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It is important to assess the likelihood of different types of precipitation, particularly those of a hazardous nature. The method of assessment of assessment of the type of surface precipitation by the IFS models depends critically upon the temperature structure of the model atmosphere, which includes the layers through which the model precipitation falls. But modelling of this can be difficult, especially if the structure of the model atmosphere is imprecisely defined.
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- the kind of surface in question and its probable temperature before assessing the result of any indicated freezing precipitation.
- the various physical effects of precipitation types when deposited on surfaces and the way IFS handles them.
Users should be aware of the potential for freezing precipitation but also note uncertainty regarding the extent, detail and changing limits of the layer with positive temperatures. 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.
Histograms of Types of Precipitation
HRES/Ensemble Control gives no information on the probability of a type of precipitation and should not be used on its own; reference should always be made to ensemble output, particularly by use of the histogram, to assess the probability of other types of precipitation. Histograms of the ensemble "Types of Precipitation" are available as an option on ecCharts and on Opencharts. This gives easy to use information about the probability of alternative types of precipitation from the results of all ensemble members.
Fig8.1.8.10-1: Method to display histograms of probability of types of precipitation from ecCharts. The location of the histogram may be selected using the probe tool on the chart.
Instantaneous, single member HRES/Ensemble control
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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 shows forecast type:
- 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.
Users should consider the impact of a particular precipitation type. A low probability of rain may not be important to customers but a low probability of freezing rain might be very important.
Fig8.1.10-2: Method to display histograms of probability of types of precipitation from ecCharts. The location of the histogram may be selected using the probe tool on the chart.
Instantaneous, single member HRES/Ensemble control
A layer is availableon ecCharts to show the type of precipitation for precipitation rates greater than 0.1mm/hr. The precipitation types are shown by colouration.
These types of precipitation charts
These types of precipitation charts give no information on probability of the type of precipitation shown, or on the probability of any alternative precipitation types.
It is important to assess the temperature structure of the lower layers of the atmosphere to enable a tentative probability, or at least the risk, of potential severe weather types.
Fig8.1.8.2: ecChart forecast chart showing HRES forecast of instantaneous type of precipitation forecast DT 00UTC 20 January 2018, T+60 VT 12UTC 22 January 2018. A type of precipitation is shown wherever the forecast rate of precipitation is greater than 0.1mm/hr. The types of precipitation are represented by colours: Green-Rain, Red-Freezing Rain, Blue-Snow, Dark Blue-Wet Snow, Cyan-Sleet, Orange-Ice Pellets.
At Straubing, Lower Bavaria (shown by the pin) if the precipitation area is moving eastwards then the initial snow will probably turn to freezing rain followed by rain. However, this type of precipitation chart give no information on probability of the type of precipitation shown at the given time nor on precipitation shown, or on the probability of any alternative precipitation types of precipitation.
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It is important to assess the temperature structure of the lower layers of the atmosphere to enable a tentative probability, or at least the risk, of potential severe weather types.
Fig8.1.10-3 Fig8.1.8.3: ecChart forecast chart showing HRES /Ensemble Control forecast of instantaneous type of precipitation forecast DT 00UTC 20 February January 2018, T+36 60 VT 12UTC 21 February 22 January 2018. A A type of precipitation is shown wherever the forecast rate of precipitation is greater than 0.1mm/hr. The types of precipitation are represented by colours: Green-Rain, Red-Freezing Rain, Blue-Snow, Dark Blue-Wet Snow, Cyan-Sleet, Orange-Ice Pellets. The type of precipitation is also given in the "probe" information (top of diagram) according to a code - in this case for Quebec City (shown by the pin), 3 represents freezing rain at this time.
The forecast shown on "'Precipitation Type" charts is based on HRES/Ensemble Control alone and is different to guidance from the ensemble. The HRES/Ensemble Control (unperturbed) member of the ensemble may or may not be similar to all, or even any, of the other ensemble member solutions. It gives no information on the probability of a type of precipitation and should not be used on its own. Reference should always be made to ensemble output to assess the probability of other types of precipitation.
An indication of freezing precipitation gives no information of likely accumulation of glazed ice although there must be a serious risk, and heavier precipitation rates suggest a potential for greater accumulation.
Instantaneous most probable type of precipitation
A more informative ecChart display of instantaneous type of precipitation is based upon ensemble output and shows the most probable type of precipitation but gives no information on any, possibly hazardous, precipitation having a lower probability. Such information can be obtained by reference to the instantaneous type of precipitation meteogram/histogram product.
At Straubing, Lower Bavaria (shown by the pin) if the precipitation area is moving eastwards then the initial snow will probably turn to freezing rain followed by rain. However, this type of precipitation chart give no information on probability of the type of precipitation shown at the given time nor on any alternative types of precipitation.
Fig8.1.10-4: ecChart forecast chart showing HRES/Ensemble Control forecast of instantaneous type of precipitation forecast DT 00UTC 20 February 2018, T+36 VT 12UTC 21 February 2018. A type of precipitation is shown wherever the forecast rate of precipitation is greater than 0.1mm/hr. The types of precipitation are represented by colours: Green-Rain, Red-Freezing Rain, Blue-Snow, Dark Blue-Wet Snow, Cyan-Sleet, Orange-Ice Pellets. The type of precipitation is also given in the "probe" information (top of diagram) according to a code - in this case for Quebec City (shown by the pin), 3 represents freezing rain at this time.
The forecast shown on "'Precipitation Type" charts is based on HRES/Ensemble Control alone and is different to guidance from the ensemble. The HRES/Ensemble Control (unperturbed) member of the ensemble may or may not be similar to all, or even any, of the other ensemble member solutions. It gives no information on the probability of a type of precipitation and should not be used on its own. Reference should always be made to ensemble output to assess the probability of other types of precipitation.
An indication of freezing precipitation gives no information of likely accumulation of glazed ice although there must be a serious risk, and heavier precipitation rates suggest a potential for greater accumulation.
Instantaneous most probable type of precipitation
A more informative ecChart display of instantaneous type of precipitation is based upon ensemble output and shows the most probable type of precipitation but gives no information on any, possibly hazardous, precipitation having a lower probability. Such information can be obtained by reference to the instantaneous type of precipitation meteogram/histogram product.
Fig8.1.10-5: ecChart forecast chart showing HRES/Ensemble Control forecast of instantaneous type of precipitation forecast DT 00UTC 20 January 2018,Fig8.1.8.4: ecChart forecast chart showing HRES/Ensemble Control forecast of instantaneous type of precipitation forecast DT 00UTC 20 January 2018, T+60 VT 12UTC 22 January 2018 (Same ecChart as Fig8.1.8.2). 10-3). A type of precipitation is shown wherever the forecast rate of precipitation is greater than 0.1mm/hr. The types of precipitation are represented by colours: Green-Rain, Red-Freezing Rain, Blue-Snow, Dark Blue-Wet Snow, Cyan-Sleet, Orange-Ice Pellets. Areas where precipitation rates are less than 0.1 are coloured grey.
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The ensemble chart that at Straubing at 12UTC 22 January 2018 several possibilities of types of precipitation are forecast by the ensemble members: ~26% rain (of which ~4% light, ~10% moderate, ~12% heavy intensity), ~2% sleet, ~2% wet snow, ~14% Snow (varying intensities), ~18% ice pellets (varying intensities), ~34% freezing rain (of which 4% light, ~20% moderate, 10% heavy intensity), ~6% no precipitation. The histogram for Straubing shows the greatest probability (34%) is for freezing rain (shown as red on the "Most Probable Type of Precipitation" chart based on the ensemble). But the probability of rain is 26% and probability of snow 14% (shown as blue on the "Type of Precipitation" chart based on HRES/Ensemble Control).
Fig8.1.8.510-6: ecChart forecast chart showing ensemble forecast of instantaneous type of precipitation forecast DT 00UTC 20 February 2018, T+36 VT 12UTC 21 February 2018. A type of precipitation is shown wherever the forecast rate of precipitation is greater than 0.1mm/hr. The types of precipitation are represented by colours: Green-Rain, Red-Freezing Rain, Blue-Snow, Dark Blue-Wet Snow, Cyan-Sleet, Orange-Ice Pellets with shades graded according to the scale above the chart. Areas where precipitation rates are less than 0.1 are coloured grey.
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- 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 will 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).
Fig8.1.8.610-7: 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.
Interpretation of colouring on charts
However, IFS models is able to 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.
Interpretation of colouring on charts
It is important to note that the colour scales on type of precipitation 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.
- The colours in the chart show the most probable type of precipitation
- The colours in the histogram show the probability of each intensity of precipitation at the chosen probe location.
Fig8.1.8.710-9: Opencharts Probabilities. Most probable precipitation type DT 00UTC 31 June 2023, VT 12UTC 01 July 2023. To help users to have a better understanding of the situation, where the total precipitation probability is less than 50% the type of precipitation is not shown, but the total probability of the precipitation (between 10-30 or 30-50%) is shown by the shades of grey. The histogram location is for the SE of Iceland and shown by the arrow.
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Freezing rain is a very hazardous type of precipitation which brings a significant risk to aviation, transport, communication and indeed to life. There are outputs to highlight the probability and amounts of freezing rain. It should be remembered that an indication of freezing precipitation gives no information of likely accumulation of glazed ice although there must be a serious risk, and heavier precipitation rates suggest a potential for greater accumulation.
Fig8.1.8.8A (left)10-10A: HRES forecasts freezing rain (total over 12hr). DT 00UTC 23 February 2018, T+72 VT 00UTC 26 February 2018. Colour scale: Light Blue 0.0-0.2mm, Mid Blue 0.2-0.5mm, Dark Blue 0.5-1.0mm, Dark Red 1.0-2.0mm, Red 2.0-5.0mm, Pink 5.0-10.0mm, Orange >10.0mm. The pin marks the location of Quebec City with the forecast 12hr total of freezing rain given in the probe information frame. Freezing rain total values are not the same as accumulation of glaze or glazed ice although some proportional accretion must be expected.
Fig8.1.8.8B (right)10-10B: The ensemble forecasts freezing rain probability (>1mm over 12hr). DT 00UTC 23 February 2018, T+72 VT 00UTC 26 February 2018. Colour scale: Yellow 5-35%, Green 35-65%, Darker Green 65-95%, Blue >95%. The pin marks the location of Quebec City with the forecast probability of >1mm 12hr total of freezing rain given in the probe information frame.
Fig:8.1.8.910-11: ecCharts presentation of HRES forecast freezing rain accumulations over the previous 6hr, DT 00UTC 16 February 2023, T+42 VT 18UTC 17 February 2023. The vertical profile at Bangor, Maine, USA shows the classic freezing rain structure of a very moist and precipitating frontal zone overlying the sub-zero layer near the surface.
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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.
Fig8.1.8.10-12: 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 of precipitation falling due to orographically forced ascent).
Fig8.1.8.1110-13: Typical histograms of probabilities of instantaneous precipitation type for locations within the airstream. Colours in Fig8.1.8.10-12 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.
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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.
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Example of freezing drizzle
Freezing drizzle can be as much of a hazard as freezing rain. The "Probability of Freezing Rain" option on ecCharts can show 0% when the default threshold is used. Freezing drizzle can be seen if a very low threshold (say >0.01mm in previous 6hr) is applied.
Fig:8.1.8.12:10-14 ecCharts presentation of ensemble forecast of probability of freezing rain accumulations, threshold 0.01 mm in previous 6 hr, DT 12UTC 24 January 2023, T+21 VT 09UTC 25 January 2023. The vertical profile at Reading (located by the pin) shows a saturated layer of stratus at about 950 hPa with temperature above 0°C. Very slight precipitation produced from the stratus by the model falls through an underlying sub-zero layer near the surface. The model gave above 65% probability of very light freezing precipitation. Freezing drizzle was observed at Reading during the period.(located by the pin) shows a saturated layer of stratus at about 950 hPa with temperature above 0°C. Very slight precipitation produced from the stratus by the model falls through an underlying sub-zero layer near the surface. The model gave above 65% probability of very light freezing precipitation. Freezing drizzle was observed at Reading during the period.
Example of freezing rain, SE Europe
Northward advection of warm moist air over Ukraine and above an undercut of very cold air from southern Russia and the North Caucasus region is depicted on charts as an upper warm front. Vertical profiles at Kyiv and Odesa show the typical structure of the cold and warm airmasses. Freezing rain at the surface occurs where the precipitating warm saturated air falls into the underlying cold relatively dry air. The structure of the atmosphere at the intersection of the air masses is typified by the vertical profile at Kryve Ozero. The precipitation area extended from Kyiv in the north to the Black Sea in the south. Ice pellets were forecast on the northern flank of the freezing rain where the temperature in the warm air only just exceeded zero deg C. Significant hazard may be expected where high extreme forecast precipitation indices intersect areas where freezing rain is forecast.
Fig:8.1.10-15: Forecast of freezing rain over Ukraine VT 06UTC 08 January 2024, DT 00UTC 06 January 2024. Colours show 6hr totals of freezing rain. Arrows show 10m winds. 2m isotherms in red. Vertical profile and type of precipitation histogram for Odesa, in the warm air.
Fig:8.1.10-16: Forecast of freezing rain over Ukraine VT 06UTC 08 January 2024, DT 00UTC 06 January 2024. Colours show 6hr totals of freezing rain. Arrows show 10m winds. 2m isotherms in red. Vertical profile and type of precipitation histogram for Kryve Ozero, in the area of freezing rain. The vertical profile shows temperatures above 0°C in a layer about 1000m thick.
Fig:8.1.10-17: Forecast of freezing rain over Ukraine VT 06UTC 08 January 2024, DT 00UTC 06 January 2024. Colours show 6hr totals of freezing rain. Arrows show 10m winds. 2m isotherms in red. Vertical profile and type of precipitation histogram for Kyiv, in the cold air.
Fig:8.1.10-18: Forecast of freezing rain over Ukraine VT 06UTC 08 January 2024, DT 00UTC 06 January 2024. Colours show 6hr totals of freezing rain. Coloured spots show type of precipitation. Vertical profile and type of precipitation for a location near Uman, on the northern flank of the area of freezing rain. The vertical profile shows temperatures just above 0°C in a thin layer. Orange spots show ice pellets.
Fig:8.1.10-19: Forecast of freezing rain over Ukraine VT 06UTC 08 January 2024, DT 00UTC 06 January 2024. Blue/purple colours show 6hr totals of freezing rain. Red/orange colours show precipitation extreme forecast index for precipitation. Arrows show 10m winds. 2m isotherms in red. Insets show EFI and CDF for 24hr precipitation for point with pin west of Odesa.
Fig:8.1.10-20: Surface analysis from Met Office VT12UTC 08 January 2024. The area of freezing rain is shown as an upper warm front.
Example of freezing rain, France
An intensifying slow-moving frontal zone was slow-moving across northern France and southern Germany. The front separated very cold air to the north from warm moist air to the south. Precipitation type histograms from a series of ensemble forecasts consistently identified the risk of freezing rain. The structure of the forecast atmosphere at the intersection of the air masses is typified by the vertical profile at Paris. Ice pellets were forecast at Bonn on the northern flank of the freezing rain where the temperature in the warm air only just exceeded zero deg C.
Fig:8.1.10-21: Precipitation type histograms for Paris from a series of ensemble forecasts.
Fig:8.1.10-22: Precipitation type histograms for Paris from a series of ensemble forecasts.
Fig:8.1.10-23: Forecast vertical profile at Paris to the north of the surface front with typical zone of positive temperatures above an undercutting sub-zero cold zone. The vertical profiles compare the forecast profile (red) and observed profile (black). The forecast vertical profile captured the underlying cold dry air and overlying warm air quite well.
Fig:8.1.10-24: Forecast vertical profile at Idar-Oberstein to the north of the frontal zone with typical zone of positive temperatures above an undercutting sub-zero cold zone. The vertical profiles compare the forecast profile (red) and observed profile (black). The forecast vertical profile didn't capture the observed structure completely. It wasn't cold enough in the lowest layers (observed surface temperature -4°C but forecast 2m temperature -1°C) and the layer with temperatures above 0°C was forecast to be too deep. Nevertheless the forecast temperature structure was sufficient for the HRES to produce freezing rain and ice pellets.
Further information in the forecaster user guide
For more information on freezing precipitation see:
- Section 2.1.5.6 Types of Precipitation
- Section 9.7 Types of Precipitation - interpretation and effects
Additional sources of informationAdditional Sources of Information
(Note: In older material there may be references to issues that have subsequently been addressed)
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- Read more information on the updates to ecCharts regarding type of precipitation products.
- Read more information on the verification of type of precipitation products.
- Read more on the probability of precipitation type products (from page 2).
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