RapidScat wind Product User Manual
Contents
1. 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 Page 20 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual 10 Appendix B NetCDF data format The wind products are also available in the NetCDF format with the following characteristics e The data format meets the NetCDF Climate and Forecast Metadata Convention version 1 4 http cf pcmdi lin gov e The data contain contrary to the BUFR data only level 2 wind and sea ice information no sigma0 information The aim was to create a compact and easy to handle product for oceanographic and climatological users e The data contain only the selected wind solutions no ambiguity information e The wind directions are in oceanographic rather than meteorological convention see section 5 1 e The format is identical for SeaWinds ASCAT and any other scatterometer data e The data has file sizes somewhat smaller than those of the corresponding BUFR data e g one orbit file of 25 km wind data is 6 5 MB in BUFR and 2 MB in NetCDF When compressed with gzip the size of one file in NetCDF reduces to 0 7 MB e The number of cells per row for the 25 km product may be 41 or 42 depending on the orbit height2. These Surface Flagged Sigma0s and Attenuations in 25 km Swath Grid data are so called egg data each ol is based on the sum of the echo energies measured among the eight centre high resolution slices in a single scatterometer pulse The product contains geo located backscatter measurements on a satellite swath WVC grid of 25 km size Geographical definition The altitude of the International Space Station s orbit is 375 to 435 kilometres and its inclination is 51 6 The observation swath of approximately 1100 kilometres covers the majority of the ocean between 56 north and south latitude in 48 hours The orbit is not sun synchronous Swath width is 42 25 km size WVCs or 21 50 km size WVCs Depending on the ISS orbit height the leftmost and rightmost WVCs may be empty in the 25 km product Products are organised in files containing one orbit starting at the southernmost location of the orbits Output product The input product in HDF is processed into 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 10 for more details 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 error
3. 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 Uncertainty In Wind Speed m s 048 11011 Wind Direction At 10 M Degree True 049 11053 Formal Uncertainty In Wind Direction Degree True 050 21104 Likelihood Computed for Wind Solution Numeric 051 02104 Antenna Polarisation Code Table Page 18 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual Number Descriptor Parameter Unit 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 0
4. International Space Station ISS and the data are acquired at several ground stations The National Aeronautics and Space Administration NASA Jet Propulsion Laboratory JPL collects the data and produces two orbit based 25 km level 2a scatterometer products with different timeliness One product is available approximately 2 hours after the sensing time of the middle of an orbit file and contains approximately 80 of the theoretical data amount The other product is available approximately 3 hours after the sensing time of the middle of an orbit file and contains approximately 98 of the theoretical data amount Hence the user can choose between a timely but less complete product and a less timely but more complete product The level 2a files from the 2 hours and 3 hours data streams are retrieved from JPL by KNMI and processed into 25 km and 50 km level 2 wind products This results in four different available wind products two timeliness s and two resolutions 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 The Product Requirements Document 1 provides an overview of the committed products and their characteristics in the current OSI SAF project phase the Service Specification Document 2 provides specifications
5. TTTTTT a ii Scatterometer v component m s 10 0 Model v component m s Figure 2 Two dimensional histograms of wind speed direction w r t wind coming from the North u and v components of 25 km top and 50 km bottom RapidScat wind product versus the ECMWF model forecast winds from 25 and 26 January 2015 Page 14 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual Figure 2 shows two dimensional histograms of the retrieved winds versus ECMWF 10 m wind background for the 25 km and 50 km wind product after rejection of Quality Controlled KNMI QC flagged wind vectors The top left plot in each panel 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 only ECMWF winds larger than 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 10 m winds whereas the scatterometer winds are equivalent neutral 10 m 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 11 m s for 25 km and 50 km close to the expected value of 0 2 m s The wind component standard deviations are below 1 3 m s for the 25 km product and below 1 2 m s for the 50 km product Much more validation information can be found in 5 Page 15 of 22 SAF OSI CDOP2 KNM
6. and detailed information on the services committed towards the users by the OSI SAF in a given stage of the project KNMI is involved in the 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 RapidScat wind product More general information on the OSI SAF project is available on the OSI SAF web site http Awww 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 More information about this product can also be found on http www 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 3 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 Oceansat 2 and RapidScat scatterometers Scatterometer processing software is developed in the EUMETSAT Numerical Weather Prediction Satellite Application Facility NWP SAF whereas wind processing is performed operationally in the O
7. be found in the Algorithm Theoretical Basis Document ATBD 3 3 1 Backscatter egg averaging The level 2a backscatter data from JPL are organised in eggs each o is based on the sum of the echo energies measured among the eight centre high resolution slices in a single scatterometer pulse The eggs are beam wise accumulated to a WVC level before wind inversion is done The egg weights are proportional to the estimated transmitted power contained in an egg i e inversely proportional to the Kp value The Sigma0 Quality Flag present in the level 2a data is evaluated and egg data with one of the following flags set are skipped e Bit 0 Measurement not usable e Bit 1 Signal to Noise Ratio level low e Bit 3 Data outside acceptable range e Bit 4 Pulse quality unacceptable e Bit 5 Location algorithm does not converge e Bit 6 Frequency shift beyond range e Bit 7 Temperature outside range e Bit 8 No applicable attitude records e Bit 9 Ephemeris data unacceptable 3 2 Backscatter calibration No absolute instrumental instrument calibration exists for Ku band pencil beam scatterometers Ku band pencil beam backscatter distributions should however be matched to achieve wind intercalibration of all space borne scatterometer instruments We thus developed methods that calibrate the winds of each scatterometer effectively to the mean winds at collocated moored buoys No signs of azimuth or WVC dependent instrument biases have been
8. pencil beam scatterometers is very similar The wind processing software which is used the Pencil beam Wind Processor PenWP is the successor of the SeaWinds Data Processor SDP and the OSCAT Wind Data Processor OWDP PenWP is capable to process data from SeaWinds OSCAT RapidScat and HY 2A scatterometers and will eventually replace all former pencil beam Ku band wind processing software packages in the NWP SAF Differences between the various rotating pencil beam scatterometers are to a great extent on a technical data formats and handling level Moreover due to different orbits and antenna geometries incidence angles differ PenWP utilises the NSCAT 4 Geophysical Model Function GMF 3 which is available for all prevailing incidence angles In order to handle instrument differences well particularly noise characteristics some parts of the processing were re tuned for RapidScat mainly the normalisation of the Maximum Likelihood Estimator MLE and the tuning of the Quality Control 4 Page 7 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual 3 Processing scheme KNMI has a processing chain running in near real time with RapidScat data including visualisation on the internet The processing software is developed in the NWP SAF and runs in the KNMI operational environment The processing includes monitoring and archiving functionalities General information about the scatterometer wind processing algorithms can
9. with respect to time and linearly interpolated with respect to location and put into the level 2 information part of each WVC see section 5 2 Note that the ECMWF winds stored in the wind products are real winds rather than equivalent neutral winds 3 4 Quality control and monitoring In each WVC the o data is checked for quality and completeness and the inversion residual 3 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 have been produced to visually display the confidence levels of the products and their evolution with time Any deviations from nominal behaviour would be immediately visible as steps in these graphs Data quality is also available to the users within the products see section 5 for a description of quality flags Page 9 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual 4 Helpdesk and data availability 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 The BUFR products are disseminated on EUMETCast Please consult http www eumetsat int under Access to Data for more information on EUMETCast dissemination and how to receive these and other EUMETSAT meteo
10. 8022 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 Variance 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 Fla
11. CTULMAKS i588 ests bila i ees eae ise NGIGING eas PANG NALNG BINA ha 5 1 5 Limitations and remaining ISSUE Snare ennn a a r E A E EA 5 1 6 History of product changes c csccccccesccccesnseceeeensecenssnnacenssnnaaeensnnaaeensnaaanensnnasennsnsaaeenspnasennen es 5 2 The RapidScat scatterometer cccsecceseeesseeenseeeeeseeesnaesesneeeneeeeseaeseseeesnseeeseeeaeseaesaseeenseaes 6 3 Processing scheme veau NEEN AGAR a 8 3 1 Backscaltter egg averaging anan nnnawaaaannawwaaannaawananNGGO0NRKNG ENGR aaa EEEE SEERA ERR E 8 3 2 Backscatter Calibration wis ssc aaa Apan 8 33 sNWP CONOCATION a oth ser nit arbi eit AN Ga PADALA PAA Apa pba TOS 8 3 4 Quality control and monitoring 2 1ecccceecceeseeceececnneeeesecseneeceuececaaesseaaeceeneecsentesaseseneeseneeecsenetsnaees 9 4 Helpdesk and data availability mma aaa 10 5 Data El In E 11 5 1 Wind product characteristics Sia aa EE EEEE E E A 11 S2 Eer TEE 11 6 CR UU 14 7 Referentes gege eeh Eet ege 16 8 Abbreviations and acronyms nnmnnn nnmnnn nnmnnn nnmnnn 17 9 Appendix A BUFR data descriptors ss ssssusnnennnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn ununun nnnnnn nnmnnn nnmnnn 18 10 Appendix B NetCDF data format cccsceseseesseeeeseeeeeeeeeseaeeeseaesnseeeeeeeeessaeseseaeenseeeeeenaess 21 Page 3 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual 1 Introduction 1 1 Overview RapidScat is mounted on the
12. I TEC MA 227 RapidScat wind Product User Manual 7 References 1 OSI SAF Product Requirements Document SAF OSI CDOP2 M F MGT PL 2 001 2015 2 OSI SAF Service Specification Document SAF OSI CDOP2 M F MGT PL 2 003 2015 3 OSI SAF Algorithm Theoretical Basis Document for the OSI SAF wind products SAF OSI CDOP2 KNMI SCI MA 197 2015 4 Verhoef A and A Stoffelen Quality Control of Ku band scatterometer winds OSI SAF report SAF OSI CDOP2 KNMI TEC RP 194 2012 5 Verhoef A J Vogelzang and A Stoffelen RapidScat wind validation report SAF OSI CDOP2 KNMI TEC RP 228 2015 6 JPL RapidScat mission website http winds jpl nasa gov missions RapidScat 7 JPL QuikSCAT Science Data Product User s Manual version 3 0 Jet Propulsion Laboratory D 18053 Rev A September 2006 8 Leidner M R Hoffman and J Augenbaum SeaWinds scatterometer real time BUFR geophysical data product version 2 2 0 NOAA NESDIS February 2000 available on ftp www scp byu edu data gscat docs bufr pdf 9 Thesis Scatterometry by Ad Stoffelen 1998 10 Thesis Wind Field Retrieval from Satellite radar systems by Marcos Portabella 2002 References marked with a are available on httip www knmi nl scatterometer publications Page 16 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual 8 Abbreviations and acronyms 2DVAR ATBD AR ASCAT BUFR DLI ECMWF ERS EUMETSAT GMF HDF HH I
13. SS JPL KNMI MLE MSS NASA NetCDF NOAA NSCAT NWP OSI SAF OWDP PenWP RMDCN OC QuikSCAT SAF SDP SeaWinds SSI SST u V VV WMO WVC Two dimensional Variational Ambiguity Removal Algorithm Theoretical Basis Document Ambiguity Removal Advanced Scatterometer Binary Universal Format Representation Downward Long wave Irradiance European Centre for Medium Range Weather Forecasts European Remote Sensing Satellite European Organisation for the Exploitation of Meteorological Satellites Geophysical Model Function Hierarchical Data Format Horizontal polarisation of sending and receiving radar antennas International Space Station Jet Propulsion Laboratory Royal Netherlands Meteorological Institute Maximum Likelihood Estimator Multiple Solution Scheme National Aeronautics and Space Administration USA Network Common Data Form National Oceanic and Atmospheric Administration USA NASA Scatterometer Numerical Weather Prediction Ocean and Sea Ice SAF OSCAT Wind Data Processor Pencil beam Wind Processor Regional Meteorological Data Communication Network Quality Control US Quick Scatterometer mission carrying the SeaWinds scatterometer Satellite Application Facility SeaWinds Data Processor Scatterometer on board QuikSCAT platform USA Surface Solar Irradiance Sea Surface Temperature West to east zonal wind component South to north meridional wind component Vertical polarisation of sending and receivin
14. The EUMETSAT Network of is ee OSI SAF amp EUMETSAT 49 Meteorological Institute Facilities ka Ministry of Infrastructure and the Environment ean and Sea Ice Ocean and Sea Ice SAF RapidScat wind Product User Manual pa c EUMETSAT KNMI 25 and 50 km wind products OSI 109 Version 1 1 March 2015 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual DOCUMENT SIGNATURE TABLE Prepared by O amp SI SAF Project Mar 2015 Team Approved by O amp SI SAF Project Mar 2015 Manager DOCUMENTATION CHANGE RECORD esuo Revision oae ee See Version 1 0 Feb 2015 First version Version 1 1 Mar 2015 Comments from ORR included KNMI De Bilt the Netherlands Reference SAF OSI CDOP2 KNMI TEC MA 227 Cover illustration RapidScat wind field over the Atlantic Ocean and western European seas at 25 km Wind Vector Cell WVC spacing on 20 January 2015 approximately 4 00 UTC overlaid on a METEOSAT infrared satellite image at 3 45 UTC The orange dots are rejected WVCs most likely due to rain or confused sea state The purple dots indicate WVCs for which the land flag is set The blue arrows show a 3 hour forecast of the winds by the KNMI High Resolution Limited Area Model HiRLAM Page 2 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual Contents 1 Ge e Te TEE 4 Lashes SCOVOIVIGW seis ea Bana NAG coni adt NA ann GP PA APANG LA a Da 4 Lili ee E e EE 4 L GER Ee 4 Lt MIS
15. cean and Sea Ice SAF OSI SAF The OSI SAF products are delivered on request through the KNMI FTP server and through EUMETCast 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 RapidScat winds on 25 km and 50 km grid spacing OSI 109 Section 2 presents a brief description of the SeaWinds 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 The product quality is elaborated in section 6 and in the validation report to these products 5 1 2 Acknowledgement JPL kindly provides the near real time RapidScat level 2a data which are used as input for the OSI SAF wind products 1 3 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 credit must be shown by displaying the words copyright year EUMETSAT on each of the products used Page 4 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual 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
16. ed 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 sea surface temperature below 1 0 C are assumed to be covered with ice and no wind information is calculated Although the freezing Page 8 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual temperature of sea water is around 1 7 we keep some margin to prevent any ice contamination in the wind computation Note that due to the ISS orbit data will not be acquired close to the poles However frozen seas will also occur at lower latitudes in the winter season e g near Newfoundland and east of Russia 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 50 km 60 km in the 50 km products of the WVC centre is calculated The weight of each grid point scales with 1 r2 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 quadratically interpolated
17. erefore depend on the position in the swath A detailed discussion is provided in 3 Here we only summarise some issues specific to RapidScat In the outer swath where only VV beam data are available the individual backscatter measurements eggs contributing to the VV fore or aft beam in a specific WVC are re distributed to form four more or less independent backscatter observations Eggs are accumulated and averaged based on their azimuth angles The outer swath winds have slightly reduced quality but they are still very well usable These winds are flagged in the product and can be filtered out easily if requested see section 5 2 Page 6 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual The RapidScat scatterometer operates at a Ku band radar wavelength 13 5 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 QC step Wind rain discrimination is easiest to manage in the sweet swath but still performs acceptably in nadir and outer swath The processing algorithms for the RapidScat wind processing are heavily based on the algorithms as developed for SeaWinds on QuikSCAT and OSCAT on Oceansat 2 3 3 When calibrated geophysical backscatter measurements are available the wind processing of the different Ku band
18. evelopment version of RapidScat global wind product PenWP version is 19 01 20 Jan 2015 Change in backscatter corrections resulting in small wind speed reduction quality control improvement PenWP version is 1_9 02 Page 5 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual 2 The RapidScat scatterometer The RapidScat scatterometer instrument is a speedy and cost effective replacement for the NASA QuikSCAT satellite which provided a decade long ocean vector wind observations used in a o oceanography Numerical Weather Prediction NWP and nowcasting QuikSCAT s measurements were essential and when the satellite stopped collecting continuous swath based wind data in late 2009 NASA was challenged to quickly and cost effectively conceive of a replacement NASA s Jet Propulsion Laboratory and the agency s station program came up with a solution that uses the framework of the International Space Station ISS and reuses hardware originally built to test parts of QuikSCAT to create an instrument for a fraction of the cost and time it would take to build and launch a new Satellite RapidScat was launched on 20 September 2014 and mounted on the International Space Station The altitude of the space station s orbit is 375 to 435 kilometres and its inclination is 51 6 The observation swath of approximately 1100 kilometres covers the majority of the oceans between 56 north and south latitude in 48 hours The orbit which i
19. found for RapidScat Also the beam incidence angles are constant and hence we have chosen to apply backscatter corrections that are only dependent on the beam polarisation The goal of applying backscatter corrections was to minimise wind speed biases between scatterometer winds on the one hand and buoy and NWP winds on the other hand With this in mind the calibration correction for VV has been obtained by looking at the outer swath data where no HH data are available and choosing a calibration amount that yields minimum wind speed biases Subsequently the swath part containing both HH and VV was considered the VV correction was applied and a HH correction was obtained that again yields minimum biases In this simple way within a few iterations the two calibration coefficients could be obtained e For HH 0 45 dB has been added in the 25 km products and 0 38 dB in the 50 km products e For VV 0 45 dB has been added in the 25 km products and 0 38 dB in the 50 km products Note that the calibrated backscatter values are only available within the wind processing software the o data in the BUFR 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 bi linearly interpolat
20. g 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 Sigma0 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 21117 Sigma 0 Variance Quality Control Numeric 104 21113 Number of Outer Beam Sigma0 aft of sat Numeric 105 05002 Latitude Coarse Accuracy Degree 106 06002 Longitude Coarse Accuracy Degree Page 19 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual Number Descriptor Parameter Unit 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
21. g radar antennas World Meteorological Organisation Wind Vector Cell Page 17 of 22 SAF OSI CDOP2 KNMI TEC MA 227 9 Appendix A BUFR data descriptors RapidScat wind Product User Manual 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 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
22. idScat software identification level 1 1902 instrument calibration version 0 software identification wind 1902 pixel size on horizontal 25 0 km service type N A processing type O contents ovw granule_name rapid_20150121_053025_iss 01860 o 250 1902 ovw 12 nc processing level L2 orbit number 1860 start date 2015 01 21 start time 05 30 25 stop date 2015 01 21 stop time 07 02 59 equator crossing longitude 272 834 equator crossing date 2015 01 21 equator crossing time 04 20 49 rev orbit period 5567 1 orbit inclination 51 6 history N A references RapidScat Wind Product User Manual http www osi saf org http www knmi nl scatterometer comment Orbit period and inclination are constant values All wind directions in oceanographic convention 0 deg flowing North creation date 2015 01 21 creation time 08 50 39 Page 22 of 22
23. lity_flag NUMROWS NUMCELLS 7 Page 21 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual wvc_quality_flag long_name wind vector cell quality wvc_quality_flag flag_masks 64 128 256 512 1024 2048 4096 8192 16384 32768 65536 131072 262144 524288 1048576 2097152 4194304 wvc_quality_flag flag_meanings distance_to_gmf_too_large data_are_redundant no_meteorological_background_used rain_detected rain_flag_not_usable small_wind_less_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_portion_of_wvc_is_over_land variational_quality_control_fails knmi_quality_control_fails product_monitoring_event_flag product_monitoring_not_used any_beam_noise_content_above_threshold poor_azimuth_diversity not_enough_good_sigma0_for_wind_retrieval short wind_speed NUMROWS NUMCELLS wind_speed long_name wind speed at 10 m wind speed units m s 1 short wind dir NUMROWS NUMCELLS 7 wind dir long name wind direction at 10 m wind dir units degree short bs distance NUMROWS NUMCELLS bs distance long name backscatter distance bs distance units 1 global attributes title ISS RapidScat Level 2 25 0 km Ocean Surface Wind Vector Product title short name RSCAT L2 25km Conventions CF 1 4 institution EUMETSAT OSI SAF KNMI source ISS Rap
24. n BUFR bit Fortran bit Data 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 BUFR bit 3 is active in the outer part of the swath WVCs 1 5 and 38 42 at 25 km WVCs 1 2 and 20 21 at 50 km 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 backscatter data present in the level 2a product are distributed to two WVC beams based on their azimuth angle such that maximum azimuth dispersion is obtained This generally results in slightly less optimal wind retrieval users assimilating the data into NWP models may consider to reject WVCs for which this flag is set If the product monitoring not used bit is set to zero the product is monitored If the product is monitored and the product monitoring flag bit is set to zero the product is valid otherwise it is rejected by the product monitoring 3 This is based on a statistical check of the number of WVC QC rejections the wind speed bias with respect t
25. o the NWP background and the wind vector RMS difference with respect to the NWP background The product monitoring bits have the same value 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 The 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 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 KNMI quality control flag or the variational quality control flag set See 3 for more information on produc
26. of any publication about the application of the data Your feedback helps us in maintaining the resources for the OSI SAF wind services 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 OSI SAF wind product documentation on http www osi saf org NWP SAF website http nwpsaf eu RapidScat visual products http www knmi nl scatterometer rscat_nrt_ 25 prod 25 km 2 hours http www knmi nl scatterometer rscat_del 25 prod 25 km 3 hours http www knmi nl scatterometer rscat_del 50 prod 50 km 3 hours EUMETSAT Data Centre http www eumetsat int website home Data DataDelivery EUMET SAT DataCentre index html RapidScat information on JPL web site http www jpl nasa gov missions iss rapidscat 1 5 Limitations and remaining issues 1 There are gaps in the data due to interruptions in data transmission from the International Space Station These gaps appear in particular over the Indian Ocean 2 Entire orbits of data are also missing periodically due to events such as vehicle dockings or astronaut cosmonaut space walks that require the RapidScat to stop operations An overview of past and planned events can be found under RapidScat scheduled outages in the web product viewers 1 6 History of product changes Here is an historical overview of the changes in the RapidScat wind products 08 Dec 2014 First d
27. optional field is the service it is missing for the 3 hours product and set to 2hr for the 2 hours product T is the processing type o for operational t for test SMPL is the WVC sampling cell spacing 250 for the 25 km and 500 for the 50 km product CONT refers to the product contents always ovw for a product containing Ocean Vector Winds Examples of file names are rapid_20150120_031543_iss 01843 o 250 ovw 2 bufr for a 25 km 3 hours product rapid 20150120 031543 iss 01843 2hr o 500 oww 2 bufr for a 50 km 2 hours product The file names on EUMETCast are different from those on the FTP server and according to the WMO conventions W NL KNMI DeBilt SURFACE SATELLITE ISS RAPIDSCAT EHDB YYYYMMDDHHMMSS ORBIT SRV T SMPL CONT bn The meaning of the acronyms in the file names is the same as for the files on FTP Example file names are W_NL KNMI DeBilt SURFACE SATELLITE ISS RAPIDSCAT_C_EHDB_20150120031543 01843 o 250 ovw I2 bin W_NL KNMI DeBilt SURFACE SATELLITE ISS RAPIDSCAT_C_EHDB_20150120031543 01843 2hr o 500 oww 12 bin File contents In each node or wind vector cell WVC 118 data descriptors are defined In addition some extra information alterations have been put in place In the 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 After the wind inversion step we initially s
28. rological satellite products or contact ops eumetsat int The BUFR and NetCDF products are also made available on a password protected ftp site data from the last three days only The access details are provided to new users by email request Please send your requests to scat knmi nl The four products can be distinguished w r t timeliness and product swath grid spacing by their file names see section 5 2 A BUFR reader is available at www knmi nl scatterometer bufr_reader ee ee EE eer also keeps an off line archive of the products You can send a request to er rat Page 10 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual 5 Data description 5 1 Wind product characteristics Physical definition Horizontal equivalent neutral wind vector at 10 m height obtained using the NSCAT 4 GMF see 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 3 In the BUFR products the wind direction is in meteorological World Meteorological Organisation WMO convention relative to North 0 degrees corresponds to a wind flowing to the South with a clockwise increment In the NetCDF products the wind direction is in oceanographic convention 0 degrees corresponds to a wind flowing to the North with a clockwise increment Input satellite data The RapidScat level 2a input data 7 are kindly provided by NASA s JPL
29. s not sun synchronous gives different overpass times for each day and as such allows cross calibration with other instruments on polar satellites like ASCAT and HY 2A For detailed information on the RapidScat instrument and data we refer to 6 and 7 A brief description is given below The RapidScat instrument is a conically scanning pencil beam scatterometer It uses a 0 75 meter dish antenna rotating at 18 rom with two spot beams on the ground a horizontal polarisation beam HH and a vertical polarisation beam VV at incidence angles of 49 and 56 respectively The beams sweep the surface in a circular pattern as depicted in Figure 1 The ground swath of the HH beam is approximately 900 km wide and the ground swath of the VV beam is approximately 1100 km wide Due to the conical scanning a Wind Vector Cell 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 WVC RapidScat ob track surface Be i normal Ur i nadir track incidence P angle 7 angle Figure 1 RapidScat pencil beam geometry source JPL web site The wind retrieval from RapidScat data is not trivial In contrast with side looking scatterometers like ASCAT the number of measurements and the beam azimuth angles vary with the sub satellite cross track location The wind retrieval skill will th
30. s of all NWP model winds in situ data and scatterometer winds are computed in a triple collocation exercise 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 standard deviation smaller than 2 m s and a bias of less than 0 5 m s in wind speed More validation information is available in 5 showing that the actual products are much more accurate 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 OSI SAF wind product is stored in exactly the same BUFR format as described in the SeaWinds BUFR manual from NOAA 8 a list of descriptors fields contained in each WVC is provided in section 9 Data are organised in files containing approximately one orbit 93 minutes of data File name conventions The file name convention for the level 2 BUFR product on the KNMI FTP server is rapid YYYYMMDD HHMMSS iss ORBIT SRV T SMPL CONT 12 bufr e YYYYMMDD denotes the acquisition date year month and day of the first data in the file e HHMMSS denotes the acquisition time hour minute and second of the first data in the file Page 11 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual ORBIT is the orbit number of the first data in the file 00000 99999 SRV
31. t reliability Page 13 of 22 SAF OSI CDOP2 KNMI TEC MA 227 6 Data quality RapidScat wind Product User Manual As stated in the OSI SAF product requirements 1 the accuracy should be better than 2 m s in wind component standard deviation with a bias of less than 0 5 m s in wind speed Collocation result speed 1067680 wind vectors Scatterometer wind speed m s 10 15 20 25 Model wind speed m s Collocation result u 1067691 wind vectors TT TT TT Tr rr Try TT Scatterometer u component m s 20 10 0 10 20 Model u component m s Collocation result speed 279148 wind vectors 25 Scatterometer wind speed m s 5 a 8 a 10 15 20 25 Model wind speed m s Collocation result u 279151 wind vectors WR RRE SEENEN 20f tok Scatterometer u component m s o 0 Model u component m s Collocation result direction 910680 wind vectors Scatterometer wind direction deg 0 90 180 270 360 Model wind direction deg Collocation result v 1067686 wind vectors T TT Terpo TTTT YET EI o TITI a EI O AAABABABAN LABABABANI EA Scatterometer v component m s 20 10 0 10 20 Model v component m s Collocation result direction 238464 wind vectors Scatterometer wind direction deg 0 90 270 360 180 Model wind direction deg Collocation result v 279150 wind vectors TTTTTTTIT TT
32. tore the up to four solutions corresponding to the inversion residual Maximum Likelihood Estimator MLE relative minima However subsequently the wind speed and wind direction of the after 2DVAR selected Multiple Solution Scheme 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 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 Product monitoring not used 4 12 Product monitoring flag 5 11 KNMI Quality Control including rain data rejection 6 10 Variational QC data rejection 7 9 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 Page 12 of 22 SAF OSI CDOP2 KNMI TEC MA 227 RapidScat wind Product User Manual Descriptio
33. which is variable for the space station The file name convention for the gzipped NetCDF product is rapid YYYYMMDD HHMMSS iss ORBIT T SMPL VERS CONT 12 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 PenWP software version A file name example is rapid 20150121 053025 iss 01860 o 250 1902 ovw I2 nc gz Below are some meta data contained in the NetCDF data files dimensions NUMROWS 1624 NUMCELLS 41 variables int time NUMROWS NUMCELLS 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 NUMCELLS 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 short ice_prob NUMROWS NUMCELLS ice_prob long_name ice probability ice_prob units 1 short ice age NUMROWS NUMCELLS ice age long name ice age a parameter ice age units dB int wvc_qua
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