Water vapour intercomparison effort in the frame of the Convective and Orographically-induced Precipitation Study

The main objective of this work is to provide accurate error estimates for the different water vapour profiling sensors based on an intensive inter-comparison effort. The inter-comparison, performed in the framework of COPS - Convective and Orographically-induced Precipitation Study (01 June-31 August 2007), involves airborne and ground-based water vapour lidar systems, radiosondes with different humidity sensors, GPS and Microwave radiometers (MWR). 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 from three dedicated flights performed in the frame of the H2Olidar EUFAR project indicate a mean relative bias between the two sensors of 3.9% (0.11 g/kg) and a mean RMS deviation of 13.7% (0.97 g/kg) in the altitude region 0-4.5 kin above ground level. A specific inter-comparison between radiosondes with different humidity sensors (Vaisala RS80-A, RS80-H and RS92) was also performed during COPS. Results from the radiosonde inter-comparison indicate that RS80-A and RS80-H are affected by several systematic sources of error (contamination error, time-lag error, etc.), which have been corrected through established algorithms [1, 2, 3]. After correction for these error sources, mean bias between RS80 (A&H) and RS92 is found to be reduced to -4.5%.

Based on the 3 comparisons between BASIL vs airborne DLR DIAL, the mean relative bias is about -3.5% in the altitude region 0-3 Km, while the RMS is approx. 13%. There are also ongoing comparisons between BASIL vs GPS, MWR and radiosondes and between the water vapor sensors located at different sites and the airborne DIALs which will be discussed at the symposium. Thus on the present statistics of comparisons between BASIL vs both the 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-3.6% and GPS 0.6%. More ongoing comparisons between water vapor profiling sensors, especially benefiting from the extraordinary performances of the ground-based UHOH DIAL system, will be discussed at the symposium.