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. 
  • Topographical barriers are generally too low in the IFS because of resolution limitations.  Both orographic enhancement of precipitation and the rain shadow effect tend to be underestimated.  The medium range ensemble benefits from higher spacial resolution (~9km) and performs best.  Extended range ensemble with less spacial resolution (~36km) 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 could be from:
    • a short interval of heavy precipitation early in the period followed by dry conditions (e.g. as a cold front clears).
    • 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 because of under-evaporation.  Incorrect modelling of the boundary layer can make low cloud (stratus or stratocumulus) too persistent and give too much drizzle. 

  • Precipitation from large scale processes is advected laterally during descent.  The advection distance is:

    • inversely proportional to the fall-speed of the hydrometeor or precipitation type.  

    • proportional to the wind strength at the given level.   

For a given wind strength:

    • Snow has lower fall-speeds so advects further downwind before reaching the surface.
    • Rain has higher fall-speeds so advects no so far downwind before reaching the surface.

Correct structure of the lower atmosphere and melting level altitude is important in deducing precipitation type.  Usually, incorrectly forecast melting levels will be rather close to the surface.  Deep layers with near zero temperatures also cause problems in forecasting precipitation type.  An incorrectly forecast precipitation type could drift downwind less or more than forecasted.

In practice, the resolution of the space and time scales of the IFC mean the impact will be low.  However, forecasters 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)