Page History

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• Limitations of the portrayal of convective precipitation:
  • Precipitation is the grid box average value, not a point value.  Detail is lost within the grid box due to sub-grid variability, particularly in convective situations when the individual showers might be heavy but the displayed average precipitation is low. 
  • Localised extreme values in precipitation totals are systematically underestimated in IFS output because of the resolution, and also the related parametrisation of convection.   Differences of about one order of magnitude are possible although verification that integrates totals over areas that are the same size as the effective grid box size suggests the agreement is generally much better.  Convective precipitation tends to have much greater sub-grid variability than large scale precipitation.
  • Only rain or snow is produced by the precipitation scheme.  Hail is not considered nor developed in the IFS convection scheme, no matter how unstable is the model atmosphere.
  • Night-time convective precipitation remains underestimated.
    
• 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 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.
    
    

• Hail is not considered nor developed in the atmospheric model convection scheme, no matter how unstable is the model atmosphere.  Only rain or snow is produced by the precipitation scheme.=

Examples

Shower advection

Non-advection inland of showers (equilibrium convection)

Fig9.6.2:  This old example was with "equilibrium convection", that is no longer present in the IFS: 30h convective precipitation totals (mm) with mean sea level pressure verifying at 18UTC 29 Nov 2010, forecast data time 12UTC 28 Nov 2010.  The convection scheme is diagnostic and works on a grid box column, so cannot produce large amounts of precipitation over the relatively dry and cold  (stable) wintery land areas.  Showers are shown as limited to the sea alone while in nature these showers penetrate inland on the brisk easterly wind.

Non-advection inland of showers (non-equilibrium convection)

Fig9.6.3: This newer example was with non-equilibrium convection, introduced into the IFS in 2013: precipitation fields when showers developed over the Great Lakes in very cold air on a westerly wind (mm rainfall equivalent).  Significant showers are shown where strong convection is initiated over the relatively warm waters of  the Great Lakes, but give very small amounts of showery precipitation down-wind where instability is weakly initiated or not initiated over the cold land.  In reality the showers developing over the Great Lakes persisted long enough to be blown well inland as active convection. Note in particular the difference in precipitation east of Lake Michigan.

Inland penetration of wintertime maritime showers

Wintertime maritime convection can penetrate further inland than indicated by IFS forecasts.

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The schematic Fig9.6.4C illustrates the biases in the IFS forecast precipitation totals, and how they would likely change with stronger winds, reflecting in particular the situation of wintry precipitation in the example over NW England and Wales. 

Generally showers forming over the sea are:

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Fig9.6.4C: Schematic illustration of systematic precipitation biases in onshore maritime convection.  Too much precipitation is forecast for windward coastal zones and too little precipitation is forecast for areas leeward of high ground.  These areas expand and move downwind with stronger winds. 

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