Bolton in the North- West of England, showed that it was possible to use dual-frequency microwave links to obtain reliable estimates of path-averaged rainfall rates under most weather conditions. The findings have been confirmed through the subsequent EU-funded MANTISSA project, for which additional links have been installed in Germany and Italy. Microwave links are well suited to urban areas, and (straight) valleys and are thereby complementary to raingauges and radar which face difficulties in providing reliable estimates in such areas. Raingauges face the problems of vandalism and urban chasms, radar has problems of clutter and obscuration. Rainfall Measurement using the d-f link A dual-frequency link consists of a pair of microwave transmitters and receivers, usually located up masts. Fading will occur if the signal from the transmitter is attenuated by scattering from particles along its path. The most severe fading is caused by precipitation. Specific attenuation, K (dB/km), is related to rain rate, R (mm/hr), by the non-linear formula K = aR b. The parameters a and b depend critically on frequency and are sensitive to the (unknown) drop shape, the dropsize distribution, and the temperature. However, there exist pairs of frequencies for which the difference in attenuations is nearly linearly related to rain rate and is relatively insensitive to the unknown rain characteristics. Thus accurate path- averaged rainfall estimates can be obtained. Typical results Rainfall for the link path is estimated using a weighted average from gauges under the link. This is compared with the estimate obtained using the link method. For each event and for each frequency, estimates of the baseline attenuation level are made before and after the event using a standard computer procedure. Figure 1 summarises the results for 157 events, for a 23km link in the Bolton region for which the frequencies are 17.6GHz and 12.8GHz (both vertically polarized). The link has its transmitter at 400m above sea level, with the receiver at an altitude of about 130m. In the figure the events subsequently identified as containing more than 5 minutes of sleet are indicated with blue crosses. The generally excellent results are marred by occasional over-estimates, mostly due to sleet. Robert Cummings, Graham Upton, & Anthony Holt University of Essex [Email: gupton@essex.ac.uk; Website: http://prswww.essex.ac.uk/];gupton@essex.ac.uk. Acknowledgements It is a pleasure to acknowledge the support of the European Union through contract EVK1-CT-200060. We are also very grateful for the suggestions and support provided by our colleagues on the MANTISSA and Bolton projects. The effect of sleet on a link A snow-flake with its surface covered by a thin film of water is “seen” by a microwave as though it were an enormous water droplet. The result is an attenuation that is disproportionate to the amount of water present. We now present a method for detecting sleet, without knowledge of temperature. We verify the results by comparison with temperature. Sleet detection using the link Our method uses the single frequency estimates of rainfall derived from the non-linear K = aR b relations. For a given polarization, over the range of frequencies of interest, the value of b is always greater at the lower frequency. In the presence of apparently massive raindrops this greater value for b translates into a greater overestimate from the lower frequency. Let H and L denote the single-frequency rainfall estimates from the higher and lower frequencies, respectively. Studying the time series of gauge-based and link-based estimates (without reference to temperature) we suggest that (for this pair of attenuations) minutes for which L /H> 1.3 and H > 6 (mm/hr) should be judged as being affected by sleet. A typical sleet event The first diagram in Figure 2 shows the time series for an event on February 17 th -18 th, 2000. The estimated path-averaged rain rate based on five nearby gauges (black line) diverges markedly from the corresponding red/green line (showing the corresponding estimates based on attenuation difference) during the period 18:00 to 21:00. The part indicated in green is that estimated by the algorithm as being sleet-affected. The temperatures at the lower end of the link are indicated in the second diagram. The sleet- affected period corresponds to a dip below 3.5°C. The relation with temperature Over the 21 months to which Figure 1 relates, there were 722 minutes with H/L > 1.3 and H > 6 mm/hr. Of these (see Figure 3) about 95% correspond to minutes for which the temperature is believed to have been less than 5°C at the lower end of the link: a reassuring validation of the procedure.