EUREC4A-UK: Elucidating the role of cloud-circulation coupling in climate

Lead Research Organisation: University of Leeds
Department Name: School of Earth and Environment


EUREC4A-UK is a programme of observational and modelling research which aims
to study the detailed aerosol and cloud processes in the life cycle of shallow trade
cumulus clouds and the two-way interactions between the cloud processes and the
large-scale dynamics. The different responses of these clouds to warming in global
climate models (GCM) explain most of the inter-model differences, yet the physics
of these responses remains poorly constrained. The programme is focussed on
the participation of UK scientists and the BAS Twin Otter aircraft in EUREC4A
(Elucidating the Role of Clouds-Circulation Coupling in Climate). EUREC4A is
a coordinated international campaign that aims to address the current lack of understanding of
the processes controlling the response of trade-wind cumulus clouds to changing environmental
conditions in a warmer climate. The goal of EUREC4A is to examine the interplay between the
clouds, atmospheric circulations and climate sensitivity. EUREC4A-UK will make a unique and self-
contained contribution to the international programme by: (i) providing observational facilities which
are needed as part of the coordinated field campaign; (ii) conducting and leading the analysis of
the aerosols, cloud microphysics and boundary-layer processes in the life cycle of shallow trade
cumulus clouds; (iii) placing the analysis in the context of the EUREC4A problems by modelling the
two-way interactions between the cloud processes and the large-scale dynamics; and (iv) applying
the results by testing the new convection scheme in the UM and using the improved model to determine
the dominant processes controlling the cloud fields. International partners will complement the re-
search with a focus on observing and modelling the macrophysical properties and the environment
of trade-cumulus clouds in order to determine: (i) what controls the convective mass flux, mesoscale
organization and depth of shallow-cumulus clouds; (ii) how the trade-cumulus cloud fraction varies
with turbulence, convective mixing and large-scale circulations; and (iii) the impact this variation has
on atmospheric radiation.

The radiative properties of the trade-wind cumulus clouds that are ubiquitous over the tropical oceans have a major influence on the Earth's radiation budget. The response to global warming of these clouds is therefore critical for global mean cloud feedbacks. It is the differing response to warming that explains most of the spread of climate sensitivity in climate models. Hence, a better understanding is required of the mechanisms that control the low-level cloud fraction. The urgency of the research is made clear by the fact that the World Climate Research Programme endorses the EUREC4A field project which supports the Grand Challenge on Clouds, Circulation and Climate Sensitivity.

There is a clear need for EUREC4A-UK because the aerosol,
cloud and precipitation processes influence the macrophysical properties of the clouds in different
environments. For example, the vertical distribution of rain
can affect the concentration and size of cloud drops in the upper detrainment layers, which influ-
ences cloud radiative properties. The intensity of rain and evaporation of raindrops influences the
strength of gust fronts and hence secondary cloud-production. However, model calculations of the
rate of production of rain and hence the quantity of rain are uncertain due to the complex interactions
of aerosols, entrainment, turbulence and giant cloud condensation nuclei (GCCN). These processes
depend on the environment conditions, controlled by the large-scale dynamics. Equally, the aerosol-
cloud-precipitation processes can influence the larger-scale dynamics, for example through radiative
transfer. Indeed there are many interactions between processes on a range of scales that need to be
understood and represented in models.

Planned Impact

Governments and businesses world wide, and the general public will benefit greatly from this research because of the greater accuracy (reduced uncertainty) in climate model predictions that will result from this research. Specifically, EUREC4A will help to reduce the uncertainty in climate sensitivity, or estimates of aerosol-radiative forcing by advancing our understanding of cloud processes and their feedbacks. Improved planning for climate change will deliver major economic benefits. Advancing understanding and modelling of clouds and circulation in the trade-wind regions is also very important for improving Numerical Weather Prediction models.

The UK addition would also contribute to the goals of the NCAS-led project the North Atlantic Climate System Integrated Study (ACSIS). Furthermore, the Global Energy and Water Cycle Exchanges Project (GEWEX) Aerosols, Clouds, Precipitation and Climate (ACPC) programme want to include EUREC4A since the cloud systems and circulation will be measured so well. This proposal provides a unique opportunity to add urgently needed measurements of aerosol and cloud processes.


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