Marine boundary-layer height estimated from the HIRLAM model

Two-weeks of measurements of the boundary-layer height over a small island (Christianso) in the Baltic Sea is discussed. The meteorological conditions were characterised by positive heat flux over the sea. The boundary-layer heights derived from radiosonde measurements were compared to Richardson-number estimates based on output from the operational numerical weather prediction model HIRLAM (a version of SMHI with a grid resolution of 22.5 km x 22.5 km). For southwesterly winds it was found that a relatively large island (Bornholm) lying 20 km upwind of the measuring site influences the boundary-layer height. In this situation Richardson-number based methods with the HIRLAM data fail most likely because the island of Bornholm and the water fetch to the measuring site are about the size of the grid resolution of the HIRLAM model and therefore poorly resolved. For northerly winds the water fetch to the measuring site is about 100 km and the Richardson methods reproduce the height of the marine boundary layer. This suggests that the HIRLAM model adequately resolves a water fetch of 100 km with respect to predictions of the height of the marine boundary layer.