Statistics of sea-effect snowfall in Finland based on ERA5 and FMIClimGrid

Convective sea-effect snowfall (snow band) can develop in the Baltic Sea when cold air masses are advected from the mainland over a relatively warm open sea. Snow bands may last for several days over the Baltic Sea and, depending on the wind direction, move towards the Finnish coast. To investigate the spatial and temporal characteristics of snow bands in Finland and statistics of conditions favoring their formation, we used a set of detection criteria together with ERA5 reanalysis at a spatial grid spacing of 0.25° (~31 km) for a 48-year time period (1973–2020). Daily changes in snow depth over land areas were studied from FMIClimGrid gridded observational data. Only snow band cases when snow fell over the Finnish mainland was considered. Based on the ERA5 and FMIClimGrid data, we found on average 16 snow band days (SBD) per year. On average, the accumulated snow depths during SBD were moderate, daily mean varied between 2 cm/day to 5 cm/day in the studied regions along the coast of Finland. The largest daily mean snow accumulation (3.5–5 cm) during SBD was observed over the southern coast, but the largest daily snow depth increase (67 cm in January 2016) in the gridded data set was detected in the western coast of Finland. Neither the annual number of snow band days nor the daily snow accumulation revealed statistically significant changes due to large variations between years. The months of November and December showed the highest frequency of SBD. However, the seasonal cycle of SBD seemed to be shifting one month forward as the decrease in the number of SBD during December as well as the increase during January and February were statistically significant in Finland. The long-term changes in sea surface temperature (SST) and air temperature at atmospheric level of 850 hPa (T850) were in line with the changes in occurrence of SBDs. SST increased in all months during 1973–2020 in northern Baltic Sea. In December, when the decrease in snow band days was largest, also the T850 increased indicating less cold air masses occurring in Finland. So, even with increased SST the temperature difference favoring snow band formation might not reach the minimum threshold (13 °C) to produce snow bands due to too warm air temperatures. On the contrary, during January and February the increased SST together with no changes in T850 could favor the formation of snow bands.