LAPBIAT* Upper Troposphere Lower Stratosphere Water VapourValidation Project: LAUTLOS WAVVAPEsko Kyrö, Arctic Research Centre (FMI/ARC), Sodankylä, Finland (esko.kyro@fmi.fi)Ulrich Leiterer, Meteorological Observatory Lindenberg, GermanyVladimir Yushkov, Central Aerological Observatory Moscow, RussiaRoland Neuber, Alfred Wegener Institute for Polar and Marine Research, Potsdam, GermanyPaul Ruppert, Meteolabor AG, Wetzikon, SwitzerlandAri Paukkunen, Vaisala Oyj, Helsinki, FinlandHolger Vömel, University of Colorado, Boulder, USAThe focus of this project is the improvement of water vapour measurement techniques in the Upper Troposphere and LowerStratosphere (UT/LS). Routine measurements of water vapour with high accuracy at these altitudes are an unsolved problem up tonow despite many activities in the past ten years. Water vapour is a dominant greenhouse gas in the earths atmosphere. Recentmodel calculations show that observed water vapour increases in the stratosphere contribute significantly both to surface warmingand stratospheric cooling. In addition to climate change, both direct chemical and indirect radiative effects of stratospheric waterchanges on ozone chemistry are important as well. Therefore one of the aims of the forthcoming EU COST Action 723 The Roleof the Upper Troposphere and Lower Stratosphere in Global change is to improve balloon sounding and remote sensingtechniques of water vapour measurements (see http://www.sat.uni-bremen.de/cost/). Another example of the work focusing onwater vapour is proposed by the GEWEX Water Vapour Project (GVaP) (see SPARC Report No. 2, December 2000 and thereferences therein).The idea of LAUTLOS-WAVVAP is a comparison/validation experiment, which brings together lightweight hygrometersdeveloped in different research groups, which could be used as research-type radiosondes in the UTLS region. These include:Meteolabor Snow White hygrometer, NOAA frostpoint hygrometer, CAO Flash Lyman alpha hygrometer, Lindenberg FN sonde(a modification of the Vaisala radiosonde) and the latest version of the regular Vaisala radiosonde with the humicap-polymersensor. The experimental plan is based on regular launches of multi-sensor payloads at Sodankylä in January February 2004. Theaim is to study the effect of atmospheric parameters such as ambient temperature, water vapour content or relative humidity, airpressure and solar radiation on each participating hygrometer/radiosonde records. Both night and daytime launches are planned.The campaign also aims at studying PSC occurrence and their dependence on local temperature and water vapour content. It willbe hosted by the FMI Arctic Research Centre Sodankylä assisted by Vaisala Oyj and is part of the planned Finnish contribution toCost 723 project. The campaign is partly funded from the LAPBIAT Facility, which belongs to the EU program: Access toResearch Infrastructures (see: http://www.sgo.fi/lapbiat/).* Lapland Atmosphere-Biosphere Facility Improving the Human Research Potential and the Socio-Economic knowledge Base