Reconstruction of the precipitation series in Gdansk, Poland during the period of instrumental measurements

Gdansk, the coastal city in Northern Poland is characterized by an unusually early start of instrumental precipitation measurements. The first isolated measurement series, over 30 years old, dates back to 1739-1772 and was the work of the physicist and mathematician Michael C. Hanow. In the second decade of the 19th century, measurements of precipitation totals were occasionally carried out by a medicine doctor Johann G. Kleefeld. The next period of precipitation measurements, continuing uninterrupted until the present day, began in 1851 and were initiated by Friedrich Strehlke – a head of the St. Peter and Paul School in Gdansk.

Due to the disappearance, most probably irreversible, of Strehlke's original records, it has become impossible to reconstruct the daily precipitation totals from the initial period of measurements. It is possible to do so for the time from 1876 onwards, from the start of the Deutsche Seewarte meteorological station in the Nowy Port (New Harbour/Neufahrwasser) district. However, the complete record of annual and monthly precipitation totals still exist.

The average annual total precipitation in Gdansk in the period 1851-2024 was about 560 mm. The wettest months are July and August, with an average of over 70 mm of precipitation, and the driest are February and March (less than 30 mm of precipitation). No statistically significant changes in precipitation totals were found in Gdansk in the considered period. There are on average 170 days with precipitation of at least 0.1 mm per year. Approximately five times a year more than 20 mm falls per day, while once in four years there is precipitation exceeding 50 mm. The absolute maximum daily precipitation, recorded in Gdansk in 2016, reached 160 mm.

Particularly important task of the paper was a reconstruction of the variability of daily precipitation totals carried out in relation to the earliest of the Gdansk measurement series, i.e. Hanow’s measurements. In addition to daily data, information on the type of hydrometeorological phenomena observed by the researcher was also used, which allowed to improve the reconstructed series values. Several significant incidents of very heavy rainfall were identified during the analyzed period. Another observed feature was the significant dominance of autumn precipitation totals over spring totals, demonstrating the strengthening of the oceanic characteristics of the city's climate in the 18^th century.

The work was done within a NCN project entitled The occurrence of extreme weather, climate and water events in Poland from the 11th to 18th centuries in the light of multiproxy data, supported by the National Science Centre, Poland, project No. 2020/37/B/ST10/00710.