Description of Moist Convection and Large Scale Condensation in AOM
Each hour, moist convection is performed followed by large scale condensation, radiation, surface interaction, and dynamics in that order. Using the mean and gradients, potential enthalpy and water vapor are calculated for each horizontal quarter cell for moist convection and for each eighth cell for large scale condensation. The Model does not have the ability to store condensate in the atmosphere; condensate is reevaporated or precipitated each time step. Cloud optical depths, used by radiation, are determined from the amount of condensate leaving each layer and the local temperature. Only one of the four quarter columns is used to determine cloud optical depths that radiation applies to whole grid cells.
For moist convection, a plume is formed with 30% of the mass of a quarter cell of layer 1, the lowest atmospheric layer. The plume temporarily rises to the layer above it where water vapor that exceeds 99% of saturation is condensed. If there is no condensation, nor if the plume, with the weight of the condensate, is less buoyant than that of the environmental air, then the plume is abandoned. If the plume is more buoyant, then the condensate is removed, and the plume temporarily rises to the next higher layer, continuing the process until it finds a layer above which it is stable. Subsidence of the environment occurs using the vertical gradients of heat and water vapor downward layer by layer until the base layer is filled. The plume is then depositied into the top convection layer. Condensate from each layer falls downward but may be reevaporated over a fraction of the grid cell as it falls. After this process is completed, another plume is formed with one fourth of the mass of a quarter cell of layer 2, and the process is repeated. Initial plume layers continue up to the third highest layer in the model. The whole process is repeated for all horizontal quarter columns and for all grid columns. Note that plumes do not entrain adjacent air from their environment as they are rising, and that plumes are slightly under saturated after condensation. Rising and subsidence mix horizontal momentum by following the parcels of air. Moist convection exactly conserves static energy including the geopotential energy of condensate on the ground.
For large scale condensation and starting from the top, vapor that exceeds 99% of saturation is condensed and falls into the eighth cell below it, where it may be fully or partially reevaporated. Super saturation conditions are checked for each eighth cell in the column and for each quarter cell column. Large scale condensation also conserves static energy.
Cloud optical depths depend on temperature and type of condensation and are proportional to the square root of the mass of condensate times the square root of the grid cell air mass (in order to make them independent of vertical resolution). Optical depths are proportional to four global constants, for water and ice clouds, for moist convection (0.06 and 0.025) and large scale condensation (1 and 0.1). The water cloud constants are used when temperature is greater than 0°C, the ice cloud constants are used when the temperature is less than -30°C, and interpolation is performed between these limits.
