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Fig9.4-1: An example of HRES forecast fog distribution on right hand chart compared with the observed visibility on left hand chart (visibility as in coloured scale). Forecast run data time 00UTC 23 January 2017 T+30 verifying at 06UTC 24 January 2017. Thick fog, less than 100m visibility in places, caused problems over southern England. Many flights were cancelled and others delayed. In the early morning hours the fog was more widespread over England and Benelux countries. HRES forecast visibility was too poor in some areas but provided good guidance to forecasters regarding areal extent of the fog risk.
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.
Fig9.4-2: Fog probability in the early medium-range from an earlier forecast run, data time 00UTC 21 January 2017 T+78 verifying at 06UTC 24 January 2017. The ensemble also highlighted England and Benelux countries as areas where fog was likely.
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.
Considerations: Fog and Freezing fog
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Sub-grid variability in visibility (due to patchy fog) can potentially be "greater" than it is in for any other meteorological parameter. Users should always remember that raw IFS forecasts are delivered on the model grid resolution (e.g. currently ~9km boxes for the ensemble). Compare this with the dimensions of a fog patch, which may be ~10m. These dimensions differ by 3-4 orders of magnitude.
Visibility is still a relatively new product and undergoing continuing assessment. Initial perceptions are that:
- radiation fog tends to be:
- rather too dense,
- rather too deep
- to form too slowly
- to clear too quickly (by about 1-3hr).
- hill fog is rather under-represented,
- visibility in precipitation falls rather too low.
Users should recognise that:
- Fog can be very patchy and 10km-scale models (such as IFS) cannot be expected to handle this well.
- Areal extent of fog is broadly captured by HRES and Ensemble Control Forecast (ex-HRES) and ensemble member forecasts but at best these are only indicative of areas at risk. Such predictions do not preclude occurrence outside of the apparent "risk areas".
- Local skill in forecasting the occurrence of fog (e.g. for a specific site) can be very low. Local skill in predicting very low visibilities (e.g. meeting motoring warnings threshold criteria) can also be very low. Beware!
- Jumpiness in fog forecasts from HRES and Ensemble Control Forecast (ex-HRES) must be expected. Results may well not show convergence towards the correct solution. Forecasts with shorter lead times will not necessarily be more skilful that those from longer lead times.
Errors in forecast near-surface data associated with cases of thick fog.
Fig9.4-8: An example of incorrect temperature and dew point forecasts in a case of predicted fog. Imperfectly modelled mixing processes near the surface induce errors in 2m temperature and humidity.
See also potential cause of poor stratus forecast.
Example of patchy fog prediction
IFS forecasts of reduced visibility can be difficult when fog is patchy or shallow.
Fig9.4-6: Forecast visibility at 30hr lead time (left) and 6hr lead time (right) with verifying observations, in a freezing fog situation. Although there is a good deal of consistency between the forecast fog distribution, there is slightly more fog forecast on the T+6 forecast chart than on the T+30 chart. However, consistency should not necessarily be taken as a good guide to the true extent of fog. The large area of fog over north France is captured quite well, but over Britain observations of fog do not in general match either of the forecast locations at T+30 or T+6 (e.g. East Anglia). However, fog is often quite patchy and may not be captured by the observations (observations are reports of fog at an individual location, not an area). Additionally it should be noted that forecast visibility charts show poor visibility where cloud covers hills.
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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.
Users should recognise that:
- Fog can be very patchy and 10km-scale models (such as IFS) cannot be expected to handle this well.
- Areal extent of fog is broadly captured by HRES and Ensemble Control Forecast (ex-HRES) and ensemble member forecasts but at best these are only indicative of areas at risk. Such predictions do not preclude occurrence outside of the apparent "risk areas".
- Local skill in forecasting the occurrence of fog (e.g. for a specific site) can be very low. Local skill in predicting very low visibilities (e.g. meeting motoring warnings threshold criteria) can also be very low. Beware!
- Jumpiness in fog forecasts from HRES and Ensemble Control Forecast (ex-HRES) must be expected. Results may well not show convergence towards the correct solution. Forecasts with shorter lead times will not necessarily be more skilful that those from longer lead times.
Errors in forecast near-surface data associated with cases of thick fog.
Fig9.4-8: An example of incorrect temperature and dew point forecasts in a case of predicted fog. Imperfectly modelled mixing processes near the surface induce errors in 2m temperature and humidity.
See also potential cause of poor stratus forecast.
An example of fog prediction by HRES
Example of prediction of unexpected fog
HRES and HRES and Ensemble Control Forecast (ex-HRES) can give good guidance on the development of fog and signal to users the possibility of otherwise unexpected hazardous conditions. Sea or coastal fog is relatively rare in June in the Mediterranean but HRES was able to predict well ahead of time a significant event in June 2022.
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