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A major weather hazard is freezing rain (and freezing drizzle) which requires a particular and relatively rare type of temperature (and humidity) structure in the vertical. It is important to appreciate that diagnosis of freezing rain in IFS 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. The user should consider the kind of surface in question and its probable temperature before assessing the result of any indicated freezing precipitation.
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ECMWF products, currently available in ecCharts, aim to help with these forecasting challenges in different ways. One can examine charts that relate to the instantaneous type of precipitation, and its instantaneous intensity (in map and meteogram/histogram formats), and also to maps that display accumulations of a certain type of precipitation over a period, or probabilities thereof. However. Freezing drizzle may often be identified on ecCharts freezing rain charts by setting the accumulation over 6hr to a very low threshold (0.01mm). However, the user should also consider the various physical effects of precipitation types when deposited on surfaces and the way IFS handles them.
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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.
Probability of freezing precipitation charts modified to show drizzle???
Fig: ecCharts presentation of ENS forecast of probability of freezing rain accumulations, threshold 0.01 mm in previous 6 hr, over the 6hr period ending 09UTC 25 Jan 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.
Fig: ecCharts presentation of HRES forecast freezing rain accumulations over the 6hr ending 18UTC 17 Feb 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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