Precipitation fields over the Baltic Sea derived from ship rain gauge measurements on merchant ships

Precipitation over the Baltic Sea has been estimated from satellite measurements, ground-based weather radars and synoptic observations made at coastal and island stations. The obtained estimates do not use any in situ measurements over the Baltic Sea. Thus, a validation of the estimates over the sea proper is required. Here we present a method to analyse precipitation measurements over the Baltic Sea from the ship observations for the period 1996–2000. In order to measure precipitation over the Baltic Sea, several merchant ships have been equipped with specially designed ship rain gauges. The measurements are stored at 8-minute intervals. More than 20000 instrumental measurements were collected during several months. An interpolation scheme based on the kriging method has been used to estimate spatial precipitation distributions on a 1° [xi] 1° grid. This method is particularly used to minimise sampling errors due to the low data density. All estimations are presented on a seasonal time resolution. The estimated spatial rain fields give reasonable results, especially in areas along the main shipping routes characterised by a high data density.