Evaluation of the ECHAM4 cloud-turbulence scheme for Stratocumulus

Abstract

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 closure that is used is sensitive to instabilities. This is caused by the formulation of the turbulent length scale / in terms of the moist Richardson number, which gives unrealistic results in the case of low wind shear. It is further 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 is able to sustain a cloud cover of more than 90 %. At operational resolution the cloud cover decreases to typically 60-70 %. A modified version of the condensation scheme is presented which does not display this strong dependency on the resolution. A three dimensional simulation with this scheme for a low level Stratocumulus cloud deck shows a significant improvement compared to the standard ECHAM4 scheme.

Keywords

• cloud and turbulence scheme
• stratocumulus
• wolken- und turbulenzschema