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Deciding between rain and snow in marginal conditions is difficult for current atmospheric models; 1 or 2°C can make all the difference.
Fig97Fig9.A7-1: Snowfall in marginal conditions. An example of the evolution of the atmospheric model temperature structure during an event where the forecast rain turned to snow. Evaporation of precipitation during descent induced cooling of the model air temperature structure (between T+66 and T+72) and allowed downward penetration of snow and sleet. Cooling of the air via latent heat absorption during the melting process probably also contributed. It is likely that sleet reaching the ground has also been treated as an accumulation of snow giving a greater snow depth on the ground than was justified. In wet snow or sleet conditions there is well-documented over-accumulation on the ground of snow in the IFS.
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A surge of warm air overrunning rather than replacing a pre-existing area of stagnated cold air (e.g. by differential advection in the vertical, or where warm air from the south overruns stationary very cold low-lying air) can deliver the special conditions needed for freezing rain generation. Users should also consider the likely local temperature structure in mountainous areas where sub-zero layers may be trapped in valleys while not in evidence over adjacent more open areas.
A schematic cross-section on a transect through a precipitating warm front, based on a real case in Eastern Europe, is shown in Fig97Fig9.B 7-2 and Fig97Fig9.C7-3.
Currently IFS output will only signify freezing rain when the vertical atmospheric structure is as shown in Fig 979.B7-2. Note, however, in conditions of shallow (generally non-frontal) cloud that is supercooled but not glaciated, drizzle or light rain can fall and also take on the characteristics of freezing rain or drizzle. This is by virtue of the fact that the temperatures of the liquid droplets and the air through which they fall are both below zero. The key difference relative to the depiction in Fig97Fig9.C 7-3 is that there is no elevated warm layer, and no snow higher up - the airmass temperature will be below zero from the surface up to cloud top at least. This scenario is often know as 'freezing drizzle', though sometimes light (freezing) rain may arise in the same way.
Fig97Fig9.B7-2: Chart of the north Adriatic and adjacent countries showing assignment of precipitation type represented by colours: Green-Rain, Blue-Snow, Yellow-Ice Pellets, Red-Freezing Rain, Pink-Sleet, Turquoise-Wet snow. Shading of each colour denotes intensity - darker for more intense.
Fig97Fig9.C7-3: Schematic cross-section north to south along the black line in the chart Fig 979.B 7-2 (in many cases the ice pellet zone will be much narrower in the horizontal direction than shown here). The section intersects a warm front zone with an elevated layer where temperatures are above 0ºC. Precipitation is assigned to each precipitation type according to the structure of the model atmosphere.
Accretion of glaze or glazed ice on surfaces
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- will increase snow depth according to the proportion of snow in the mix - acting as snow falling onto snow as given above.
- will increase snow density according to the rain in the mix - acting as rain falling onto snow as given above.
- commonly snow depth/mass on the ground increases too much when ice pellets and/or sleet are falling (i.e. melting of the evolving snowpack is underdone)
Further information in the forecaster user guide
For more information on freezing precipitation see:
Additional Information
(Note: In older material there may be references to issues that have subsequently been addressed)
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