ParaCon - Representation of convection in models

Weather and climate models are critical to society's ability to reduce the impacts of hazardous weather and inform decisions regarding mitigation of and adaptation to climate change. The representation of convection remains the key error in these models, which limits our confidence in predictions and thus their value for decision-making on timescales from days to decades.

The key issue in representing convection in global models is that the resolutions of these models are too coarse to represent individual convective systems. Instead, models rely on physically based parametrization of convection. However, these parametrizations are based on paradigms developed 30-40 years ago in which convection was represented as a one dimensional, balanced problem between atmospheric instability and the convection required to remove that instability. Much more is now known about convection, how it is intimately related to the local dynamics and how it is organised on a range of space and timescales from the diurnal cycle of precipitation to synoptic scales such as tropical storms or the Madden-Julian Oscillation. These outdated paradigms have been identified as a major blockage to more skilful and reliable weather forecasts and climate predictions, in which realistic simulations of convection and the regional water cycle are of fundamental importance.

ParaCon was initiated as part of a Joint Strategic Response between NERC and the Met Office, and started in May 2016. It involves researchers from the Universities of Cambridge, Exeter, Leeds, and Reading. Each participating university will shortly be recruiting for their projects. For more information please contact:

Programme phases

There are two phases to the programme:

  1. A three-year exploratory phase, where new ideas in convection parametrization are investigated, and research is undertaken to understand the key elements required of a new scheme.
  2. This will be followed by a two-year integration phase, where we will aim to bring together the different strands of the programme towards a single parametrization.

Project areas

The overall programme is led by Alison Stirling at the Met Office, and is divided into five project areas:

1. Triggering of convection

Led by Doug Parker, this project aims to develop a new, scale-aware genesis scheme for convective triggering and updraughts. It aims to provide sizes and spectra of triggering structures, taking organisation, temporal coherence, and stochasticity into account.

2. Fluid dynamics of convective flows

Led by Michael Herzog, this project aims to identify and quantify the dynamical processes associated with individual clouds that need to be considered in convective parametrizations.

3. Mass flux and beyond

Led by Bob Plant, this project will provide a critical examination of parametrization methods that are based around the concept of conditional averaging. This will include the traditional bulk mass flux approach, but also more general approaches, such as multi-plume, and multi-fluid formulations. The research will include analyses of how well such approaches represent a fully-resolved flow through to test-of-concept and prototype parametrizations.

4. Convection-dynamics coupling

Led by John Thuburn, this project comprises two linked strands. The first will develop and test a multi-fluid scheme, derived from ideas of conditional averaging, for representing convection in an atmospheric model. The second will use theory and numerical experimentation to improve understanding of the mechanisms by which convection couples to the larger-scale circulation, and to quantify the sensitivity of aspects of the coupled circulation to the formulation and parameters in convection schemes.

5. Turbulent approaches for the grey zone

Led by Peter Clark, this component is concerned with the development of improved cloud and turbulence schemes for convection permitting models on scales (100 m - 10 km). It will provide a critical examination of volume-averaged formulations of the Navier-Stokes equation, assessing the performance of different levels of complexity in the formulation against high-resolution simulations of convection.

These research areas are underpinned by high-resolution modelling, which is being coordinated across the programme by Steve Woolnough, and Simon Vosper to ensure consistency of modelling tools, data availability and efficient use of the HPC. This working group will also develop a joint plan for the evaluation of convection parametrization schemes, including a hierarchy of test cases.

Working groups

In addition, there will be two working groups that cut across the individual projects:

Unifying conceptual framework for the formulation of convection schemes

Led by John Thuburn, we will seek to formulate an overarching mathematical framework from which each of the proposed approaches can be derived by a clearly stated set of approximations.

Numerical methods within parametrizations

Led by Hilary Weller, the aim of this activity is to share knowledge and methods in order to improve the numerical implementation of the physical parametrizations arising from ParaCon.

Reading positions

As part of ParaCon, advertisements will shortly appearing for 4 PDRA posts within the Meteorology Department of the University of Reading.

Two will be associated with the 'Mass flux and beyond' project area. One will take responsibility for the performance and analysis of high-resolution reference simulations, the other for implementation and testing of new parametrization ideas; these roles will need to work closely together to ensure that the appropriate analyses are performed in response to key parametrization issues, and that the results are used appropriately in formulating and evaluating the new parametrizations. For further details contact Bob Plant (email).

Two will be associated with the 'Turbulent approaches for the grey zone' project area. One will take responsibility for the implementation and testing of an improved parametrization of turbulence in convection-permitting models, the other for running and analysing reference simulations, and working with others to implement a new parametrization optimisation method. Both will need to work closely together. For further details contact Peter Clark (email).

All the posts require experience of numerical modelling, preferably computation fluid dynamics development (CFD) and/or analysis, and all the PDRAs will be expected to actively work together with the rest of the project team.