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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

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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.   Users should be aware of the potential for freezing precipitation but 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.

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Fig:8.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.

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Fig:8.1.10-23: Forecast vertical profile at Paris to the north of the frontal zone 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 doesn't capture captured the underlying cold air but is sufficient for the HRES to produce freezing rain and  ice pelletsdry 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 doesndidn't capture the cold air but is 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 ice pellets.

Further information in the forecaster user guide

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