A 6-yr Climatology of Fronts Affecting Helsinki, Finland, and Their Boundary Layer Structure

Abstract

AbstractA 6-yr climatology of the frequency, characteristics, and boundary layer structure of synoptic-scale fronts in Helsinki, Finland, was created using significant weather charts and observations from a 327-m-tall mast and from the Station for Measuring Ecosystem–Atmosphere Relationships III. In total, 855 fronts (332 cold fronts, 236 warm fronts, and 287 occluded fronts) affected Helsinki during the 6-yr period, equating to one front every 2.6 days. Seasonal and diurnal cycles were observed, with frontal frequency peaking during the cold season and during daytime. Composites of warm and cold fronts were developed to provide observationally based conceptual models of the low-level structure of fronts at the end of the North Atlantic Ocean storm track. The composite warm front displays a temperature increase of 4.0°C; a broad, forward-tilting frontal zone; and prolonged, weak-to-moderate precipitation. The composite cold front is characterized by a temperature decrease of 4.4°C, a narrow and slightly rearward-tilting frontal zone, and moderate precipitation collocated with the surface front. Relationships between frontal characteristics and the direction from which fronts approached, the season, time of day, prefrontal boundary layer lapse rate, and the location of the wind shift relative to the thermal gradient were investigated. The prefrontal lapse rate was the single most important variable in determining the temperature change, the height of the maximum temperature change, and the near-surface tilt of both warm and cold fronts. This result demonstrates the interaction between boundary layer and synoptic-scale processes that must be captured by numerical weather prediction models to accurately forecast surface fronts.