Triggering Deep Convection with a Probabilistic Plume Model

A model unifying the representation of the planetary boundary layer and dry, shallow and deep convection, the Probabilistic Plume Model (PPM), is presented. Its capacity to reproduce the triggering of deep convection over land is analysed in detail. The model accurately reproduces the timing of shallow convection and of deep convection onset over land, which is a major issue in many current general climate models. The PPM is based on a distribution of plumes with varying thermodynamic states (potential temperature and specific humidity) induced by surface layer turbulence. Precipitation is computed by a simple ice microphysics, and with the onset of precipitation, downdrafts are initiated and lateral entrainment of environmental air into updrafts is reduced. The most buoyant updrafts are responsible for the triggering of moist convection, causing the rapid growth of clouds and precipitation. Organization of turbulence in the subcloud layer is induced by unsaturated downdrafts, and the eect of density currents is modeled through a reduction of the lateral entrainment. The reduction of entrainment induces further development from the precipitating congestus phase to full deep cumulonimbus. Model validation is performed by comparing cloud base, cloud top heights, timing of precipitation and environmental proles against cloud resolving models and large-eddy simulations for two test cases. These comparisons demonstrate that PPM triggers deep convection at the proper time in the diurnal cycle, and produces reasonable precipitation. On the other hand, PPM underestimates cloud top height.

Plain English Discussion

When the sun rises, sunlight heats the ground and evaporates water. Convection carries this heat and water upwards into the atmosphere. This heated layer is the Earth's daytime boundary layer, which first grows deeper for several hours without clouds. Then clouds usually form, and often this convective boundary layer keeps growing until rain starts in the afternoon. As raindrops fall, they evaporate and this cools the air and drives downward convection currents (downdrafts). These downdrafts organize deep convective clouds (cumulonimbus) which can rain for longer. This is the third paper in a series (following two in 2013), developing a new model to describe in simplified form this sequential progression of the daytime growth of the boundary layer over land.

Related Topics

• Boundary Layer,
• Clouds,
• Convection,
• Idealized models