Long-term ELBARA-II assistance to SMOS land product and algorithm validation at the Valencia Anchor Station (MELBEX Experiment 2010-2013)
Résumé
The main activity of the Valencia Anchor Station (VAS) is currently now to support the validation of SMOS (Soil Moisture and Ocean Salinity) Level 2 and 3 land products (soil moisture, SM, and vegetation optical depth, TAU). With this aim, the European Space Agency (ESA) has provided the Climatology from Satellites Group of the University of Valencia with an ELBARA-II microwave radiometer under a loan agreement since September 2009. During this time, brightness temperatures (TB) have continuously been acquired, except during normal maintenance or minor repair interruptions. ELBARA-II is an L-band dual-polarization radiometer with two channels (1400-1418 MHz, 1409-1427 MHz). It is continuously measuring over a vineyard field (El Renegado, Caudete de las Fuentes, Valencia) from a 15 m platform with a constant protocol for calibration and angular scanning measurements with the aim to assisting the validation of SMOS land products and the calibration of the L-MEB (L-Band Emission of the Biosphere) -basis for the SMOS Level 2 Land Processor- over the VAS validation site. One of the advantages of using the VAS site is the possibility of studying two different environmental conditions along the year. While the vine cycle extends mainly between April and October, during the rest of the year the area remains under bare soil conditions, adequate for the calibration of the soil model. The measurement protocol currently running has shown to be robust during the whole operation time and will be extended in time as much as possible to continue providing a long-term data set of ELBARA-II TB measurements and retrieved SM and TAU. This data set is also showing to be useful in support of SMOS scientific activities: the VAS area and, specifically the ELBARA-II site, offer good conditions to control the long-term evolution of SMOS Level 2 and Level 3 land products and interpret eventual anomalies that may obscure sensor hidden biases. In addition, SM and TAU that are currently retrieved from the ELBARA-II TB data by inversion of the L-MEB model, can also be compared to the Level 2 and Level 3 SMOS products. L-band ELBARA-II measurements provide area-integrated estimations of SM and TAU that are much more representative of the soil and vegetation conditions at field scale than ground measurements (from capacitive probes for SM and destructive measurements for TAU). For instance, Miernecki et al., (2012) and Wigneron et al. (2012) showed that very good correlations could be obtained from TB data and SM retrievals obtained from both SMOS and ELBARA-II over the 2010-2011 time period. The analysis of the quality of these correlations over a long time period can be very useful to evaluate the SMOS measurements and retrieved products (Level 2 and 3). The present work that extends the analysis over almost 4 years now (2010-2013) emphasizes the need to (i) maintain the long-time record of ELBARA-II measurements (ii) enhance as much as possible the control over other parameters, especially, soil roughness (SR), vegetation water content (VWC) and surface temperature, to interpret the retrieved results obtained from both SMOS and ELBARA-II instruments.