Impact of Ice Data Quality and Treatment on Wave Hindcast Statistics in Seasonally Ice-Covered Seas

The seasonal icecover has significant effect on the wave climate of the Baltic Sea. We used the third-generation wave modelWAM to simulate the Baltic Sea wave field during four iceseasons (2009†-2012). We used data from two different sources: daily icecharts compiled by FMI’s IceService and modeled daily mean iceconcentration from SMHI’s NEMO†Nordic model. We utilized two different methods: a fixed threshold of 30 % iceconcentration, after which wave energy is set to zero, and a grid obstruction method up to 70 % iceconcentration, after which wave energy is set to zero. The simulations run using icechart data had slightly better accuracy than the simulation using NEMO-Nordic icedata, when compared to altimetermeasurements. The analysis of the monthly mean statistics of significant wave height(SWH) showed that the differences between the simulations were relatively small and mainly seen in the Bothnian Bay, the Quark, and the eastern Gulf of Finland. There were larger differences, up to 3.2 m, in the monthly maximum values of SWH. These resulted from individual high wind situations during which the iceedge in the icechart and NEMO-Nordic was located differently. The two different methods to handle iceconcentration resulted only in small differences in the SWH statistics, typically near the iceedge. However, in some individual cases the two methods resulted in quite large differences in the simulated SWH and the handling of iceconcentrations as additional grid obstructions could be important, for example, in operational wave forecasting.