^{image © sheilades/iStock/Thinkstock}

• 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.

Allocation of Precipitation Type.

The structure of the lower atmosphere governs diagnosis of the type of precipitation at the surface. The structure, depth and elevation of any melting layer is vitally important.   At temperatures just above 0ºC in the melting layer the model physics transfers mass from the snow category to the rain category.  Thus a mix of rain and snow exists in the melting later.  The proportion of each type depends upon time spent in the melting layer.  The precipitation is diagnosed as:

• Rain: Temperature >0C in lower atmosphere.
• Dry snow: No melting layer. Temperature <0C in lower atmosphere.  
• Wet snow: Melting layer in lower atmosphere.  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.
• Sleet, Mixed rain and 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.
• Ice pellets: If there is a warm layer (> 0ºC) above a near-surface sub-zero (< 0ºC) layer.  The warm layer may be thin or only just above 0C.  The snow aloft is only partially melted in the warm layer (20-80% snow at its base) and re-freezes in the underlying cold air.  A higher proportion of snow penetrates through the melting layer promoting ice pellet growth on the more abundant nuclei.
• Freezing rain: If there is a warm layer (> 0ºC) above a near-surface sub-zero (< 0ºC) layer.  The snow aloft is almost all melted in the warm layer (<20% snow at its base) and becomes super-cooled in the underlying cold air.   Freezing rain with higher precipitation rates (say 5-10mm/hr) can be become diagnosed and forecast as ice pellets.  The diagnostic assumes more snow penetrates through the melting layer rather than melting into liquid.  Heavy freezing rain can be difficult to forecast.
• Freezing drizzle: This can occur from relatively thin stratus clouds in a near-surface sub-zero (< 0ºC) layer.  Cloud tops may just be into a warm layer (> 0ºC). 

Note:

• IFS forecast of freezing precipitation indicates the likely presence of super-cooled droplets in the lower atmosphere.  It does not forecast the effect of their impact on a surface to produce glaze or glazed ice.
• Wet snow or sleet tends to over-accumulate as snow on the ground. 
• Forecast verification of precipitation amounts and snow depths is particularly challenging when there is blowing snow.
• The default scheme for the ensemble control (Ex-HRES) on ecCharts does not include freezing drizzle!

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.  No melting layer.  b: Sleet (rain and snow mixed) or rain (depending upon the depth of the surface warm layer and the amount of melting of the snow).  c: Ice Pellets. Falling snow penetrates the melting layer (in reality each particle contains melted liquid around an ice core) before re-freezing in the underlying cold layer.  d: Freezing Rain. Falling snow penetrates the melting layer (each particle contains melted liquid) before becoming super-cooled droplets in the underlying cold layer.  

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.  The "most probable precipitation type" is available as a layer in ecCharts.  

For a given station, use the precipitation type at the nearest grid point.  Interpolation of the precipitation type field is misleading and meaningless.

 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.  2m temperature or screen level temperature is a diagnostic, not a direct output from the atmospheric model.

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 derivation of screen level temperature see:

• Section 9.2 Screen Level Temperatures and Moisture

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

(FUG Associated with Cy49r1)