Large-scale cloud and precipitation parameterization

Clouds interact strongly with solar and infrared radiation and may produce precipitation, hence their representation in models is required. Latent heat is also released or required when water changes phase, and this heating or cooling plays a critical role in the way air moves in the atmosphere.

Clouds form in air that contains more water vapour than the saturation vapour content. The saturation vapour content increases dramatically as the temperature is increased, hence most clouds form when air is cooled. This is usually achieved when air is lifted (when air cools due to adiabatic expansion) although thermal radiative cooling and mixing of air may also produce cloud.

Water in the model is assumed to be in the form of vapour, liquid cloud droplets, ice crystals or aggregates, or raindrops. The purpose of the large-scale cloud and precipitation schemes are:

• to transfer water between these categories as a result of the most significant cloud physics processes that occur in the atmosphere
• to let ice and rain fall downwards towards the surface
• to calculate the fractional coverage of cloud in a gridbox

The cloud physics processes that are represented include:

• the condensation of water vapour to cloud droplets and the evaporation of these droplets
• the deposition of water vapour to ice crystals or aggregates and the evaporation of these particles
• the riming of supercooled cloud droplets by ice particles
• melting of ice particles to produce raindrops
• evaporation of raindrops
• accretion ('sweep-out') of cloud droplets by raindrops
• the collision/coalescence mechanism to form raindrops from cloud droplets
• the fall downwards of ice particles and raindrops

The quantitative calculation of these transfers is performed using standard cloud physics theory assuming raindrop and ice particle size distributions and fall speed/diameter relationships, etc.

Cloud fractions for liquid clouds are calculated assuming a distribution of water vapour across a gridbox — the regions where the water vapour exceeds the saturation value will be condensed to liquid cloud droplets, the other regions will remain free of liquid cloud. Cloud fractions for ice cloud are calculated assuming a relationship between the cloud fraction and the amount of ice present in the gridbox.