Abstract
Precipitation and evaporation budgets over the Baltic Sea were studied in a concerted project called PEP in BALTEX (Pilot study of Evaporation and Precipitation in the Baltic Sea), combining extensive field measurements and modelling efforts. Eddy-correlation-measurements of turbulent heat flux were made on a semi-continuous basis for a 12 month period at four well-exposed coastal sites in the Baltic Proper (the main basin of the Baltic Sea). Precipitation was measured at land-based sites with standard gauges and on four merchant ships travelling between Germany and Finland with the aid of specially designed ship rain gauges (SRGs). The evaporation and precipitation regime of the Baltic Sea was modelled for a 12 month period by applying a wide range of numerical models: the operational atmospheric High Resolution Limited Area Model (HIRLAM, Swedish and Finnish versions), the German atmospheric REgional-scale MOdel, REMO, the operational German Europe Model (only precipitation), the oceanographic model PROBE-Baltic, and two models that use interpolation of ground-based data, the Swedish MESAN model of SMHI and a German model of IFM-GEOMAR Kiel. Modelled precipitation was compared with SRG measurements on board the ships. A reasonable correlation was obtained, but the regional-scale models and MESAN gave some 20% higher precipitation over the sea than is measured. Bulk parameterisation schemes for evaporation were evaluated against measurements. A constant value of C HN and C EN with wind speed, underestimated large fluxes of both sensible and latent heat flux. The limited area models do not resolve the influence of the height of the marine boundary layer in coastal zones and the entrainment (on the surface fluxes), which may explain the observed low correlations between modelled and measured latent heat fluxes. Estimates of evaporation, E, and precipitation, P, for the entire Baltic Proper were made with several models for a 12 month period. While the annual variation was well represented by all predictions, there are still important differences in the annual means. Evaporation ranges from 509 to 625 mm year−1 and precipitation between 624 and 805 mm year−1 for this particular 12 month period. Taking the results of model verification from the present study into account, the best estimate of P–E is about 100 ± 50 mm for this particular 12 month period. But the annual mean of P–E varies considerably from year to year. This is reflected in simulations with the PROBE-Baltic model for an 18 year period, which gave 95 mm year−1 for the 12 month period studied here and 32 mm year−1 as an average for 18 years.
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Acknowledgements
The PEP-project was financed by a grant from the European Union, ENV4-CT97-0484. Additional data have been provided by the Umweltbundesamt in Zingst, Germany, GKSS in Geesthacht, Germany, the BALTEX Data Centre of the Deutscher Wetterdienst, Germany, and the Swedish Meteorological and Hydrological Institute, Sweden. Wave-rider buoys were operated by the GKSS near Zingst and the Finnish Institute of Marine Research close to Östergarnsholm.