Analysis of prognostic cloud scheme increments in a climate model

An analysis of a cloud scheme is presented, which can aid the development of climate and weather forecasting models. Budget tendency terms, or cloud increment diagnostics, are analysed to show the relative importance of the various physical processes in creating the cloud fields simulated by a climate model. When looking at time-averaged diagnostics, the use of mean positive and mean negative increments separately, as well as mean net increments, is shown to be informative in understanding how the model is behaving. Maps of vertically integrated tendency terms illustrate how the dominant cloud production terms vary geographically. A cross-section through the Hadley Circulation shows how the tendency terms vary across a transition region as the clouds change from stratocumulus to shallow cumulus and then to cumulonimbus. Although there are no observations against which these increment diagnostics can be compared, identification of the key cloud formation and dissipation processes highlights the parts of the cloud scheme where further work should be focussed in order to improve the climatological cloud fields. Simulations with changes to a part of the cloud parametrization are analysed using the increment diagnostics and the change in the relative importance of the different processes highlights some of the feedbacks present in the model. While our detailed findings are model specific, the analysis technique could be used in the development of any weather forecasting or climate model. (C) Crown Copyright 2010. Published by John Wiley & Sons, Ltd.