-SMOS Level 3 SSS Research products - Product User Manual Reprocessed Year 2010

Transcription

1 CATDS-CECOS-L3-PUDOC Issue 1 Rev 0 - products - Product User Manual Reprocessed Year 2010 Function Name Signature Date Project Manager Nicolas REUL (IFREMER C-ECOS) CATDS -CECOS Team 11 September 2011 Centre Aval de Traitement des Données SMOS (CATDS)- Expertise Center -Ocean Salinity (CEC-OS) Laboratoire d Océanographie Physique et Spatiale Z.I. Pointe du Diable-B. P. 70, Plouzané France Tél. : +33 (0) Fax : +33 (0) nreul@ifremer.fr

2 Products Product User Document Page 2 Document status Title Products -Products User Manual Issue Revision Date Reason for the revision /09/2011 Initial version Modification status Issue Rev Status * Modified pages Reason for the modification * I = Inserted D = deleted M = Modified

3 Page 3 Table of contents 1. INTRODUCTION PRODUCTS CHARACTERISTICS, FORMATS, FILE NAMING CONVENTIONS & DATA CITATION Products general characteristics Product format and content File Naming convention File Name convention for the Monthly Composite products File Name convention for the 10 Day Composite products File Name convention for the Daily running 10 days Composite products Data citation PRODUCT ALGORITHM, VALIDITY, TEMPORAL COVERAGE AND KNOWN FLAWS Product Algorithm and Validity Input Data and temporal coverage Several known flaws Solar contaminations RFI Land Contamination Major Geophysical correction issues: Sky noise Roughness-induced biases along the SEC in the Eastearn Pacific... 15

4 Page 4 1. Introduction This document is the product user manual for the CATDS/CEC-OS SMOS Level 3 SSS research products. 2.1 describes the composite products major characteristics, 2.2 presents the product format, 2.3 describes the file naming convention, 2.4 explains how to aknowledge the use of these data, 3 gives an overview of the products coverage (e.g. input data features, missing output data,..) and describe several known flaws in these datasets.

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6 Page 6 2. Products characteristics, formats, file naming conventions & data citation 2.1 Products general characteristics The CATDS/CEC-OS SMOS Level 3 SSS research products are of three types depending on the temporal window used to generate the Level 3 sea surface salinity composite. The temporal windows we used to generate the products are: monthly composite 10 days composite daily composite based on a running +/-5 days temporal window For each temporal resolution, the composite products are gridded at several spatial resolution varying from 0.25 to 1, depending on the temporal window: Monthly composite products are available at a spatial resolution of: 0.25 x x x days composite products are available at a spatial resolution of: 0.25 x x x 1.00 Daily composite products based on a running +/-5 days temporal window are for the moment only available at 0.25 x 0.25

7 Page Product format and content All products are available in netcdf format complient with version CF1.4. The name, dimension and description of the variables included into all files are given in Table 1. Table 1: Variable Name Dimension and Description Variable Name Dimension Description time 1 Central date of the time period over which the SMOS data were combined to generate the composite product. Number of days since :00:00 latitude nlat Vector of the latitude of the grid nodes over which the composite product is derived. Expressed in degrees North from -90. to +90. longitude nlon Vector of the longitude of the grid nodes over which the composite product is derived. Expressed in degrees East from to sss nlon nlat Gridded Sea Surface Salinity [Practical Salinity Scale]. Missing Values= NaN date_start 1 Start date of the time period over which the SMOS data were considered to generate the composite product date_stop 1 End date of the time period over which the SMOS data were considered to generate the composite product Important Note: as precised in the above table, the dimension of the variable sss is nlon nlat. However the header contents of the netcdf files erroneously indicate a transposed dimension for sss of nlat nlon. This is a bug that shall be corrected in future versions. An example results of the netcdf file content for a 10-day composite product at 0.5 x0.5 resolution is given here below using the "ncdump -h" command under linux: netcdf SSS_SMOS_L3_10day_0.50deg_CATDS_CECOS_ _ _V01 { dimensions: time = 1 ; latitude = 360 ; longitude = 720 ; date_start = 1 ; date_stop = 1 ; variables: double time(time) ; time:time = "central time of sss file" ; time:units = "days since :00:00" ; double latitude(latitude) ; latitude:long_name = "latitude" ; latitude:units = "degree_north" ;

8 Page 8 latitude:valid_min = -90. ; latitude:valid_max = 90. ; double longitude(longitude) ; longitude:long_name = "longitude" ; longitude:units = "degree_east" ; longitude:valid_min = ; longitude:valid_max = 180. ; double sss(latitude, longitude) ; sss:long_name = "Practical Sea Surface Salinity" ; sss:units = "P.S.S." ; sss:_fillvalue = nan ; double date_start(date_start) ; date_start:long_name = "Start Date of the composite Product" ; date_start:units = "days since :00:00" ; double date_stop(date_stop) ; date_stop:long_name = "End Date of the composite Product" ; date_stop:units = "days since :00:00" ; // global attributes: :Conventions = "CF-1.4" ; :title = "SMOS L3 SSS 10 Day Composite" ; :summary = "Composite of CATDS produced Sea Surface Salinity granule data from SMOS satellite on a 0.5 degree resolution grid over global ocean" ; :license = "CATDS-CECOS data use is free and open. Users must register at support@catds.fr" ; :institution = "IFREMER-CNES" ; :project_name = "CATDS C-EC-OS Research products" ; :scientific_support_contact = "Nicolas-Reul (nreul@ifremer.fr)" ; :technical_support_contact = "support@catds.fr" ; :spatial_resolution = "1/2 degrees" ; :temporal_resolution = "10 days" ; :platform = "SMOS" ; :sensor = "MIRAS" ; :product_version = "V01" ; :history = "composite file produced by CATDS/CEC-OS with SSS Chain Prototype V01" ; :cdm_data_type = "grid" ; :southernmost_latitude = "-90." ; :northernmost_latitude = "89.5" ; :easternmost_longitude = "-180." ; :westernmost_longitude = "179.5" ; :geospatial_lat_units = "degrees_north" ; :geospatial_lat_resolution = "0.5" ; :geospatial_lon_units = "degrees_east" ; :geospatial_lon_resolution = "0.5" ; :keywords = "Oceans > Ocean Salinity > Sea Surface Salinity" ; :keywords_vocabulary = "NASA Global Change Master Directory (GCMD) Science Keywords" ; :standard_name_vocabulary = "NASA Global Change Master Directory (GCMD) Science Keywords" ; :acknowledgment = "Please acknowledge the use of these data with the following statement: these data were obtained from the Ocean Salinity Expertise Center (CECOS) of the CNES-IFREMER Centre Aval de Traitemenent des Donnees SMOS (CATDS), at IFREMER, Plouzane (France)." ; :creator_name = "Nicolas Reul" ; :creator_ = "nreul@ifremer.fr" ;

9 Page 9 } :creator_url = " ; :project = "CATDS" ; :publisher_name = "CATDS CECOS" ; :publisher_url = " ; 2.3 File Naming convention File Name convention for the Monthly Composite products For the Monthly composite products the generic filenames are defined as follows: SSS_SMOS_L3_Monthly_xxxdeg_CATDS_CECOS_xxxx.xx _Vxx.nc in which red crosses indicate digital variables. The first variable (3 digits) indicate the spatial resolution: The second (4 digits) and third (2 digits) variables indicate the year and the ordinal month of the year, respectively. Example: for the monthly composite product at 0.25 degree resolution for the month of may 2010:xxxx=2010 and xx=05: SSS_SMOS_L3_Monthly_0.25deg_CATDS_CECOS_ _Vxx.nc The last 2 digits variable indicate the version number of the product processing File Name convention for the 10 Day Composite products For the 10 day composite products, the generic filename is given as defined below:

10 Page 10 Red crosses indicate digital variables defined as follows: The first variable (3 digits) indicate the spatial resolution as defined in The second (4 digits) and third (3 digits) variable indicate the year and the ordinal number of the day in year corresponding to the start date of the time period over which the SMOS data were considered to generate the composite product. The fourth (4 digits) and fifth (3 digits) variable indicate the year and the ordinal number of the day in year corresponding to the end date of the time period over which the SMOS data were considered to generate the composite product. The last variable (2 digits) indicate the product processing version. Example: for the 10-day composite product at 0.5 degree resolution generated for the first 10 days of january 2010 using the processing version 1, the file name is: SSS_SMOS_L3_10day_0.50deg_CATDS_CECOS_ _ _V01.nc File Name convention for the Daily running 10 days Composite products For the daily running 10 day composite products, the generic filename is given as defined below: and the variables follow the same convention than in

11 Page Data citation The following example show how to cite the use of these data sets in a publication. List the data set title (time and space resolution), the producing center, the year of data set release, the version number, and the dates of the data you used (for example, March to June 2010): "These Monthly Composite, quarter degree resolution SMOS SSS data were obtained from the Ocean Salinity Expertise Center (CECOS) of the CNES-IFREMER Centre Aval de Traitemenent des Donnees SMOS (CATDS), at IFREMER, Plouzane (France) V01, [list the dates of the data used]."

12 Page Product Algorithm, Validity, coverage and known flaws 3.1 Product Algorithm and Validity Algorithms used to generate these datasets are described in a dedicated document: the Algorithm Theoretical Breadboard Document. For the date of the first data realease (24 Sep 2011) the ATBD is not fully finalized and it will therefore be provided on the ftp server and catds web site not later than the 15th of october A preliminary assessment of the validity of these products is provided in the Product Validation document, respectively. For the moment, it does only include an in depth validation of the monthly composite products. As for the ATBD, the complete product set validation will be given in an updated validation report around mid-october Both documents shall be downloadable in the Document directory of the CATDS/CECOS ftp server. 3.2 Input Data and coverage The input data used to generate the present version of the CATDS CECOS-research Level 3 products are the ESA reprocessed L1A data for year The products period covers almost the full year. We used both dual polarization mode and full polarisation L1A data as inputs. No data were reprocessed the first ten days of January 2010 as this was just after launch. Therefore the first 10-day Level 3 composite of the year exhibits no SSS data content (file name: SSS_SMOS_L3_10day_0.25deg_CATDS_CECOS_ _ _V01.nc) During the commissioning phase (from January to end of June) the instrument was operated alternatively in both polarisation modes. The number of half orbits per day in each mode that was used as input to generate the L3 products is given in the plot below:

13 Page 13 Figure 1: Number of half orbits data per day in each polarization mode (blue= Full Pol; black=dual pol) that were used as input to generate the L3 SSS products. As can be seen, the Level 3 products were not generated using a uniform type of aquisition throughtout the year and while we have not yet found a direct correlation between Level 3 SSS errors and the change of polarization mode in the data we used as input to our algoritms, it is possible that the data quality is affected by these changes. 3.3 Several known flaws 3.1 Solar contaminations As we found after analyzing the ESA reprocessed L1B data that a large part of the spatiotemporal drifts observed in the operational DPGS SSS Level 2 product could be attributed to an erroneous correction of the direct sun impact at the reconstruction processing level from L1A to L1B. In reprocessing this step, we decided not to apply a "direct sun" correction. Because the direct sun aliases contamines the Extended Field of View of the antenna, in our re-processing, the SSS retrievals were limited to the Alias-free Field of view domain (AF-FOV) of the instrument. Nevertheless, the direct sun aliases are sometimes located at the border of the Alias Free domain, particulary at the end of the year (November to December) in descending passes. Moreover, as a very strong local source, the imaged sun disk and its aliases induce spurious brightness temperature tails after interferometric image reconstruction. These solar contributions that were not corrected for in our processing may have produced some spurious signal in the SSS data at the end of the year. In particular, the sun disk alias propagating on the bottom left border of the AF-FOV may explain the presence of stripe-like too fresh anomalies detected in the composite SSS data that are seen to progressively amplify from october to december. 3.2 RFI SMOS multi-angular brightness temperature measurements at 1.4 GHz are strongly affected by radio frequency interferences (RFI) from radar networks, TV and radio links in what shall be a protected band. These intereferences are numerous over land in Europe and Asia, but can be also encountered in some other areas of Africa, America and Greenland and in some numerous islands over the world. Over the oceans, the signals emanating from land sources can extend very far away from the coasts and have dramatic consequences on the accuracy of sea surface salinity remote sensing in some key oceanic areas like the north atlantic, north pacific and north indian oceans. The signature of RFI in SMOS data is highly variable in time and space and strongly depends on the instrument probing polarization and observation angles. Because of the interferometric principle, local strong RFI signals in the physical space can pollute a very extended area in the Fourier domain of the synthetic antenna and contaminate large portion of the SMOS reconstructed brightness temperature images. In particular, RFI sources located in the aliased regions of the image can impact the data in the (extended) alias-free field of view. Ideally, detection and Mitigation techniques of these spurious signals in SMOS data shall therefore be performed from the raw data at the visibility level prior image reconstruction and shall consider instantaneous acquisitions (snap-shot information) and deal with the whole field of view images. Nevertheless, because of the strong amplitude of the RFI contamination with respect geophysical signal over the ocean, simple detection

14 Page 14 algorithm can be applied to the reconstructed brightness temperature data, identifying samples that deviate anomalously from the average of their neighbors in space, time and probing angles. Collecting acquired data over the full year 2010, we were in a position to re-analyze the spatiotemporal characteristics of these signals and their varying signature as function of the instrument probing configuration (incidence angle, ascending versus descending passes). A global RFI analysis over the world ocean was performed from that data ensemble. The large number of data acquired during the one year time series at a given location on earth allowed us to clearly establish robust threshold detection criteria for these contaminations to best filter out the major contaminations using a multiple criteria mitigation approach. Nevertheless, it is clear that residual RFI-induced structures remain in our products, particularly in the Northern Latitudes, North Indian Ocean, along the coast of Asia and south of Madagascar. RFI density in the Mediteranean sea and Artic Sea induce a low quality retrieval in these area. We advise not to use our data for oceanographic studies in these zones. 3.3 Land Contamination Systematically, SMOS operational and reprocessed Level 2 SSS data for 2010 were found too salty within a ~1500 km wide belt along the world coasts and sea ice edges, with a contamination intensity and spread varying from ascending to descending passes. This spurious signal in the ESA products was found to be present when brighter land or ice masses are located in the extended field of view domain of the antenna. A bug in the weight of the visibility at zero baseline V(0,0) was found to be responsible for most of the very significant land-contamination observed in the operationnal L2 products. Our new SSS products were therefore derived based on the reprocessed L1A data but using a corrected L1B processor in which the V(0,0) bug was corrected. Apparently, the land contamination effect is much reduced in intensity and spread after correcting the bug but too fresh signal are still detected along some of the world coast. 3.4 Major Geophysical correction issues: Sky noise Modeling studies conducted by several teams prior to SMOS launch indicated that the downwelling celestial radiations at L-band that are scattered back by the ocean surface toward the upper hemisphere can be a source of brightness contamination affecting the quality of sea surface salinity retrieval. For sun-synchronous polar-orbiting satellite measurements of upwelling L-band radiation over the ocean, like with SMOS, this so-called sky noise depends strongly on pass direction (ascending or descending), time of year and surface roughness (wind speed). Based upon the modeling studies for SMOS sensor, the impact is expected to be strongest for descending passes in September because for these passes the reflections of the instrument viewing directions over the field of view tend to lie along the galactic equator where L-band galactic emission is maximum. If the contribution to the brightness temperatures measured by SMOS from celestial sky noise is underpredicted, the residual brightness temperatures will be overpredicted and the resulting sea surface salinity will be underpredicted (because brightness temperature increases as surface salinity decreases). SSS retrievals produced by the ESA ground segment (DPGS) thus show along-track strips of low SSS in descending orbits that are aligned with the expected maximum in galactic noise contribution

15 Page 15 to the brightness temperatures. More accurate corrections for these geophysical effects are still under development in the frame of the ESA level 2 processor improvment studies. Erroneous correction of this effect in our alogorithm might have cause some non-geophysical variability in the Level 3 composite products from august to october Roughness-induced biases along the SEC in the Eastearn Pacific The SMOS SSS products from Level 2 to Level 3 all exhibit a too salty signal along the South Equatorial Current (SEC) in the Eastearn Pacific when compared to in situ observations acquired at depths varying from 10 to 1 meter. As illustrated in the Figure 2, this is seen as very clear salty tongue along the equator that spreads up to 180 W and a band of ~±2 along the equator. The signal exhibits a seasonal cycle with minimum effect in the period Januray to April. The source for this apparent flaw is not yet firmly identified but we strongly suspect a bad roughness correction along the current, potentially induced by insufficient and/or inaccurate roughness state auxilliary information (e.g. ECMWF wind speed errors and atmospheric stability effects not well accounted for). Figure 2: the problem of the too salty tongue along the South Equatorial current in the Eastern Pacific. Potentially, this effect can also be associated with some errors in the SST fields (the UK Metoffice OSTIA L4 SST products that are used at ECMWF) along that cold equatorial tongue.