Section 9.4 Visibility

• Section9.4.1 Reduction of visibility in fog
• Section9.4.2 Reduction of visibility in dust and sand

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Visibility in IFS

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 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 not resolved by the atmospheric model.  For these reasons a probabilistic approach using the ensemble members is more beneficial.

The visibility diagnostic is an experimental product introduced in 2016.  The quality of this product is being continuously evaluated with the aim of improving the diagnostic and its usefulness.  It should be used with caution; expectations regarding the quality of this product should remain low.  Visibility products are available so that they can be evaluated and also to allow users to gain experience.  Users are advised to keep themselves updated about visibility products through the ECMWF Newsletter and web site. 

In IFS:

• Visibility in IFS is defined as the near surface horizontal visibility within the lowest 20m layer above the surface.  It is calculated using an exponential scattering law.
• Visual range is taken to be the distance at which, despite extinction of light by the atmospheric model fog, it would be possible to distinguish between a theoretical object and the model's foggy background.  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 from:

• the contributions from precipitation and cloud water droplets in the lowest layer. 
• 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.
• the effects caused by local deviations of the aerosol fields from climatological values are not modelled.
• the interactions of fog and aerosol particles are not modelled.

Horizontal resolution is relatively low. and the lowest level in the vertical is 10m.  Thus capturing the detailed extent and composition is difficult, especially in rugged areas.  Where fog is forecast due to cloud water drops in the lowest model layer, visibility can fall below 500m.  

Convective precipitation is not taken into account for the visibility calculation, only large-scale precipitation, so visibility may often be too high in convective situations.

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 visibility in clean air seems rather too great.

Visibility calculation using a ‘tuned’ CAMS aerosol climatology was introduced in mid 2017 in Cycle 43R3 and may alter or reduce the deficiencies in visibility outlined above.