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L-Band Brightness Temperature at Dome-C Antarctica: Intercomparison Between DOMEX-3, SMOS and Aquarius Data
[15-Apr-2013] Macelloni, G., Brogioni, M., Pettinato, S., Monti, F., and Casal, T.
Presented at the 2013 SMOS-Aquarius Science Workshop
With the purpose of evaluating the brightness temperature long-time temporal stability of a possible SMOS external calibration reference target an experimental activity, called DOMEX, was carried out in the past years at the Italian-French base of Concordia (Antarctica). The activity consists in the observation of the plateau using a ground based L-band radiometer installed on an observation tower. The first experiments, called DOMEX-1 and DOMEX-2 respectively, featured a ground-based L-band radiometer operating 24 hours per day for one month in 2004-2005 and for two years from 2008 to 2010. The data collected during these experiments confirmed that the microwave emission of the Dome C site at L-band can be considered stable (especially at V polarization) for periods several months long. The results obtained demonstrated that DOME-C represents a unique "high- temperature" extended target that provides a temporally-stable reference which potentially meets existing requirements for assessing the long-term stability of space-borne L-band radiometric instruments. Long-term monitoring of DOME-C is also crucial to investigating further and to modeling variations in H-pol observed during the DOMEX-2 experiment. In addition, the data set will be instrumental for satellite sensor product intercalibration ensuring a harmonized Level 1 data set of passive microwave observations at L-band covering the SMOS - Aquarius - SMAP era and for future climate applications. In order to meet this objective, a new experiment called DOMEX-3 started in December 2013. The instrument used in the campaign is an improved version of the Radomex radiometer built by the Institute of Applied Physics in Florence, Italy, which was used successfully in 2008-2010 period. The newly-designed Radomex radiometer is able to operate autonomously and remotely transmitting data daily to Europe. The instrument was accurately tested and calibrated before the shipment using standard procedures.
Data acquired in the first months of the experiment confirm the results obtained in previous campaign, in particular the temporal stability of the site at V polarization. DOMEX-3 time-series data, collected at 42° of incidence angle, were also compared to SMOS and Aquarius data collected over the same area. The last version of reprocessed SMOS data was used here. The L1C data was transferred from Antenna to Top of Atmosphere (TOA) level (XY to HV) by applying Faraday and geometric rotations and the Brightness temperatures were averaged within an incidence angle of between 37.5 and 47.5 degrees, and only for nodes within the alias-free part of the SMOS field of view. The Aquarius dataset consisted of Level-2 data (version 1.3) without land correction collected at 45.6 deg of incidence angle. Although DOMEX-3 collects measurements at a fixed incidence angle (42°) it occasionally performs incidence angle scans in the 20°-60° range. These data were compared to SMOS and Aquarius data collected in the same period. For Aquarius data acquired at the three different incidence angle were considered. Besides the area of Concordia another large area, exhibiting a triangular-shape and located 200 Km far from the station in the west direction, was observed in both SMOS and Aquarius data. From a preliminary investigation, this area appears to be very homogenous in space and able to contains several SMOS and Aquarius pixels. Time series of both satellite sensors collected over that area were considered and compared to the data acquired at Concordia. For SMOS the mean difference of the Tbv time series between the triangular area and Concordia base was 2.6 K (is higher over Concordia). Moreover, this difference remained stable in time (the temporal standard deviation was 0.24K), and it was thus possible to correlate the measurements collected at Concordia to those acquired over this large area. For Aquarius data the difference between the triangle and Concordia was 2 K at V polarization.

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