Understanding and Representing Atmospheric Convection across Scales - ParaCon Phase 2

Cumulus clouds are produced by the vigorous ascent of buoyant air, a process known as convection. The weather and climate of the tropics are dominated by cumulus clouds, and severe weather at all latitudes involves convection. Convection communicates heat and moisture 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 is essential for both numerical weather prediction (NWP) and climate projections. 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, the energy sector, the emergency services, and government policy on climate risks. Computer models for NWP and for climate projection divide the atmosphere into boxes with typical horizontal sizes of 10km and 100km respectively. Convective elements such as thunderstorms, on the other hand, are typically only around 1km in size so they cannot be explicitly represented in the models. Instead we must somehow 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.

Cumulus parameterization is a stubborn and difficult problem and is 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. These include errors in the distribution, timing, and intensity of convective rainfall, as well as the behaviour of larger-scale weather systems that are coupled to convection.

ParaCon Phase 2 is a wide-ranging plan to redesign the convection parameterization for the Met Office Model, to demonstrate clear improvements in model fidelity and performance, and to lay the groundwork for the next generation of parameterization research.

In Phase 1 we have developed a new convection scheme infrastructure called CoMorph, which enables many of the assumptions that are made in such parameterizations to be relaxed, removed or generalized and we have begun the process of developing a formulation based on alternative and more general assumptions. Also in Phase 1 we have performed promising investigations into radically different formulations based on modelling convection as a manifestation of turbulence, and on a multi-fluid approach that relaxes the usual assumptions even further than CoMorph does.

In Phase 2 we will continue the development of CoMorph with a view to its adoption for operational forecasting. Building on the work in Phase 1, improved formulations for the components of the scheme will be developed and implemented. The performance of CoMorph will be evaluated in a wide range of test cases. These will include comparison with a suite of high-resolution simulations of idealized convective archetypes conducted in Phase 1, as well as a range of operational-style configurations.

In Phase 2 we will also continue to develop the turbulence-based and multi-fluid-based approaches and to evaluate their potential for representing convection in atmospheric models. A key goal will be to clarify the relationship between the three approaches and to understand the extent to which some unification or combination of the approaches might be possible and beneficial.

A parameterization of convection is an essential ingredient in operational weather forecasting (including extreme events such as flooding), in seasonal weather forecasts and in regional and global climate models. A reliable and soundly-physically-based parameterization is therefore essential for the quality of weather forecasts and for reducing uncertainties in climate projections. Improved parameterization methods will ultimately impact end-users from government, industry and the public, through significant improvements to Met Office products and advice. The Met Office, in common with operational prediction centres worldwide, has made this issue a major priority.

The primary impacts of ParaCon Phase 2 will be:

1. directly on the accuracy and utility of Met Office weather forecasts and climate predictions via implementation of improved convection parameterization in the Met Office's Unified Model (UM);

2. indirectly on other weather and climate prediction centres through promulgation of the specific methodology implemented, and of alternative strategies for future parameterization developments and evaluation techniques;

3. indirectly on other weather and climate prediction centres through access to the reference data that the new methodology is based upon.

ParaCon targets the first of these impacts in a completely natural and straightforward way, since the programme has been designed as a joint venture between the Met Office and the university partners with this impact at the very heart of the design. The partners and the Met Office have a long history of collaboration in model development projects, which has only been enhanced through our joint work in Phase 1. Thus, strong relationships between staff already exist that we will draw upon extensively in ensuring that our parameterization developments will be translated into operational practice.

The scientific understanding gained through this project will lead to substantial benefits for the second and third impacts within other operational centres, and will drive forward the fundamental understanding of convection within the academic, operational and broader WGNE (Working Group on Numerical Experimentation) communities. This will be further facilitated by the availability of reference datasets from the ParaCon project which will form the basis of further exploitation. A good example from Phase 1 is our involvement in both the design and delivery of the international RCEmip project, which was led by Jones (a PDRA from the Circle-A project) and Holloway (a co-I from the RevCon project).

Ensuring the project's very high impact will require close interactions with scientists working on operational weather and climate prediction and a full engagement with the international community. We will achieve this by continuing to build on new links and further developing and exploiting the existing links of the senior investigators with operational centres (eg, ECMWF, DWD, Meteo France, RNMI, SAWS).

With respect to other centres, we also note that the Met Office has formal arrangements with a substantial number of national meteorological services and research institutes around the world, concerning the use by these services of the UM. Therefore, these international partners, including Australia, Korea, New Zealand, Norway, India and South Africa, will also directly benefit from improvements to the UM. Members of the ParaCon university teams are already engaged in some of the knowledge transfer and working group activities between the Met Office and its partners: for example, Plant, Clark and Birch have participated in meetings of the Convection Working Group.