Progress in Improving Upper Air Moisture Measurements over the Uk

1. INTRODUCTION The Met Office UK has been progressing towards an automated upper air network for several years. In 2002, the number of radiosonde stations reporting on a regular basis has been reduced from 8 to 6, but upper air observations are now augmented by the introduction of operational observations from 4 boundary layer wind profilers operating near 1 GHz and the MST radar, Aberystwyth, operating at 46 MHz. Work on the installation of a tropospheric profiler, operating at 64 MHz, in the Hebrides to the west of Scotland is in progress. The European AMDAR project is also contributing significant numbers of temperature and wind profiles near London and at some sites in Scotland and northern England. This paper will discuss the testing of new radiosonde relative humidity sensors, results associated with the European COST 716 GPS Total water vapour project and studies looking at the relationship between signal to noise measured by boundary layer wind profilers and relative humidity derived from microwave radiometer measurements. 2. NEXT GENERATION RADIOSONDES Before 2001, the Met Office had been operating a radiosonde network with two systems, Vaisala PC-CORA with Vaisala RS80 Loran radiosondes and Sippican ground stations with MKII Loran radiosondes. With the reduction in the number of UK stations, operations have now reverted to a single supplier. However, two independent GPS radiosonde systems (Sippican and Dr. Graw) were purchased to provide reference winds and heights for test purposes at the Camborne test centre, replacing the obsolete Cossor tracking radar. In addition, the Met Office has purchased Meteolabor Snow white chilled mirror hygrometers flown using the Sippican ground system to provide a reliable method of measuring relative humidity independent of the Vaisala systems. The new generation Vaisala Digicora-3 ground system was introduced into operation in 2001. Although this was extensively tested before introduction (no failures in 100 test flights), at a later stage associated with introduction of various system peripherals and also with problems when the PC hard discs became relatively full. Rectification of these faults is still in progress. The Vaisala RS90 radiosonde was to be supplied with the new ground system. However, the absolute calibration of the RS90 relative humidity sensor and the protection from contamination for the relative humidity sensor when passing through supercooled water cloud were found to be outside of the procurement specification. The Met Office was relatively happy with the relative humidity measurements of the RS80 H-Humicap …