PEP in BALTEX is short for Pilot study of Evaporation and Precipitation in the Baltic Sea. BALTEX is one out of 5 GEWEX regional experiments which will explore, model and quantify the various processes determining the space and time variability of the energy and water cycle of the Baltic Sea and its cachment area. It is envisaged to have a main BALTEX experiment phase in 1999-2000 to integrate observations by numerical modelling and thus achieve a description of the Energy and Water cycle in all of the Baltic Sea Catchment area. PEP is designed as a pilot experiment to the main experiment, with the specific scope to study precipitation and evaporation over sea. For determination of precipitation at sea the main experiment needs to rely heavily on radar measurements. To test, improve and validate radar estimates of precipitation against in situ measurements, a pilot study prior to the main experiment is necessary. At the same time, PEP will provide data sets for the improvement of numerical models over the sea. PEP in this way is designed to sharpen our tools for the main experiment. The measurements in PEP will be concentrated along a transect from the northern coast of Germany to southwestern Finland.
For an 18 month period PEP will provide a comprehensivee set of actual evaporation data measured with the eddy correlation technique, at 4 well exposed sites in the area as well as precipitation, measured with micro rain radars at 3 of these sites. This data set will be used within PEP for validation of routines for evaporation in 2 regional scale models and in a meso-gamma-scale model and for improvements of those routines. The precipitation data will also be used for validation and calibration of weather radars for estimation of precipitation over the Baltic Sea. The micro rain radars will be calibrated against measurements with an optical disdrometer and a rain gauge which is designed for use on board ships.
During a one month period, intensive measurements will be performed at the 4 sites and from research vessel Alkor. These measurements will include:
(i) Detailed boundary layer studies with a Differential Absorption Lidar (DIAL) operated side by side with a UHF RADAR/PASS at one site, providing not only profiles of meteorological parameters but also profiles of dissipation and turbulent flux of water vapour and momentum.
(ii) Radio soundings at all 4 sites.
(iii) RV Alkor will cruise the transect covering the four sites, measuring i.e. precipitation, with disdrometer and ship rain gauge.
Six models will be employed to evaluate and compare areal evaporation (3 models) and precipitation (also, 3 models) over the Baltic Sea for the year 1998. This comprehensive intercomparison of a set of methods and models to determine the areal evaporation and precipitation over the Baltic Sea means taking an important step forward towards the solution of one of the most tricky issues of the BALTEX project, i.e. that of determining the net precipitation minus evaporation balance for the sea itself.
Funding SchemeCSC - Cost-sharing contracts