Development of a numerical short range forecast model of fog and visibility
Final Report Abstract
The presence of fog and low clouds in the lower atraosphere can have a critical impact on both airborne and ground transports and is often connected with serious accidents. An improvement of fog forecasts in terras of localisation, duration and variations in visibihty therefore holds an immense operational value for the field of transportation in conditions of low visibility. However, fog is generally a small scale phenomenon which is mostly affected by local advective transport, turbulent mixing at the surface as well as its microphysical structure. Therefore, a detailed description of the microphysical processes within the three-dimensional dynamical core of the forecast model is necessary. For this purpose, a new microphysical parametrisation based on the one-dimensional fog forecast model, PAFOG was implemented in the Lokal Modell (LM), the nonhydrostatic mesoscale model of the German Meteorological Service. The implementation of cloud water droplets as a new prognostic variable allows a detailed definition of the sedimentation processes and the variations in visibility. A horizontal resolution of 2.8 km and a vertical resolution of 4 m describe the boundary layer processes, forecasted by LM-PAFOG. In the framework of the COST 722 intercomparison campaign, the evaluation of the LM- PAFOG forecasts, based on statistical study and case studies, points out the variability of the model performance between day and night time periods. LM-PAFOG has some difficulties to form fog in the afternoon, while it forecasts the observed fog episodes in the calm atmosphere accurately. Despite the variations of the prediction accuracy, 20% of fog episodes have been correctly forecasted. The turbulent mixing parametrisation adjusted to the high vertical resolution has shown evident shortcomings of the chosen turbulence scheme. It has been clearly identified as responsible for the inaccurate afternoon fog forecasts. Current researches and sensitive tests concerning a new turbulent mixing parametrisation are in progress. Moreover, the comparisons with other fog forecast systems highlight the decisive influence of an adapted data assimilation scheme for the high grid resolution model, as well as the necessary calibration of a visibility parametrisation. The research models such as LM-PAFOG heed more computing experience in order to adjust their new physical parametrisation to the high spatial grid distance of this new models and finally to be competitive or better than the current fog forecast, supplied by the operational mesoscale models. Finally, due to the lack of information concerning the observed fog spatial extension, a verification scheme with MSG satellite products for fog and low stratus is tested. Case studies outlined the good appraisal of forecasted fog extension and the potential of advanced satellite products which can be used for verification purposes. However, an evaluation only based on the satellite product comparison is not sufficient in presence of multilayer clouds since the lower atmosphere can be hidden from the satellite. A product combining surface observations and satellite products for fog and very low stratus is necessary to evaluate the gains in spatial distribution rendition by our model. The development of such a product has to be investigated.
Publications
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COST 722 Model Intercomparison on Lindenberg area. COST-722 Final Report, 2007
Masbou, M., A. Bott, H. Petithomme, C. Petersen, N.W. Nielsen, H. Seidl, A. Kann, M.D. Müller, J. Cermak and W.K. Adam
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LM-PAFOG- a new three-dimensional fog forecast model with a parametrised microphysics. Ph.D. thesis, Meteorological Institute, University of Bonn, 2008
Masbou, M.
