Water Vapour Intercomparison Effort in the Frame of theConvective and Orographically-Induced Precipitation Study

An intensive water vapour intercomparison effort, involving airborne and ground-based water vapour lidar systems, radiosondes with different humidity sensors, GPS and microwave radiometers (MWR), was performed in the frame of COPS (01 June - 31 August 2007). The main objective of this work is to provide accurate error estimates for the different water vapour profiling sensors. Simultaneous and co-located data from different sensors are used to compute relative bias and root-mean square (rms) deviations as a function of altitude. Comparisons between airborne CNRS DIAL and ground-based Raman lidar BASIL (25 in total) indicate a mean relative bias between the two sensors of 2.1 % (0.12 g/kg) in the altitude region 0–3.5 km a.g.l. Based on the 3 comparisons between BASIL vs airborne DLR DIAL, the mean relative bias is -3.5 % (-0.24g/Kg) in the altitude region 0–3 Km. On the present statistics of comparisons between BASIL vs both airborne DIALs and GPS and putting equal weight on the data reliability of each instrument, it results in the bias values of: BASIL Raman Lidar 0.3 %, DLR DIAL -3.2 %, CNRS DIAL 2.4 % and GPS 2.0 %. An inter-comparison between radiosondes indicates that RS80-A and RS80-H are affected by several systematic sources of errors. After correction for these error sources, mean bias between RS80 (A&H) and RS92 is found to be reduced to -4.5 %. Comparisons (5 in total) between the two airborne DIAL’s (CNRS DIAL and DLR DIAL) over the COPS region result in mean relative bias of 6.0 % (0.53 g/Kg) in the altitude 0-3 Km. The ongoing comparisons between BASIL vs GPS, MWR and radiosondes and between the water vapor lidars located at different sites especially benefiting from the extraordinary performances of the ground-based UHOH DIAL system, will be discussed at the conference.