The Advanced Infra-Red Water Vapour Estimator Prototype Processor (AIRWAVE-PP): tool design and ATSR processing

The Advanced Infra-Red Water Vapour Estimator, or AIRWAVE, is a novel algorithm developed by CNR and ESA for the accurate and precise estimate of ocean/cloud-free Total Column Water Vapour (TCWV). The algorithm is based on the exploitation of Thermal Infra-Red channels of the ATSR-like instruments, such as SLSTR on board Sentinel 3, in combination with advanced radiative transfer models and a sea surface spectral emissivity database. In particular, the simultaneous use of forward and nadir measurements minimise the impact of changes in sea surface temperature and in atmospheric radiation on the quality of retrieved TCWV for all illumination conditions (day and night). The AIRWAVE-PP prototype processor has been designed and integrated in the ESA GRID environment (GPOD) for testing purposes, and for the bulk processing of the ATSR missions (1991-2012). It is written in Python 2.7, and based on standard Python libraries, such as Numpy and Matplotlib. The BEAT-CODA software is used for the decoding of ESA Level 1 data format. The tool's logical operation flow can be summarised as follows: 1) ATSR Level 1 product decoding and pre-processing; 2) sea and cloud-free data screening using L1 product flags; 3) brightness temperature to radiance conversion; 4) TCWV retrieval at 1x1 km2 and 0.25°x0.25° spatial resolution; 5) exporting of TCWV results into NetCDF-CF (Climate Forecast) file products. The NetCDF-CF convention has been adopted in order to facilitate the readability of dataset by means of freely available software. For each ATSR orbit file, AIRWAVE-PP produces two outputs: a) NetCDF-CF file containing the TCWV data; b) a PNG file (image) showing the TCWV vs. latitude plot (quick-look), the latter used for a fast check on the quality of the retrieval. The AIRWAVE-PP implementation will be discussed in detail, along with the preliminary results of the ATSR vs. SSM/I TCWV inter-comparison exercise.