Session: Atmosphere II

A key issue in assessing climate change is the precise knowledge of distributions and variability of Total Column Water Vapour (TCWV). We developed a novel algorithm, named Advanced Infra-Red WAter Vapour Estimator (AIRWAVE), for the retrieval of TCWV form Along Track Scanning Radiometer (ATSR) missions measurements.

The retrieval method does not require any tuning or adjustments to independent water vapour datasets and is based on: 1) calculations from a radiative transfer model that was specifically developed to simulate ATSR radiances, 2) informations on instrumental parameters, 3) sea surface spectral emissivity. AIRWAVE has been applied to ocean cloud-free ATSR radiances in the Thermal Infra-Red channels (11 and 12 microns) and exploits the simultaneous use of forward and nadir measurements. It allows for accurate and precise day-time and night-time TCWV retrievals as shown by preliminary inter-comparisons between ATSR and Special Sensor Microwave/Imager (SSM/I) coincident TCWVs.

The algorithm was developed for ATSR-like instruments having channels in the TIR and exploiting dual viewing geometries. The Sea and Land Surface Temperature Radiometer (SLSTR), on board the European Copernicus Sentinel 3, fully satisfies these requirements. AIRWAVE can, therefore, be applied to SLSTR measurements to retrieve TCWV.

Here we discuss the physical derivation of the AIRWAVE retrieval method and present preliminary calculations (based on available instrument characteristics) for the application of AIRWAVE to SLSTR measurements.

The ESA-SEOM S3 'advanced Clouds, Aerosols and WAter vapour products for Sentinel-3/OLCI' CAWA project aims to the development and improvement of advanced atmospheric retrieval algorithms for Sentinel-3/OLCI mission, and will be prepared on the Envisat/MERIS dataset. First results of a sensor comprehensive and consistent 1D-Var water vapour algorithm applied to the MERIS, MODIS and upcoming OLCI measurements will be presented. An innovative and consistent cloud top pressure 1D-Var procedure as defined for MERIS and all three OLCI O2 A-band channels will be discussed, based on radiative transfer simulations. The challenging and innovative GRASP algorithm for the retrieval of aerosols and surface properties has already shown its advantage in comparison to conventional aerosol retrieval methods. The intention of the CAWA team is to establish new and improved procedures to estimate atmospheric properties, which also improve the retrieval of land and ocean properties. The algorithms will be implemented in the free available ESA Sentinel 3 toolbox.

The Optimal Retrieval of Aerosol and Cloud (ORAC) - formerly the Oxford-RAL Aerosol and Cloud retrieval - is a generalized optimal estimation system that retrieves cloud, aerosol and volcanic parameters using visible to infrared measurements from a wide range of instruments. For cloud and aerosol the results have been shown to be good.  Retrieval of volcanic ash is currently experimental with results that are encouraging.  Use of the same algorithm for different parameters and sensors leads to radiative consistency.

ORAC currently supports the AATSR, AVHRR, MODIS, and SEVIR measurements.  ORAC has been designed to easily support other measurement sources,  Use of AATSR measurements in the aerosol retrieval benefits from the dual view, leading to the ability to characterise the surface relative to the aerosol signal.

In this presentation we well discuss the ORAC retrieval algorithm in general as applied to cloud, aerosol and ash and we will discuss new developments related to application of ORAC to SLSTR. SLSTR will provide both an increased spatial resolution and additional channels.  A benefit of the increased spatial resolution is better discrimination of properties in the combined cloud and aerosol retrieval problem.  As well, SLSTR measures an additional infrared channel relative to AATSR providing further information on particle size, which can be used to better characterise clouds. Finally these differences, in relation to climate studies will be addressed.