Page History

...

A major weather hazard is freezing precipitation (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 or freezing rain in drizzle 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.

...

The ensemble gives an effective way of assessing possible variations in the temperature and humidity structure of the lower troposphere and hence the probabilities of the types of precipitation that may occur.  Probabilities are naturally assigned according to the distribution of types of precipitation among the ensemble members.  Even a small probability of hazards such as heavy snowfall or, more especially, freezing rain/drizzle can be useful information for the forecaster and ultimately for the customer, even at quite short lead-times.

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.  

Freezing drizzle may often can sometimes be identified on ecCharts freezing rain charts by setting the accumulation over 6hr to a very low threshold (0.01mm).  "Type of precipitation" charts and meteograms give an indication However, the user should also consider the various physical effects of precipitation types when deposited on surfaces and the way IFS handles them.

...