A new mixing length formulation for the parameterization of dry convection: Implementation and evaluation in the COAMPS mesoscale model

A realistic representation of the evolution of the dry convective boundary layer in mesoscale and large-scale atmospheric models has been an elusive goal for many years. In this paper we evaluate the performance of a new mixing length formulation for the dry convective boundary layer in the context of the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS-TM). In this new formulation, the mixing length is proportional to a time-scale and to the square root of the turbulent kinetic energy. The model results are tested against observations from the CICLUS observational campaign in the South of Portugal. We show that COAMPS with the new formulation produces a more realistic simulation of the boundary layer growth. A data assimilation experiment performed with COAMPS shows that the improvements provided by the new formulation are significant, particularly in terms of the humidity vertical distribution. Finally, one-dimensional simulations are used to confirm that the new formulation provides more accurate results due to a more realistic representation of the entrainment and of the vertical mixing in general.