Revolutionizing Convective Parameterization

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The weather and climate of the tropics is dominated by cumulus clouds. These clouds are produced by vigorous convection currents within the atmosphere. The convection communicates heat and evaporation from the Earth's surface throughout the atmosphere. It is the main process controlling the change of temperature and moisture content with height in the tropical atmosphere. On the global scale cumulus clouds are responsible for the majority of the rainfall, and convection is a crucial component in the overall pattern of the Earth's atmospheric flows. Computer modelling of the atmosphere, both for global numerical weather prediction (NWP) and for climate projection, divides the atmosphere into boxes with typical horizontal sizes of 20 and 100km respectively. This means that the models are not able to represent convective elements (such as thunderstorms) properly, because these elements are typically only around 1km in size. However, as convection has a crucial role to play in the atmosphere, it must be represented within the models. We have somehow to estimate what cumulus clouds will be present in each of the boxes and what their collective effects will be on the larger-scale atmosphere. This is known as a cumulus parameterization. Computer modelling of the atmosphere is essential for reliable climate projections and weather forecasts. Society benefits enormously from their outputs to inform decision making on all scales from the individual member of the public to weather-sensitive business activities to the insurance sector to the emergency services to government policy on climate risks. Convection parameterization is a stubborn and difficult problem and the largest single uncertainty that we face. It is a severe and unforgiving test of just how well we understand the fundamental science of convection and its role in the atmosphere. Defects in the existing parameterizations are known to translate into serious deficiencies in weather and climate models. To give just one example, in many models the predicted convective rainfall is too frequent and too light.RevCon is so named because the project aims at a revolution in convective parameterization by challenging many of the key assumptions that have been made within convective parameterizations for decades. Some of these assumptions are very limiting and known to be poor approximations in many circumstances. We are also convinced that they are unnecessary and that better parameterizations can be achieved without them. This project will establish what the mathematical structure of convection parameterization really should be, with that structure being informed and carefully justified through the detailed analysis of very high-resolution simulations of convective cloud systems. It is a critical contribution to a NERC / Met Office programme in this area because it will provide a necessary starting point for Phase 2 of the programme to build a new-generation parameterization for the Met Office weather forecast and climate model.