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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 Fig9.7-2 and Fig9.7-3.
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Fig9.7-3: Schematic cross-section north to south along the black line in the chart Fig 9.7-2 (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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- is considered to accumulate where the surface temperature is below 0°C and is added the previous snow lying on that surface. the full snowpack is represented with one (evolving) density, as currently there is only surface snow layer.
- predicted snow depth will be in error whenever snow of one density falls onto snow that has a different density; if the density difference is large errors in forecast depth can be substantial.
- is considered to immediately melt where the surface is above 0°C, although associated latent heat release will generally cool the surface leading soon to accumulation.
- in practice, melting is often underdone, which commonly results in too much snow on the ground in 'wet snow' situations.
- when convective component dominates, the model tends to produce high precipitation rates with a significant snow fraction event when surface temperatures are well above freezing;
- the IFS tends to produce rather too active convection with too much wintry precipitation or snow (e.g. a mixture of snow and rain (sleet) at the surface while the 2-metre temperature is 8.2°C). These showers sometimes leave snow on the ground (mainly <1 cm) which is then slow to thaw. This can lead to some false alarms, especially if users are not aware of this tendency.
Freezing rain falling onto snow on the ground:
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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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