Correction of Daily Rain Gauge Measurements in the Baltic Sea Drainage Basin

Abstract

Within the framework of BALTEX regular measurements made at about 4,200 rain gauge sites have been collected by the BALTEX Meteorological Data Centre. This network of rain gauges is about 10 times denser than the synoptic network; here it is used for the objective analysis of daily precipitation fields on the grids of the mesoscale models developed in the research project NEWBALTIC during the pilot period PIDCAP (August to October 1995). The observations were corrected for systematic measuring errors with the Dynamic Correction Model. Its main purpose is to correct for the wind-induced losses which is the largest error. The correction formulae use the synoptic observations of wind speed, temperature and rain intensity at the rain gauge station considered. For non-synoptic stations the values of the closest synoptic station are used as estimates; the mean distance of the synoptic stations within the drainage basin is 30 km. For evaporation and wetting losses, which represent the second largest error of the precipitation measurements, climatological corrections are applied. The formulae in the Dynamic Correction Model take instrument-specific properties into account; these comprise HELLMANN UNSHIELDED, SMHI SHIELDED, H&H-90 SHIELDED and TRETJAKOV SHIELDED. The spatial distribution of corrected precipitation values were objectively analysed according to Rubel (1998). They yield daily gridded precipitation fields over the drainage basin which are systematically higher than the uncorrected fields. The corresponding increase of the freshwater input into the Baltic Sea drainage basin is 4.7% in August, 5.8% in September and 9.1% in October 1995. Finally, the perspective for further developments and the generation of a ten-year data base of the BALTEX main intensive observational and modelling period BRIDGE is introduced.