Convective Precipitation

Considerations regarding model convective precipitation

In the current configuration of the convection scheme any showers that are developed are considered to remain within the model grid box column, with precipitation, of whatever type, always falling vertically downwards. These and other aspects have consequences when forecasting shower activity.

In particular users should be aware of:

• The IFS currently shows:
  • a bias towards:
    • insufficient convective precipitation in arid regions (e.g. parts of West Africa, the Middle East, and central Australia).
    • excessive convective rainfall near orography.

• • a tendency to:
    • under-forecast precipitation amounts from large scale convection (e.g. MCSs),
    • over-forecast convective precipitation amounts otherwise.

• • imprecision in the diurnal cycle of convection.  In particular:
    • the convective inhibition (CIN) tends break too easily.  CIN is not currently well evaluated and consequently there can be unreliable timing of release convection – generally too soon. 
    • there is too rapid increase to a peak in convection (by about 3-4hrs, i.e. around local noon rather than in the afternoon).  
    • there is too rapid decay in convection.  CAPE is destroyed too quickly and showers die away too soon.  With active convection some showers may be expected to persist much longer and linger into the night.  In general, showers die out:
      • 2-3hrs too early in west Europe,
      • 1-2hrs too early in east Europe,
      • about right in USSR.

• 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 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 MUCAPE and MUCAPE-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 MUCAPE, MUCAPE-shear and precipitation values as necessary.
  • Convective cells are not modelled as such by IFS as they are below the resolution of operational IFS models.   Observed precipitation can vary considerably over short distances so may appear suspect.  Incorrect observations and some observations in convective areas, even if correct, and can so modify the initialised precipitation field that the forecast is adversely affected.  Users should critically assess the initial precipitation fields. Gross errors affecting precipitation fields is a real issue.  

• 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.

• 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.

Note: ECMWF output consists of the most unstable convective available potential energy (MUCAPE) and the associated convective inhibition to release this (MUCIN) and ancillary derived products (MUCAPE-shear). 

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

• See the impact of Cycle 47R3 on representation of rainfall.
• See the impact of Cycle 47R3 on representation of tropical convective rainfall.
• Read more on the Convective Scheme in the Atmospheric Physics page (scroll down to the subsection: Convection). 
• Read more on the Convective Scheme (Quasi-Equilibrium and Non-Equilibrium Convection) in Breakthrough in Forecasting Convection.
• Read more on Atmospheric Moist Convection.

• Read more about the EFI parameters for Severe Convection (page 27).

• Read more on Early Warnings of Severe Convection using the ECMWF Extreme Forecast Index.
• Further information is available on derivation of CAPE and Most Unstable CAPE (MUCAPE).

(FUG Associated with Cy49r1)