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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 shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Types of precipitation - charts and diagrams

IFS gives information about the intensity and probability of different types of precipitation.  It is especially important to assess the likelihood of hazardous precipitation and its impact.  Even a small probability of hazards such as heavy snowfall or freezing rain/drizzle can be useful information forecaster and customer, even at quite short lead-times.  The type of surface precipitation critically depends upon the temperature structure of the model atmosphere.  Especially important are the layers in the lower atmosphere through which model precipitation falls. However, modelling these can be difficult, especially if the structure of the model atmosphere is imprecisely defined.  However, the ensemble gives possible variations in the temperature and humidity structure of the lower troposphere.   The distribution of types of precipitation among the ensemble members gives an assessment of probability of the types of precipitation that may occur.  

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Rain at the surface can sometimes change to snow perhaps because of evaporative cooling of the airmass as the precipitation falls.

Instantaneous type of precipitation single member (Ensemble Control Forecast (ex-HRES))

The precipitation types are shown by colours.

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Precipitation type is indicated, even if precipitation rate is minuscule.

Instantaneous most probable type of precipitation (ensemble)

ecCharts show the type of precipitation for precipitation rates greater than 0.1mm/hr.  The precipitation types are shown by colours.

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• predominantly dry snow (shades of purple) with fairly high probabilities.
• some indication of wet snow (shades of blue) with low probability and a few indications of heavy wet snow (black) with very low probability.
• some indication of rain (shades of green) with low probability.
• a few indications of ice pellets (brown/orange) with low probability.
• a few indications of freezing rain (red) with low probability.

Interpretation of colouring on charts

It is important to note that the colour scales on type of precipitation charts and the accompanying histograms indicate different things - they are not interchangeable. 

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• Colours show precipitation types at the chosen probe location at the chosen probe location.
• Shading shows precipitation rates (heavy, medium, light - scale shown above histograms) at the chosen probe location.

Importance of investigating the range of precipitation types.

Users should inspect the histogram of precipitation types and not rely only on the instantaneous precipitation types shown by Ensemble Control Forecast (ex-HRES) charts, or the instantaneous probability of precipitation type (ensemble charts alone).  They give no information on any alternative types of precipitation.   It should not be assumed that:

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It is always wise to use histograms of precipitation type in conjunction with charts of most probable precipitation type to assess the most likely precipitation type and the probability of alternatives.  The histograms are readily available on ecCharts.

Freezing precipitation

A major weather hazard is freezing precipitation (freezing rain and freezing drizzle) which requires a particular and relatively rare type of temperature and humidity structure in the vertical.  In IFS, diagnosis of freezing precipitation takes no account of the temperature of the surface, only on the temperature structure of the boundary layers.  IFS indication of freezing rain/drizzle indicates only that super-cooled droplets are likely to exist in the boundary layer but gives no guidance upon the likely formation of glaze or glazed ice on exposed surfaces.  Nevertheless, the forecast rate of precipitation does give some indication of the amount of ice accretion that may be expected.  In assessing the impact forecast of freezing precipitation the user should consider:

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An indication of freezing precipitation gives no information of likely accumulation of glazed ice although there must be a serious risk.  Heavier precipitation rates suggest a potential  for greater accumulation.

Model diagnosis of freezing precipitation 

Freezing rain and drizzle form in different ways:

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The precipitation type a reasonable precipitation rate threshold (e.g. 0.1 mm/h) all severe precipitation types basically disappear

Examples of Instantaneous charts and corresponding histograms

The figures below (Fig8.10.1 and Fig8.10.2)  illustrate the significant differences that can occur between data shown by instantaneous type of precipitation charts and the important detail that can be concealed.   Histograms show probabilities of each type of precipitation measured as the proportion among the ensemble members of each type of precipitation forecast at the selected location.  The probabilities of all forecast precipitation types are shown, including potentially hazardous precipitation, even at low probabilities.

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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 shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Variation of probability of precipitation type with forecast time

The "Probability of Type of Precipitation" chart shows:

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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 shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Modulation of Precipitation Type Probabilities by Sea Track and Topography

The following plots illustrate how the handling of precipitation type by ensemble forecasts in a marginal rain-snow situation can make physical sense; and specifically how user-relevant subtle changes are predicted with some precision.

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Fig8.1.10-13: Typical histograms of probabilities of instantaneous precipitation type for locations within the airstream.  Colours in Fig8.1.10-12 represent the most likely (i.e. produced by the greatest percentage of ensemble members) but don't show other types, even if they are marginally less likely.  The use of histograms gives a better overview of precipitation types and enables a more confident forecast to be made by the user.  Considering 12UTC 31 Jan 2019: In Belgium there is high confidence of dry snow with very small probability of any alternative precipitation type.  

Further information in the forecaster user guide

For more information on freezing precipitation see:

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• Read more information on the verification of type of precipitation products.
  

(FUG Associated with Cy49r1)