Numerical simulation of fog and radiation in complex terrain : results from COST-722

Two high resolution numerical 1D models, namely COBEL and PAFOG, have been adapted to compute a probabilistic fog forecast. Major modifications were made to the COBEL model. It was coupled to the NOAH land surface model to take into account the effects of soil and vegetation and furthermore a parameterization of precipitation was added. To deal with the large uncertainty inherent to fog forecasts, a whole ensemble of 1D runs is computed using the two different numerical models and a set of different initial conditions in combination with distinct boundary conditions. Initial conditions are obtained from variational data assimilation, which optimally combines observations with a first guess taken from operational 3D models. The design of the ensemble scheme computes members that should fairly well represent the uncertainty of the current meteorological regime. Verification reveals that the probabilistic forecast can significantly improve the current methods used at Z¨rich u Unique airport. The complex topography in Switzerland further complicates fog forecasting. In order to simulate processes like advection, cold air drainage flows and cold air pooling, the NMM 3D model of NOAA/NCEP is modified and extended with detailed fog microphysics. The resulting 3D fog model runs at a horizontal resolution ofkm and a vertical resolution comparable to the 1D models. First results look very promising and are able to reproduce the spatial distribution of fog as it is seen by satellite. With increasing horizontal resolution of numerical weather prediction models, topographical effects on radiation gain importance. With a newly developed parameterization it is possible to consider slope angle, aspect angle, shadows and restricted sky view on the subgrid scale and with negligible computational costs. Verification reveals that RMS and mean error ofm temperature forecasts are generally improved by 0.5 toK.