Cloud Radar - Initial Measurements from the 94ghz Fmcw Radar

A new solid state Frequency Modulated Continuous Wave (FMCW) cloud radar operating at 94GHz has been developed in the UK by the STFC Rutherford Appleton Laboratory, for the UK Met Office. This vertical-profiling cloud radar incorporates technological and operational advances over a previous demonstration instrument, which operated at 78GHz. In addition to providing independent high spatial (order 10m) and temporal (order 10s) resolution of cloud structure, the radar will also be used in testing integrated networks of surface-based remote sensors, with the aim of improving cloud observation for input into numerical weather prediction models. A further requirement of the instrument is that it should be relatively inexpensive, allowing several instruments to be purchased and deployed around the UK. An account of the technological aspects and initial performance of this new cloud radar will be provided, as will examples of data gathered during different meteorological conditions, including comparisons with other vertical profilers. INTRODUCTION The cloud radar was built by the Millimetre Wave Technology Group at the STFC Rutherford Appleton Laboratory in the UK for and in collaboration with the UK Met Office. The radar operates at 94GHz: this frequency is ideal for observing cloud as it lies within a part of the spectrum which experiences relatively low absorption in the atmosphere, whilst being high enough to resolve small particles like cloud droplets by Rayleigh scattering (Lhermitte, 1987). The 94GHz frequency is also assigned for observing cloud from satellites. The radar is sufficiently compact and lightweight to be readily transported: Figure 1. It exploits the frequency modulated continuous wave (FMCW) approach, which uses generally less expensive, more durable and lower power components than the more common pulsed radars. In addition, the radar is Doppler capable, so the vertical hydrometeor velocity can also be measured, although this feature has not been investigated at present. The 94GHz radar beam is capable of penetrating dense, thick cloud layers allowing a complete scan of deep clouds from base to top. A key feature of the cloud radar is the ability to provide information on multiple cloud layers, even if the lowest layer completely obscures the sky. This ability will be useful for routine observation of overhead cloud at meteorological reporting sites and for aviation, where knowledge of the heights of cloud base and tops are important. Cloud profiles also benefit the meteorological research community, such as by providing observations to evaluate meteorological models by, either directly or as part of an integrated profiler system to retrieve information on cloud liquid and ice water contents (Illingworth et al., 2007). The cloud radar produces a new vertical profile at a temporal resolution of up to 5 seconds, with a vertical resolution of 4m for a range up to 2km, or 16m for a range of 8km, allowing fine cloud structures to be resolved. Despite the low power output, the sensitivity of the radar allows cloud to be detected to a height of at least 8km (the maximum height of initial trials, although the radar is capable of detecting cloud higher than this, with a limit of 16km). The radar can detect the presence of cloud as low as ~30m. Below which the signal is somewhat ambiguous due to ground