The effect of cloud top entrainment on the aerosol indirect effect

Entrainment in stratocumulus-topped planetary boundary layers is one of the most challenging problems in boundary layer research. Not only is there a limited understanding in the physical processes that control entrainment, but their parameterization in GCMs has been difficult or non-existent.

A recent paper by Chlond et al. (2004) compared Large-Eddy Simulations and the ECHAM Single Column Model (SCM) from MPI, Germany. They find that the liquid water path in the SCM is much too low. Further, the turbulent kinetic energy is unrealistically large within the cloud layer. Both these findings are found to be due to a numerical instability arising from a decoupling or radiative and diffusive processes. Chlond et al. (2004) make improvements to the SCM results by adding an explicit entrainment closure at the boundary layer top.

Here, entrainment is added to the full ECHAM GCM, extending the idea of Chlond et al. (2004). The parameterization of del Genio et al. (1996) for simplicity.

Chlond, A., F. Mueller, and I. Sednev, Numerical simulation of the diurnal cycle of marine stratocumulus during FIRE - An LES and SCM modelling study, Q. J. R. Meteorol. Soc. 130, 3297-3321, 2004.

del Genio, A. D., M. -S. Yao, W. Kovari, and K. K. -W. Lo, A prognostic cloud water parameterization for global climate models, J. Climate 9, 270-304, 1996.