Cold air outbreaks over high-latitude sea gulfs

Abstract

A B S T R A C T Wintertime cold outbreaks were studied via a 2-D numerical model set across an 80 km wide non-frozen sea gulf along 608N ('Gulf of Finland'). In calm conditions, land breezes develop over both coasts with relatively large along-shore wind components. The mid-gulf convergence of the colliding land breezes leads to a moderate rising motion at about 600 m height, forcing bands of low cloud and snowfall along the gulf, whereas the near-surface horizontal wind shear may induce 'mini-hurricanes'. A weak large-scale cold outbreak across the gulf distorts the land breeze cells, damping the rising motion, whereas a moderate cross-coast outbreak modifies them into a typical heat island circulation pattern with only a modest rising motion over a flat windward shore. A cold outbreak along the non-frozen gulf leads to strong heat transfer from the sea. This maintains the embedded coastal land breeze circulations that contribute to a double low-level jet structure. The strongest rising motion was obtained for surface winds blowing along the gulf. It is suggested that the Swedish Ga¨vle snowstorm of December 1998 was such a case.

Snow bands over the Gulf of Finland in wintertime

“…The relatively warm and wet maritime air was forced to ascend over, leading to strong convergence and vertical motion, which apparently drove the cellular convection inland. By comparing with the results of the flat-coast experiments of Savija¨rvi (2012) it can be concluded that the role of the coastal ridge was important whereas.…”

“…He proposed that the collision of two land-breeze cells from nearby, opposing shores is the main trigger of along-shore snow bands in calm weather. In addition, Savija¨rvi (2012) suggested that this mechanism caused the snow band recorded between the Estonian and Finnish coasts in January 2006 over the Gulf of Finland, as well as the Ga¨vle snow band that hit the Swedish coast in December 1998 being caused by this mechanism. However, a full 3D numerical study of such snow bands over the Gulf of Finland has not been performed yet.…”

“…1 and 2), cold-air outbreaks over the partly ice-covered sea have been studied by Andersson and Gustafsson (1994), Gustafsson et al (1998), andBrummer (2002). Savija¨rvi (2012) applied a 2D numerical model for an idealised study over the sea gulf, and concluded that a cold-air outbreak along an ice-free gulf leads to a strong heat transfer from the sea, maintaining a land-breeze circulation over both coastlines of the gulf, and contributing to a double low-level-jet (LLJ) structure. He proposed that the collision of two land-breeze cells from nearby, opposing shores is the main trigger of along-shore snow bands in calm weather.…”

“…The airÁwater temperature difference is the most important forcing mechanism for snow bands (Lavoie, 1972). Besides this sea-air thermal difference, the formation of snow bands may be supported by various factors, among them land-breeze cells formed as a consequence of the large thermal differences between the sea/lake and land (Passarelli and Braham, 1981;Simpson, 1994;Savija¨rvi, 2012). Heavy precipitation events related to snow bands are commonly called lake-effect snow storms, which have been studied extensively over the *Corresponding author.…”

Snowbands over the English Channel and Irish Sea during cold‐air outbreaks

“…Passarelli and Braham, 1981;Savijärvi, 2012). In accordance with this relaxed definition, the American Meteorological Society (AMS) Glossary states that the land breeze 'usually' blows at night (http://amsglossary.allenpress.com/glossary).…”