Evaluation of the ECHAM4 cloud-turbulence scheme for Stratocumulus

The behavior of the ECHAM4 cloud and turbulence scheme is evaluated for an idealized case of Stratocumulus. It is: shown that the E-l turbulence closu re that is used is sensitive to instabilities. This is caused by the formul ation of the turbulent length scale I in terms of the moist Richardson numb er, which gives unrealistic results in the case of low wind shear. It is fu rther shown that the ECHAM4 condensation scheme is highly dependent on both spatial and temporal resolution. It is only in the limit of high vertical resolution (25 meters) and of small time steps (1 second) that the scheme i s able to sustain a cloud cover of more than 90 %. At operational resolutio n the cloud cover decreases to typically 60-70 %. A modified version of the condensation scheme is presented which does not display this strong depend ency on the resolution. A three dimensional simulation with this scheme for a low level Stratocumulus cloud deck: shows a significant improvement comp ared to the standard ECHAM4 scheme.