Cloud System Resolving Modelling of the Tropical Atmosphere

Lead Research Organisation: University of Reading
Department Name: Meteorology

Abstract

The tropics are often described as the engine room of the Earth's climate system, powering the global circulations of the atmosphere and oceans. Absorption of sunlight heats the land and ocean surfaces strongly at low latitudes, producing convection that carries the energy from the surface into the atmosphere. Except over the deserts, the convection generates deep clouds that transport moisture evaporated from the ocean into the upper atmosphere. When these clouds rain, the release of 'latent heat' by condensing water produces further heating that drives the weather systems of the tropics and influences the winds all around the globe. But such deep convective clouds rarely exist in isolation; they are almost always organised into structures ranging from squall lines and cloud clusters to tropical storms, hurricanes and super-clusters; and convection varies on a wide range of timescales from that of an individual cloud element (hours), through the daily cycle to a plethora of waves with periods ranging up to the intra-seasonal oscillation, which can propagate around the world in 30-60 days. The tropical atmosphere thus organises itself on a huge range of space and timescales; the effect on the climate system is very different from that of random, or disorganised turbulence so that all these scales should be represented in a computer model if it is to reproduce the real world accurately. Until now, many of these effects have had to be represented in a highly simplified way, through a process known as parametrization. This tries to mimic the effect of the convection on the scales smaller than those represented explicitly in the model. The trouble is that across the cloud system from a few to a few hundreds of kilometres there is no preferred scale, and these scales span the range from those that are 'sub-grid' and therefore need parametrization to those which are resolved by the model. Even with the availability of modern supercomputers, this transition from parametrization to resolved motion takes place at around 100km in the latest climate models. All of the structures that occur in the real world below this scale are parametrized, so a completely artificial break occurs between what is resolved and what is parametrized. To compound the problem, the parametrization assumes that organization on scales which cannot be resolved is not important for determining the properties of the convection or its influence on the large-scale flow. But there is mounting evidence that this creates all sorts of problems in the models. Several decades have been invested in developing convection parametrizations and even after all this time and effort no fully satisfactory solution has been found. We therefore propose to take a radical but entirely logical approach to this problem. It is now possible to run models that do resolve convective systems explicitly (at least down to scales of around 1km) over very large domains that encompass all of the important scales mentioned above. Such cloud system resolving models provide a new tool for understanding how convection really works and organises itself, and how it should be parametrized in climate models. Our proposal links these models with new data from satellites and from the surface that will give us an unprecedented view of the evolution of clouds and rain-producing systems. We will bring this unique combination of modelling and observations to bear on what is regarded as one of the most fundamental problems in weather and climate. The results of the work will inform the development of a new generation of more accurate atmospheric models that will find employment in both climate prediction and weather forecasting.

Publications

10 25 50

publication icon
Garcia-Carreras L (2015) The Impact of Parameterized Convection on the Simulation of Crop Processes in Journal of Applied Meteorology and Climatology

publication icon
Love B (2011) The diurnal cycle of precipitation over the Maritime Continent in a high-resolution atmospheric model in Quarterly Journal of the Royal Meteorological Society

publication icon
Pantillon F (2015) A Parameterization of Convective Dust Storms for Models with Mass-Flux Convection Schemes in Journal of the Atmospheric Sciences

 
Description The objectives of this project can be summarized as addressing problems in the understanding and simulation of tropical convection and in particular the role of scale-interactions in both time and space on the evolution of large-scale tropical weather systems, to inform the development of new approaches to the parametrization of convection in weather and climate models.

This project used ground breaking simulations of large tropical domains (of order 10,000km by 4,000km) at resolutions which were capable of simulating individual cloud systems, in comparison to simulations at the coarser resolutions of traditional weather and climate models, with convective parametrizations.
Across the project two common themes emerged. Firstly the simulation of organzied tropical convection is more sensitive to the representation of the convection (i.e. explicit simulated vs parametrized) than the resolution per se. That is whilst aspects of the simulation with explicit convection improve on moving from 12km resolution to 1.5km resolution, the characteristics of the large-scale organization in these simulations is more similar than comparisons between simulations at 12km resolution with explicit or parametrized convection. In particular the representation of the diurnal cycle of tropical convection, the spatial organization of convection, the distribution of precipitation intensity and the relationship between convection and soil moisture are all improved with explicit convection.
Secondly, whilst the explicit convection simulations show better spatial and temporal organization, there are still weakness in many aspects of the simulation of convection, and the detailed behaviour is sensitive to apsects of the model formulation. In particular even at these highest resolutions individual convective updrafts are still not resolved and the simulations are sensitive to choices in the representation of cloud microphysics and turbulent mixing. There is still a clear need to improve the representation of convective clouds at these resolutions.
Exploitation Route These findings can be used to inform the parametrization of convection in weather and climate models by operational centres. In particular in understanding the relationship between the small scale convective processes and the large-scale organization.
Sectors Environment

 
Description The modelling infrastructure developed by this project is now being widely used by the UK Met Office both for its own research and in the development of high resolution weather forecast models for international partners.
First Year Of Impact 2012
Sector Environment
Impact Types Policy & public services