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Table of Contents

Note: The Ensemble Control Forecast is identical to HRES output in Cycle49 and earlier cycles.  They have the same resolution and are scientifically, structurally and computationally identical.  Ensemble Control Forecast output and HRES output are fully equivalent where shown in diagrams.   At the time of some older diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Types of precipitation - interpretation and effects

The method of  assessment of the type of surface precipitation by the atmospheric model depends primarily upon the temperature structure of the model atmosphere although humidity is relevant too.  Small differences in structure in the region where the precipitation is being generated and in the layers through which the droplets fall can both have a major impact.  A change from rain at the surface to snow can be brought about by cooling of the airmass by evaporation or melting as the precipitation falls.  A major weather hazard is freezing rain which requires a very specific and rare lower tropospheric temperature and humidity structure.  Modelling of all these processes can be difficult, especially if the structure of the model atmosphere is initially imprecisely defined.  Forecasters need to assess the structure of the lower layers of the atmosphere, particularly when warm air is advancing over a very much colder airmass.

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  • High impact considerations

    • Snow cover analyses rely on snow reports, background forecasts and satellite-derived snow-cover products.  Any or all could be in error.
    • Incorrect analyses and forecasts of snow are quite possible at altitudes above 1500m, or in data sparse areas, or after a prolonged period without observations.
    • Incorrect reports of shallow snow (say by thick deposition of frost) may be assimilated.  This can be persistent in the model and give misleading forecasts.

    • IFS tends to melt snow away too slowly.  Snow cover and associated colder surface temperatures may persist for longer than they should.  This could influence other parameters.  

    • IFS convection and associated precipitation do not advect.  Marine convection may not penetrate sufficiently far inland.  Lower snowflake fall speed also can drift further inland.  Repeated active cells developing in a given location, perhaps on high ground or upslopes, can lead to high totals (by factor of 2 or 3).

    • The direction and strength of the low level winds can have a strong effect on snowfall:

      • Surface wind from land - temperatures can be lower and snowfall deeper.

      • Surface wind from sea – temperatures slightly higher and snow more sleety, at least at lower levels.

    • IFS snowfall can be insufficient (by factor of ~2) when IFS dewpoint depression is >~4C.  This is due to under-evaporation of falling particles in IFS.
    • IFS snowfall often accumulates incorrectly on the ground when IFS develops rain and snow.  
    • Snow may accumulate then melt (e.g. with rain, or as as a warm front advances over a cold area).
    • The extent and thickness of cloud or freezing fog has a strong influence energy fluxes into and from the snowpack.  Consider possible cloud formation, persistence or clearance and assess possible changes in energy transfer between cloud and snowpack.  Thick cloud at any level will reduce solar radiation.  But low cloud could be warmer than the underlying snow surface resulting in a net increase in downwards long wave radiation. 
    • The characteristics of each grid box and areal extent of each tile type are updated through the forecast period and can vary in a rapid and interactive way.  Tiles in HTESSEL may be incorrectly assigned.
      • Model forecast snowfall might increase the area or depth of snow cover incorrectly.  Partial cover of snow may incorrectly become full cover as the accumulated model snow depth becomes >10cm.
      • The statistical information on the slope and aspect of orography within each grid box is not detailed enough for forecasts at an individual location.  This can be important in mountainous areas and HTESSEL may under- or over-estimate solar heating and runoff.
      • A valley in a mountainous area may allow low-level moisture into the sheltered side.  This can enhance model precipitation rates there instead of evaporating snow in dry air (in a fohn area).  
    • Differing snowfall among the ENS members can cause increasing differences in evolution during the remainder of the model forecast period.  Nevertheless each member remains equally probable.

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(FUG

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associated with Cy50r1)