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Double-layered structures found over the Baltic Sea are investigated using radiosoundings and lidar measurements. Situations with double-layer structures are also simulated with the regional model REMO in a realistic manner. The double layer consists of two adjacent well-mixed layers, with a sharp inversion in between.

Results from radiosoundings show that the double-layer structure over the Baltic Sea mainly occurs during the autumn with thermally unstable stratification near the surface. The structure is present in about 50% of the radiosoundings performed during autumn. The presence of the double-layer structure cannot be related to any specific wind direction, wind speed or sea surface temperature.

The lidar measurements give a more continuous picture of the time evolution of the double layer structure, and show that the top of the lower layer is not a rigid lid for vertical transport.

Two possible explanations of the double-layer structure are given, (i) the structure is caused by ‘advection’ of land boundary-layer air over the convective marine boundary layer or, (ii) by

development of Sc clouds in weak frontal zones connected to low pressure systems. Also the forming of Cu clouds is found to be important for the development of a double-layer structure.