As a weather hazard, fog is extremely important, but a difficult variable to predict. The visibility diagnostic includes information on the reduced visibility in fog, but correctly predicting very low visibility is dependent on predicting the correct dynamic and thermodynamic conditions in the boundary layer. These can be highly variable in space and time and are often tied to orographic features that are not resolved by the model. For these reasons a probabilistic approach using the ensemble members is more beneficial.
The visibility diagnostic is an experimental product introduced in 2016 and and the quality of this product is undergoing continued evaluation with the aim of improving the diagnostic and its usefulness as a forecast product. It should be used with caution; expectations regarding the quality of this product should remain low. It is in the operational system so that it may be evaluated and experience gained with the aim of improving the diagnostic and its usefulness as a forecast product.
The model visibility is defined as the near surface horizontal visibility within the lowest 20m layer above the surface and is calculated using an exponential scattering law. The visual range is taken to be the distance at which, despite extinction of light by the model fog, it would be possible to distinguish between a theoretical object and the model's foggy background. In the model the contrast between object and foggy background is taken as 2% difference in luminosity (a liminal contrast of 0.02). The extinction coefficients are calculated for the contributions from precipitation and cloud water droplets in the lowest model layer, and also the presence of climatologically and seasonally varying aerosol species.
The current technique for visibility forecasting has several limitations; it uses a fixed particle size for cloud and precipitation particles, and the effects caused by local deviations of the aerosol fields from climatological values and the interactions of fog and aerosol particles are not modelled yet. Model horizontal resolution is relatively low, and the lowest level in the vertical is 10m so capturing the detailed extent and composition is difficult, especially in rugged areas. Where fog is forecast by the model due to cloud water drops in the lowest model layer, visibility can fall below 500m. Visibility is still a relatively new product and undergoing continuing assessment. Initial perceptions are that radiation fog tends to be rather too dense and to form too slowly and clear too quickly (by about 1-3hr), that hill fog is rather under-represented and that visibility in precipitation falls rather low. The extinction coefficient of clean air is taken to be equivalent to a visibility of 100km so values can be no greater than this, and in general model visibility seems rather too great.
Visibility calculation using ‘tuned’ CAMS aerosol climatology has been introduced in mid 2017 in Cycle 43R3 and may alter or reduce the deficiencies in visibility outlined above.
It should be stressed that this is a preliminary implementation of a visibility diagnostic. Users are advised to keep themselves updated about visibility products through the ECMWF Newsletter and web site.