Oceansat-2 Wind Product User Manual
Contents
1. equa equator_crossing_date 2012 0 equator_crossing_time 23 58 rev_orbit_period 5958 6 98 3 r _inclination N A references Oceansat 2 Wind P http www knmi nl scatterometer comment orbi history Orbit period and inc directions in oceanographic conventi creation_date 2012 09 04 creation_time 06 03 42 OSI SAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 399 189 9 03 58 roduct User Manual http www osi saf org lination are constant values All wind on 0 deg flowing North r 212. December Calibration of sigma0 values for the low wind problems is not necessary any more and the calibration is removed OWDP version is 1_0 01 23 Oct 2012 Upgrade of the wind product new MLE normalisations correction for latitude dependent wind speed biases inclusion of outer swath processing and introduction of NSCAT 3 Geophysical Model Function to correct biases at high wind speeds Product status is operational now OWDP version is 1_0_03 23 May 2013 OWDP version 1_1_02 to accommodate the upgrade to ISRO data version 1 4 OSISAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 6 SAF OSI CDOP2 KNMI TEC MA 140 2 The OSCAT scatterometer The scatterometer is one of the three instruments carried on board the Oceansat 2 polar satellite launched and operated by the Indian Space Research Organisation ISRO It was launched on 23 September 2009 A similar instrument is planned to be launched in 2013 on ScatSat 2 1 Instrument description The OSCAT instrument is a conically scanning pencil beam scatterometer as depicted in figure 1 It uses a 1 meter dish antenna rotating at 20 rom with two spot beams of about 25 km x 55 km size on the ground a horizontal polarisation beam HH and a vertical polarisation beam VV at incidence angles of 43 and 49 respectively that sweep the surface in a circular pattern Note that the egg shaped beam footprints are divided into slices by applying a modulated chirp signal Due
3. R and Augenbaum J SeaWinds scatterometer real time BUFR geophysical data product version 2 3 0 NOAA NESDIS June 2000 available on ftp metroweb nesdis noaa gov seawinds bufr_v2 3 0 ps gz Ref 9 Thesis Scatterometry by Ad Stoffelen 1998 Ref 10 Thesis Wind Field Retrieval from Satellite radar systems by Marcos Portabella References marked with a are available on http www knmi nl scatterometer publications 1 4 Useful links KNMI scatterometer web site http www knmi nl scatterometer Information on OSI SAF activities at KNMI http www knmi nl scatterometer osisaf OSCAT visual products http www knmi nl scatterometer oscat_50 prod Information on EUMET Cast http www eumetsat int OSI SAF wind product documentation on http www osi saf org Scientific documents Technical documents NWP SAF website http Awww nwpsaf org ISRO website http www isro gov in 1 5 Limitations of the OSCAT winds The following restrictions and limitations hold 1 Although the calibration of the backscatter values in the level 2a product is already quite good further improvements in the wind calibration and quality control can be achieved using ocean calibration methods that have been developed for ASCAT OSI SAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 5 SAF OSI CDOP2 KNMI TEC MA 140 2 A correction is applied to the backscatter values to correct for the latitude dependent w
4. 4 m s The bottom plots show the u and v wind component statistics bins of 0 5 m s The contour lines are in logarithmic scale Note that the ECMWF winds are real 10m winds whereas the scatterometer winds are equivalent neutral 10m winds which are on average 0 2 m s higher From these results it is clear that the spread in the distributions is small The wind speed bias is 0 16 m s close to the expected value of 0 2 m s and we obtain wind component standard deviations of 1 37 in u and 1 30 in v directions More validation information can be found in Ref 3 OSI SAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 15 SAF OSI CDOP2 KNMI TEC MA 140 2DVAR AR ASCAT BUFR ERS EUMETCast EUMETSAT GMF HDF HH KNMI MSS NASA NetCDF NOAA NSCAT NWP OSCAT OSI SAF OWDP QC QuikSCAT RMDCN SAF SeaWinds SST u V VV WVC OSI SAF 7 Glossary Two dimensional Variational Ambiguity Removal Ambiguity Removal Advanced Scatterometer Binary Universal Format Representation European Remote Sensing Satellite EUMETSAT s Digital Video Broadcast Data Distribution System European Organisation for the Exploitation of Meteorological Satellites Geophysical Model Function Hierarchical Data Format Horizontal polarisation of sending and receiving radar antennas Royal Netherlands Meteorological Institute Multiple Solution Scheme National Aeronautics and Space Administration USA Network Common Data Form National Oce
5. 21117 Sigma 0 Variance Quality Control Numeric OSISAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 18 SAF OSI CDOP2 KNMI TEC MA 140 Number Descriptor Parameter Unit 104 21113 Number of Outer Beam Sigmao aft of sat Numeric 105 05002 Latitude Coarse Accuracy Degree 106 06002 Longitude Coarse Accuracy Degree 107 21118 Attenuation Correction On Sigma 0 dB 108 02112 Radar Look Azimuth Angle Degree 109 02111 Radar Incidence Angle Degree 110 02104 Antenna Polarisation Code Table 111 21105 Normalized Radar Cross Section dB 112 21106 Kp Variance Coefficient Alpha Numeric 113 21107 Kp Variance Coefficient Beta Numeric 114 21114 Kp Variance Coefficient Gamma dB 115 21115 Seawinds Sigma 0 Quality Flag Flag Table 116 21116 Seawinds Sigma 0 Mode Flag Flag Table 117 08018 Seawinds Land Ice Surface Flag Flag Table 118 21117 Sigma 0 Variance Quality Control Numeric OSISAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 19 SAF OSI CDOP2 KNMI TEC MA 140 9 NetCDF data format The OSCAT wind products are also available in the NetCDF format with the following characteristics The data format meets the NetCDF Climate and Forecast Metadata Convention version 1 4 http cf pcmai In gov The data contain contrary to the BUFR data only level 2 wind and sea ice information no sigma0 nor soil moisture information The aim was to create a compact and easy to handle prod
6. Code Table 018 04001 Time to Edge Second 019 05034 Along Track Row Number Numeric 020 06034 Cross Track Cell Number Numeric 021 21109 Seawinds Wind Vector Cell Quality Flag Flag Table 022 11081 Model Wind Direction At 10 M Degree True 023 11082 Model Wind Speed At 10 M m s 024 21101 Number of Vector Ambiguities Numeric 025 21102 Index of Selected Wind Vector Numeric 026 21103 Total Number of Sigma0 Measurements Numeric 027 21120 Seawinds Probability of Rain Numeric 028 21121 Seawinds NOF Rain Index Numeric 029 13055 Intensity Of Precipitation kg m 2 sec 030 21122 Attenuation Correction On Sigma 0 from Tb dB 031 11012 Wind Speed At 10 M m s 032 11052 Formal Uncertainty In Wind Speed m s 033 11011 Wind Direction At 10 M Degree True 034 11053 Formal Uncertainty In Wind Direction Degree True 035 21104 Likelihood Computed for Wind Solution Numeric 036 11012 Wind Speed At 10 M m s 037 11052 Formal Uncertainty In Wind Speed m s 038 11011 Wind Direction At 10 M Degree True 039 11053 Formal Uncertainty In Wind Direction Degree True 040 21104 Likelihood Computed for Wind Solution Numeric 041 11012 Wind Speed At 10 M m s 042 11052 Formal Uncertainty In Wind Speed m s 043 11011 Wind Direction At 10 M Degree True 044 11053 Formal Uncertainty In Wind Direction Degree True 045 21104 Likelihood Computed for Wind Solution Numeric 046 11012 Wind Speed At 10 M m s 047 11052 Formal Uncertaint
7. E E a a a aa 4 My VETS Wes OE E E a T 4 AERA A E E EET E E EATE a A oa 4 de m EEE O E E E E E E dann ele 5 A Useful liN S cosido is 5 5 Limitations of the OSCATWIMOS vestida id 5 6 History of product CHANGES iii dea 6 The OSCAT scatterometer aa de ondas qeda 7 2 1 Instrument descriptio ss dd it 7 2 2 Oceansat 2 versus QUIKSCA Tussoorapia iia 8 Processi g SCNOMIC meinaa a aa haan a a a aa aO AET A An a A 9 3 1 Backscatter slice averaging siii 9 3 2 Backscatter corrections and Calibrati0N oooononncccnnnnnninncnononcccnnnnnncnnnnanannnnnnnnnnnnnnnnos 9 Bae A a a aA a a am 9 3 4 Quality control and MONO Uvssrartraea tii letrada fa 10 Helpdesk product dissemination and archive ccccceceeceeeeeeeeeeceeeeeeeeseeeeenceeeeeeeees 11 Data descripta ends as ase awe EPET 12 5 1 Wind product characteriSticS econsioa id tes 12 52 gt E NT 13 Data Quads eaa a A Ora 15 OSS ANY A O en sete ede nadtna tees 16 BUFR data descriptor S csi dois 17 NelCDR Gata to Tatiana do eto ada 20 OSISAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 3 SAF OSI CDOP2 KNMI TEC MA 140 1 Introduction 1 1 Overview The EUMETSAT Ocean and Sea Ice Satellite Application Facility OSI SAF produces a range of air sea interface products namely wind sea ice characteristics Sea Surface Temperatures SST and radiative fluxes Surface Solar Irradiance SSI and Downward Long wave Irradiance DLI KNMI is involved in the
8. EUMI The IETSAT Network of ais e OSI SAF 2 EUMETSAT DGF Meteorological Institute Facilities mE Ministry of Infrastructure and the Ocean and Sea le Environment Oceansat 2 Wind Product User Manual 1 ew c EUMETSAT KNMI Ocean and Sea Ice SAF OSI SAF 50 km wind product OSI 105 Version 1 3 May 2013 SAF OSI CDOP2 KNMI TEC MA 140 DOCUMENT SIGNATURE TABLE Prepargdby O amp SI SAF Project May 2013 Team Approved by 08 SI SAF Project May 2013 Manager DOCUMENTATION CHANGE RECORD Issue Revision Date Change Description _______ Version 1 0 Dec 2011 Draft version Version 1 1 Jun 2012 Minor editorial changes comments of PCR included Version 1 2 Sep 2012 Version for ORR removed information which is now in ATBD Version 1 3 May 2013 OWDP update for ISRO data v1 4 KNMI De Bilt the Netherlands Reference SAF OSI CDOP2 KNMI TEC MA 140 Cover illustration OSCAT wind field of hurricane Katia retrieved in the western Atlantic at 50 km WVC spacing on 7 September 2011 approximately 4 15 UTC overlaid on a GOES IR satellite image The orange dots are rejected WVCs the purple dots indicate WVCs for which the land flag is set The two orange arrows near the hurricane centre failed the 2DVAR spatial consistency check OSISAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 2 SAF OSI CDOP2 KNMI TEC MA 140 ne oND b bo h b i d Contents Introduci n eiere a E
9. OSI SAF as the centre where the level 1 to level 2 scatterometer wind processing is carried out This document is the Product User Manual to the Oceansat 2 wind product More general information on the OSI SAF project is available on the OSI SAF web site http www osi saf org The user is strongly encouraged to register on this web site in order to receive the service messages and the latest information about the OSI SAF products Information about and status of this product can be found on http Awww knmi nl scatterometer The scatterometer is an instrument that provides information on the wind field near the ocean surface and scatterometry is the knowledge of extracting this information from the instrument s output Space based scatterometry has become of great benefit to meteorology and climate in the past years This is extensively described in the Algorithm Theoretical Baseline Document see Ref 1 KNMI has a long experience in scatterometer processing and is developing generic software for this purpose Processing systems have been developed for the ERS NSCAT SeaWinds ASCAT and Oceansat 2 scatterometers Scatterometer processing software is developed in the EUMETSAT Numerical Weather Prediction Satellite Application Facility NWP SAP whereas wind processing is performed operationally in the Ocean and Sea Ice SAF OSI SAF Oceansat 2 scatterometer OSCAT data are acquired at the Svalbard ground station and sent to India for further
10. SI CDOP2 KNMI TEC MA 140 6 Data quality As introduced in section 5 1 the accuracy should be better than 2 m s in wind component RMS with a bias of less than 0 5 m s in wind speed Collocation result speed 383193 wind vectors Collocation result direction 328663 wind vectors T r T T T T T v a 5A Figure 5 Contoured o E g histograms of the 50 Ss eo 3 km OSCAT wind JN product v 8 T o 1 a T o fi 180F Scatterometer wind speed m s El T Scatterometer wind direction deg a L L J L 0 5 20 25 0 90 360 10 15 180 Model wind speed m s Model wind direction deg Collocation result u 383257 wind vectors Collocation result v 383284 wind vectors component m s component m s Scatterometer u Scatterometer v 20 10 0 10 20 20 10 0 10 Model u component m s Model v component m s Figure 5 shows two dimensional histograms of the retrieved winds versus ECMWF 10m wind background for the 50 km wind product with the backscatter calibration see section 3 2 applied and after rejection of Quality Controlled KNMI QC flagged wind vectors The data for these plots are from 28 consecutive orbits from 9 and 10 February 2012 level 2a data version 1 3 The top left plot corresponds to wind speed bins of 0 5 m s and the top right plot to wind direction bins of 2 5 The latter are computed for ECMWF winds larger than
11. UMCELLS lon long_name longitude lon units degrees_east short wvc_index NUMROWS NUMCELLS wvc_index long_name cross track wind vector cell number wvc_index units 1 short model_speed NUMROWS NUMCELLS model_speed long_name model wind speed at 10 m model_speed units m s 1 short model_dir NUMROWS NUMCELLS model_dir long_name model wind direction at 10 m model_dir units degree OSI SAF ice_prob lon ice_prob uni short ice_age NU short ice_prob NU g_name ice ts ROWS NUMCEL Oceansat 2 Wind Product User Manual version 1 3 May 2013 ROWS NUMCEL 5 probability iS 5 20 SAF OSI CDOP2 KNMI TEC MA 140 ice_age long_nam ice ag ice_age units VaBe ty_flag NUMROWS ty_flag NU int wvc_quali wvc_quali long_name wvc_quality_flag coordinates wvc_quality_flag flag_masks 32768 65536 131072 262144 wvc_quality_flag flag_meanin data_are_redundant no_meteorological rain_flag_not_usable small_wind_less 16384 a parameter MCELLS wind vector cell quality lat lon 64 128 256 512 1024 2048 4096 524288 1048576 2097152 4194304 gs distance_to_gmf_too_large 8192 _background_used rain_detected than_or_equal_to_3_m_s large_wind_greater_than_30_m_s wind inversion_not_successful some_portion_of_wvc_is_over_ice some variational
12. _quality_control_fails kn product_monitoring_event_flag produc any_beam_noise_content_above_thresho not_enough_good_sigma0_for_wind_retr wind speed NUMROWS NUMCEL wind_speed long_name wind snor m s 1 A NUMCELLS wind d degree NUMCI bac wind_speed units t wind_dir NUMROWS wind_dir long_name nor wind_dir units t bs_distance NUMROWS bs_distance long_name ry nor bs_distance units wI global attributes PEEL Oceansat 2 OSCAT title_short_name OSCAT CF 1 4 EUMETSAT OSI SAF Oceansat 2 OSCAT software_identification _level_1 Level L2 50 Conventions institution sourc instrument_calibration_version software_identification_wind 50 pixel_size_on_horizontal N A won service_type processing_type contents ovw oscat_20120904 granule_name _portion_of_wvc_is_over_land mi_quality_control_fails t_monitoring_not_used ld poor_azimuth_diversity ieval LS speed at 10 m irection at 10 m LLS kscatter distance Y Y 2 50 0 km Ocean Surface Wind Vector Product km KNMI 0 1002 0 km 1002 r 002126_ocsat2_15614_0_500_ovw_12 nc processing_level EZT 15614 2012 09 04 00 21 33 2012 09 04 02 00 39 tor_crossing_longitude t_number t_date t_time date istop orbi star star stop time
13. ance Quality Control Numeric 074 21111 Number of Outer Beam Sigma0 fwd of sat Numeric 075 05002 Latitude Coarse Accuracy Degree 076 06002 Longitude Coarse Accuracy Degree 077 21118 Attenuation Correction On Sigma 0 dB 078 02112 Radar Look Azimuth Angle Degree 079 02111 Radar Incidence Angle Degree 080 02104 Antenna Polarisation Code Table 081 21105 Normalized Radar Cross Section dB 082 21106 Kp Variance Coefficient Alpha Numeric 083 21107 Kp Variance Coefficient Beta Numeric 084 21114 Kp Variance Coefficient Gamma dB 085 21115 Seawinds Sigma 0 Quality Flag Flag Table 086 21116 Seawinds Sigma 0 Mode Flag Flag Table 087 08018 Seawinds Land Ice Surface Flag Flag Table 088 21117 Sigma 0 Variance Quality Control Numeric 089 21112 Number of Inner Beam Sigmao aft of sat Numeric 090 05002 Latitude Coarse Accuracy Degree 091 06002 Longitude Coarse Accuracy Degree 092 21118 Attenuation Correction On Sigma 0 dB 093 02112 Radar Look Azimuth Angle Degree 094 02111 Radar Incidence Angle Degree 095 02104 Antenna Polarisation Code Table 096 21105 Normalized Radar Cross Section dB 097 21106 Kp Variance Coefficient Alpha Numeric 098 21107 Kp Variance Coefficient Beta Numeric 099 21114 Kp Variance Coefficient Gamma dB 100 21115 Seawinds Sigma 0 Quality Flag Flag Table 101 21116 Seawinds Sigma 0 Mode Flag Flag Table 102 08018 Seawinds Land Ice Surface Flag Flag Table 103
14. anic and Atmospheric Administration USA NASA Scatterometer Numerical Weather Prediction Scatterometer on board the Oceansat 2 satellite India Ocean and Sea Ice SAF OSCAT Wind Data Processor Quality Control USA dedicated scatterometer mission Regional Meteorological Data Communication Network Satellite Application Facility Scatterometer on board QuikSCAT platform USA Sea Surface Temperature West to east wind component South to north wind component Vertical polarisation of sending and receiving radar antennas Wind Vector Cell Oceansat 2 Wind Product User Manual version 1 3 May 2013 16 SAF OSI CDOP2 KNMI TEC MA 140 8 BUFR data descriptors The BUFR format used for the OSCAT data is identical to the format which is used for SeaWinds data Number Descriptor Parameter Unit 001 01007 Satellite Identifier Code Table 002 01012 Direction of Flight Degree True 003 02048 Satellite Instrument Identifier Code Table 004 21119 Wind Scatterometer GMF Code Table 005 25060 Software Identification Numeric 006 02026 Cross Track Resolution m 007 02027 Along Track Resolution m 008 05040 Orbit Number Numeric 009 04001 Year Year 010 04002 Month Month 011 04003 Day Day 012 04004 Hour Hour 013 04005 Minute Minute 014 04006 Second Second 015 05002 Latitude Coarse Accuracy Degree 016 06002 Longitude Coarse Accuracy Degree 017 08025 Time Difference Qualifier
15. anised in slices see Ref 2 The slices need to be beamwise accumulated to a WVC level before wind inversion can be done The slice weights are proportional to the estimated transmitted power contained in a slice The Sigma0 Quality Flag present in the level 2a data is evaluated and slice data with one of the following flags set are skipped e Bit 4 Sigma0 is poor e Bit 5 K noise value is poor e Bit 6 Invalid footprint e Bit 7 Footprint contains saturated slice 3 2 Backscatter corrections and calibration The Ku band radiation from OSCAT is attenuated by the atmosphere Climatological values of this attenuation were determined as a function of location and time of the year Ref 6 The attenuation is based on a climatology water vapour The attenuation includes atmospheric oxygen water vapour and nominal cloud A mean global cloud cover of 0 1 mm is assumed A table containing the monthly climatological attenuations was kindly provided by NOAA The attenuations are the same that were used for SeaWinds The one way nadir looking values in dB in the table are transformed into an attenuation correction taking the beam incidence angle into account The attenuation correction is added to the beam backscatter value The two way nadir looking values i e without the incidence angle correction are stored in the BUFR output data The backscatter values in the level 2a product can be further calibrated by adding a WVC and beam dependent
16. bias in dB to the incoming os The calibration table can be obtained by fitting the actual measurements to the theoretical GMF More details are provided in Ref 7 Currently this is not yet implemented and a constant beam and WVC independent o correction of 0 65 dB is used yielding an average wind speed bias of 0 2 m s against the ECMWF forecast winds This value is expected since the model winds are real winds and the scatterometer winds are equivalent neutral winds which are know to be 0 2 m s higher on average Ref 1 Note that the calibrated backscatter values are only available within the wind processing software the o data in the wind product are uncorrected values 3 3 NWP collocation KNMI receives NWP model data from ECMWF twice a day through the RMDCN NWP model sea surface temperature SST data are used to provide information about possible ice presence in the WVCs The SST values of the four surrounding model grid points around the WVC location are bilinearly interpolated Note that the ECMWF model data do not contain SST values over land if one or more of the four surrounding grid points has missing SST data the SST value of the grid point closest to the WVC is taken WVCs with a OSI SAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 9 SAF OSI CDOP2 KNMI TEC MA 140 sea surface temperature below 272 16 K 1 0 C are assumed to be covered with ice and no wind information is calculated Although the freezing t
17. ctor at 10 m height obtained using the NSCAT 3 GMF see Ref 1 Ref 3 Units and range Wind speed is measured in m s The wind speed range is from 0 50 m s but wind speeds over 25 m s are generally less reliable Ref 1 In the BUFR products the wind direction is in meteorological WMO convention relative to North O degrees corresponds to a wind flowing to the South with a clockwise increment In the NetCDF products the wind direction is in oceanographic convention O degrees corresponds to a wind flowing to the North with a clockwise increment Input satellite data The generation of OSCAT level 2a data by ISRO is described in their technical documentation Ref 2 The global OSCAT data are acquired in Svalbard Norway and transmitted to India where they are processed up to level 2a with a backup at the EUMETSAT central processing facilities in Darmstadt The product contains geo located slice measurements on a satellite swath WVC grid of 50 km size Geographical definition The Oceansat 2 satellite flies in a near polar sun synchronous orbit at 98 degrees inclination at approximately 720 km orbit height Swath width is 36 50 km size WVCs Products are organised in files containing one orbit Coverage The OSCAT product has a global coverage The actual coverage is available on the OSI SAF product visualisation website see http www knmi nl scatterometer osisaf Output product The input product in HDF5 is processed in
18. e are the same for all WVCs in one BUFR output file If the KNMI QC flag is set in a WVC then the backscatter information is not useable for various geophysical reasons like rain confused sea state etc resulting in a too large inversion residual WVCs in which the KNMI QC flag is set are not used in the calculation of the analysis field in the ambiguity removal step However after the ambiguity removal the wind solution closest to the analysis field is chosen if wind solutions are present in the WVC This means that such a WVC may contain a selected wind solution but it is suspect Land presence flag is set if a land fraction see section 3 3 larger than zero is calculated for the WVC As long as the land fraction is below the limit value a reliable wind solution may however still be present so there is normally no reason to reject WVCs with the land flag set Ice presence flag is set if the SST calculated for the WVC see section 3 3 is below 272 16 1 0 C No winds are computed in this WVC If the variational QC flag is set the wind vector in the WVC is rejected during ambiguity removal due to spatial inconsistency A wind solution is present but it may be suspect It is recommended not to use WVCs with the monitoring flag the KNMI quality control flag or the variational quality control flag set See Ref 1 for more information on product reliability OSI SAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 14 SAF O
19. e the links on http www knmi nl scatterometer The OSI SAF wind product is stored in exactly the same BUFR format as described in the SeaWinds BUFR manual from NOAA Ref 8 a list of descriptors fields contained in each WVC is provided in section 8 Data are organised in files containing approximately one orbit 100 minutes of data The file name convention for the level 2 BUFR product is W_NL KNMI DeBilt SURFACE SATELLITE OCEANSAT2 0SCAT_C_EHDB_YYYYMMDDHHMMSS_ORBIT_T_SMPL_CONT_12 bin YYYYMMDD denotes the acquisition date year month and day of the first data in the file HHMMSS denotes the acquisition time hour minute and second of the first data in the file ORBIT is the orbit number of the first data in the file 00000 99999 T is the processing type o for operational t for test SMPL is the WVC sampling cell spacing always 500 for the 50 km product CONT refers to the product contents always ovw for a product containing Ocean Vector Winds An example of a file name is W_NL KNMI DeBilt SURFACE SATELLITE OCEANSAT2 OSCAT_C_EHDB_20120102101848_12053_0_500_ovw_l2 bin In each node or wind vector cell WVC 118 data descriptors are defined In addition some extra information alterations have been put in place Inthe BUFR header the value for generating centre is set to 99 representing KNMI The products contain up to four ambiguous wind solutions with an index to the selected wind solution Af
20. emperature of sea water is around 1 7 C we keep some margin to prevent any ice contamination in the wind computation Land presence within each WVC is determined by using the land sea mask available from the model data The weighted mean value of the land fractions of all model grid points within 80 km of the WVC centre is calculated The weight of each grid point scales with 1 7 where r is the distance between the WVC centre and the model grid point If this mean land fraction value exceeds a threshold of 0 02 no wind retrieval is performed NWP forecast wind data are necessary in the ambiguity removal step of the processing Wind forecasts are available twice a day 00 and 12 GMT analysis time with forecast time steps of 3h 6h 36h The model wind data are linearly interpolated with respect to time and location and put into the model wind part of the WVC Note that the ECMWF winds stored in the wind products are real winds rather than neutral winds 3 4 Quality control and monitoring In each WVC the o data is checked for quality and completeness and the inversion residual see Ref 1 is checked Degraded WVCs are flagged see section 5 2 for more details An information file is made for each product The content of the file is identical whatever the product and results from a compilation of all the global information concerning this product From these files various graphs are produced to visually display the confidence leve
21. from at least one of the four possible beam view 15 1 combinations are not available Not used 16 0 Missing value All 17 set All 17 set In Fortran if the Wind Vector Cell Quality Flag is stored in an integer I then use BTEST I NDW NB 1 to test BUFR bit NB where NDW 17 is the width in bits of the data element in BUFR The flag indicating that more than two beams contain VV polarised data is active in the outer part of the swath WVCs 1 4 and 33 36 It indicates that outer beam data is used to obtain four independent o values contrary to the middle part of the swath where two beams contain VV outer beam data and two beams contain HH inner beam data In the outer parts of the swath the VV slice data present in the level 2a product are distributed to two WVC beams based on their slice azimuth angle in order to obtain optimal azimuth dispersion This generally results in slightly less optimal wind retrieval users assimilating the data into NWP models may consider to reject WVCs in which this flag is set If the monitoring flag is set to zero the product is monitored If the product is monitored and the monitoring value is set to zero the product is valid otherwise it is rejected by the product monitoring Ref 1 This is based on a statistical check of the number of WVC QC rejections the wind speed bias with respect to the NWP background and the wind vector RMS difference with respect to the NWP background The monitoring flag and valu
22. ind biases which have been reported by several users A negative wind speed bias was found in the southern hemisphere most notably at latitudes south of 50 The latitude dependent correction coefficients have been derived from the orbit height as a function of latitude which can be computed from the orbit information in the OSCAT level 1b files From the orbit height the slant range R is computed and the attenuation corrections can be calculated from this assuming that the received power is proportional to A In this way we obtain latitude dependent wind speed biases that are comparable to those found for QuikSCAT in the past The issue is reported to ISRO 3 The ascending passes are sometimes not complete due to an issue in the near real time data acquisition This leads to reduction of the coverage over the North Atlantic region These restrictions and limitations are subject to further study 1 6 History of product changes Here is an historical overview of the changes in the OSCAT wind products 28 Jun 2011 Oceansat 2 wind products are available in BUFR format to European users with status in development 29 Aug 2011 Improvement of wind retrieval performance at low wind speeds This is done by a calibration of the sigma0 values to correct issues in the backscatter noise subtraction Products are available in NetCDF format as well OWDP version is0_2 00 20 Dec 2011 Changes to prepare for ISRO data version 1 3 implemented on 21
23. ls of the products and their evolution with time These graphs are available in near real time if you click on the Monitoring information link on the product visualisation web pages Data quality is also available to the users within the products see section 5 for a description of quality flags OSI SAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 10 SAF OSI CDOP2 KNMI TEC MA 140 4 Helpdesk product dissemination and archive For a swift response management procedure user requests on the OSI SAF data products should be issued at the Ocean and Sea Ice SAF website http www osi saf org The BUFR products are disseminated on EUMETCast Please consult http Awww eumetsat int under Access to Data for more information on EUMETCast dissemination and how to receive these and other EUMETSAT meteorological satellite products or contact opsOeumetsat int The BUFR and NetCDF products are made available on a password protected ftp site This password is provided to new users by Email request Please send your requests to scat knmi nl A BUFR reader is available at www knmi nl scatterometer bufr_reader KNMI keeps an off line archive of the global products You can send a request to scat knmi nl OSI SAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 11 SAF OSI CDOP2 KNMI TEC MA 140 5 Data description 5 1 Wind product characteristics Physical definition Horizontal equivalent neutral wind ve
24. processing with a backup facility at the EUMETSAT headquarters EUMETSAT then makes available near real time level 2a scatterometer products through EUMETCast These products are used as basis for further processing at KNMI The wind products are distributed in one resolution with 50 km cell spacing The product has a timeliness of approximately 1 1 5 hours from the last sensing time in a product file The OSI SAF products are delivered on request through the KNMI FTP server to the users and through EUMETCast Currently the OSI SAF OSCAT wind product is available only to European non commercial users this is due to limitations in the data exchange agreement between ISRO and EUMETSAT See also http www knmi nl scatterometer for real time graphical examples of the products and up to date information and documentation This user manual outlines user information for the OSI SAF wind OSCAT product OSI 105 Section 2 presents a brief description of the OSCAT instrument and section 3 gives an overview of the data processing configuration Section 4 provides details on how to access the products Detailed information on the file content and format is given in section 5 while in section 6 the product quality is elaborated 1 2 Disclaimer All intellectual property rights of the OSI SAF products belong to EUMETSAT The use of these products is granted to every interested user free of charge If you wish to use these products EUMETSAT s copyright c
25. redit must be shown by displaying the words copyright year EUMETSAT on each of the products used OSI SAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 4 SAF OSI CDOP2 KNMI TEC MA 140 The OSI SAF is much interested in receiving your feedback would appreciate your acknowledgment in using and publishing about the data and like to receive a copy of any publication about the application of the data Your feedback helps us in maintaining the resources for the OSI SAF wind services 1 3 References Ref 1 OSI SAF Algorithm Theoretical Basis Document for the OSI SAF wind products SAF OSI CDOP2 KNMI SCI MA 197 Ref 2 Padia K Oceansat 2 Scatterometer algorithms for sigma 0 processing and products format version 1 1 ISRO April 2010 Ref 3 Verhoef A and A Stoffelen OSCAT winds validation report OSI SAF report SAF OSI CDOP2 KNMI TEC RP 196 2012 Ref 4 Belmonte Rivas M and A Stoffelen New Bayesian algorithm for sea ice detection with QuikSCAT EEE Transactions on Geoscience and Remote Sensing 49 6 1894 1901 2011 Ref 5 Verhoef A and A Stoffelen Quality Control of Ku band scatterometer winds OSI SAF report SAF OSI CDOP2 KNMI TEC RP 194 2012 Ref 6 Wentz F J Climatology of 14 GHz Atmospheric Attenuation Remote Sensing Systems May 20 1996 Ref 7 Verspeek J A Verhoef and A Stoffelen ASCAT NWP Ocean Calibration OSI SAF report 2011 Ref 8 Leidner M Hoffman
26. s OWDP uses HDF5 level 2a backscatter data on slice level these data are averaged to WVC level by OWDP see section 3 1 SeaWinds and OSCAT are comparable in many ways but they are different instruments built of different components Moreover they utilise different incidence angles due to the lower Oceansat 2 orbit 720 km versus 800 km In order to handle these differences well some parts of the processing were re tuned for OSCAT mainly the normalisation of the MLE and the tuning of the Quality Control Ref 5 Furthermore the OSCAT processing utilises the NSCAT 3 Geophysical Model Function GMF rather than NSCAT 2 that was used for QuikSCAT NSCAT 3 has smaller wind speed biases for scatterometer winds versus buoys and NWP winds than NSCAT 2 Ref 1 Ref 3 OSI SAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 8 SAF OSI CDOP2 KNMI TEC MA 140 3 Processing scheme KNMI has a processing chain running in near real time with OSCAT data including visualisation on the internet This processor is based on the NWP SAF software and runs in the KNMI operational environment The processing includes monitoring and archiving functionalities A brief overview of some of the specific processing steps of the OSCAT scatterometer processor is given below General information about the scatterometer wind processing algorithms can be found in the ATBD Ref 1 3 1 Backscatter slice averaging The level 2a backscatter data from ISRO are org
27. ter the wind inversion step we initially store the up to four solutions corresponding to the inversion residual MLE relative minima However subsequently the wind speed and wind direction of the after 2DVAR selected MSS wind solution is put at the index of the selected wind solution This index is set to the initial wind vector solution which is closest to the MSS wind vector selection obtained after 2DVAR Thus the former wind vector is not provided in the product but rather the MSS selected wind vector The Formal Uncertainty in Wind Direction does not contain the uncertainty but the normalised inversion residual referred to as Rn in Ref 10 The Wind Vector Cell Quality Flag table 021109 is redefined and now has the following definitions Description BUFR bit Fortran bit Not enough good sigma 0 available for wind retrieval 1 15 Not used 2 14 VV polarised data in more than two beams 3 13 Monitoring flag 4 12 Monitoring value 5 11 KNMI Quality Control including rain 6 10 Variational QC 7 9 OSI SAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 13 SAF OSI CDOP2 KNMI TEC MA 140 Description BUFR bit Fortran bit Land presence 8 8 Ice presence 9 7 Not used 10 6 Reported wind speed is greater than 30 m s 11 5 Reported wind speed is less than or equal to 3 m s 12 4 Not used 13 3 Rain flag algorithm detects rain 14 2 Data
28. ter works at a Ku band radar wavelength 13 515 GHz The atmosphere is not transparent at these wavelengths and in particular rain is detrimental for wind computation In fact moderate and heavy rain cause bogus wind retrievals of 15 20 m s wind speed which need to be eliminated by a quality control step Wind rain discrimination is easiest to manage in the sweet swath performs acceptable in nadir but is problematic in the outer swath Due to the availability of VV and HH polarisation measurements discrimination of water and ice surfaces is generally well possible as was shown for SeaWinds Ref 4 The SeaWinds ice screening algorithm needs to be adapted for OSCAT in the current OSI SAF product a filter based on NWP Sea Surface Temperature is applied to prevent erratic winds over sea ice surfaces see section 3 3 2 2 Oceansat 2 versus QuikSCAT The processing algorithms for the OSCAT wind processing are heavily based on the algorithms as developed for SeaWinds Ref 1 The wind processing software which was developed for OSCAT the OSCAT Wind Data Processor OWDP shares its main functionality with the SeaWinds Data Processor SDP Processing steps like wind inversion and ambiguity removal are done in the same way and with the same generic code Differences between SDP and OWDP are to a great extent on a technical data handling level SDP uses SeaWinds BUFR data as input which already have backscatter data accumulated to WVC level wherea
29. to a BUFR output product including a unique wind solution chosen its corresponding ambiguous wind solutions and quality information distance to cone quality flag The products are also available in NetCDF format see section 9 for more details Delivery time A wind product is available for distribution within 10 minutes after the input product reception at KNMI The latency between acquisition of the last data in a file and availability for the user is about 1 to 1 5 hour Expected accuracy The expected accuracy is defined as the expected bias and standard deviation of the primary calculations The accuracy is validated against in situ wind measurements from buoys and against NWP data Even better the errors of all NWP model winds in situ data and scatterometer winds are computed in a triple collocation exercise Ref 9 The performance is pretty constant over the globe and depends mainly on the sub footprint wind variability The performance of the products issued by the OSI SAF is characterised by a wind component RMS error smaller than 2 m s and a bias of less than 0 5 m s in wind speed OSISAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 12 SAF OSI CDOP2 KNMI TEC MA 140 5 2 File formats Wind products are in BUFR Edition 4 or in NetCDF format A complete description of BUFR can be found in WMO publication No 306 Manual on Codes The graphical displays of the wind products are available and explained on the web se
30. to the conical scanning a WVC is generally viewed when looking forward fore and a second time when looking aft As such up to four measurement classes called beam here emerge HH fore HH aft VV fore and VV aft in each wind vector cell WVC The 1800 km wide swath covers 90 of the ocean surface in 24 hours and represents a substantial improvement compared to side looking scatterometers like ERS NSCAT and ASCAT Figure 1 OSCAT wind scatterometer geometry source Ref 2 Nadir Track 918 Kms On the other hand the wind retrieval from OSCAT data is not trivial In contrast with the side looking scatterometers the number of measurements and the beam azimuth angles vary with the sub satellite cross track location The wind retrieval skill will therefore depend on the position in the swath A detailed discussion is provided in Ref 1 In the outer swath where only VV beam data are available the two looks result in an ambiguous set of generally four wind solutions with an equal probability of about 25 Measurement noise here results in systematic wind direction errors It was shown in Ref 3 that the outer swath winds have slightly reduced quality but they are still very well usable especially for nowcasting These winds are flagged in the product and can be filtered out easily see section 5 2 OSI SAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 7 SAF OSI CDOP2 KNMI TEC MA 140 The OSCAT scatterome
31. uct for oceanographic and climatological users The data contain only the selected wind solutions no ambiguity information The wind directions are in oceanographic rather than meteorological convention see section 5 1 The format is identical for OSCAT ASCAT QuikSCAT and any other scatterometer data The data has file sizes somewhat smaller than those of the corresponding BUFR data e g one orbit file of 50 km wind data is 2 2 MB in BUFR and 1 MB in NetCDF When compressed with gzip the size of one file in NetCDF reduces to 300 kB The NetCDF data in near real time are only available on the KNMI FTP server but EUMETCast dissemination can be considered on user request The file name convention for the gzipped NetCDF product is oscat_ YYYYMMDD_HHMMSS_ocsat2_ORBIT_T_SMPL_VERS_CONT_l2 nc gz where the meaning of the fields is identical to those in the BUFR file names see section 5 2 The VERS part of the file name denotes the software version A file name example is oscat_20120904 002126 ocsat2_15614 o 500 1002 ovw_l2 nc gz Below are some meta data contained in the NetCDF data files dimensions NUMROWS 816 NUMCELLS 36 variables int time NUMROWS UMCELLS time long_name time time units seconds since 1990 01 01 00 00 00 int lat NUMROWS NUMCELLS lat long_name latitude lat units degrees_north int lon NUMROWS N
32. y In Wind Speed m s 048 11011 Wind Direction At 10 M Degree True OSI SAF Oceansat 2 Wind Product User Manual version 1 3 May 2013 17 SAF OSI CDOP2 KNMI TEC MA 140 Number Descriptor Parameter Unit 049 11053 Formal Uncertainty In Wind Direction Degree True 050 21104 Likelihood Computed for Wind Solution Numeric 051 02104 Antenna Polarisation Code Table 052 08022 Total Number w r t accumulation or average Numeric 053 12063 Brightness Temperature K 054 12065 Standard Deviation Brightness Temperature K 055 02104 Antenna Polarisation Code Table 056 08022 Total Number w r t accumulation or average Numeric 057 12063 Brightness Temperature K 058 12065 Standard Deviation Brightness Temperature K 059 21110 Number of Inner Beam Sigma0 fwd of sat Numeric 060 05002 Latitude Coarse Accuracy Degree 061 06002 Longitude Coarse Accuracy Degree 062 21118 Attenuation Correction On Sigma 0 dB 063 02112 Radar Look Azimuth Angle Degree 064 02111 Radar Incidence Angle Degree 065 02104 Antenna Polarisation Code Table 066 21105 Normalized Radar Cross Section dB 067 21106 Kp Variance Coefficient Alpha Numeric 068 21107 Kp Variance Coefficient Beta Numeric 069 21114 Kp Variance Coefficient Gamma dB 070 21115 Seawinds Sigma 0 Quality Flag Flag Table 071 21116 Seawinds Sigma 0 Mode Flag Flag Table 072 08018 Seawinds Land Ice Surface Flag Flag Table 073 21117 Sigma 0 Vari
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