CLouds and Aerosol Radiative Impacts and Forcing: Year 2016 (CLARIFY-2016)
The CLARIFY-2016 project was consortium of 5 university partners and the UK Met Office who aimed to measure and understand the physical, chemical, optical and radiative properties of BBAs (biomass burning aerosol) in the key South East Atlantic region. This project was funded by the Natural Environment Research Council (NERC) with the grant reference - NE/L013797/1 - and was led by Professor James Haywood (University of Exeter).
Biomass burning aerosol (BBA) exerts a considerable impact on climate by impacting regional radiation budgets as it significantly reflects and absorbs sunlight, and its cloud nucleating properties perturb cloud microphysics and hence affect cloud radiative properties, precipitation and cloud lifetime. However, BBA is a complex and poorly understood aerosol species as it consists of a complex cocktail of organic carbon and inorganic compounds mixed with black carbon and hence large uncertainties exist in both the aerosol-radiation-interactions and aerosol-cloud-interactions, uncertainties that limit the ability of our current climate models to accurately reconstruct past climate and predict future climate change.
The African continent is the largest global source of BBA (around 50% of global emissions) which is transported offshore over the underlying semi-permanent cloud decks making the SE Atlantic a regional hotspot for BBA concentrations. While global climate models agree that this is a regional hotspot, their results diverge dramatically when attempting to assess aerosol-radiation-interactions and aerosol-cloud-interactions. Hence the area presents a very stringent test for climate models which need to capture not only the aerosol geographic, vertical, absorption and scattering properties, but also the cloud geographic distribution, vertical extent and cloud reflectance properties. Similarly, in order to capture the aerosol-cloud-interactions adequately, the susceptibility of the clouds in background conditions; aerosol activation processes; uncertainty about where and when BBA aerosol is entrained into the marine boundary layer and the impact of such entrainment on the microphysical and radiative properties of the cloud result in a large uncertainty. BBA overlying cloud also causes biases in satellite retrievals of cloud properties which can cause erroneous representation of stratocumulus cloud brightness; this has been shown to cause biases in other areas of the word such as biases in precipitation in Brazil via poorly understood global teleconnection processes.
These challenges can now be addressed as both measurement methods and high resolution model capabilities have developed rapidly and are now sufficiently advanced that the processes and properties of BBA can be sufficiently constrained. This measurement/high resolution model combination can be used to challenge the representation of aerosol-radiation-interaction and aerosol-cloud-interaction in coarser resolution numerical weather prediction (NWP) and climate models. Previous measurements in the region are limited to the basic measurements made during SAFARI-2000 when the advanced measurements needed for constraining the complex cloud-aerosol-radiation had not been developed and high resolution modelling was in its infancy.
The main aims of CLARIFY-2016 were to deliver a suite of ground and aircraft measurements which measure, understand, evaluate and improve:
a) the physical, chemical, optical and radiative properties of BBAs
b) the physical properties of stratocumulus clouds
c) the representation of aerosol-radiation interactions in weather and climate models
d) the representation of aerosol-cloud interactions across a range of model scales.
The main field experiment took place during September 2016, based in Walvis Bay, Namibia. The UK large research aircraft (FAAM) was used to measure in-situ and remotely sensed aerosol and cloud properties while advanced radiometers on board the aircraft measured aerosol and cloud radiative impacts. This project was written on a stand-alone basis, but there was close collaboration and coordinating with both the NASA ORACLES programme (5 NASA centres, 8 USA universities) and NSF-funded ONFIRE programme (22 USA institutes).
Key objectives of CLARIFY-2016 were:
KO1: Measure and understand the physical, chemical, optical and radiative properties of BBAs in the key SE Atlantic region.
KO2: Understand, evaluate and improve the physical properties of the SE Atlantic stratocumulus clouds and their environment in a range of models.
KO3: Evaluate and improve the representation of BBA-radiation interactions over the SE Atlantic when clouds are absent/present at a range of model scales and resolutions.
KO4: Evaluate and improve the representation of BBA-cloud interactions over the SE Atlantic at a range of model scales and resolutions.
These objectives were be achieved by conducting an intensive airborne field campaign with supporting surface and satellite measurements. The measurements were used to challenge, and develop improved models at different spatial scales from the cloud scale to the global scale that couple aerosols, clouds and radiation.
The enabling objectives were:-
EO1: To use forecast and observations to optimise scheduling of the flight plans, balancing our operations to ensure data provision for all our enabling objectives.
EO2: To characterise chemical, microphysical, optical and radiative properties of BBA over the SE Atlantic region, focussing on black carbon, absorption and single scattering albedo.
EO3: To investigate the geographic and vertical profile of BBA over the region.
EO4: To characterise the vertical thermodynamic structure of the MBL, residual continental polluted layer, and free troposphere and diurnal and synoptic scale variations.
EO5: To characterise broad-band and spectral reflectance of the ocean surface and stratocumulus clouds when overlying BBA is present/absent from the atmospheric column.
EO6: To characterise key cloud processes and parameters such as entrainment, cloud dynamics, cloud-base updraft velocities, cloud condensation nuclei (CCN), cloud droplet number concentrations (CDNC), cloud droplet effective radius, cloud liquid water path and optical depth.
EO7: Use CLARIFY campaign data with representative statistical sampling as well as high-resolution models to establish robust relationships between sub-grid scale variables and large-scale model parameters suitable for constraining aerosol-cloud interactions.
EO8: To use synergistic observations/model simulations to investigate impacts on NWP and climate model performance, feedback mechanisms, regional and global climate impacts and teleconnections.