Das Projekt "Measurement and Intercomparison of Active, Passive and In-situ Sensors during the International Water Vapour Project for the Verification of the Specifications of the WALES Experiment" wird vom Umweltbundesamt gefördert und von Universität Hohenheim, Institut für Physik und Meteorologie durchgeführt. The performance of state-of-the-art water vapor DIAL systems operated during the International Water Vapor Project (IHOP 2002) shall be investigated. Interesting results have been achieved which allow for important conclusions for WALES. Introduction: Atmospheric water vapour plays a crucial role in the Earth climate and weather system. It is a key element controlling the global radiation budget, the atmospheric circulation, as well as microphysical processes leading to cloud formation and precipitation (e.g. OStarr 1991, AGU 1995). Though atmospheric water vapour is essential for weather and climate prediction, its global distribution and development are by far not understood. On the one hand, this is due to the large range of scales from turbulence and cloud development to global atmospheric waves which need to be accurately modelled. This will overstrain not only to date but also for a long time in the future each computer system. On the other hand, this is certainly due to a severely limited amount of data. Current water vapour observation systems are by far not able to observe water vapour with the required temporal and spatial scales as well as the accuracy required to improve our knowledge of its global 4-dimensional distribution. Furthermore, only a few techniques are available to measure water vapour profiles from ground to the lower stratosphere with high time and spatial resolution. Particularly, passive remote sensing systems suffer from high biases in the estimates of water vapour which may even be time dependent. A detailed overview of current observation systems and its limitations are given in ESA 2001 and ESA 2002.( ) The goals of this study: Consequently, the data collected during IHOP represent a unique source to investigate the performance of space borne DIAL systems. The goals of this study are the use of IHOP data to - investigate the performance of state-of-the-art water vapour DIAL systems with respect to systematic and noise errors, - intercompare different DIAL systems, - intercompare different water vapour measurement techniques, - investigate the role of sub-grid scale atmospheric variability on DIAL performance, - estimate the WALES water vapour sampling error on a scale of 100 km x 100 km, - use the results to predict the performance of WALES under different atmospheric conditions also in the presence of clouds, - provide a data base for an end-to-end performance model for WALES in order to enable the forward simulation of WALES performance under different atmospheric conditions, - confirm or specify in more detail the WALES mission requirements, - provide general conclusions and recommendations for the WALES missions and support future work. As the key personnel of this study was extensively involved in the IHOP campaign and consists mainly on IHOP Principle Investigators (PIs), access to this unique data set is ensured according to the IHOP data policy.