Studies of upper tropospheric humidity using airborne in-situ sensors

An on-going study of the performance and measurements of the Fluorescence W ater Vapour Sensor (FWVS), on board the British C-130 Hercules research air craft is presented. The instrument has a very fast time response (raw data at 8 Hz) and the potential of measuring the in-situ water vapour volume mix ing ratio at high altitudes. Nevertheless, compar isons with a frost-point hygrometer mounted on the same aircraft yield a pressure dependent differen ce between the two sensors, the FWVS giving higher humidities. Possible cau ses of the discrepancies are investigated indicating the necessity of labor atory tests in order to develop a deeper understanding of the instrument re sponse. Furthermore, comparisons between the airborne hygrometers data and coincident NCEP/NCAR reanalysis profiles are presented. In general, the res ults show a very good agreement between the two datasets. Finally, a case w hen the two profiles differ considerably, due to specific weather condition s in the area of interest, is discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.