A cloud model is used to represent cumulus and cumulonimbus convection, in which an updraught and precipitation-induced downdraught are considered. A test is made for convective instability and if the potential temperature of any level is higher than that of the level above convection is initiated. Convection will continue as long as the air within the cloud continues to be buoyant. Dilution of the cloud is represented by entrainment of environmental air.
Before the cloud detrains completely at the level where the parcel of air ceases to be buoyant, the remaining mass, heat, water vapour and cloud water/ice are completely mixed into the environment at the cloud top.
A single cloud model is used to represent a number of convective plumes within the grid square, and precipitation is diagnosed within that square if:
- cloud liquid/ice content exceeds a critical amount, and
- the cloud depth exceeds a critical value
Typically this value is set to 1.5 km over the sea and four kilometres over land. However, if the cloud-top temperature is less than -10 °C, the critical depth is reduced to one kilometre over land or sea. As with large-scale precipitation, the convection scheme allows for evaporation and melting of precipitation. Cloud physics research is leading to scientific development in this area of our modelling.
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