DESIGN AND PERFORMANCE OF A NEW CLOUD MICROPHYSICS SCHEME DEVELOPED FOR THE ECHAM GENERAL-CIRCULATION MODEL

A new cloud microphysics scheme including a prognostic treatment of cl oud ice (PCI) is developed to yield a more physically based representa tion of the components of the atmospheric moisture budget in the gener al circulation model ECHAM. The new approach considers cloud water and cloud ice as separate prognostic variables. The precipitation formati on scheme for warm clouds distinguishes between maritime and continent al clouds by considering the cloud droplet number concentration, in ad dition to the liquid water content. Based on several observational dat a sets, the cloud droplet number concentration is derived from the sul fate aerosol mass concentration as given from the sulfur cycle simulat ed by ECHAM. Results obtained with the new scheme are compared to sate llite observations and in situ measurements of cloud physical and radi ative properties. In general, the standard model ECHAM4 and also PCI c apture the overall features, and the simulated results usually lie wit hin the range of observed uncertainty. As compared to ECHAM4 only slig ht improvements are achieved with the new scheme. For example, the ove restimated liquid water path and total cloud cover over convectively a ctive regions are reduced in PCI. On the other hand, some shortcomings of the standard model such as underestimated shortwave cloud forcing over the extratropical oceans of the respective summer hemisphere are more pronounced in PCI.