Convective storms, tropical circulation and climate change

This is a mathematical and theoretical project which will explore the relationship between severe storms and large-scale circulation in the atmosphere. Convective storms dominate the energy balance of the tropics, but their coupling with the circulation represents one of the foremost uncertainties in global prediction. Numerical predictions of rainfall over the tropical continents have substantial errors, in regions such as India and Africa where large populations are vulnerable to fluctuations in the climate.

We will first use the classic Rayleigh-Benard convection model, modified to include a simple representation of cloudy atmospheric convection (“Rainy-Benard”, RyB), to explore deep questions. This simple “Rainy-Benard” problem shares a number of characteristics seen in real tropical convection, and will allow us to explore the underlying mathematical controls convection-circulation interaction. What are the budgets and equilibrium states of RyB? How do these respond to changes in boundary conditions (climate-change forcings like thermal radiation, sea surface temperate or land use)? Are there regional differences, when representing convection and rainfall over land and sea?

Rainy-Benard equations
Rayleigh-Benard equation set modified to include humidity, q, condensation, latent heat release and precipitation. See Vallis et al. (2019).

We will exploit the new generation of very high-resolution atmospheric simulations with the Met Office’s operational weather and climate prediction model, to test the theories developed from RyB. These model simulations will be used to test and refine our mathematical models and describe the ways in which latent heating in clouds, on scales of a few kilometres, influences patterns of rainfall over continents and oceans on scales of many thousand kilometres.

We have an established partnership with the UK Met Office, and this project will be co-supervised by Lorenzo Tomassini from the Met Office, with opportunities to visit periodically. The project will also benefit from links with several UK and international projects in which we are partners, such as EUREC4A.

The primary project outcome will be improved theoretical understanding of the role of cloud latent heating functions in fields of tropical convection, and the way in which this influences the capacity of the convection to drive and interact with large-scale circulations. This knowledge is needed to understand the way in which the climate system responds to changes in its climatic boundary conditions. Ultimately, if the work is successful, it will influence policy and decision-making around regional climate change. It is expected that the work will involve the following activities.

RyB solutions
Time-distance plots showing the relative humidity half way up the model domain, for different values of Rayleigh number, Ra. For low Ra, steady chimneys of convection are maintained. For higher Ra, the solutions become transient and chaotic, with storms splitting and merging, much as observed in nature.
  • Idealised modelling of theoretical convective regimes in the Rainy-Benard system.
  • Extension of the idealised modelling to consider physical processes such as: planetary rotation; radiative cooling; evaporative cooling; variable surface conditions; or interaction with tropical waves and jets.
  • Quantitative analysis of statistical properties of convection-permitting model simulations of tropical convection (e.g. equilibria and energy budgets), using the same theoretical framework.
  • Potential to explore the use of machine learning for parameter estimation of statistical properties.
  • Analysis of low-resolution versions of both the theoretical/idealised and convection-permitting models, leading to recommendations for better future representation of convection in low-resolution climate models.
Satellite image of storms over Africa.
Satellite image showing an example of convective storms over West Africa. We are routinely observing and analysing these systems: this project will aid in the understanding of their behaviour.