trade wind cumulus clouds over the western Atlantic

Grey Zone Project for model parameterizations

With the ever-increasing computer resources, more and more weather and climate numerical models are operating in the "grey zone" at horizontal grid spacings in the range of 200 m to 10 km. As these resolutions are finer than the depth of the troposphere they, in principle, allow for numerically resolved vertical turnover behaviour and, at these resolutions, phenomena such as turbulence, convective transport, and clouds are partly resolved. However, this has led to the false perception that models with grey zone mesh sizes might operate satisfactory without convective parameterizations. Many weather and climate models do switch off their convection parameterizations at an arbitrary resolution within the grey zone and use different variants of turbulence parameterizations compared to respective coarser resolution model configurations. Hence, there is an urgent need to systematically explore the behaviour of  models operating in the grey zone, as well as support and compare the development of different scale-aware convection and boundary layer parameterizations. As a response to this need, the Working Group on Numerical Experimentation (WGNE) has initiated the Grey Zone Project, which is also endorsed by the GEWEX Global Atmosphere System Studies (GASS) panel.

By defining the grey zone as the range of resolutions in which both the resolved and the sub-grid contributions to turbulent and convective transport give a non-negligible contribution, it is clear that the resolution range of the grey zone is dependent on the type of turbulence and convection. Consequently, the Grey Zone Project investigates this issue under different atmospheric conditions using a case study approach, which includes both observations as well as ambitious ultra-high resolution simulations. In coordinated numerical experiments, model resolutions are systematically varied in order to explore the behaviour of the models across a range of spatial scales.

It is not only the practical need for appropriate parameterizations which makes the investigation of the grey zone interesting, it is also the scientific questions about the nature of convective and boundary layer processes, and their relation to the larger scales. Can a seamless transition between parameterized and resolved turbulence and convection be achieved? What are the key characteristics of turbulence and convection that parameterizations need to be able to represent?

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