Surface Winds and Development of Thunderstorms along Southwest–Northeast Oriented Mountain Chains

Abstract

Northern Switzerland is characterized by a plateau (Swiss Mittelland) between two parallel southwest?northeast-running mountain chains: the Alps to the southeast of the plateau and the Jura to its northwest. Under the same large-scale conditions, thunderstorms form sometimes over the Jura and not over the Alps and vice versa. This paper investigates whether the different regions of storm formation can be related to differences in the surface wind field on the plateau. Fourteen years (1983?96) of wind data from the Swiss Mesonet (ANETZ) show for fair weather a thermal circulation between the plateau and the mountains. The interaction of the channeled surface flow on the plateau (usually NE or SW) with the thermal circulation is investigated by dividing the fair weather days into days with strong southwesterly, weak southwesterly, weak variable, weak northeasterly, and strong northeasterly surface flow on the plateau. During northeasterly surface flow, the thermal upslope flow from the plateau toward the Jura is stronger than from the plateau toward the Alps. During southwesterly surface flow, the thermal upslope flow from the plateau toward the Alps is stronger than from the plateau toward the Jura. These flow patterns are consistent with the preferred origin regions of thunderstorms. A total of 329 hail days (1983?96) with 1262 radar-derived hail cells are divided according to the same surface wind criteria as the fair weather days. During northeasterly surface flow, hail cells form preferably over the western Jura while during southwesterly flow hail cells form preferably over the eastern Jura, the plateau, and the Alps. Hence, using the surface flow on the plateau is a first step toward thunderstorm forecasts on a regional scale in Switzerland.