The Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) instrument onboard SMOS is a Y-shaped antenna with a total of 72 receivers distributed along its three arms and central body. Each receiver captures the thermal radiation in the microwave L-band, more specifically in the protected passive band comprised between 1400 and 1427 MHz. Since the emission within this band is prohibited by the International Telecommunications Union (ITU), no relevant external interferences were expected before SMOS launch (2009). Nevertheless, the real situation is that the Radio Frequency Interferences (RFI) are present in large areas of Europe and Asia leading to low quality measurements. Moreover, due to the MIRAS interferometric processing, RFI sources located far away, even beyond the MIRAS Field of View (FOV), can contaminate large portions of the MIRAS image.
Figure 1. Map of the number of retrieved L2 SSS values during 2012 for ascending passes. White areas have no valid L2 SSS values along 2012.
For a given zone, RFI signals can be classified in terms of the mean life time of the interference as transient emissions or permanent emissions. The former have a limited temporal influence and are mainly produced by mobile sources (for instance ships in open ocean). The latter have a strong effect and may even systematically prevent the retrieval of salinity or soil moisture.
Our Web Map Server service (based on ncWMS and Godiva2 developed by Reading e-Science Centre at the University of Reading) can be used to reveal the spatial distribution of persistent RFI over ocean. The presence of a RFI source reduces the number of valid measures in the zone. Thus, affected zones can be detected by mapping the L2 used measures parameter.
Research vessel Sarmiento de Gamboa
The R/V Sarmiento de Gamboa arrived to Ponta Delgada, Azores, on April 12, and R/V Endeavor is expected to be at Narragansett a few days later. The SPURS spring 2013 cruise is finished and both vessels have achieved the collection of an impressive amount of high resolution oceanographic data, as well as the deployment of several autonomous sampling devices. The SPURS blog (Cruises, SPURS-March 2013) has reported several aspects of the work done. It has been one month of intensive sampling of the high salinity region in the central convergence of the North Atlantic subtropical gyre. The data we have recorded will contribute understanding how this maximum is formed and sustained.
The Spanish in-situ contribution to the international SPURS (Salinity Processes in the Upper ocean Regional Study) experiment is taking place on board the R/V Sarmiento de Gamboa from March 16, 2013. A team of scientists from SMOS-BEC, plus other researchers and technicians from ICM and UTM-CSIC Barcelona, NUI Galway, LOCEAN Paris, LDEO-U. Columbia New York and U. Vigo are performing a wide range of mesoscale and submesoscale measurements to contribute understanding the mechanisms of formation and permanence of the largest ocean salinity maximum in the centre of the North Atlantic subtropical gyre. Several standard and prototype instruments are used in measuring sea surface salinity and other ocean variables.
Underway near-surface salinity along Sarmiento de Gamboa track from the Canary Islands to the SPURS site (figure: O. Hernandez, LOCEAN)
Since March 5th 2013, SMOS-BEC distributes new level 3 and level 4 products derived from level 2 data processed by ESA
The SMOS data used to compute the new level 3 and level 4 Ocean products come from level 2 (L2) Ocean Salinity User Data Product (UDP) and Ocean Salinity Data Analysis Product (DAP). These UDP and DAP files are generated by ESA and include geophysical parameters, a theoretical estimate of their accuracy, and flags and descriptors for the product quality for three different roughness models. These new products developed by BEC are based on the roughness model described in Guimbard et al IEEETrans. Geeosci. Remote Sens. (2012).
The new maps have been created using an improved filtering technique over L2 products. Also the resolution has been modified: current maps are generated at 0.25 degree resolution (rather than 1 degree resolution as for the previous products). The variety of averaging periods has also been increased: three days, nine days (generated every three days), monthly, seasonal, and annual averages (see SMOS-BEC Ocean and Land Products Description for additional information) are now available.
SPURS field experiment. Click on image to enlarge
The SPURS-MIDAS cruise (Las Palmas de Gran Canaria 16 March 2013 – Ponta Delgada, Açores 17 April) on board the Spanish R/V Sarmiento de Gamboa is a contribution to the SPURS experiment (Salinity Processes in the Upper ocean Regional Study) aimed at understanding the processes that drive the upper ocean dynamics and the role that salinity plays on them in the area of maximum salinity in the center of the North Atlantic subtropical gyre. The experiment is coordinated by WHOI (R. Schmitt) and sponsored by NASA (E. Lindstrom), and includes intensive field work with a large variety of state-of-the-art instrumentation, the use of satellite remotely sensed salinity information (Aquarius and SMOS), as well as dedicated numerical modeling. The SMOS BEC team is one of the participants in SPURS where it will contribute with in situ data acquisition, processing and mapping SMOS salinity data, and a regional implementation of the NEMO model with assimilative and process-based simulations to complement and analyze the processes suggested by the observations.
All ocean L4 products distributed by CP34 BEC are obtained by the application of singularity-based fusion. We will discuss this technique in greater detail in this blog when the paper presently under revision is available. So far, it suffices to comment that with this technique a template variable (Sea surface temperature, SST, in our case) of good quality is used to restore the multifractal structure of singularity fronts on a noisy variable (SSS in our case). To know more about the multifractal structure of ocean scalars please consult the 2009 Ocean Science paper.
Sequence of binned L3 SSS maps
The animation above represents the sequence of binned L3 SSS maps; each frame is a 10-day average, which a time lag of three days between the beginning of consecutive averaging periods. This map has a resolution of 1 degree X 1 degree, what is a rather coarse time and space resolution when phenomena like Tropical Instability Waves or the onset of a El Nino are sought. To make things worse, present levels of accuracy on SMOS products make even harder to characterize this large scale phenomena. This is a typical situation in which L4 products can come to rescue!
Since January 14th, 2013, CP34 distributes singularity exponent fields derived from OSTIA SST
For any given ocean scalar (SST, SSS, SSH, Chlorophyll Concentration and even Water Leaving Radiances) singularity exponents can be calculated. Singularity exponents are dimensionless (i.e., no units) measures of the degree of regularity or irregularity of a function at each of its domain points. They extend the concept of Holder exponents, so positive exponents imply that the function is continuous and has a given number of derivatives, while negative exponents imply that the function is irregular and it experiences transitions, jumps and eventually divergences to infinity.
For obtaining singularity exponents we follow the theory explained in Turiel et al., Remote Sensing of Environment (2008) and Turiel et al, Journal of Physics A (2008). The modulus of the gradient of the scalar is evaluated at each point in the domain. The resulting field is projected on a given wavelet at different resolution scales, such that the dependence of the projection on the resolution scale can be assessed by means of a log-log regression, the slope of which is the singularity exponent.
Christmas is surely an appropriate moment to talk about the “El Niño / Southern Oscillation” (ENSO). Today there is no doubt that ENSO is the largest source of inter-annual climate variability at regional and planetary scales. Although its ocean-atmosphere coupled nature was postulated in 1969, the quasi-periodic oceanic and atmospheric anomalous behavior has been observed for centuries. For more than five hundred years, Peruvian fishermen and farmers have been aware that a periodic warm surface counter-current off the Peruvian Coast reduces the anchovy catch, while, at the same time, increased rainfalls transform barren lands onto fertile ones. This counter-current was termed as the current of the “El Niño” (the Child Jesus) because it usually appears around Christmas. On the other hand, several tens of thousands of kilometers to the west, over the Asian continent, other climate events also have a strong impact on society. For example, the failure of monsoons resulted in the Great Drought (1876-1877) that contributed to cause more than seven million deaths in the British-controlled India. Since then, various efforts were made to predict the interannual variability of the Indian monsoons. In 1904 Sir Gilbert Walker was appointed as the director-general of Observatories in India to lead such task. Although Walker was not aware of the El Niño current, he did know about the existence of synchronized interannual pressure fluctuations over the Indian Ocean and eastern tropical Pacific (fluctuations that Walker called the “Southern Oscillation”). His research team evidenced that monsoons are part of a global phenomenon, and that the Southern Oscillation is correlated with major changes in the rainfall patterns and wind vents over the tropical Pacific and Indian Oceans.
New Year, new challenges!
The SMOS BEC team wishes you Merry Christmas and a Happy 2013 full of achievements.
SSS measured by a volunteer yatch participating in the Barcelona World Race.
This is the first post in the new CP34 BEC blog.
The SMOS Barcelona Expert Center (BEC) is an ESA Expert Support Laboratory for the Soil Moisture and Ocean Salinity (SMOS) mission, an innovative Earth Observation satellite devoted to the remote sensing of soil moisture over land and sea surface salinity over the oceans. This is the first time that both variables are measured by a single spaceborne instrument.
The SMOS single payload consists of the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS), a L-band interferometric radiometer. This new instrument allows taking the maximum profit from the capabilities of L-band to infer the geophysical variables of interest, but it is also a very challenging device, requiring sophisticated calibration, correction, pre- and post-processing algorithms. As an ESA ESL, the main commitment of BEC is to develop and test new algorithms to improve the quality of SMOS Level 2 products. However, the goal of BEC is also to generate higher added-value products of interest for a broad range of users. This led to the creation of CP34 as a dedicated production center for SMOS Level 3 and Level 4 products a few years ago. Since the beginning of the mission, CP34 has provided L3 products for sea surface salinity and soil moisture in an operational way.