A Multiple Mass-Flux Parameterization for the Surface-Generated Convection. Part I: Dry Plumes

Abstract

The present study introduces a parameterization for the mixing in the surface-generated free convection regime (including dry and moist convection cases). The parameterization explicitly represents the plumes that drive the convective turbulence. Ascending parcels that have similar properties in the surface layer are grouped into updrafts, with a determined fractional area. The updrafts ascend according to an entraining plume model, and may eventually overturn. The plume model can either use a time or a length scale for the lateral entrainment closure. The plumes' environment is assumed to be horizontally uniform. A one-dimensional framework is used to evaluate the parameterization in the case of a dry convective boundary layer (CBL). The predicted variances are underestimated (presumably due to the uniform environment assumption). The contrast between the strong upward motions and the compensating subsidence is simulated, and the skewness of w increases realistically with height. The plumes' dynamics agrees with experimental data throughout the entire CBL. The vertical entrainment is reproduced, and as expected, it is driven by the strongest motions. Sensitivity tests on atmospheric forcings and model parameters show the robustness of the results. The number of updrafts can be decreased to less than 10 without altering the heat transport prediction.