Endorsement of SMOS Brightness Temperature Using In-Situ Buoy Data over Indian Ocean
Calla, OPN¹; Dadhich, Harendra Kumar²; Singhal, Shruti^2
1ICRS, INDIA; ²International Centre for Radio Science, INDIA

The European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) satellite mission has been launched in November 2009 with the aim at providing global and regular observations of Soil Moisture and Ocean Salinity in which they are variables in order to understand and predict the evolution of the water cycle on our planet. In particular, it provides over the oceans, synoptic sea surface salinity (SSS) measurements with frequent temporal coverage and good spatial resolution. The single payload embarked on SMOS is the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS); it is a 2D interferometric radiometer operating at the protected L-band with a nominal frequency of 1413.5 MHz. Since SMOS is the first 2D interferometric radiometers put in orbit so far, the interpretation of the interferometrically measured brightness temperatures (BT) is increasingly being acknowledged among scientific community.

This paper aims to validate the SMOS BT with the estimated BT using Radiative Transfer Model (RTM). There are many challenges in measuring SSS from space and obtaining a targeted accuracy of 0.1 psu on a 200 km grid over a 10-day period. The accuracy of the retrieved SSS is better when the correct observation of BT has been made by SMOS. So, now it has become essential to ratify the BT values of SMOS. For estimating BT the in-situ SSS and sea surface temperature (SST) is obtained from the Research Moored Array for African-Asian-Australian Monsoon Analysis and prediction (RAMA) buoys available over Indian Ocean. The BT obtained from SMOS over Bay of Bengal (BoB) and Arabian Sea (AS) is found corrupted due to the presence of radio frequency interference (RFI) and land contamination. Thus one cannot use this data for validation purpose; as a result the analysis has been done over Indian Ocean.

The BT obtained from SMOS level 2 data has been compared and validated with the BT estimated using Radiative Transfer Model (RTM). The estimated BT is largely governed by sea emissivity which is a complex function of the dielectric constant of sea water, polarization, and viewing angle. The complex dielectric constant, in turn, is affected by the electrical conductivity and the microwave frequency of observation, where conductivity is directed by SSS and SST, thus one can express BT as a function of SSS and SST. The complex dielectric constant of sea water is estimated using Klien and Swift (K&S) theoretical model, in case of non availability of in-situ dielectric constant. BT so obtained is compared and analyzed over a couple of months with different RAMA buoy location over Indian Ocean. Further analysis is planned region wise so as to avoid the local area effects on the Indian Ocean characteristics. The result obtain from this analysis is presented and discussed in the paper.