Dilution of the cloud is represented by entrainment of environmental air. This value is set to 1.5 km over the sea and 4 km over land. However, if the cloud top temperature is less than -10 C, the critical depth is reduced to 1 km over land or sea. As with large-scale precipitation, the convection scheme allows for evaporation and melting of precipitation. The Unified Model includes a scheme to parametrize gravity wave drag, whereby flow over mountains in stable conditions excites waves.
Stress exerted is proportional to the sub-gridscale variance of topography and the wind speed.
The waves are allowed to propagate vertically, reducing static stability by ascent and increasing wind-shear, until the Richardson Number falls below a critical value. Where it does so 'breaking' of the wave is diagnosed and a drag is exerted, leading to a brake on atmospheric flow. Breaking has been shown to occur most commonly in the boundary layer and in the lower stratosphere, where isentrophic surfaces are tilted and overturned. The effect is largest over mountains in winter, where it reduces the strength of jet streams and causes surface depressions to be less deep and quicker to fill than would otherwise be the case. Long-wave fluxes depend upon the amount and temperature of the emitting medium and its emissivity.