A probabilistic-bulk model of coupled mixed layer and convection: 2) Shallow convection case.

The probabilistic bulk convection model (PBCM) developed in a companion paper is here extended to shallow non-precipitating convection. The PBCM unifies the clear-sky and shallow convection boundary layer regimes, by obtaining mixed-layer growth, cloud fraction and convective inhibition from a single parameterization based on physical principles. The evolution of the shallow convection PBCM is based on the statistical distribution of the surface thermodynamic state of convective plumes.

The entrainment velocity of the mixed layer is related to the mass flux of the updrafts overshooting the dry inversion capping the mixed layer. The updrafts overcoming the convective inhibition generate active cloud base mass flux, which is the boundary condition for the shallow cumulus scheme. The subcloud layer entrainment velocity is directly coupled to the cloud base mass flux through the distribution of vertical velocity and fractional cover of the updrafts. Comparisons of the PBCM against large-eddy simulations from the Barbados Oceanographic and Meteorological Experiment (BOMEX) and from the Southern Great Plains Atmospheric Radiation Measurement (ARM) facility demonstrate good agreement in terms of thermodynamic structure, cloud base mass flux and cloud top.

The equilibrium between the cloud base mass flux and rate of growth of the mixed layer determines the equilibrium convective inhibition and cloud cover. This process is an important new insight on the coupling between the mixed-layer and cumulus dynamics. Given its relative simplicity and transparency, the PBCM represents a powerful tool for developing process-based understanding and intuition about the physical processes involved in boundary layer-convection interactions, as well as a testbed for diagnosing and validating shallow convection parameterizations.

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, but this convective boundary layer keeps growing till late afternoon. This is a new model (in two parts) to describe this boundary layer growth in simplified form, so we can understand how all the processes involved are linked together.

Related Topics

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