A model-intercomparison study was conducted, the first of its kind for the Baltic Sea, whose aim was to systematically simulate the basic three-dimensional hydrographic properties of a realistic, complex basin. Simulations of the hydrographic features of the Gulf of Finland for the summer autumn of 1996 by six three-dimensional hydrodynamic models were compared. Validation was undertaken using more than 300 vertical hydrographic profiles of salinity and temperature. The analysis of model performance, including averaging of the ensemble results, was undertaken with a view to assessing the potential suitability of the models in reproducing the physics of the Baltic Sea accurately enough to serve as a basis for accurate simulations of biogeochemistry once ecosystem models are incorporated. The performance of the models was generally satisfactory. Nevertheless, all the models had some difficulties in correctly simulating vertical profiles of temperature and salinity, and hence mixed layer dynamics, particularly in the eastern Gulf of Finland. Results emphasized the need for high resolution in both vertical and horizontal directions in order to resolve the complex dynamics and bathymetry of the Baltic Sea. Future work needs to consider the choice of mixing and advection schemes, moving to higher resolution, high-frequency forcing, and the accurate representation of river discharges and boundary conditions.

A combination of a circulation anda diffusion model has been developed to be used for dispersion studies in the Baltic Sea. As the time scale of interest is from months up to several years a straightforward way to model the dispersion would require the circulation model to be run for a very long time, which would be impractible. Instead atypical meteorological year has been constructed for which the circulation model has been run. The circulation model is three-dimensional, uses six layers in the vertical and hasa horizontal resolution of 10 kilometres. The diffusion model is of Monte Carlo type in all three dimensions. So far only passive pollutants have been treated but the model will be extended to include biochemical interaction and sedimentation processes. Results of applications to three outlets are shown.

Continuous current and temperature data were taken from one station in Lake Vättern during the 1977 surnrner. Further temperature data were collected from two stations by continuous recordings at eleven depths during a fortnight period.

A three-dimensional hydrodynamic model has been applied to the lake. Observations of current and temperature show a good agreement with corresponding model results.

Wave like motions have been studied through a method of rotary spectral analysis. It is suggested tha tobserved waves can be related to different modes of Poincaré waves. Motions with a period of several days were observed and have been interpreted in terms of internal Kelvin-waves. There also is evidence of motions with a frequency lower than the inertial value pointing towards nonlinear interactions between inertial motions and lower frequency currents.

Föroreningssituationen i Vättern har sedan början på 60-talet mer i detalj kunnat föijas genom mätningar av biologiska och kemiska parametrar. Främst är det Vätterns växande betydelse som vattentäkt som föranlett en kontinuerlig provtagning. Även förändringar i fisket har aktualiserat en fortlöpande kontroll av föroreningssituationen.

I en vattenvårdsplan (Kommitten för Vätterns vattenvård,1970) beskrivs utförda mätningar såväl före 1962 som under den mer intensiva perioden 1962-67.I vattenvårdsplanen påpekas också behovet av en noggrann kartläggning av strömningsförhållandena i sjön. Därigenom skulle möjligheterna ökas att bestämma spridningen och utspädningen av avloppsvatten från industrier. Samtidigt skulle detta underlätta en övergripande och långsiktig planering av Vätterns utnyttjande som vattenresurs.

De enstaka mätningar av skiktningsförhållanden och strömhastigheter som tidigare utförts har indikerat komplicerade strömningsmönster. I denna rappor tredovisas kontinuerliga mätningar av ström och temperatur, vilka också jämförs med genererade strömmar och skiktningsförhållanden från en hydrodynamisk modell.

Current, temperature and water-level measurements in Lake Vänern from two epdisodes in 1971, representing a stratified resp anunstratified situation, have been used for the verification of an multilayer hydrodynamic model. Concentrating on wind-driven lake circulation it is shown that, aside from the currents along the western shore of the eastern basin, the correspondence between model results and observations is satisfactory. The computed currents could therefore be used for studies on the transport and dilution of wasterwater in the lake. It is also concluded that neither avariation of model parameters nor an increased vertical or horizontal resolution will make the model reproduce the strong and mainly south-going current along the western shore of the eastern half of the lake.

Verifiering av en flerlagers hydrodynamisk modell tillämpad på Vänern har utförts med hjälp av ström-, temperatur- och vattenståndsvärden från två perioder 1971 (representerande skiktade resp oskiktade förhållanden). De beräknade strömmarna visas stämma väl överens med observerade data och bör därför kunna utgöra ett bra underlag vid studier av transportvägar och utspädningsgrad för olika typer av utsläpp. De relativt starka strömmarna i västra delen av Värmlandssjön har inte kunnat beskrivas i modellen trots försök med flera kombinationer av modellens parametervärden och ökad horisontell och vertikal upplösning.