Structure of a Warm Front: Helsinki Testbed Observations and Model Simulation

Abstract

The purpose of this study is to document the structure of a warm front in northeast Europe, identify the effects that the Finnish coastline has on the evolution of the front, and investigate factors that influence the speed that the warm front moves at within, and above, the boundary layer. The warm front formed over Estonia, traveled northward across the Gulf of Finland, and then crossed the southern coastline of Finland. Surface-based measurements from the Helsinki Testbed are analyzed together with output from a high-resolution numerical weather prediction model, Application of Research to Operations at Mesoscale (AROME). During the early stages of development, the warm front interacted with a stationary baroclinic zone and, consequently, evolved into an S shape. As the front approached the southern coast of Finland, the temperature gradient at 1000 hPa increased, as it merged with a diabatically generated temperature gradient. At 1000 hPa, the front stalled at the coastline due to friction-enhanced convergence, while the front’s speed at 860 hPa was almost uniform and unaffected by the coastline. At both 860 and 1000 hPa, the front moved slower than the wind speed. Hence, the front’s movement had a propagating component that was directed in the opposite direction to that of the front’s movement. The distribution of the ageostrophic winds showed that the front’s propagation component was produced by the front’s secondary circulation and surface friction. These results highlight the importance of surface sensible heat fluxes and friction on the evolution and movement of warm fronts.