Mixed Layer Variability in the Salinity Processes in the Upper Ocean Regional Study (SPURS) Area
Contact author: Jessica Anderson, <email@example.com>
Jessica Anderson, University of Washington
Stephen Riser, University of Washington
The first Salinity Processes in the Upper Ocean Regional Study (SPURS) I field campaign took place in the approximate center of the north Atlantic surface salinity maximum (~ 25°N, 38°W). During SPURS I 16 Argo-type profiling floats were deployed in a high-density, 4 by 4 grid surrounding the central mooring with approximate 0.5 degree spacing. The floats were programmed to profile from 2000 meters to the surface every 5 days and remained remarkably close to the heavily instrumented central mooring for the duration of the primary study period from October 2012 to October 2013. Mixed layer salinity was at a maximum when the floats were deployed and the depth of the maximum slowly increased through early spring. Multi year observations display variability in both the mixed layer depth and salinity. We present observations of the formation, evolution, and erosion of the mixed layer in time and space.
Global Surface Alkalinity from Aquarius Satellite
Contact author: Rana Fine, <firstname.lastname@example.org>
Rana Fine, RSMAS/University of Miami
Debra Willey, RSMAS/University of Miami
Frank Millero, RSMAS/University of Miami
The unprecedented salinity coverage from the Aquarius satellite (version 3) has provided the opportunity to calculate surface alkalinity globally. In the oceans, total alkalinity (TA) is a gauge on the ability of seawater to neutralize acids. Alkalinity is a major component of seawater, and there is a strong correlation between TA and salinity. In earlier work, the ocean was divided into five regions where an empirical relationship has been used to represent surface TA as a function of SST and SSS. Monthly and annual 2012-14 global surface ocean maps of satellite TA will be presented using the empirical relationship with Aquarius SSS and satellite Reynolds SST. The maps are used to document variability in TA on time scales of months to years. Differences in TA between satellite and ‒ climatology and in situ data are mapped. In addition, satellite TA and in situ pCO2 are used to calculate Total CO2 and pH. Our overall objective is to increase understanding of the spatial and temporal variability of surface ocean alkalinity, which has direct significance for understanding ocean acidification.
Isohaline salinity budget of the North Atlantic salinity maximum
Contact author: Frank Bryan, <email@example.com>
Frank Bryan, NCAR
Scott Bachman, Cambridge University
The choice of control volume influences the processes that dominate budgets of ocean properties. In this study we analyze the salinity budget of the North Atlantic subtropical salinity maximum region for control volumes bounded by isohaline surfaces. We provide closed budgets based on output from a high-resolution numerical simulation, and partial budgets based on climatological analyses of observations. With this choice of control volume, advection is eliminated from the instantaneous volume integrated salt budget, and time mean advection eliminated from the budget evaluated from time-averaged data. In this way, the role of irreversible mixing processes in the maintenance and variability of the salinity maximum are more readily revealed. By carrying out the analysis with near instantaneous and time-filtered model output, the role of mesoscale eddies in stirring and mixing for this region is determined. We find that the small-scale mixing acting on enhanced gradients generated by the mesoscale eddies is approximately equal to that acting on the large-scale gradients estimated from climatological mean conditions. The isohaline salinity budgets can be related to water mass transformation rates associated with surface forcing and mixing processes in a straightforward manner. We find that the surface net evaporation in the North Atlantic salinity maximum region accounts for a transformation of 7 Sv of water into the salinity maximum in the simulation, whereas the estimate based on climatological observations is 9-10 Sv.
Diurnal Sea Surface Salinity Variation Detection in Aquarius Data
Contact author: Effie Fine, <firstname.lastname@example.org>
E. C. Fine, NCAR
F. O. Bryan, NCAR
W. G. Large, NCAR
Small, but measurable, diurnal variations in sea surface salinity (SSS) have been observed at a few select locations with adequate in situ instrumentation. These variations result primarily from imbalances on diurnal timescales between surface freshwater fluxes and vertical mixing of deeper water to the surface. The diurnal variations should be reflected as differences between ascending and descending pass SSS retrievals from the Aquarius satellite. However, the true diurnal signal can be masked by errors in the geophysical corrections (e.g. galactic reflection) used in processing the Aquarius measurements and other spurious sources such as radio frequency interference. In this study we quantify the expected range of diurnal salinity variations on a global scale using an extension of a model developed for predicting diurnal sea surface temperature variations. Based on this guidance, we search for regions where Aquarius may be detecting a true diurnal SSS signal. We also use an independent approach based on the near surface salinity balance and observed precipitation by isolating regions where the surface buoyancy input is such as to suppress vertical mixing. This study should help shed light on the processes contributing to upper ocean salinity variations, as well as provide a better target for assessing remaining errors in the Aquarius processing algorithms.
Basin-scale Exchange of Salt and Fresh Water between the Arabian Sea and the Bay of Bengal during Monsoons
Contact author: Joseph D’Addezio, <email@example.com>
Joseph D’Addezio, The University of South Carolina at Columbia
Subrahmanyam Bulusu, The University of South Carolina at Columbia
Ebenezer Nyadjro, NOAA Pacific Marine Environmental Laboratory
V.S.N Murty, Council of Scientific and Industrial Research (CSIR)
The Northern Indian Ocean (NIO) presents a unique dipolar sea surface salinity (SSS) structure with the salty Arabian Sea (AS) on the west and the fresher Bay of Bengal (BoB) on the east. At the surface, the largest driver of seasonal salinity variability is the monsoonal riverine runoff and winds and their ability to transport volume between the two basins. Aquarius and Argo capture SSS anomalies during each monsoon and provide an observational baseline to assess model performance for a number of different calculations. Using Argo, various reanalyses, and model simulations over the 4 year period from January 2008 to December 2011, the upper 200 m layer salinity structure of this contrasting, yet interconnected, region is quantified. Salt and freshwater fluxes show a strong semi-annual zonal variation between the two basins along (or south of) Sri Lanka twice a year. Regional salt budgets reveal the seasonality of each advection term. The BoB shows the largest seasonal variability in salinity with changes up to ~0.5 psu month-1 during the northeast and southwest monsoons. Meridional depth-integrated salt, freshwater, and volume transports along a slice of each basin at 6°N reveal the advective processes at depths greater than the mixed layer. In the AS, maximum volume transport of ~3 Sv (1 Sv = 106 m3 s-1) occurs towards the north in July when the Somalia current is at its peak. In the BoB, southward volume transport of ~3 Sv occurs during the same period. Neither the southwest nor northeast monsoon currents dominate the transport profile of either basin at this latitudinal cross-section.
Tools, Services and Support of Aquarius/SAC-D Salinity Data Archival and Distribution through PO.DAAC
Contact author: Vardis Tsontos, <firstname.lastname@example.org>
Vardis Tsontos, NASA/Jet Propulsion Laboratory
Jorge Vazquez, NASA/ Jet Propulsion Laboratory
The Physical Oceanography Distributed Active Center (PO.DAAC) serves as the official NASA repository and distribution node for all Aquarius/SAC-D data products in close collaboration with the project. Here we report on the status of Aquarius data holdings at PO.DAAC, the range of data services and access tools that we provide in support of this mission, and new efforts underway to support in-situ salinity datasets from the SPURS field campaign. Particular emphasis is placed on new tools and services that have come online recently for the Aquarius v3.0 dataset release. These range from OPeNDAP and THREDDS data access services, to web-based visualization via PO.DAAC’s State of the Ocean (SOTO) tool and LAS, to PO.DAAC’s new, advanced L2 subsetting tool called HITIDE (High-level Tool for Interactive Data Extraction). Dataset discovery via the PO.DAAC web-portal, and user services are also described. A demonstration of these capabilities could accompany this poster presentation.
A wide-spread freshening during passage of a typhoon captured by Aquarius : a case study
Contact author: Hiroto Abe, <email@example.com>
Hiroto Abe, Hokkaido University
Naoto Ebuchi, Hokkaido University
The present study examined ocean salinity responses to a passage of a typhoon using Aquarius Level-2 swath SSS data and Level-3 weekly-averaged SSS data. We targeted at one of typhoons generated in the east of Philippines in 2012; after the formation, it moved to north-east direction by crossing western North Pacific basin, passed through south of Japan, and went into central North Pacific. Analyses of the Aquarius data were conducted by focusing on a region near Japan.
We picked up three of the Aquarius subtracks at nearly the same locations in order to examine Aquarius SSS 1) before, 2) during, and 3) after the passage of the typhoon. In case 1), typical value of the Aquarius along-track salinity was 34 – 35 psu in this region. In case 2), Aquarius captured a freshened sea water with a value of less than 30 psu, which is quite low compared to the typical value. In case 3), the lower-value SSS was recovered up to the normal level. This SSS freshening signal, found in the Level-2 data, was also detected even in the Level-3 weekly-averaged data. The Level-3 Aquarius SSS averaged over 7 days for a peak phase of the typhoon, was lower by 1 psu than that in a week ahead of the peak phase.
According to the analysis using satellite data from TRMM/TMI, a precipitation event was observed at the location where the negative temporal change of the weekly-averaged SSS was detected. Evaporation minus precipitation flux (or E – P flux), calculated using the data, was -150 mm/week over the negative change area. This fresh water has to be mixed with sea water within an ocean mixed layer with the scale of 5 m, in order to explain 1 psu of SSS freshening.
Evaluation of sea surface salinity observed by Aquarius
Contact author: Hiroto Abe, <firstname.lastname@example.org>
Hiroto Abe, Hokkaido University
Naoto Ebuchi, Hokkaido University
Sea surface salinity (SSS) fields from three different Aquarius products (Aquarius Official Release version 3.0 (V3.0), Combined Active-Passive (CAP) algorithm version 3.0, Remote Sensing Systems (RSS) test bed algorithm version 3) were evaluated by comparing them to in situ SSS measurements from Argo floats and tropical moored buoys, as well as with global gridded SSS fields produced by JAMSTEC and JMA/MRI.
Level-2 Aquarius SSS was collocated with in situ near-surface salinity with spatial and temporal separations of less than 200 km and 12 hours, respectively. In general, the Level-2 Aquarius SSSs showed good agreement with the Argo near-surface salinity. The root-mean-square (rms) difference was 0.42 psu, 0.52 psu, and 0.41 psu for the Aquarius V3.0, CAP, and RSS products, respectively. The same calculation was conducted using the near-surface salinity from tropical moored buoys. The rms difference was 0.35 psu, 0.39 psu, and 0.35 psu for each of the Aquarius products. In any of these cases, the CAP product had the largest rms difference. A significant SSS difference between ascending and descending paths was detected in all of the Aquarius products, although it has been reduced in the V3.0 and CAP V3.0 products.
Level-3 monthly-averaged Aquarius SSS was compared to the outputs from an ocean data optimal interpolation system produced by JAMSTEC and an ocean data assimilation system by JMA/MRI. The monthly-averaged SSSs were collocated at the same longitude-latitude grids with 1 degree spacing at each month. The Level-3 SSS products showed good agreement with the global gridded SSS field. A systematic negative bias was detected in the Asian Pacific in the Aquarius products. The V3.0 was biased positively (negatively) in the high (low) latitude while the CAP product was less biased. The rms difference, calculated for 40°S–40°N, was 0.26 psu for the V3.0. The CAP product showed a lower value of the rms difference, 0.22 psu, which is close to Aquarius mission goal.
CONAE Microwave Radiometer (MWR) Counts to Tb Algorithm and On orbit Validation
Contact author: Linwood Jones, <email@example.com>
Linwood Jones, University of Central Florida
Zoubair Ghazi, UCF
Linwood Jones, UCF
Maria Jacob, CONAE
Andrea Santos-Garcia, UCF
The Central Florida Remote Sensing Lab (CFRSL), in collaboration with CONAE, have developed two improved algorithms to convert the radiometric counts to brightness temperature (Tb) for production of MWR level-1 science data. This poster presents a description of these algorithms and shows results for MWR Cal/Val activities during the past 15 months. Version 6.0 provides corrections to the previous MWR Tb algorithm (V5.0S) that remove a small nonlinearity in the radiometer transfer function. This algorithm is based upon: 1) a reanalysis of pre-launch thermal vacuum radiometric calibration testing to derive the antenna switch matrix loss coefficients, and 2) on orbit inter-satellite cross calibrations (XCAL) with the Naval Research Lab’s WindSat radiometer. Results are presented that demonstrate excellent radiometric stability for a 12 month data set over both oceans and land.
Also a second version (V7.0) of MWR counts to Tb algorithm is presented. This algorithm is essentially V6.0 that has been normalized to WindSat as a radiometric transfer standard. By using the CFRSL XCAL double-difference technique, small radiometric biases between channels have been removed as well as systematic radiometric calibration drift over the entire MWR time series (~ 3 years). Comparisons between V6.0 and V7.0 are presented relative to WindSat.
Comparison of Level-2 Aquarius SSS with Argo Near-surface Salinity
Contact author: Eric Bayler, <Eric.Bayler@noaa.gov>
Li Ren, NOAA/NESDIS/STAR & Cooperative Institute for Climate Studies (University of Maryland)
Eric Bayler, NOAA/NESDIS/STAR
Comparing sea-surface salinity (SSS) observations from the U.S. National Aeronautics and Space Agency’s (NASA) Aquarius mission and in-situ Argo float data at global, zonal and regional scales reveals interesting and unexpected insights on data quality and uncertainty. The Aquarius official data (Aquarius Data Processing System (ADPS)) Version 3.0 and NASA Jet Propulsion Laboratory’s (JET PROPULSION LABORATORY) Combined Active-Passive (CAP) Version 3.0 were compared in this study. For the ADPS data, differences between Aquarius’ ascending and descending nodes are also analyzed. Regional comparisons are used to examine dominant underlying influences, in particular SSS biases/differences resulting from precipitation, sea-surface temperature (SST) and wind speed (surface roughness).
Comparison of Argo and Aquarius sea surface salinity probability distributions
Contact author: Frederick Bingham, <firstname.lastname@example.org>
Elizabeth Mannshardt, North Carolina State University
Katarina Sucic, North Carolina State University
Montserrat Fuentes, North Carolina State University
Frederick Bingham, University of North Carolina Wilmington
Using Argo in situ and Aquarius version 3.0 sea surface salinity, we compare measured salinity across metric distributions. In addition to traditional comparisons between mean or median values, the 1%, 10%, 25%, 50%, 75%, 90% and 99% quantiles of the statistical distributions are compared. The datasets compare well at the median, which is to be expected as the Aquarius retrieval algorithm is based on calibration with Argo central characteristics. The datasets are less similar at the tails of the distributions, especially in the lower tail. In general, Argo data are much peakier, with more of the observations concentrated in the center of the distribution. This is true across seasons, ocean basins and hemispheres. Aquarius is better able to capture the low and high tail values which are most prevalent during periods when the surface salinity is rapidly changing.
To what extent Aquarius surface salinity measurements can detect large-scale, low-frequency climate signals?
Contact author: Tong Lee, <email@example.com>
Tong Lee, Jet Propulsion Laboratory
Existing evaluations of the uncertainty of Aquarius sea surface salinity (SSS) measurements often include the errors both on small scales (e.g., at the Aquarius footprint) and short time scales (e.g., month-to-month time scales) as well as errors on the larger scales and longer time scales. The former errors often overwhelmed the latter ones, making it difficult to assess Aquarius’ ability to detect large-scale signals associated with climate variability. The latter assessment is important to justify the continuation of Aquarius-like measurements to monitor decadal variability and climate change signals and to the planning of NASA’s decadal survey missions. Historical in-situ salinity observations showed that the changes of SSS in the past few decades have typical scales on the order of 1000 km and with a typical magnitude of 0.2 psu over 50 years (e.g., Durack and Wijffels 2010). These changes imply an acceleration of the global water cycle, making fresh regions fresher and salty regions saltier. This presentation will present results of the analysis of uncertainty of Aquarius SSS data on scales of 1000 km with a focus on year-to-year changes currently resolved by Aquarius in order to shed light on the extent to which Aquarius can detect climate variability and change signals like these.
Aquarius radiometers cold sky calibration
Contact author: Emmanuel Dinnat, <firstname.lastname@example.org>
Emmanuel Dinnat, Chapman University/NASA-GSFC
David Le Vine, NASA-GSFC
Aquarius is a NASA instrument comprising three L-band (1.4 GHz) radiometers dedicated to the remote sensing of global sea surface salinity. During the nominal operation, Aquarius’ radiometers are calibrated using an internal hot target. An additional empirical adjustment is performed by comparing measured and simulated antenna temperatures (Ta) over oceans. The simulation utilizes a radiative transfer model that integrates contributions from all directions around the antenna and weights them by the antenna gain patterns. The ocean calibration was used at the beginning of the mission to correct biases of a few Kelvin, and a drift of -1K during the first year of the mission. This resulted in measured Ta being within +/-0.2K of the simulations over oceans.
In order to assess and improve the calibration, Aquarius’ beams are pointed toward the celestial sky about once a month. These cold sky calibration (CSC) maneuvers started seven months into the mission and have been performed continuously since then. The brightness temperature (Tb) of the Sky is relatively well know and very stable in time. The Sky also has very large areas of homogenous Tb, offering flat scenes to Aquarius’ relatively large footprints. The CSC is performed when the spacecraft is above the oceans in order to limit the uncertainties due to land emissivity and radio frequency interferences (more prevalent over land).
During the first year of the mission, the CSC identified a bias of the order of -2K at the low end of the Tb range. A significant source of bias is the uncertainty on the antenna gain pattern. Several versions of Aquarius gain pattern model exist (derived from numerical simulations and measurements on a scale model of the spacecraft) and there is significant disagreement regarding the spillover ratio (SR). The SR is the off-Earth fraction of antenna power when the antenna is in its normal mode pointing toward the Earth. SRs differ by ~1.5% between various models, enough to create a ~1.5K/4K Ta bias over ocean/land. A first step in improving the calibration was to validate the SR as accurately as possible. For that purpose, we use measurements performed when the spacecraft was in the cold sky calibration mode (i.e. inverted) and crossing an ocean/land transition: The very large change in scene temperature makes the uncertainty on the emissivity model negligible when estimating the SR. With the new estimate of SR, the existing gain pattern is adjusted to create a hybrid model for use in the calibration.
The use of the hybrid pattern results in significant improvements in the agreement between measured and simulated TA at the cold end. We will report on the CSC performed between March 2012 and September 2014 and discuss the result regarding the absolute calibration and the stability of the radiometers.
On-orbit Validation of Microwave Radiometer (MWR) Beam-Pointing for the Aquarius/SAC-D Mission
Contact author: Linwood Jones, <email@example.com>
Bradley Clymer, UCF
Catherine May, UCF
Larry Schneidar, UCF
Felipe Madero, CONAE
Martin Labanda, CONAE
Maria Jacob, CONAE
Linwood Jones, UCF
This poster concerns the on-orbit validation of the antenna beam pointing and corresponding instantaneous field of view (IFOV) earth location for the CONAE Microwave Radiometer (MWR). The MWR is a three-channel radiometer operating at 23.8 GHz (H-Pol) and 36.5 GHz (V- and H-Pol), which has two multi-beam parabolic reflector antennas in a pushbroom configuration, with eight beams per frequency (36.5 GHz looking forward and 23.8 GHz looking aft producing 24 simultaneous beams). The scene brightness temperature is dependent upon the earth incidence angle, and MWR retrieval algorithms require good IFOV collocation between channels. Thus, knowledge of the MWR antenna beam footprint geolocation is important to mission success.
As a result, this poster presents results of an on-orbit validation of the MWR antenna beam pointing and comparison with observed and calculated (geometric) MWR IFOV centers. This procedure compares CONAE-calculated IFOV centers at land/water crossings against high-resolution coastline maps. MWR IFOV locations versus time are computed from knowledge of the satellite’s ephemeris, attitude (roll, pitch and yaw) and a priori measurements of antenna gain pattern boresight directions and mounting geometry.
Previous conical scanning microwave radiometer missions (e.g., SSMI and WindSat) have utilized the observation of rapid change in T_B at land/water boundaries to determine the antenna beam-pointing location. In this paper, results of an algorithm to quantify the geolocation error of MWR beam center is presented, based upon two-dimensional convolution between each beam’s gain pattern and land-water transition based upon a 1 km coastline map. The analysis procedures have been applied to on-orbit datasets that represent selected land-water boundaries bearing specific desirable criteria. The goal of this research is to estimate the satellite radiometer beam-pointing error and thereby to improve the geolocation (latitude and longitude) given in the MWR level-1 science data. Examples of colocation comparisons are presented for selected “super sites” with favorable geometry.
RAIN ACCUMULATION (RA) PRODUCT FOR AQUARIUS
Contact author: Maria Jacob, <firstname.lastname@example.org>
Andrea Santos-Garcia, CFRSL
Maria Jacob, CONAE
Linwood Jones, CFRSL
The influence of precipitation on the sea surface salinity (SSS) measurements is an important issue for users of the Aquarius data (AQ L2) as discussed in a companion paper. This poster presents a new ancillary Rain Accumulation (RA) data product developed by the Central FL Remote Sensing Lab that integrates the collocated precipitation with the AQ Level 2 (L2) science dataset. We use the NOAA precipitation product CMORPH (Climate Prediction Center – Morphing Technique) that has a global coverage between ±60° latitude, a spatial resolution of 0.2425° and a sampling window of 30 minutes. In addition to the instantaneous rain rate, this overlay product to the AQ L2 science data provides rain accumulation for the previous 24 hours in time steps of 30 minutes. The spatial integration of the precipitation product over the AQ IFOV is performed using the weighted average based on the antenna beam efficiency. The purpose of this poster is to describe the RA product and to present validation results using independent environmental data records (EDR) WindSat rain retrievals. The WindSat EDR’s have a large number of rain event collocations (< 30 min) with the AQ observation.
Validation of MWR Marine Surface Wind Speed
Contact author: Maria Jacob, <email@example.com>
Carolina Tauro, CONAE
Yazan Hejazin, CFRSL
Maria Jacob, CONAE
Linwood Jones, CFRSL
The CONAE MWR team has developed an improved algorithm for retrieving ocean wind speed using the newest MWR V7.0 brightness temperature data. This poster presents a description of the algorithm and results of a comprehensive on-orbit validation using coincident ocean wind speed retrievals provided by Remote Sensing Systems, Inc (RemSS).
The previous algorithm was based on MWR ocean Tb measurements @ 37 GHz (52° and 58° incidence angles), ancillary Aquarius (GDAS) SST and the microwave radiative transfer theory developed by Wentz, 1992. A linear regression was used to translate the MWR Tb @ 37 GHz V-&H-pol to match WindSat brightness temperature at 53° incidence angle. During the on-orbit Cal/Val period, a simple regression was applied to remove the mean wind speed biases compared to collocated WindSat wind speed retrievals
The new model is also based on the same microwave radiative transfer theory, but now all coefficients have been tuned using one year of MWR brightness temperature at 23 H (H-pol) and 36.5 GHz (H- & V-pol) and auxiliary data, such as sea surface temperature obtained from GDAS (AQ L-2 product) and wind direction from NCEP. Unlike the previous algorithm, this version makes a wind direction correction to the surface Tb’s before retrieving atmospheric transmissivity and the isotropic ocean surface wind speed at 10 m height, and has two different sets of coeffecients for each incidence angle.
For on-orbit validation, results for selected MWR single pass comparisons with independent collocated ocean wind speed retrievals provided by RemSS are presented. Also presented are statistical results of global MWR wind speed averages in different seasons. Results demonstrate significant improvements in the random differences between MWR retreivals and the RemSS surface truth
Comparison of L-band Radio Frequency Interferences from Aquarius and SMOS observations
Contact author: Yan Soldo, <firstname.lastname@example.org>
Yan Soldo, GESTAR-Goddard Space Flight Center
Paolo de Matthaeis, GESTAR-Goddard Space Flight Center
David Le Vine, NASA-Goddard Space Flight Center
Philippe Richaume, CESBIO
The ESA (European Space Agency) SMOS (Soil Moisture and Ocean Salinity) satellite and the NASA (National Aeronautics and Space Administration) Aquarius instrument have been measuring Earth’s natural emissions in the protected part of the L-band (1400-1427 MHz) since January 2010 and August 2011, respectively. SMOS is an interferometric radiometer designed to retrieve surface salinity over oceans and moisture over landmasses, whereas Aquarius is composed by three real-aperture radiometers and its main purpose is the retrieval of sea surface salinity.
Even though the spectral band used by the two instruments has been preserved for passive measurements by ITU (International Telecommunication Union) regulations, both instruments see the quality of their observations degraded by significant Radio Frequency Interference (RFI), especially over land.
In this work, the RFI contamination in both instruments have been compared through the “RFI probability” associated to points on Earth’s surface, i.e. the ratio between the number of times RFI were detected on a certain point and the number of times the same point was observed. The comparison of RFI probabilities from both instruments has been done: 1) on a single, isolated and strong source, and 2) globally, over one month.
In both instruments, the contamination by RFI sources originates in the same way: the energy coming from artificial sources emitting in the protected band reaches the instrument, and the instruments registers brightness temperatures higher than the expected natural thermal noise. But, of course, there are also several differences in the way Aquarius and SMOS are affected by the RFI sources. These differences include: the times of observation of the same sources, the spectral bandwidths used, the antenna directivities, the size of the radiometric pixels, the capability of making multi-angular observations, the acquisition sequences and the approaches used to detect RFI.
Because of these differences, the RFI experienced by both instruments are compared not only through the nominal RFI probabilities, that can be obtained from both instruments independently, but also with a more elaborate approach that aims at making Aquarius and SMOS data as similar as possible, i.e. addressing the differences listed above. In this approach the Aquarius data is projected onto the Discrete Global Grid (DGG) used in SMOS Level-1C and the SMOS field of view is matched to the Aquarius beams.
This comparison shows generally a good agreement between the two sets of RFI probabilities, but also some cases in which the instruments give very different results (Japan and France). It also shows how SMOS suffers because of its wider field of view, while Aquarius suffers because of the wider bandwidth it uses.
Another approach has been to consider only the RFI sources that are always detected by one of the two instruments and to assess the corresponding detections by the other instrument. The results of this approach show that high RFI probabilities in SMOS correspond to high RFI probabilities in Aquarius, though not conversely, because of the higher spatial heterogeneity of SMOS RFI probabilities.
Sea surface salinity variability in the East China Sea observed by the Aquarius instrument
Contact author: Seungbum Kim, <email@example.com>
Jae Hak Lee, KOIST
Paolo de Matthaeis, GSFC
Simon Yueh, Jet Propulsion Laboratory
Chang-su Hong, KOIST
Joon-Ho Lee, Jeju Natl Univ
Gary Lagerloef, ESR
This study demonstrates that the spaceborne Aquarius instrument is able to monitor the sea surface salinity (SSS) variations in the East China Sea (ECS) with the spatial resolution of about 150 km at 7-day interval, where routine observations are difficult. The two geophysical contaminants enter the sidelobes of the Aquarius antenna and bias the coastal SSS low: the emission from the land surface and the radiofrequency interference (RFI). Away from about one Aquarius pixel (150 km) from the coastline, the Aquarius SSS is fairly insensitive (less than about 0.2 psu) to the radiometric details of the method to correct for the land emission. The ascending orbits appear to be affected by unfiltered RFI much less than the descending tracks. The Aquarius SSS along the ascending tracks is low over the ECS by 0.40 to 0.93 psu (with respect to the in situ data during the two separate 7-day periods) and is biased low by 0.41 to 1.07 psu (accuracy, or the time-mean of difference from the regional model along three Aquarius tracks over a 18-month period). The presence of the ascending and descending differences in the Aquarius SSS, and the spatially widespread bias suggest that the bias is attributed to the unfiltered RFI originating from strong point sources (rather than to the land contamination from weak distributed sources, or to other seasonally-varying geophysical contaminants). Despite the bias, the Aquarius data describe well the temporal and spatial variability of the ECS SSS. The temporal trend and magnitude of salinity changes agree remarkably between Aquarius and a regional numerical model, during both the freshwater discharge season from the Yangtze river and the rest of the year. The precision of the Aquarius observation in the ECS is comparable with the Aquarius mission requirement (0.2 psu one-sigma for a monthly average over the open ocean). The river discharge rate correlates with the Aquarius SSS with the coefficient of 0.71 on a seasonal scale with the discharge leading the SSS changes. The Aquarius SSS increases away from the coast, in response to the river outflow. The interannual changes in the Aquarius SSS capture the effect of the regional drought in summer 2013.