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 uponthe 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.
ecCharts and Opencharts products aim to help with the challenges of forecasting the types of precipitation. Charts are available for:
- instantaneous type of precipitation (in map and meteogram/histogram formats).
- accumulation of each type of precipitation during a period (in map format).
- most probable type of precipitation during a period (in map format).
- probability of rain/snow amount occurring during a period (in map format).
- probability of rain/snow rate occurring during a period (in map format).
- probability of combined events ((e.g. of wind gust and total snowfall).
Chart and histogram products regarding type of precipitation incorporate some post-processing related to the minimum permissible precipitation rate. This varies according to type and has been incorporated to try to deliver relatively "bias-free" products for the user. In tests this helped greatly to minimise net under- or over-prediction of the occurrence of the different types of precipitation.
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 HRES/Ensemble control)
The precipitation types are shown by colours.
These charts show only the type of precipitation indicated by a single IFS model. They gives no information on:
- the probability of any alternative precipitation types.
The temperature structure of the lower layers of the atmosphere enable a tentative probability, or at least the risk, of potential severe weather types.
The single member charts should not be used on their own; ensemble output should always be consulted.
Precipitation type is shown:
- on Opencharts when forecast precipitation rates greater than 0.1mm/hr.
- on ecCharts with user defined minimum rate threshold. This can be very useful when investigating the possibility of freezing drizzle.
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.
These charts show the most probable type of precipitationbased upon ensemble output. It gives no information on:
- any precipitation type (possibly hazardous) having a lower probability. Precipitation type is indicated, even if precipitation rate is minuscule.
Sometimes the probabilities of two or more precipitation types may be very similar. For example, the difference in probability between problem-causing wet snow and less hazardous sleet or rain may be small and it may be may be difficult to decide between them. The charts of most probable precipitation type only show the precipitation type that has the greatest probability, no matter how small the probability is greater than the next most probable precipitation type.
Precipitation type is shown:
- on Opencharts when forecast precipitation rates greater than 0.1mm/hr.
- on ecCharts with user defined minimum rate threshold. It can be useful to use:
- a very low threshold when investigating the possibility of freezing drizzle.
- a higher threshold to remove confusing indication of extremely light but hazardous model precipitation.
Histograms of type of precipitation
The types of precipitation histogram shows 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 types of precipitation are shown in cumulative histogram form as an option on ecCharts and on Opencharts.
In each column on the histogram:
- the colours show the type of precipitation.
- the shading shows the intensity of precipitation.
- the extent within each column shows the probability of each type and intensity of precipitation.
The ensemble grid point altitude is displayed at the top of the histogram. The difference in altitude between of the location of interest and the model grid point altitude might have a significant impact.
Histograms are calibrated to include a minimum precipitation rate for display for each type of precipitation. This is there to deliver zero net bias in terms of frequency of occurrence versus synoptic precipitation reports. The legend shows what this is for each type.
Fig8.1.10-1: An example of Types of Precipitation chart for Moscow DT 00UTC 23 Nov 2023. The forecast shows the probabilities of different types of precipitation in cumulative histogram form with the shades indicating different intensities. The diagram showsforecast type:
- predominantly dry snow (shades of purple) withfairly high probabilities.
- someindicationof wet snow (shades of blue) with lowprobabilityand a few indications of heavy wet snow (black) with verylow probability.
- some indicationof rain (shades of green) with low probability.
- a few indicationsof ice pellets (brown/orange) with low probability.
- a few indicationsof 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.
On charts:
- Colours show most probable precipitation types of precipitation.
- Shading shows percentages of the most probable precipitation.
On Histograms:
- 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 (HRES) charts or instantaneous probability of precipitation type (ENS) charts alone. They give no information on any alternative types of precipitation. It should not be assumed that:
- the type shown clearly predominates above other types. The probability of another type may only be a few percent lower than the most probable.
- there isn't a probability of hazardous precipitation type at lower probability. This often occurs at longer lead-times. It is very important to capture these events.
The types of precipitation histogram gives much greater information than instantaneous charts alone and should always be consulted. Users should inspect the histogram of precipitation types and not rely only on the "Precipitation Types" charts or "Probability of Precipitation Type" charts alone.
The histograms separate intensities into heavy, moderate and light. However, it might be useful to sum the forecast probabilities of all intensities of the precipitation type.
It is always wise to use histograms of precipitation type in conjunction withcharts 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:
- the probable temperature of the surface.
- the temperature and humidity structure of the lowest layers of the atmosphere.
- the various physical effects of precipitation types when deposited on surfaces.
- uncertainty regarding the extent, detail and changing limits of the warm layer.
Much depends upon the detail of the structure of the airmass. Forecast vertical profiles should be compared with available observations of the different air masses.
The impact of a particular precipitation type should be considered. A low probability of rain may not be important to customers but a low probability of freezing rain might be very important. Freezing precipitation is such a significant hazard that even a small probability might be worth a warning to customers sensitive to this type of precipitation even though rain or snow might be more probable and shown as such on the charts.
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:
- Freezing rain is precipitation falling from a higher level through a warm layer (temperature >0C), then becoming super-cooled falling through an underlying cold layer (temperatures <0C).
- Freezing drizzle is super-cooled liquid drizzle drops formed from super-cooled cloud droplets within high moisture in the lower atmosphere (temperature <0C).
It can be difficult for IFS to identify the distinction between freezing rain and freezing drizzle. The freezing precipitation algorithm is invoked where the model 2m temperature is below 0C and the model precipitation is >50% liquid. The 2m temperature is computed by interpolation between surface temperature and the level of the lowest atmospheric model level (level 137, ~10m).
The algorithmsearches upwards from the lowest model level (L137, ~10m) at each model level for the base of a warm layer. If the algorithm:
- finds a warm (>0C) layer at or above the lowest model level (~10m) and precipitation falls from higher levels. Freezing rain is diagnosed.
- does not find a warm (>0C) layer. Either:
- the lowest model level (~10m) has temperature >0C. No warm layer is identified the algorithm diagnoses freezing drizzle. IFS does not capture the freezing level (i.e. the base of the warm layer) because that lies below the lowest model level (~10m). Freezing rain should be diagnosed in precipitation falls from above; freezing drizzle if very moist super-cooled moist layer exists in lowest layers.
This is a fairly a rare occurrence, inspect the temperature and humidity structure of lowest layers to determine the correct type of precipitation.
- there is no warm (>0C) layer at all, and all temperatures at and above the lowest model level (~10m) are <0C and precipitation falls from higher levels. Snow, ice pellets etc are normally diagnosed.
- the lowest model level (~10m) has temperature >0C. No warm layer is identified the algorithm diagnoses freezing drizzle. IFS does not capture the freezing level (i.e. the base of the warm layer) because that lies below the lowest model level (~10m). Freezing rain should be diagnosed in precipitation falls from above; freezing drizzle if very moist super-cooled moist layer exists in lowest layers.
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.
Users should always inspect the histogram of precipitation types.
Fig8.1.10-1A:HRES type of precipitation chart and ENS most probable type of precipitation. DT 12UTC 10 Mar 2024, VT T+72 12UTC 13 Mar 2024. At the arrowed location the single model (HRES) shows freezing rain (red) but ENS shows most probable precipitation is snow (50-70%).
Fig8.1.10-1B: Vertical profile at the arrowed location in Fig8.1.10-1. DT 12UTC 10 Mar 2024, VT T+72 12UTC 13 Mar 2024. The single model (HRES, solid line) shows warm layer above the surface typical of freezing rain. ENS shows a broad range of temperatures but no clear indication of the structure of the warm layer.
Fig8.1.10-1C: Type of precipitation histogram at the arrowed location in Fig8.1.10-1. DT 12UTC 10 Mar 2024, VT T+72 12UTC 13 Mar 2024. The histogram shows the main probability is for snow. Note that types of precipitation diagrams separate intensities into heavy, moderate and light and it might be useful to sum the forecast probabilities of all intensities of the precipitation type.
Detail of precipitation types: 49% snow (purple, 0.2-1.0mm/hr), 13% light snow (light purple,0.04-0.2mm/hr), 14% wet snow (blue, 0.2-1.0mm/hr), 7% light wet snow (light blue, 0.04-0.2mm/hr), 2% rain (green, 0.2-1.0mm/hr). A small chance of freezing rain (red, 0.2-1.0mm/hr) is shown during the preceding 6 hours. The probability of snow of anyintensity is about 76%.
Fig8.1.10-2A: HRES type of precipitation chart and ENS most probable type of precipitation. DT 12UTC 10 Mar 2024, VT T+72 12UTC 13 Mar 2024. At the arrowed location the single model (HRES) shows ice pellets (orange) but ENS shows most probable precipitation is uncertain (30-50%), but see also Fig8.1.10-2C.
Fig8.1.10-2B: Vertical profile at the arrowed location in Fig8.1.10-2. DT 12UTC 10 Mar 2024, VT T+72 12UTC 13 Mar 2024. The single model (HRES, solid line) shows temperature fringes the 0C isotherm layer above the surface typical of ice pellets. ENS shows a broad range of temperatures but no clear indication of the structure of the warmer layer.
Fig8.1.10-2C:Type of precipitation histogram at the arrowed location in Fig8.1.10-2. DT 12UTC 10 Mar 2024, VT T+72 12UTC 13 Mar 2024. The histogram shows a wide range of precipitation types, none individually greater than 40% probability so the most probable type of precipitation is coloured grey on the chart (Fig8.1.10-2A). There is, however, an indication of freezing rain which is a significant hazard which is not evident from the chart (Fig8.1.10-2A). Note that types of precipitation diagrams separate intensities into heavy, moderate and light and it might be useful to sum the forecast probabilities of all intensities of the precipitation type.
Detail of precipitation types: 4% heavy freezing rain (dark red, >1.0mm/hr), 15% freezing rain (red, 0.2-1.0mm/hr), 2% light freezing rain (pale red, 0.05-0.2mm/hr), 5% heavy ice pellets (brown, 0.2-1.0mm/hr), 10% ice pellets (orange, 0.2-1.0mm/hr), 32% heavy snow (dark mauve, >1.0mm/hr), 5% snow (mauve, 0.2-1.0mm/hr), 4% light snow (light mauve, 0.04-0.2mm/hr), 7% heavy sleet (dark cyan, >1mm/hr), 8% sleet (cyan green,0.2mm-1mm/hr), 8% heavy rain (dark green,>1mm/hr). The probability of freezing rain of any intensity is about 21%.
Variation of probability of precipitation type with forecast time
The "Probability of Type of Precipitation" chart shows:
- areas at greater risk for precipitation of any kind.
- areas where there is greater uncertainty.
Uncertainty varies with forecast lead-time:
- At shorter lead-times the ensemble solutions will usually be fairly similar. The probability of the types of precipitation are likely to be high. Charts normally show a lot of detail.
- As lead-time increases the spread of ensemble solutions will be greater. The development, timing and location of precipitation events will become less certain. Charts will show larger areas of grey (<50% probability of precipitation type).
Fig8.1.10-7: Multiplots of ensemble probability of type of precipitation forecasts all verifying at 12UTC 21 February 2018 from a series of ensemble forecast runs at 24hr intervals. As lead-time increases the more hazardous, less common types of precipitation are less prominent or do not appear, and greys (total probability <50%) are more prominent.
Nevertheless, IFS isableto predict freezing rain several days in advance despite the finely balanced vertical thermodynamics structure required.
Fig8.1.10-8: Sequence of precipitation types DT:00UTC 04 March 2023. VTs:00UTC 04 Mar (T+00) to 12UTC 06 Mar (T+60). Precipitation type shown by colours.
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.
Fig8.1.10-12:Illustration of the probabilities of different instantaneous precipitation types. The flow is mainly easterly over England and illustrates the impact of different length sea tracks over the relatively warm sea, and the impact of topography. Consider an approximate low level trajectory from Belgium to Wales (dashed arrow). Colour is used wherever the probability of some precipitation falling is 50% (from the ensemble).The colour itself illustrates the most likely type, whilst the darkness of the shading indicates how likely that type is. Dry snow over Belgium, wet snow and sleet over the relatively warm southern North Sea, wet snow over colder southeastern England, dry snow further away from the sea (most likely as the air re-cools partly via evaporation), then over Welsh mountains the probability of dry snow is very high (because the high probability of temperatures being below zero and of high probability ofprecipitationfalling due to orographically forced ascent).
Fig8.1.10-13:Typical histograms of probabilities of instantaneous precipitation type for locations within the airstream. Colours in Fig8.1.10-12represent 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:
- Section 2.1.5.6 Types of Precipitation
Section 8.1.10.1 Types of Precipitation - charts and diagrams
- Section 9.7 Types of Precipitation - interpretation and effects
Additional sources of information
(Note: In older material there may be references to issues that have subsequently been addressed)
- Read more information on the verification of type of precipitation products.
