ALBERT

Description / Table of Contents: This scientific assessment has been carried out by the WCRP project on Stratospheric Processes and their Role in Climate (SPARC). The objective of the report is to critically review measurements of water vapour in the stratosphere and upper troposphere, in order to consolidate our knowledge and understanding of the distribution of water vapour and its variability on time scales ranging from the seasonal to the long-term inter-annual. Considering the fundamental role of water vapour in climate, and the scarcity of information concerning its distribution, variability and long-term evolution, the need for such an assessment was recognised by the SPARC Scientific Steering Group. The lack of knowledge on water vapour also leads to a large uncertainty in the prediction of climate change. One of the objectives of the assessment was therefore to support the Third IPCC Assessment Report on Climate Change due to be published in 2001. Great effort has been made to prepare the best data sets possible, to retrieve historical data sets, and to make them all available to the assessment team. This report contains an extensive description of the measurements and their associated uncertainties, an assessment of data quality based on comparison studies of the various data sets, and a description of the understanding of the distribution and variability of water vapour in the stratosphere and upper troposphere which ensues from the data. Finally, recommendations are made to ensure that the difficulties met during this work are overcome in order that the remaining uncertainties in our knowledge and understanding can be resolved. The preceding summary also appears in SPARC Newsletter number 16 (January 2001). The outline of the assessment was determined during an international workshop held at NCAR, Boulder, Colorado, USA, 26-28 August 1998. The drafts of the chapters were prepared in the following year and a number of new data sets were produced. The first draft report was examined by an international panel of reviewers both by mail peer review and at a meeting in Paris, France in January 2000. During the review meeting the responses to the mail review comments were proposed by the authors and discussed by the participants. This rigorous review greatly improved the report, the contribution of the reviewers being significant. A second draft report was reviewed by mail review in August 2000. The success in producing the Report is the result of the intensive work and enthusiastic cooperation of a large number of scientists world-wide who have worked towards improving the quality of the measurements and our understanding of the observations. The work of the contributors and reviewers was generously supported by many organisations and agencies including WMO, WCRP, SPARC, DG Research of the European Commission, NASA, NOAA, NCAR, CNRS, CNES, Forschungszentrum Jülich, Imperial College and other national research programmes and institutions. We take this opportunity to express our gratitude to all the scientists (authors, contributors and reviewers) who helped in the preparation of this assessment and to the SPARC scientific steering group who have been supportive since its inception. Our special gratitude is due to the lead authors of the chapters. Particular thanks must be given to: Petra Udelhofen at the SPARC Data Center for setting up the data archive, Sam Oltmans who organised the workshop at NCAR, Boulder, Colorado; Computational Physics Inc., for hosting the workshop in Washington D.C.; François Dulac from the CNES for hosting the review meeting in Paris, and Céline Phillips for her co-editorship. We also thank Marie-Christine Gaucher at the SPARC office for her help in the organisation of the review meeting in Paris and in the final editing of the report and Catherine Michaut at the SPARC office for her help in editing the second peerreview draft and the final draft of the report.

Description / Table of Contents: One of the largest uncertainties in determining the effect of CFC’s on stratospheric ozone has been the magnitude of the trends in the altitude region between 15 and 20 km. In the 1994 WMO-UNEP ozone assessment, SAGE was reported as giving trends up to -0± 8% per decade at northern mid-latitudes, while the ozonesonde stations gave a trend of -± 3% per decade. In 1996 the SPARC panel on Understanding Ozone Trends and the International Ozone Commission decided to collaborate, under the auspices of the World Climate Research Programme and the World Meteorological Organisation, on a study to carefully re-evaluate the ground-based and satellite data to resolve this discrepancy. The philosophy of the study was similar to that of the International Ozone Trends Panel of 1988 which addressed the total ozone measurements. The published literature was not simply reviewed, but a critical re-analysis and interpretation of the vertical profiles of ozone was performed. One of the principal aims of the report was to determine if there was sufficient confidence in the long-term measurement systems to use them for accurate determination of ozone trends in the stratosphere and troposphere. A major purpose of the study was to validate the quality of the data including quantification of the errors and to determine if there were any limitations in altitude or latitude. The report is divided into three main chapters. Chapter 1 contains a description of how the various instruments work, and how ozone concentrations are calculated from the raw measurement. Particular attention is paid to the true vertical resolution of each instrument’s measurement and to its long term calibration drift as well as to its precision and accuracy. Chapter 2 assesses how well the various measurements agree through a series of rigorous data comparisons. Traditional techniques based on zonal averages and on close matches in time and space are augmented by new techniques which classify the air mass according to its dynamic history. Chapter 3 discusses and uses the various methods available for calculating trends, as well as investigating how well the causes of the trends can be determined by statistical approaches.