Use of Richardson Number Methods in Regional Models to Calculate the Mixed-Layer Height

Measurements of the boundary-layer height over a small island (Christiansø) in the Baltic Sea for the period 26 October - 3 November 1998 are discussed. The meteorological conditions are characterised by positive heat flux over the sea and formation of convective marine boundary layer. The boundary-layer height was simulated with two models, a simple applied high-resolution (2 km times 2 km) model, and the operational numerical weather prediction model HIRLAM (grid resolution of 22.5 km times 22.5 km). For south-westerly winds it was found that the relatively large island (Bornholm) lying 20-km upwind of the measuring site influences the boundarylayer height. In this situation the high-resolution simple applied model gives good estimate of the boundary-layer height over the measuring site while the Richardson-number methods based on HIRLAM output fail. For northerly wind the water fetch to the measuring site is about 100 km and both models reproduce the characteristics of the height of the marine boundary layer.