Boundary-layer structure of fronts observed in Helsinki, Finland

Previous numerical modelling studies have shown that boundary-layer mixing has a significant effect on the structure of synoptic-scale fronts and also that the passage of fronts can modify the structure of the boundary layer. However, there are few observations that show the vertical structure of synoptic-scale fronts in the boundary layer, and the turbulent characteristics of the boundary layer ahead of, during and behind the frontal zone. We address this deficiency by analysing six years of fronts observed in Helsinki, Finland. Helsinki (25^oE 60^oN) is an interesting location for studying boundary-layer frontal structure; it is at the end of the North Atlantic storm track, at a high latitude where stable boundary layers are common, and is on the coast. In total, 855 fronts (332 cold fronts, 236 warm fronts and 287 occluded fronts) were studied using data from two instrumented towers: basic meteorological observations from seven measurement levels on the 327m-tall Kivenlahti tower; micro-meteorological data, including turbulent fluxes of heat and momentum and standard meteorological measurements from the Station for Measuring Ecosystem-Atmosphere Relationships (SMEAR) III. Results show that within the boundary layer a vast variety of frontal structures occur and that statistically significant correlations between frontal characteristics and the pre-frontal boundary layer structure exist. Such results are likely to be useful to operational forecasters as well as developers of numerical weather prediction models.