SMALL-SCALE VARIABILITY IN THE COASTAL ATMOSPHERIC BOUNDARY-LAYER

The influence of the main large-scale wind directions on thermally dri ven mesoscale circulations at the Baltic southwest coast, southeast of Sweden, is examined. The aim of the study is to highlight small-scale alterations in the coastal atmospheric boundary layer. A numerical th ree-dimensional mesoscale model is used in this study, which is focuse d on an overall behaviour of the coastal jets, drainage flows, sea bre ezes, and a low-level eddy-type flow in particular. It is shown that s ynoptic conditions, together with the moderate terrain of the southeas t of Sweden (max. height h(0) less than or equal to 206 m), governs th e coastal mesoscale dynamics triggered by the land-sea temperature dif ference Delta T. The subtle nature of coastal low-level jets and sea b reezes is revealed; their patterns are dictated by the interplay betwe en synoptic airflow, coastline orientation, and Delta T. The simulatio ns show that coastal jets typically occur during nighttime and vary in height, intensity and position with respect to the coast; they intera ct with downslope flows and the background wind. For the assigned land surface temperature (varying +/-8 K from the sea temperature) and the opposing constant geostrophic wind 8 m s(-1), the drainage flow is mo re robust to the opposing ambient flow than the sea breeze later on. D epending on the part of the coast under consideration, and the prevail ing ambient wind, the sea breeze can be suppressed or enhanced, statio nary at the coast or rapidly penetrating inland, locked up in phase wi th another dynamic system or almost independently self-evolving. A low -level eddy structure is analyzed. It is governed by 'tilting', 'diver gence' and horizontal advection terms. The horizontal extent of the co astal effects agrees roughly with the Rossby radius of deformation.