GKSS The Baltic Sea Experiment BALTEX: A brief overview and some selected results PR 0 S 833 OSH Autoren: E. Raschke U. Karstens R. Nolte-Holube R. Brandt H.-J. fsemer D. Lohmann M. Lobmeyr B. Rockel R. Stuhlmann /M/CC n-i /c/io DISCLAIMER Portions of this document may be illegible electronic image products. Images are produced from the best available original document. GKSS 97/E/13 The Baltic Sea Experiment BALTEX: A brief overview and some selected results Autoren: E. Raschke U. Karstens R. Nolte-Holube R. Brandt H.-J. Isemer D. Lohmann M. Lobmeyr B. Rockel R, Stuhlmann (Institut fur Atmospharenphysik) Die extemen Berichte der GKSS warden kostenlos abgegeben. The delivery of the external GKSS reports is free of charge. Anforderungen/Demands: GKS S -F orschungszentrum Geesthacht GmbH Bibliothek/Library Postfach 11 60 D-21494 Geesthacht Als Manuskript vervielfaltigt. Fur diesen Bericht behalten wir uns alle Rechte vor. GKSS-Forschungszentrum Geesthacht GmbH • Telefon (04152)87-0 Max-Planck-StraBe • D-21502 Geesthacht / Postfach 11 60 • D-21494 Geesthacht GKSS 97/E/13 The Baltic Sea Experiment BALTEX: A brief overview and some selected results E. Raschke, U. Karstens, R. Nolte-Holube, R. Brandt, H.-J. Isemer, D. Lohmann, M. Lobmeyr, B. Rockel, R. Stuhlmann 28 pages with 8 figures and 2 tables Abstract The mechanisms responsible for the transfer of energy and water within the climate system are under worldwide investigation within the framework of the Global Energy and Water Cycle Experiment (GEWEX) to improve the predictability of natural and man-made climate changes at short and long ranges and their impact on water resources. Five continental-scale experiments have been established within GEWEX to enable a more complete coupling between atmospheric and hydrological models. One of them is the Baltic Sea Experiment (BALTEX). In this paper, the goals and structure of BALTEX are outlined. A short overview of measuring and modelling strategies is given. Atmospheric and hydrological model results of the authors are presented. This includes validation of precipitation using station measurements as well as validation of modelled cloud cover with cloud estimates from satellite data. Furthermore, results of a large-scale grid based hydrological model to be coupled to atmospheric models are presented. Z^....,'j , Das Regional-Experiment BALTEX: Ein kurzer Uberblick und einige ausgewahlte Ergebnisse Zusammenfassung j Im Rahmen des Programmes GEWEX (Globales Energie- und Wasserkreislauf-Experiment) werden weltweite Untersuchungen derjenigen Mechanismen untemommen, die die Ubertragung von Energie und Wasser innerhalb des Klimasystems bestimmen. Dadurch soil die Vorhersagbarkeit von naturlichen und anthropogenen Klimaanderungen in kurzen und langeren Zeitraumen und deren Wirkung auf die ver- fugbaren Wasservorrate verbessert werden. Insgesamt funf kontinentweite Experimente wurden innerhalb von GEWEX fur diese Zwecke begonnen. In ihnen soil vordringlich eine Kopplung von Hydrologiemodellen an Atmospharenmodelle erfolgen. Bines dieser Experimente ist das BALTEX (Baltic Sea Experiment). In dieser Arbeit werden die Ziele und die Struktur von BALTEX vorgestellt. Es wird auch ein kurzer Uberblick iiber die Mefl- und Modellierstrategie vermittelt. Femer werden erste Ergebnisse der Autoren vorgestellt. Diese schliefien auch einen Vergleich zwischen gemessenen und modellierten Verteilungen des Niederschlages und der Bewdlkung im BALTEX-Gebiet. Weiterhin werden erste Ergebnisse des in einem gitterorientierten AbfluGmodell berechneten Abflusses vorgestellt. Dieses Model! soil spater an das Regionalmodell fur das BALTEX-Gebiet angekoppelt werden. \ \ Manuscript received /Manuskripteingang in der Redaktion: 11. April 1997 CONTENTS 1 INTRODUCTION 7 2 BALTEX GOALS AND STRUCTURE 8 2.1 Goals and structure 8 2.2 Structure and organization of BALTEX 12 3 OBSERVATIONS 13 3.1 In-situ measurements and data centers 13 3.2 Remote sensing data from satellites, radar and other profilers 15 4 MODELLING 16 4.1 Atmospherical modelling: Precipitation 17 4.2 Atmospheric Modelling: Cloud cover 19 4.3 Hydrological modelling 20 5 CONCLUSIONS 22 6 ACKNOWLEDGEMENT 23 7 REFERENCES 23 7 1 INTRODUCTION The transport of water and energy within the climate system, which consists of the atmosphere, oceans, cryosphere and land surfaces, must be known very accurately in various spatial and tem­ poral domains to describe present and future climate states and the impact of their variations on related processes and on the water resources on the earth ’s surface. Several studies investigated the mean monthly atmospheric water budget over the Baltic Sea using observations from aerological stations in combination with precipitation measurements. The budget calculations are based on the "aerological method" in which the divergence of the water transport and the change of water content are determined from direct atmospheric soundings, and the difference between evaporation and precipitation is computed as the residuum of the budget (for a review: see Holopainen, 1996). Palmen and Soderman (1966) computed the evaporation from the central part of the Baltic Sea for the time period October 1961 - September 1962 using the aerological method. Alestalo (1983) presented the seasonal variation of the water budget over Europe based on radiosonde data for July 1969 - June 1977 in comparison to long-term averages of precipitation and evaporation. The most comprehensive study of the water budget of the Baltic Sea has been initiated by the Baltic Marine Environment Protection Commission - Helsinki Commission - (HELCOM, 1986). Here, long-term averages (1951-1970) of precipitation and evaporation as well as river inflow and sea volume changes are presented. In another study, results from numerical simulations over a period of 3 years with the Europa-Modell (EM) of the German Weather Service (DWD) have been analysed by Heise (1995). He calculated the components of energy and water cycles over the Baltic Sea catchment area from consecutive daily operational forecasts and obtained close agree­ ment with climatology. Numerical modelling with high spatial and temporal resolution often provides the only means to estimate the available water resources in future scenarios of climate states and land uses. However, even the most advanced climate models show at present quite large differences between observa ­ tions and modelled quantities (Gates, 1992; Lau et al., 1996). With controlled forcings at their lower boundary the decadal simulations of the AMIP (Atmospheric Model Intercomparison Project) show large differences in computed cloud and precipitation fields of up to 50 % from the average of all model results. But even in daily weather forecasts of the precipitation monthly aver­ ages can differ from measurements by the order of 50 %. Recently it has been demonstrated in more detail by Cubasch et al. (1995), that the precipitation values which were modelled for the present climate over Central Europe are more than 50 % smaller than observations, and over some areas the seasonal variations deviate by up to 6 months in phase. Such systematic deficiencies, which were evident since the beginning of the World Climate Research Programme (WCRP) must considerably be reduced to justify the confidence into the results of such models and their projections into the far future. An accurate validation of model results had been established already in 1987 within the WCRP-programme GEWEX (Global Energy and Water Cycle Experiment; see e.g.: WCRP, 1990, Chahine 1992) with the major objectives - to determine the hydrological cycle and energy fluxes by means of global measurements of observable atmospheric and surface properties; - to model the global hydrological cycle and its impact on the atmosphere, the ocean, and on the land surface; 8 - to develop the ability to predict variations of global and regional hydrological processes and water resources, and their response to environmental change; - to foster the development of observational techniques, data management and assimilation systems suitable for operational application to long-range weather forecasts, hydrological and climate predictions. The outcomings of GEWEX contribute to improvements of weather and climate analyses and forecasts. They are of benefit to the climate and climate impact research in general, but also to the research of various aspects within the International Geosphere and Biosphere Programme (IGBP) and the monitoring of pollutant transports within the atmosphere. Most of the involved opera­ tional agencies and organizations advice their national governments and economical sectors on climate issues. In this paper an introduction is provided into the present objectives and structure of BALTEX and its major subprojects. Some first results of the authors are shown on the modelling of energy and water transports within the BALTEX region and their validation. 2 BALTEX GOALS AND STRUCTURE Within GEWEX, which at present provides an umbrella for all climate related precipitation, atmos­ pheric water vapour, cloud and radiation projects and also for related numerical experimentations, five continental scale experiments (CSEs) have been established. Their geographical areas and major specific tasks are outlined in Table 2.1. Besides of several specific applications, these regional CSEs serve the major common goal to improve the modelling of hydrometeorological processes in numerical forecast schemes for weather and climate. Table 2.1: Characteristics of the