Studying aerosols can be useful in several scientific subjects, including radiative transfer, cloud formation, air quality, visibility, atmospheric stability, the hydrological cycle, human health and especially, climate change. The concept of aerosol-radiation-climate interactions was first developed around 1970. Since then and especially over the past 10 years, the determination of mechanisms and magnitudes of these interactions have considerably progressed. Recently, extensive field experiments associated with ground-based network measurements, satellite remote sensing and its integration with model simulations have substantially improved the characterisation of aerosols.
According to the Intergovernmental Panel on Climate Change (IPCC), the role of aerosols in climate is one of the biggest uncertainties in understanding the current climate system and in predicting further climate change. Because of spatial and temporal variability, the establishment of the mechanism of aerosol direct effects is more challenging. Insufficient comprehension of the distribution and physical and chemical properties of aerosols and aerosol-cloud interactions leads to significant uncertainties in current estimates of aerosol forcing.
From both the observational and modelling point of view, aerosol indirect effects on clouds represent a huge challenge. Because of the advancement in satellite and modelling techniques, recent studies were able to concentrate on the estimation of the aerosol indirect radiative forcing. For instance, thanks to the comparison between forward and inverse model calculations, Anderson et al, (2003) suggest an overestimation of this forcing in current climate models. In addition, Quaas and Boucher (2005) evaluate and improve the representation of the Aerosol Indirect Effect (AIE) in a general circulation model by:
- deriving statistical relationships of cloud-top droplet radius and aerosol index (AOD) using MODIS and POLDER data from satellite retrievals
- fitting an empirical parameterisation in a general circulation model to suit the relationships.
The inverse calculation is coherent with their result on the aerosol indirect radiative forcing effect.
Current knowledge of cloud dynamic and radiative properties is known and accepted to be insufficient for modern climate analysis and prediction schemes. This results from high spatial and temporal variability of aerosol loading and from a complex link between aerosols and cloud properties. As a consequence, the total aerosol forcing of the climate system remains indeterminate.
As well as affecting the global climate, aerosols have an impact on the climate of certain regions and their water cycle. Concerning SLSTR and the retrieval of surface temperature, the first practical priority is to detect the presence of clouds. Once this objective is satisfied, SLSTR's multi-angle multi-wavelength viewing geometry can be used to characterise and investigate the properties of clouds.
The development of operational SLSTR cloud product is recommended to be supported and implemented (as it was suggested in AATSR Exploitation Plan - vol. 6). These products, including cloud optical depth, cloud phase, cloud particle size, cloud top pressure, cloud fraction and cloud water path, need to be supported by accurate validation activities.
For further information about atmosphere applications and services available, see: Copernicus website.