Observing Fog And Low Cloud With A Combination Of 78GHz Cloud Radar And Laser Ceilometer

Results from two demonstration tests of a FMCW (Frequency Modulated Continuous Wave) 78GHz cloud radar are used to examine the radar’s ability to measure low cloud and fog. The first test took place at Payerne, Switzerland (from Nov 2003 to Feb 2004), as part of the COST 720 TUC (Temperature, Humidity & Cloud Profiling) Project; and the second test took place at the Chilbolton Facility for Atmospheric and Radio Research (CFARR), UK (Aug 2004). The FMCW radar was of similar sensitivity to the pulsed cloud radars at Chilbolton and the good vertical resolution of the system allowed accurate detection of fog top and cloud top in layered clouds. _______________________________________________________________________________________________________________________ INTRODUCTION In partnership with the Met Office a vertically pointing 78.2GHz FMCW cloud radar has been developed by the Space Science and Technology Department of the Rutherford Appleton Laboratory (RAL), to demonstrate the potential capability of the system in cloud detection and the benefits it can offer in comparison to existing observing systems (i.e. Laser Cloud Base Recorders – LCBR). Figure 1 shows a picture of the antenna of the system along with the specification details. Frequency: 78.2 GHz Transmitted Power:250mW Operated at 16km Range (30m spatial resolution) at Chilbolton Operated at 8km Range (15m spatial resolution at Payerne) Temporal Resolution: 1s Radiometer Sky Temperature Accuracy: +/1K Pulse duration: 800 μs Pulse separation: 1.5 2.0 ms Polarisation: linear Total chirp excursion: 10 MHz Dynamic range of receiver:50dB Viewing Angle: Zenith Beam width: ~0.5 degree (half power) Figure 1 – RAL 78.2GHz Cloud Radar and system specification. As part of the development and assessment of this system, it has been deployed at several UK observing sites since early 2001. Major improvements to the sensitivity and data range/resolution have been added to the system through a complete redesign of the antenna and the backend processing. As such only results from the later deployments are relevant in the assessment of the systems capabilities. This paper concentrates on the results from two deployments, firstly at Chilbolton, UK ( Aug 2004) and secondly, Payerne, Switzerland ( Nov 03 to Feb 04). CHILBOLTON EXPERIMENT The Chilbolton experiment was designed to provide absolute calibration of the reported cloud radar signals, which was necessary to satisfy requirements from participants in the COST TUC experiment. As well as comparing directly with the pulsed 35GHz Copernicus Cloud Radar, it was also possible to confirm that the calibration was consistent with historical records of the signals observed with the 94GHz Galileo Cloud Radar at Chilbolton. The Vaisala 905nm CT-75K ceilometer, at Chilbolton, was used to check for low-level clouds with small drop size distributions (eg. Stratocumulus), which cloud radars find difficulty in detecting. Figure 2 provides an example of the cloud radars performance on a day with several layers of cloud and with little precipitation. (Note Data from the FMCW radar has been range corrected.) The dashed purple line in both the cloud radar plots indicates the maximum in the ceilometer signal, which is associated with relatively thin cloud, at about 2km in height. The FMCW radar was more sensitive to the lower thin cloud, than the pulsed radar. Both cloud radar’s were able to detect the higher cloud at around 5km, although due to its higher resolution, cloud structure is more evident in the FMCW profile. The ceilometer was unable to detect any of the higher cloud, due to obscuration of the signal by the lower thin cloud. 78GHz FMCW cloud radar Figure 2. Time-Height cross-section plots from fmcw radar, 35GHz pulsed radar & Lidar: 28/8/04 Figure 3 illustrates the FMCW radar’s ability to resolve cloud structure close to the ground, in a region where the pulsed radar is less reliable. The dashed red line in both the cloud radar plots indicates the maximum in the ceilometer signal on this day. Note that the Chilbolton ceilometer has relatively poor resolution in the boundary layer. For these lower altitude signals the ceilometer returns are very strong, and are well matched to the cloud base from the FMCW radar. However these low level clouds were not resolved very well by the 35GHz pulsed radar. Figure 3. Time-Height cross-section plots from FMCW radar, 35GHz pulsed radar & Lidar: 25/8/04 FMCW radar