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A probabilistic-bulk model of mixed layer and convection: 1) Clear-sky case.

A new bulk model of the convective boundary layer, the probabilistic bulk convection model (PBCM), is presented. Unlike prior bulk approaches that have modeled the mixed- layer-top buoyancy flux as a constant fraction of the surface buoyancy flux, PBCM implements a new mixed-layer-top entrainment closure based on the mass ux of updrafts overshooting the inversion. This mass flux is related to the variability of the surface state (potential temperature and specific humidity q) of an ensemble of updraft plumes. We evaluate the model against observed clear-sky weak and strong inversion cases and show that PBCM performs well. The height, state and timing of the boundary layer growth are accurately reproduced. Sensitivity studies are performed highlighting the role of the main parameters (surface variances, lateral entrainment). The model is weakly sensitive to the exact specification of the variability at the surface and is most sensitive to the lateral entrainment of environmental air into the rising plumes. Apart from allowing time-dependent top-of-the boundary-layer entrainment rates expressed in terms of surface properties, which can be observed in situ, PBCM naturally takes into account the transition to the shallow convection regime, as described in a companion paper. Thus, PBCM represents an important step towards a unified framework bridging parameterizations of mixed layer entrainment velocity in both clear-sky and moist convective boundary layers.

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.

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Gentine, P., A.K. Betts, B.R. Lintner, K.L. Findell, A. Tzella and F. D’Andrea (2013), A probabilistic-bulk model of mixed layer and convection: 1) Clear-sky case. J. Atmos. Sci. http://dx.doi.org/10.1175/JAS-D-12-0145.1