Snow bands over the Gulf of Finland in wintertime

Abstract

A B S T R A C T Large shore-parallel, quasi-stationary snow bands are occasionally observed over the Gulf of Finland during wintertime when the sea is not frozen. On the basis of Weather Research and Forecasting mesoscale model experiments and radar observations of snow bands formed in January 2006 and February 2012, we show that their dynamics share common characteristics: (1) the sea gulf that produces the known lake effect, (2) cold easterly large-scale flow along the gulf and (3) a cold local flow from the two near and opposite coastlines of Estonia and Finland in the form of two land-breeze cells which collide offshore. The associated fronts, which have strong rising motions, are maintained by the convergence of the land-breeze cells. In addition to these factors, the concave shape of the coast in the eastern part of the Gulf of Finland promotes offshore convergence and the formation of several secondary bands of precipitation that are adjacent to the eastern part of the main band. When the easterlies turn to southerlies, horizontal convective rolls appear over the sea. The Estonian land breeze is enhanced while the cold air remains stagnant inland over the Finnish coast, acting as an orographic barrier lifting the marine air mass upwards. Consequently, a line of convective precipitation composed of several cells is formed along the Finnish coast. In both events, the simulations also show two low-level jets generated by the combined effects of the land-breeze cells and baroclinicity over the coast of Finland and Estonia.

Intense sea-effect snowfall case on the western coast of Finland

“…The roles of colliding land breezes from the coasts of Finland and Sweden or Finland and Estonia; orographic lifting; and a concave shape of a coastline in promoting offshore convergence and formation of snowbands have recently studied by Savijärvi (2012Savijärvi ( , 2015 and Mazon et al (2015). Based on …”

“…This produces an unstable boundary layer over the water body. As a result, the turbulent heat and moisture fluxes from the water surface are large and generate shallow convection that induces small and intensive convective precipitation which can drift to the coast as snowbands (Markowski and Richardson, 2010;Mazon et al, 2015).…”

“…Surface heating of cold-air advection over relatively warm water is the most important factor affecting the occurrence of snowbands (Mazon et al, 2015, and references therein). As a rule of thumb, the temperature difference between the 850 hPa temperature and the water surface should be at least 13 • C (Markowski and Richardson, 2010).…”

Characteristics of convective snow bands along the Swedish east coast

“…Convective snow bands in the Baltic Sea area have been studied widely using a variety of methods for the Gulf of Finland as well as other parts of the Baltic Sea (e.g. by Andersson and Gustafsson, 1993;Andersson and Nilsson, 1990;Vihma and Brümmer, 2002;Mazon et al, 2015;Savijärvi, 2012Savijärvi, , 2015. The focus of our study is on the performance of a regional climate model to represent convective snow bands affecting the Swedish east coast.…”

“…The most important element for the formation of snow bands is the thermal difference between the water surface and the overlaying air (Mazon et al, 2015), which determines the extent of the essential heat and moisture fluxes. Therefore, it is necessary that a large part of the water surface is ice-free in order to ensure sufficient sensible heat release and evaporation from the sea.…”