Section 2.1.5.6 Types of Precipitation

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• Types of Precipitation
  • Allocation of Precipitation Type.
  • Additional Sources of Information

Types of Precipitation

The type of precipitation in winter is particularly important to be able to forecast.  The IFS internally only represents two types of precipitation; rain and snow.  However, different types of precipitation at the surface can be diagnosed from the profiles of rain and snow combined with near-surface temperature from the model.

The Table below shows the different precipitation types that can be diagnosed. 

Allocation of Precipitation Type.

At temperatures just above 0ºC in the melting layer the model physics transfers mass from the snow category to the rain category and will therefore have a mix of rain and snow (in reality each particle contains melted liquid around an ice core).  The structure, depth and elevation of the melting layer is used to derive the precipitation type at the surface. 

• If the ground surface is near the top of the melting layer the precipitation is mainly snow that is just starting to melt with only a small amount (<20%) of liquid water, then the precipitation type is diagnosed as "wet snow".
• If the melting layer intercepts the ground, the precipitation is snow with rather more melting giving a larger amount (20%-80%) of liquid water, then the precipitation type is diagnosed as "mixed rain and snow".
• If there is a warm layer (> 0ºC) above a near-surface sub-zero (< 0ºC) layer, then the precipitation type is diagnosed as:
  
  • "ice pellets" if the snow is only partially melted in the warm layer.
  • "freezing rain" if the snow is completely melted in the warm layer,  (Note: An IFS forecast of freezing precipitation indicates the likely presence of super-cooled droplets in the lower atmosphere - not the effect of their impact on a surface to produce glaze or glazed ice.

The diagnosis of precipitation type is valid at a particular time.  It is consistent with the total precipitation rate at that time, calculated by summing the large-scale and convective rainfall and snowfall rates.  Users should 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.

The precipitation type should be used carefully together with the combined precipitation rates to provide indication of potential significant or hazardous events (e.g. freezing rain, heavy fall of wet snow).  A precipitation type is assigned wherever there is a non-zero value of the total precipitation rate in the model output surface precipitation field, however small.   Thus, if only the precipitation type is plotted the areal coverage of 'dry' (i.e. precipitation type = 0) may look unrealistically small.  Probabilities of different types of precipitation in ECMWF products are derived from the ENS.  Users should also note that "interpolation" of the precipitation type field should only be done by mapping the value at the nearest neighbouring grid point; mathematical interpolation operations would give misleading and indeed meaningless results.  The "most probable precipitation type" is available as a layer within ecCharts.  

 Table2.1.5.6-1: Diagnostics for precipitation type at the surface based based on IFS model temperature and moisture characteristics.  The precipitation type index is used in association with the probe tool on ecCharts presentation of the Precipitation Type.

Fig2.1.5.6-1: Schematic showing the influence of IFS model temperature structure upon forecast precipitation as it falls through each layer. 

Each frame shows temperature structures that produce at the earth's surface:- a: Snow.  b: Sleet (melting snow) or Rain (depending upon the depth of the surface warm layer).  c: Ice Pellets.  d: Freezing Rain.

Example of assessment of precipitation types.                         

Fig2.1.5.6-2:  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.  Shading of each colour denotes intensity - darker is more intense.

Fig2.1.5.6-3: Schematic cross-section north to south along the black line in the chart Fig2.1.5.6-2 (in many cases the ice pellet zone will be much narrower in the horizontal direction than shown here).  The under-cut of cold air beneath the precipitating warmer air aloft produces different types of precipitation at the surface depending upon the thickness of the warm air aloft and the under-cutting cold air.

Further information in the forecaster user guide

For more information on freezing precipitation see:

• Section 8.1.10 Types of Precipitation - charts and diagrams

• Section 9.7 Types of Precipitation - interpretation and effects