Statistics of sea-effect snowfall along the Finnish coastline based on regional climate model data

Abstract

Abstract. The formation of convective sea-effect snowfall (i.e., snow bands) is triggered by cold air outbreaks over a relatively warm and open sea. Snow bands can produce intense snowfall which can last for several days over the sea and potentially move towards the coast depending on wind direction. We defined the meteorological conditions which statistically favor the formation of snow bands over the north-eastern Baltic Sea of the Finnish coastline and investigated the spatio-temporal characteristics of these snow bands. A set of criteria, which have been previously shown to be able to detect the days favoring sea-effect snowfall for Swedish coastal area, were refined for Finland based on four case study simulations, utilizing a convection-permitting numerical weather prediction (NWP) model (HARMONIE-AROME). The main modification of the detection criteria concerned the threshold for 10 m wind speed: the generally assumed threshold value of 10 m s−1 was decreased to 7 m s−1. The refined criteria were then applied to regional climate model (RCA4) data, for an 11-year time period (2000–2010). When only considering cases in Finland with onshore wind direction, we found on average 3 d yr−1 with favorable conditions for coastal sea-effect snowfall. The heaviest convective snowfall events were detected most frequently over the southern coastline. Statistics of the favorable days indicated that the lower 10 m wind speed threshold improved the representation of the frequency of snow bands. For most of the favorable snow band days, the location and order of magnitude of precipitation were closely captured, when compared to gridded observational data for land areas and weather radar reflectivity images. Lightning were observed during one third of the favorable days over the Baltic Sea area.

Convection‐permitting modeling with regional climate models: Latest developments and next steps

“…Finally, events over Finland that were generated over the Baltic Sea were captured well in a CPRCM with good timing and location of the most intense snowstorms (Olsson et al, 2017). Olsson et al (2020) refined a set of criteria to detect lake-effect snowfall over the Finnish coast simulating four case studies with a CPRCM to gain information on atmospheric conditions favoring coastal snow band formation.…”

Natural hazards and extreme events in the Baltic Sea region

“…Nevertheless, for the two major bays (the Gulf of Bothnia and the Gulf of Finland), the sea-effect snowfall can occur on any coast with cold air outbreaks. Favourable conditions for the development of convective snow bands include an optimum strong wind, large air-sea temperature difference, low vertical wind shear, high atmospheric boundary layer height, and favourable wind directions (Jeworrek et al, 2017;Olsson et al, 2020).…”

Natural Hazards and Extreme Events in the Baltic Sea region

“…Our current knowledge is mainly based on studies from the Great Lakes in North America (Wright et al 2013, Cordeira and Laird 2008, Laird et al 2009, Niziol et al 1995, Hjelmfelt 1990). For the Baltic Sea there is an increasing number of studies concerning the formation (Olsson et al 2017b, Mazon et al 2015, Savijärvi 2015, Savijärvi 2012, Andersson and Nilsson 1990, Gustafsson et al 1998) and statistical analysis (Jeworrek et al 2017, Olsson et al 2020) of sea-effect snowfalls, as well as effects of excess snowfall to society (Juga et al 2014, Vajda et al 2014.…”

“…Nevertheless, for the two major bays (the Gulf of Bothnia and the Gulf of Finland), the sea-effect snowfall can occur on any coast with cold air outbreaks. Favorable conditions for the development of convective snow-bands include an optimum strong wind, large air-sea temperature difference, low vertical wind shear, high atmospheric boundary layer height and favorable wind directions (Jeworrek et al, 2017;Olsson et al 2020).…”

“…Using simulations conducted with the regional climate model RCA4 for the period 2000-2010, annually 4 to 7 days was seen to be favorable for snowband formation in the western Baltic Sea area and 3 days per year in the eastern Baltic Sea area (Jeworrek et al, 2017;Olsson et al 2020 The sea-effect snowfall events typically have temporal and spatial scales smaller than what is covered by the observational network and resolved by climate models. The high resolution ERA5 data was used in a case study for January 2016.…”