Sea-effect snow in Finland

Sea-effect snow (SES) is convective snowfall which can produce large snow accumulation in less than a day. Although cold weather and snowfall are frequent in Finland in wintertime, intense snowfall can cause major problems especially for the traffic sector by blocking roads, decreasing visibility, causing traffic accidents, hampering railway traffic and airport operations and thus have high impacts on society. SES is among the main natural hazards identified in the Baltic Sea region. Yet, a significant knowledge gap lies in the current and future occurrence of SES. The aim of this thesis was to add to the understanding of the occurrence of SES episodes in Finland. Finland is lacking continuous statistics of past SES episodes. The knowledge of SES occurrence in Finland is based on research articles on a few highly influential SES episodes. Typically, the studied SES episodes have caused major road traffic accidents in southern Finland. In this thesis a descriptive climatology of spatial and temporal occurrence of SES episodes in Finland was derived for the past 48 years. Firstly, the feasibility of different data to study SES episodes in Finland was investigated. The main results of this thesis were based on gridded observations (FMIClimGrid), weather radar images, a numerical weather prediction model (HARMONIE) and a reanalysis data set (ERA5). These data were used to study four past SES episodes in Finland and were found to have a good agreement on the location and the temporal occurrence of the snowfall area. The four SES episodes showed highly variable atmospheric conditions and accumulated snow amounts compared to each other. Secondly, a detection method to identify SES episodes from long-term data was created. A set of criteria used to identify SES episodes was based on atmospheric and sea surface conditions known to favor SES formation. The detection method utilized air and sea surface temperature, wind speed and direction, atmospheric boundary layer height and snowfall amount. The detection method was found to perform well as most of the identified SES episodes were verified from weather radar images. Lightning occurred during one third of the SES episodes indicating highly convective conditions. Lastly, the detection method was applied to hourly reanalysis data (ERA5) for 1973–2020. The inter-annual variation of the occurrence of SES episodes was found to be large in Finland. The annual number of SES days varied between 6–40. The annual frequency of SES days was largest on the western coast of Finland. SES episodes were detected from September to May in Finland, and the height of the SES season occurred during November and December. The largest increase in snow depth (SDI) during one day was 73 cm and occurred on the western coast of Finland. Otherwise, the SES episodes accumulated mainly moderate snowdrifts as the median SDI remained below 8 cm/day. The largest SDIs occurred most frequently on the southern coast of Finland. The results of this thesis indicate that the warmed climate has also influenced SES occurrence. Although the amount of snowfall did not show significant changes over time, a shift in the SES season towards midwinter was revealed. A better understanding of SES episodes enables society to be better prepared for convective snowfall and make adaptation and precaution plans.