An improved representation of cloud and precipitation
Forbes, Richard ; Tompkins, Adrian
A major upgrade to the parametrization of stratiform cloud and precipitation was implemented in the Integrated Forecasting System (IFS) cycle 36r4, operational from 9 November 2010. This change is part of the continuing programme of development of moist physics parametrizations in the IFS, which remains a key area for improvement for Numerical Weather Prediction and an important part of the ECMWF model development strategy. Three additional prognostic variables have been introduced to enable a more physically based representation of mixed-phase (liquid/ice) cloud and precipitating rain and snow. It is the most significant change to the structure of the cloud parametrization since the Tiedtke scheme was introduced operationally in 1995. Many aspects of the model are systematically improved including the skill of precipitation forecasts, the spatial distribution of cloud ice and precipitating snow in the troposphere, the physical processes in mixed-phase cloud, and the impact of cloud and precipitation on radiation. In addition, the new scheme provides a more physically based framework for further development of the parametrization, particularly as model resolution is projected to increase in the future. A brief history of the IFS cloud scheme is first described to put the recent changes into context, followed by the primary motivations for the upgrade and description of the new scheme. A few examples of evaluation against observations are used to highlight the improved representation of cloud and precipitation in the IFS, and the article concludes with a summary and outlook for the future.
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