Page History

...

• The effects of non-advection of showers by the convective scheme:
  
  • Convective precipitation falls out immediately, vertically downwards, as soon as convection is diagnosed (in contrast to hydrometeors classed as large-scale which follow a wind-dependant path down through the atmosphere in the IFS).  Drifting of convective particulate as it falls is not represented in the IFS.  In reality rain drops or snow flakes are likely to be blown downwind a distance proportional to:
    • the fall-speed of the hydrometeor (rain higher fall-speed, snow low fall-speed),
    • the low-level wind strength.
    • cloud depth.
  • Convective cells do not have a finite life cycle in the IFS - in effect the lifetime is zero with the model atmosphere instantly resetting itself.  In the real world showers retain some integrity in terms of their vertical circulations beyond their triggering point and this is not really represented in IFS.  The exception to this is when convection becomes so organised on the gridscale grid resolution that large scale precipitation is diagnosed.
  • The net effect of the two aspects above is that in the IFS one tends to see discontinuities in convective precipitation at the coastline, whereas in reality totals in convective situations (as seen via radar-based accumulations) generally cross coastlines unimpeded, with steady decay beyond the coastline. Therefore precipitation totals downwind of the coastline are often under-forecast. This can be:
    
    • where land-based convection moves out over the sea (daytime), and
      
    • where marine-based convection moves inland (any time).
  • Showers forming over the sea can be:
    • too few and extend insufficiently far to the leeward side of high ground.
    • too prevalent in the coastal zone and windward side of high ground.
  • Errors extend across larger distances when the wet-bulb freezing level is low, when winds are strong, and when convection is deep and active.
    
    
• Potential for incorrectly forecast convective initiation:
  • Over-active convection in the tropics is occasionally produced in the very short-range (e.g. between T+0 and T+6). In consequence, anomalous forecasts of precipitation totals can also be indicated in adjacent areas.

  • Occasionally small modifications to IFS near surface parameters can lead to convection being much more active than the IFS shows.  IFS ordinarily under-represents the heat island effect of cities and larger built-up areas where low-level temperature forecasts can be too low by a few degrees.  Consequently CAPE and CAPE-shear values can also be insufficiently large.  Just small adjustments to IFS boundary layer temperature and moisture parameters can produce much higher CAPE values.  Where relatively high shear is also present the convection could be more energetic and the associated precipitation, and precipitation rate, could be much greater than IFS shows (possibly by a factor of 5 to 10).  This does not imply that there is always more triggering of convection near cities – in many cases there is no more convection than is likely generally in the area. However, users should assess the potential for deficiencies in low-level parameters and allow for errors in CAPE, CAPE-shear and precipitation values as necessary.
    
    

• Potential effects of precipitation from medium level instability:

  • Tendency towards over-evaporation of medium level precipitation during descent through dry layers.  The effect is to modify CAPE by:

    • increasing moisture in drier levels of the model atmosphere.

    • under-forecasting precipitation reaching the ground and insufficient increase in boundary layer moisture.  

  • Possible local, possibly major, reduction in CIN and an increase in CAPE.   Further instability may then be released inducing further showery activity.

  • Forecast charts of surface precipitation are not likely to capture all such details.

  • Additional showers and increased probability of precipitation are likely within area of forecast lightning charts, even where only moderate CAPE.

...