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Stratiform Cloud Processes and Precipitation

In the IFS Stratiform (or Large-scale) Cloud scheme there are five prognostic cloud variables plus water vapour (yellow boxes in Fig2.1.22) and the modelling of changes of state or precipitation development is represented by various microphysical processes.  Separate cloud ice and cloud water variables allow the representation of supercooled liquid water and mixed-phase processes, commonly observed in clouds.  All cloud and precipitation variables are advected by the wind.

The precipitation can be either rain or snow or a mix of the two.  Once precipitation is generated it will fall and during descent it will either:

  • grow if it enters lower layers of cloud or
  • start to evaporate in sub-saturated air.

This process can occur several times before reaching the surface.

The descent of stratiform precipitation follows a pathway according to IFS wind speeds and particulate fallspeeds, so rain and snow precipitation particles can transfer between grid boxes as they descend in the IFS.  This "precipitation drift" is much more pronounced for snow because it has a much slower fallspeed than rain.

Convective precipitation is not treated in the same way (see convective cloud process and precipitation).

 Fig2.1.22:  Schematic of processes and interactions within the Stratiform Cloud Scheme.  Note: Precipitation is only produced as rain or snow.  Precipitation from stratiform cloud is referred to as "large-scale" precipitation.

Additional Sources of Information

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

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