CLouds and Aerosol Radiative Impacts and Forcing: Year 2016 (CLARIFY-2016)

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 bundary 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.

It is timely to address these challenges as both measurement methods and high resolution model capabilities have developed rapidly over the last few years 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.

We are therefore proposing a major consortium programme, CLARIFY-2016, a consortium of 5 university partners and the UK Met Office, which will deliver a suite of ground and aircraft measurements to 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 will take place during September 2016, based in Walvis Bay, Namibia. The UK large research aircraft (FAAM) will be used to measure in-situ and remotely sensed aerosol and cloud and properties while advanced radiometers on board the aircraft will measure aerosol and cloud radiative impacts. While the proposal has been written on a stand-alone basis, we are closely collaborating and coordinating with both the NASA ORACLES programme (5 NASA centres, 8 USA universities) and NSF-funded ONFIRE programme (22 USA institutes).

CLARIFY-2016: Impact Summary

1. The Beneficiaries of the Research and how they will benefit from it:-

Scientific community:
CLARIFY will be of significant scientific interest nationally and internationally as evident by the synergistic proposed research under the NASA ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) and NSF ONFIRE (ObservatioNs of Fires Impact on the southeast atlantic REgion) proposals and the level of support from the Met Office. CLARIFY data will be placed on the BADC for use initially by project partners, but will be available to the entire community subsequent to completion of the project. Specifically:-
1) The ONFIRE consortium consists of a total of 22 US universities and research institutes who plan complementary measurements with the C-130 aircraft from Sao Tome Island (2000km north of Walvis Bay). CLARIFY measurements will provide vital cloud-aerosol-radiation observations in stratocumulus, rather than cumulus-dominated cloud regimes hence extending the geographical extent and applicability of their study. They will be granted access to bespoke Numerical Weather Prediction model forecasts, and CLARIFY data (ODMP), and will provide the same to CLARIFY in a reciprocal data sharing agreement.
2)The ORACLES consortium consists of 5 NASA Research Centres and 8 US universities. They plan deployment of the NASA P3 and ER-2 high altitude aircraft to Walvis Bay. They will benefit from intercomparison/validation of aircraft data, synergistic planning of the aircraft measurements with potential dual-aircraft flights planned, NWP forecasts and CLARIFY data (ODMP). They will provide the same to CLARIFY in a reciprocal agreement.
3) The AEROCOM community. There are 22 global atmospheric models/groups contributing to Phase II of AEROCOM who will benefit from a full range of validation data, and comparisons against an optimised version of HadGEM3.
4) Project partners in the satellite community; the provision of accurate aerosol microphysical properties, cloud retrievals and biases caused by overlying BBA will be utilised by the satellite community.

Operational Weather Forecasting Centres:
5) The Met Office will benefit from (i) sharing costs of aircraft deployment costs (ii) validation and improvement of satellite retrievals, Met Office large eddy simulation model (LEM), limited area and global NWP models, and the HadGEM3 climate model leading to better weather, seasonal forecasts, and climate projections at local, regional, and continental scales.
6)The South Africa Weather Service (SAWS) provides weather forecasts for southern Africa, using a licensed version of the Met Office limited area NWP model.
7) The European Centre for Medium Range Weather Forecasting (ECMWF, PP) will collaborate and benefit from provision of forecast validation data (see letter of support).

General public / media:
8) There has been significant publicity in recent years on aerosols, clouds and climate. However, there remains a significant lack of knowledge surrounding these issues and clear information needs to be conveyed to the public to enable greater appreciation of the uncertainties in regional and global climate prediction.
9) Improved forecasts by SAWS will improve NWP forecasts for southern Africa, while the global NWP model improvements will improve forecasts in other areas by better representing global teleconnections in weather patterns.

Policymakers:
10) There is considerable interest from policy-makers on the impacts of absorbing aerosol on the radiative forcing of climate with suggestions for black carbon emission reduction as a mitigation strategy to counter global warming. The measurements will lead to a greater knowledge of the impacts of black carbon-rich absorbing aerosol above reflective cloud over the SE Atlantic hot-spot. The measurement and modelling integration will provide policy makers with evidence for potential effectiveness of mitigation strategies.