Sea surface salinity retrievals from HUT-2D L-band radiometric measurements

摘要

The interest of the scientific community in global climate has been constantly increasing in the last years. Much effort has been devoted to better understand the water cycle and its role in global climate regulation. This is one of the objectives of the European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) mission, approved in May 1999 and successfully launched in November 2nd 2009. Collecting brightness temperature measurements by means of a new type of instrument, a synthetic aperture radiometer operating at L-Band (MIRAS: Microwave Imaging Radiometer by Aperture Synthesis), the SMOS mission will provide regular and global maps of sea surface salinity (SSS) and soil moisture (SM), two key parameters to describe the water cycle. Synthetic aperture brightness temperatures have been simulated since the early 90s, during the MIRAS technological studies, and both SSS and SM retrievals have been tested with simulated data first, and later with real data to assess and improve the instrument performance. Nevertheless, except for the processing of the data collected by the SMOSillo (MIRAS Demonstrator) on June 20th, 2006, Duffo et al. (2007) and Camps et al. (2008b), these outcomes have never been validated using sea surface brightness temperatures coming from a real synthetic aperture radiometer. The aim of this study is thus to test some of the techniques proposed in previous years to improve the SSS retrieval, in particular only Level 2 techniques (i.e. converting calibrated and geo-located brightness temperature into SSS maps) will be investigated. To do so, brightness temperatures resulting from the SMOS Salinity Demonstrator Campaign held in August 2007 will be used. In that campaign the Helsinki University of Technology-2 Dimensional (HUT-2D) radiometer flown over both over land and sea. The part of the campaign conducted over the sea consisted of two series of flights over a very fresh water plume characterized by a strong SSS gradient (from 0 to 4. psu) in the Gulf of Finland. In-situ auxiliary data was collected simultaneously with the radiometer measurements. The positive outcomes in these very challenging conditions (due to the very low brightness temperature sensitivity to SSS, the lack of accurate models of the sea water dielectric constant at low SSS (Klein & Swift (1977)), and the weak radiometric sensitivity of the HUT-2D radiometer) demonstrate the importance of data pre- and post-processing to improve the results. Removing both brightness temperature biases and salinity retrieval biases and further averaging of the results, permits estimating SSS with an rms error on the order of 1. psu, which is comparable to the SMOS Level 2 expected accuracy.