Large Scale Precipitation

Users should be aware that:

  • Precipitation is the grid box average value, not a point value. 
  • "Large scale precipitation" is produced from the large-scale (stratiform) cloud microphysical processes and denotes both rain and snow. Large scale precipitation falls through the atmosphere, is advected by the wind and can evaporate before it reaches the surface. 
  • Owing to the resolution limitations, topographical barriers are generally too low in the IFS and both orographic enhancement of precipitation and the rain shadow effect tend to be underestimated.  The medium range ensemble benefits from higher resolution and performs best.  Extended range ensemble with less resolution of orography gives less detail or modulation of precipitation. 
  • Detail of probable variations in precipitation rates are hidden when examining precipitation totals over periods.  Accumulations may have occurred from moderate or heavy precipitation for a short interval early in the period followed by dry conditions (e.g. as a cold front clears) or may have arisen from continuous drizzle or light rain falling throughout the period.
  • Drizzle tends to be over predicted.  Generally too much (light) precipitation is developed in stratocumulus, and too much of it reaches the ground through under-evaporation.  Over-production of drizzle can become a problem when low cloud (stratus or stratocumulus) becomes over-persistent through incorrect modelling of the boundary layer. 

  • Precipitation from large scale processes is advected laterally during descent a distance inversely proportional to the fall-speed of the hydrometeor (rain higher fall-speed, snow low fall-speed) and proportional to the wind strength at the given level.  If the melting level or precipitation type are not forecast correctly (e.g. as might arise when deep layers with a temperature near zero are forecast) then precipitation could drift a greater or lesser distance downwind before reaching the surface.  Usually, incorrectly forecast melting levels will be rather close to the surface.   So with the relatively coarse space and time scales of the current operational model there will not be much impact in practice.  However, the forecaster should be aware of the potential impact of lateral displacement due to precipitation drift.


Additional Sources of Information

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