Page History

...

The convection processes are similarly calculated using a complex suite of programs dealing with the interactions between the phases of water.  The convection process "detrains" cloud and precipitation into the large-scale cloud scheme.  This allows representation of convective anvils and also the precipitation associated with the more stratiform part of the convective cell.  However, the main part of the precipitation from the core of the updraught is treated diagnostically in the convection scheme, and assumes all the precipitation falls out within the grid column in a timescale less than the model timestep.  Convective precipitation in the model falls vertically and immediately; it is not advected with winds underlying the model cloud.  This can cause problems in coastal areas where maritime convection and showers are kept at the coast and don't progress inland Thus, model showers are not advected with the wind during their life-cycle.  In particular, any showers that the model develops over the sea do not penetrate beyond the coast.  In reality, showers normally advect with the wind during their life-cycle.  Users should allow for:

• possible advection of any showers developed by the convection scheme. 
• penetration of maritime showers inland from windward coasts, especially in winter or with wintry precipitation because snowflakes fall more slowly than raindrops and thus advect further inland before reaching the ground.

Cloud Fraction

Cloud fraction is a prognostic model variable for each grid box.  This means at each timestep cloud fraction is advected into the grid box by the model wind and then acted upon by sources and sinks of moisture.   

...

Total cloud cover is calculated from the predicted cloud fraction within each layer for each grid column.  The layers used are: low layer (surface to 800hPa), medium layer (800hPa to 450hPa) and high layer (450hPa to model top) for each grid column.  The model clouds are assumed to extend vertically over the whole of depth of each model layer.  The calculation uses assumptions about the overlap between the subgrid clouds in the vertical (whether the layers of cloud are stacked above one another in the vertical, or whether they are displaced relative to one another.

...