ERS Scatterometer Product User Manual
Contents
1. Ambiguity removal iia dls 10 3 2 2 Quality control ines 10 4y Process Chemen rd ii 11 4 1 Node triplet completiON occcccccnccccncconiconinnnncnnncnnnnnninnnnnnnnnnnnnnnnnnnnnnnnnnnnnneninins 11 424 NWP COlOCAON vic ida 11 AS Validat O a Aa E A EEE E E 11 4 4 Quality control and monitoring icosiocmmitrs criteria deca 12 5 Dissemination neea aare a ae a e a a betas ieee 13 63 Data descripta sa sashes 14 6 1 Wind product 053 30161131168 14 6 2 7 A bol oe tease ee eee 15 T Data quals ot ae ad end 17 71 ACC sis 17 7 2 Reliability and data 086 tic ai O taria tadas 17 7 97 Ambiguity Select tn diles 17 A A A dinates sr A A Aa 18 EARS ERS Product User Manual version 1 6 November 2008 2 KNMI publicatie 212f Preface The distribution of ERS 2 scatterometer winds in the framework of the EARS EUMETSAT Advance Retransmission Service has only been made possible through a concerted effort The ERS 2 scatterometer data is provided by ESA to KNMI in the framework of the Cat 1 project Nr 1105 of 2001 KNMI is considered Principal Investigator in2. insufficient to provide good quality ERS scatterometer winds at WVCs 1 and 2 we recommend to accept these winds with degraded confidence For use in NWP these winds should be disregarded altogether WVCs 3 19 are of uniform quality and fulfil the requirements as specified in the OSI SAF User Requirements Document 7 3 Ambiguity selection A version of 2D VAR is used with minimal regional performance differences Ref 10 This improved version was obtained after taking into account the findings of Ref 8 A variational QC step is performed to reject a few WVCs which are in meteorological unbalance with their neighbours This rejection rate often called gross error rate peaks at the inside swath and falls off quickly in the middle swath The variational QC flagged WVCs are flagged in the output product EARS ERS Product User Manual version 1 6 November 2008 17 KNMI publicatie 212f 8 Glossary Ambiguity Removal Advanced Scatterometer Binary Universal Format Representation EUMETSAT Advanced Retransmission Service Eumetsat Polar System European Remote Sensing Satellite EUMETSAT s Digital Video Broadcast Data Distribution System European Organisation for the Exploitation of Meteorological Satellites Hierarchical Data Format Jet Propulsion Laboratory NASA Royal Netherlands Meteorological Institute Meteorological Operational satellite National US Air and Space Agency National US Oceanic and Atmospheric Administration
3. scatt reports pcs_cyclic The timeliness of the arrival of input data from different stations ranges from 20 to 100 min depending on the link between the stations and the Rome node at ESRIN The processing at KNMI includes a step to remove product overlaps between different stations It is based on BUFR segments which for ERS SCAT contain 19 rows each row containing in turn 19 nodes This is equivalent to a 500 km x 500 km swath and approximately 1 5 minutes Each node is processed to generate winds only once as soon as the triplet of fore mid and aft beam information is complete A triplet can be completed with data coming from more than one station If input data for that node is received after the wind node has been generated i e duplicated input data from another station it is discarded Node grids from different stations are not synchronised If a triplet is completed with o s from more than one station the node position is fixed to the latest received product and the node is filled with the closest o values available from all the stations and located within a distance of half a cell No interpolation is performed because tests at KNMI have shown that the improvement in the wind quality is not significant 4 2 NWP collocation KNMI receives NWP model data from ECMWF twice a day through the RMDCN NWP model sea surface temperature and land sea mask data are used to provide information about possible ice or land presence in the WVC
4. this project and registered users of the KNMI wind product are considered as Co Principal Investigators their names will be provided to ESA The KNMI wind processing and dissemination of the ERS 2 scatterometer winds on EUMETCast EUMETSAT s Digital Video Broadcast Data Distribution System is funded by EUMETSAT The processor software was developed in a coordinated effort between EUMETSAT ESA and KNMI EARS ERS Product User Manual version 1 6 November 2008 3 KNMI publicatie 212f 1 Introduction 1 1 Overview The EUMETSAT Advanced Retransmission Service EARS was established to provide the meteorological community with regional data with a timeliness adequate for nowcasting and regional numerical weather prediction Currently ATOVS and AVHRR data from NOAA satellites are collected by several HRPT reception stations processed and distributed on EUMETCast After the MetOp launch products generated from HRPT data from this satellite will be also disseminated In particular the EARS will be complemented with a Pilot ASCAT service The targeted EARS ASCAT product is a regional coverage Level 2 wind product on the ASCAT swath grid in BUFR format for distribution on EUMETCast The targeted timeliness is 30 minutes from data acquisition KNMI will be involved in the EARS ASCAT service as the centre where the Level 1b to Level 2 processing will be carried out As a preparation for the EARS ASCAT service KNMI has established and op
5. when the satellite flies in sight of a ground station ESA deploys a number of ground stations mainly in the North Atlantic Region KNMI combines data from all ground stations and produces one unique data set without overlap between different ground stations In case of overlap data from different ground stations are combined to obtain a result as complete as possible Hence contrary to the ESA product the KNMI wind product offers completeness with best timeliness and no redundant data Moreover the KNMI additional quality control ensures that only reliable data will be made available The KNMI products are delivered on request through the KNMI FTP server to all users and through the EUMETCast system 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 KNMI products based on the ERS scatterometer Section 2 presents a brief description of the ERS SCAT instrument section 3 the processing algorithms and section 4 gives an overview of the data processing configuration Section 5 provides details on how to access the products Detailed information on the file content and format is given in section 6 while in section 7 the product quality is elaborated EARS ERS Product User Manual version 1 6 November 2008 4 KNMI publicatie 212f 1 2 Disclaimer All intellectual property rights of the EARS products bel
6. 6 November 2008 11 KNMI publicatie 212f initialisation with a one day lead NWP forecast and validation of the ambiguity selection against NWP analyses as in Ref 8 Moreover de Vries et al Ref 10 describe subjective comparison of the 2D VAR and PreScat schemes by routine operational meteorologists 4 4 Quality control and monitoring In each WVC the o data is checked for quality and completeness and the inversion residual see section 3 2 2 is checked Degraded WVCs are flagged see section 6 2 for more details The quality of the delivered products is controlled through an ad hoc visual examination of the graphical products and the automatic production of control parameters The examination of the products is done at KNMI by experts Specific tools have been developed to help this analysis User queries obviously lead to the inspection of suspect products The ad hoc and user queried inspections are used for quality assurance 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 levels of the products and their evolution with time These graphs are available on the KNMI website Data quality is also available to the users within the products see section 6 and 7 for a description of quality flags EARS ERS Product
7. ES 2 EUMETSAT KNMI ERS Scatterometer Product User Manual EUMETSAT Advanced Retransmission Service Version 1 6 November 2008 KNMI publicatie 212f DOCUMENT SIGNATURE TABLE Prepared by KNMI EARS Team November 2008 Oo DOCUMENTATION CHANGE RECORD Issue Revision Date Change Description _ Draft version EUMETSAT comments incorporated BUFR output is generated in ASCAT format as well Adapted web location of Ref 3 Changed file name convention Correction in file name convention Abandoned product in ESA BUFR format Nov 2005 Jan 2006 Mar 2006 Apr 2006 Feb 2007 Feb 2007 Jul 2007 Version 0 9 Version 1 0 Version 1 1 Version 1 Version 1 Version 1 2 3 Version 1 4 5 6 Version 1 Nov 2008 Change to neutral winds KNMI De Bilt the Netherlands Reference KNMI publicatie 212f Cover illustration on 23 April 2004 around midnight and west of Ireland a complex low was developing The red arrows show wind speeds up to 15 m s and a cyclonic wind direction One cyclonic centre is located surrounding the group of yellow spots while another centre is located more to the southwest at the left side of the scatterometer swath Between both cyclonic centres a line of shear flow is present where the wind direction turns by 180 degrees surrounded by wind speeds of 10 m s strength The blue arrows in the background represent a 3 hour forecast of the winds by the KNMI Hi
8. NASA Scatterometer Numerical Weather Prediction Ocean and Sea Ice SAF Quality Control US dedicated scatterometer mission Regional Meteorological Data Communication Network Satellite Application Facility US scatterometer on board QuikSCAT platform Scatterometer Ocean Stress Sea Surface Temperature Software Verification and Validation Plan West to east wind component South to north wind component Wind Vector Cell ERS Product User Manual version 1 6 November 2008 18 AR ASCAT BUFR EARS EPS ERS EUMETCast EUMETSAT HDF JPL KNMI MetOp NASA NOAA NSCAT NWP OSI SAF QC QuikSCAT RMDCN SAF SeaWinds SOS SST SVVP U WVC EARS
9. User Manual version 1 6 November 2008 12 KNMI publicatie 212f 5 Dissemination The ERS 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 meteorological satellite products The products are also 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 EARS ERS Product User Manual version 1 6 November 2008 13 KNMI publicatie 212f 6 Data description 6 1 Wind product characteristics Physical definition Horizontal neutral wind vector at 10m height Units and range Wind speed is measured in m s and wind direction in meteorological WMO convention relative to North O degrees corresponds to a wind flowing to the South with a clockwise increment The wind speed range is from 0 50 m s but wind speeds over 25 m s are generally not reliable Ref 6 Input satellite data ERS SCAT BUFR data from ESA are described in their user manual Ref 11 ERS SCAT data is acquired locally at ESA stations processed locally up to level 1b and level 2 and then transmitted to ESRIN in Rome from where it is distributed to all users via the GTS The product contains geo located measurement triplets on a satellite swath grid of 25 km size Geographical definition The ERS 2 satellite flies in a near p
10. WVC As long as the land fraction is below the limit value a reliable wind solution may however still be present Ice presence flag is set if the SST calculated for the WVC see section 4 2 is below 272 16 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 EARS ERS Product User Manual version 1 6 November 2008 15 KNMI publicatie 212f It is recommended not to use WVCs with the monitoring flag the KNMI quality control flag or the variational quality control flag set See section 7 2 for more information on product reliability The likelihood computed for solution descriptor 021104 actually contains the logio of the calculated likelihood for the wind solution This is done since otherwise values close to zero will be rounded to zero in the BUFR encoding In order to recalculate the probability the user should compute 10 to the power lt value from BUFR gt EARS ERS Product User Manual version 1 6 November 2008 16 KNMI publicatie 212f 7 Data quality 7 1 Accuracy As introduced in section 6 1 the accuracy is better than 2 m s in wind component RMS with a bias of less than 0 5 m s in wind speed see Ref 3 7 2 Reliability and data use For global coverage products it is possible to generate a product monitoring flag based ona multi step check If in one product the number of WVC Qualit
11. ata z are the backscatter measurements z u yv are the model backscatter values corresponding to the measurements The local minima of J correspond to wind vector solutions The three independent measurements fore mid and aft beam well sample the azimuth variation of the GMF in order to resolve the wind direction albeit ambiguously 3 2 1 Ambiguity removal ERS scatterometer winds have a multiple ambiguity and there are up to two wind solutions in each WVC on the earth s surface These ambiguities are removed by applying constraints on the spatial characteristics of the output wind field such as on rotation and divergence Several ambiguity removal AR schemes were evaluated for ERS data Ref 8 In the OSI SAF Development Phase some schemes that were developed for the SCAT were compared In addition to the subjective comparison of AR schemes a method for the objective comparison of AR performance among the different schemes was used In Ref 8 it is shown that this way of comparison is effective to evaluate the shortcomings of AR schemes but also reveals a more general way forward to improve AR which is followed up by tuning 2D VAR For ERS this tuned version of 2D VAR is used 3 2 2 Quality control Since the scatterometer wind retrieval problem is over determined this opens up the possibility of quality control QC by checking the inversion residual J If J is normalised by the expected isotropic error variance then it is in
12. er the errors of all NWP model winds in situ data and scatterometer winds are computed in a triple collocation exercise Ref 3 The performance is pretty constant over the globe and depends mainly on the sub footprint wind variability The performance of the products issued by KNMI 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 EARS ERS Product User Manual version 1 6 November 2008 14 KNMI publicatie 212f 6 2 File formats Wind products are in BUFR 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 http www knmi nl scatterometer ers prod The file name convention is scatt YYYYMMDD_HHMMSS_ers2___ 00000_ear_o 250 ovw 2_bufr where scatt denotes the instrument ERS AMI scatterometer YYYYMMDD denotes the date of the first data in the file HHMMSS denotes the time UTC of the first data in the file 2 denotes the satellite name 00000 is reserved for the orbit number not available for ERS 2 ear denotes the EARS service o stands for operational product denotes 25 km WVC spacing 250 vw denotes that the product contains ocean vector winds and no soil moisture information 2 lis the lowercase L denotes Level 2 product bufr denotes BUFR product format An example of a file name is scatt_20070115 071325 er
13. erates a demonstration system based on the dissemination via EUMETCast of the current regional ERS SCAT KNMI value added wind product This document is the Product User Manual to this product The wind product is distributed in the BUFR format that is also used for the ASCAT wind data This data stream can be ingested by the user using the same interfacing as for the ASCAT wind products 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 see e g 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 and SeaWinds scatterometers and will be adopted for ASCAT Scatterometer processing software is developed in the EUMETSAT Numerical Weather Prediction Satellite Application Facility NWP SAF whereas wind processing is performed semi operationally in the Ocean and Sea Ice SAF OSI SAF ESA makes available a near real time Level 2 scatterometer wind product from the ERS 2 satellite through the Regional Meteorological Data Communication Network RMDCN This product is used as basis for further processing at KNMI Since the loss of the on board tape device in 2001 data can be acquired only
14. gh Resolution Limited Area Model HiRLAM Even on this short forecast range the HiRLAM model only depicts one cyclonic centre and no shear line at all As background a METEOSAT Infrared image is shown in black and white While this shows the supporting cloud structures associated with the ERS scatterometer winds it does in itself not provide much information on the surface wind conditions In the next 12 hours after midnight on 23 April the winds strengthened and the depression central surface pressure dropped by more than 10 hPa to 974 hPa In this case the timely available ERS scatterometer winds thus provide guidance on routing conditions to ships at sea See ESA press release on http www esa intlesaEO SEMOV177ESD index 0 50 EARS ERS Product User Manual version 1 6 November 2008 1 KNMI publicatie 212f Contents T Introduction ii di a A contin AA 4 11 A AEA 4 122 JDISCIAINS E T T dust 5 Aids References a a 5 T A 6 Toy OO 6 2 The ERS scatterometer instrument 7 A E E 8 3 12 Wi a 4670111900 a a a a aaa 8 3 1 1 Geophysical model UNC isso soon 9 A A teen 10 3 2 1
15. ility of a calm patch will quickly decrease and the mean microwave backscatter will increase Also natural slicks quickly disappear as the wind speed increases and as such the occurrence of these is correlated to the amplitude of the mean vector wind over the footprint as modelled by the GMF Low scatterometer wind speeds are thus providing useful information Figure 3 Overview of wind retrieval algorithm NWP collocation Quality Control Wind computation by GMF Inversion Fieldwise Ambiguity Removal At high wind speeds wave breaking will further intensify causing air bubbles foam and spray at the ocean surface and a more and more complicated ocean topography Although theoretically not obvious it is empirically found that o keeps increasing for increasing wind speed from 25 m s to 40 m s and that a useful wind direction dependency remains Ref 6 albeit gradually weakening EARS ERS Product User Manual version 1 6 November 2008 9 KNMI publicatie 3 2 Wind retrieval The GMF has two unknowns namely wind speed and wind direction so if more than two backscatter measurements are available then these two unknowns may be estimated using a maximum likelihood estimator MLE as the objective function for determining wind vector solutions Ref 7 The MLE is defined by Ref 3 Chapter II J Z z u 7 where z o are the transformed backscatter d
16. measurements for each grid point obtained using the three different viewing directions and separated by a short time delay As the backscatter depends on the sea surface roughness as a function of the wind speed and direction at the ocean surface it is possible to calculate the surface wind speed and direction by using these triplets within a mathematical model Figure 1 ERS wind scatterometer geometry source ESA web site The instrument operates at a frequency of 5 3 GHz C band which makes it rather insensitive to rain ERS 1 has been operational from 1991 to 2000 and ERS 2 is still working although since 2001 there have been problems with the satellite gyroscopes and the on board tape recorder Since August 2003 ESA distributes the Wind Scatterometer data from ERS 2 within the visibility of ESA ground stations over Europe North Atlantic the Arctic and North America The list of ground stations was extended since then with Beijing China and McMurdo Antarctica The station coverage can be seen in the latest ERS scatterometer cycle report http earth esa int pcs ers scatt reports pes cyclic EARS ERS Product User Manual version 1 6 November 2008 7 KNMI publicatie 212f 3 Algorithms Scatterometry was developed heuristically It was found experimentally that the sensitivity to wind speed and direction describe well the changes in backscatter over the ocean at moderate incidence angles due to changes in surface ro
17. n Ad Siebren de Haan Yves Quilfen and Harald Schyberg ERS Scatterometer Ambiguity Removal Comparison OSI SAF report 2000 Ref 9 Portabella M A Stoffelen Quality Control and Wind Retrieval for SeaWinds EUMETSAT fellowship report 2002 Ref 10 de Vries J Stoffelen A and Beysens J Ambiguity Removal and Product Monitoring for SeaWinds NWP SAF report NWPSAF_KN_TR_001 Ref 11 ERS PRODUCTS WMO FM94 BUFR FORMAT ER IS UKM GS 0001 Version 4 Issue 2 16 February 2001 Ref 12 ADDITIONS TO BUFR CREX TABLES FOR PRE OPERATIONAL IMPLEMENTATION ENDORSED BY CBS for full operational status on 7 November 2007 updated 04 01 07 pp55 60 available on http www wmo int web www WMOCodes Updates BUFRCREX Preoperational050 107 doc Ref 13 Portabella M and Stoffelen A Development of a Global Scatterometer Validation and Monitoring Visiting Scientist report for the Ocean amp Sea Ice SAF SAF OSI CDOP KNMI SCI RP 141 2007 Ref 14 Verhoef A M Portabella A Stoffelen and H Hersbach CMOD5 n the CMOD5 GMF for neutral winds OSI SAF report SAF OSI CDOP KNMI TEC TN 165 2008 References marked with a are available on http www knmi nl scatterometer publications EARS ERS Product User Manual version 1 6 November 2008 5 KNMI publicatie 212f 1 4 Useful links KNMI scatterometer web site http www knmi nl scatterometer ERS visual products http www knmi nl scatterometer ers_prod Informa
18. nd the amplitude of gravity or longer ocean waves depend to some degree on the strength of the wind and may to the same degree be fitted by a geophysical model function GMF Ref 3 Chapter 1 Stoffelen Ref 3 Chapter IV discusses a unique method to determine the accuracy of scatterometer buoy and NWP model winds 3 1 1 Geophysical model function For the ERS wind product the CMOD5 n geophysical model function GMF for calculating neutral winds is used Ref 14 This model function enables the calculation of wind speeds meeting the product requirements between O and 25 m s CMOD5 n is based on CMOD5 Ref 4 It is known from extensive validation work on ERS that a CMOD5 bias of 0 5 m s against buoys persists for all wind speeds Ref 13 and this is compensated moreover another 0 2 m s is added to convert from real winds to neutral winds The CMOD5 n inverted winds are thus 0 7 m s higher than CMOD5 winds At low wind speeds the wind direction and speed may vary considerably within the WVC Locally below a speed of roughly 2 ms calm areas are present where little or no backscatter occurs perhaps further extended in the presence of natural slicks that increase the water surface tension Ref 5 However given the variability of the wind within a footprint area of 25 km it is even in the case of zero mean vector wind very unlikely that there are no patches with roughness in the footprint As the mean vector wind increases the probab
19. olar sun synchronous orbit at 98 degrees inclination at approximately 800 km orbit height The satellite swath is located to the right of the satellite ground track The swath width is 19 25 km size WVCs corresponding to 500 km Products are organised in messages of 19 X 19 WVCs Coverage Data is available only when the satellite is in sight of a ground station Most ground stations are in the North Atlantic region Actual coverage is available on the KNMI ERS 2 product visualisation website see http www knmi nl scatterometer ers_prod Output product The input product in BUFR is processed into a BUFR output product including a unique wind solution chosen its corresponding ambiguous wind solution and quality information distance to cone quality flag Delivery time A wind product is available for distribution within 10 minutes after the input product reception at KNMI The delivery time between acquisition of the data and reception at KNMI ranges from 20 to 100 minutes depending on the ground station and network speed In practice approximately 40 of the KNMI output data is available within 60 minutes after acquisition and approximately 90 of the data is available after 100 minutes 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 platforms or ship and against NWP data Even bett
20. ong 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 1 3 References Ref 1 Isaksen L and A Stoffelen 2000 ERS Scatterometer Wind Data Impact on ECMWF s Tropical Cyclone Forecasts IEEE Transactions on Geoscience and Remote Sensing special issue on Emerging Scatterometer Applications 38 4 pp 1885 1892 Ref 2 Valenzuela G R Theories for the interaction of electromagnetic and ocean waves areview Bound Layer Meteor 13 612 685 1978 Ref 3 Thesis Scatterometry by Ad Stoffelen 1998 Ref 4 Hersbach H Stoffelen A and de Haan S An Improved C band scatterometer ocean geophysical model function CMODS J Geophys Res 112 2007 Ref 5 Donelan M A and W J Pierson Radar scattering and equilibrium ranges in wind generated waves with application to scatterometry J Geophys Res 92 4971 5029 1987 Ref 6 Donelly William J James R Carswell and Robert E McIntosh Revised ocean backscatter at C and Ku band under high wind conditions J Geophys Res 104 11 485 11 497 1999 Ref 7 Pierson W J Probabilities and statistics for backscatter estimates obtained by a scatterometer J Geophys Res 94 9743 9759 1989 correction in J Geophys Res 95 809 1990 Ref 8 Stoffele
21. s WVCs with a sea surface temperature below 272 16 K 1 C are assumed to be covered with ice and no wind information is calculated 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 1 where ris 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 Our processing uses stricter limits for ice and land presence than the ESA input product resulting in less WVCs containing wind data but of a better quality 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 averaged with respect to time and location and put into the appendix part of each WVC see section 6 2 4 3 Validation Each step in the processing is validated separately by a quality control and monitoring scheme The product validation step is controlled by visual inspection and a statistical analysis is performed to control the validation steps The inversion step is controlled in the same way For ambiguity removal schemes an objective scheme exists that relies on EARS ERS Product User Manual version 1
22. s2_ 00000 _ear_o 250 ovw I2 bufr The wind product is stored in the BUFR format as proposed for ASCAT and described in Ref 12 Note that this format may still be subject to small changes Any change will be notified to all registered users In Fortran if the Wind Vector Cell Quality table 021155 is stored in an integer then use BTEST I NDW NB 1 to test BUFR bit NB where NDW 24 is the width in bits of the data element in BUFR If the monitoring not used bit BUFR bit 4 is set to zero the product is monitored If the product is monitored and the monitoring flag bit BUFR bit 5 is set to zero the product is valid otherwise it is rejected by the monitoring The monitoring bits are set to the same value in all WVCs in one BUFR output file If the KNMI QC flag BUFR bit 6 is set in a WVC this means that the backscatter information is of poor quality for various reasons such as a too large inversion residual poor satellite yaw steering or a too high noise value in the input product 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 4 2 larger than zero is calculated for the
23. servations ECMWF seminar 2000 available on http www knmi nl scatterometer publications 5 Thesis Wind Field Retrieval from Satellite radar systems by Marcos Portabella available on http www knmi nl scatterometer publications EARS ERS Product User Manual version 1 6 November 2008 KNMI publicatie 212f 2 The ERS scatterometer instrument The Active Microwave Instrument AMI is one of the instruments carried on board the first European Remote Sensing Satellites ERS 1 and ERS 2 launched by the European Space Agency on 17 July 1991 and 20 April 1995 The AMI incorporates two separate radars a Synthetic Aperture Radar SAR operating in image or wave mode and a Wind Scatterometer The Earth s surface is illuminated by four antennas and backscattered energy is received either to derive data on wind fields and wave spectra or to produce high resolution images The operational requirements are such that each mode needs to be operated exclusively but the Wind and Wave Modes are also capable of interleaved operation in so called Wind Wave Mode The Wind Mode uses three antennas to generate radar beams looking 45 degrees forward sideways and 45 degrees backwards with respect to the satellite s flight direction These beams illuminate a 500 km wide swath as the satellite moves along its orbit and each provide measurements of radar backscatter from the sea surface on a 25 km grid The result is three independent backscatter
24. theory inversely proportional to the log probability that a node is affected solely by a uniform wind If Var o Kyi 0 are the measurement variances then the norm for the inversion residual is V3 times the RMS of Var 0 2 Stoffelen 1998 Generally this normalised MLE is substantial and as a consequence the inferred probability low when there is substantial wind or sea state variability within the cell As such Stoffelen Ref 3 and Portabella and Stoffelen Ref 9 found that the inversion residual is well capable of removing cases with extreme variability at fronts or centres of lows or with other geophysical variables affecting the radar backscatter EARS ERS Product User Manual version 1 6 November 2008 10 KNMI publicatie 212f 4 Processing scheme KNMI has an operational processing chain running in near real time with ERS 2 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 global overview of the modules of the ERS scatterometer processor is given below 4 1 Node triplet completion ERS SCAT data is acquired at ESA stations processed locally up to level 1b and 2 and then transmitted to ESRIN in Rome from where it is distributed to all users via the GTS The station coverage can be seen in the latest ERS scatterometer cycle report http earth esa int pcs ers
25. tion on EARS and EUMET Cast http www eumetsat int O amp SI SAF wind product documentation on http www osi saf org 1 Scientific documents 2D Variational Ambiguity Removal Technical documents Science Plan OSI SAF User Requirements Document OSI SAF Project Plan OSI SAF Report on Algorithm Development and Prototyping Activities OSI SAF top level Configuration Management Plan OSI SAF Output Products Format Document OSI SAF Software Requirements Document ICD for the wind production of the OSI SAF SVVP for the wind production of the OSI SAF NWP SAF website http www metoffice gov uk research interproj nwpsaf index html ESA ERS Documentation http earth esa int ers eeo4 144 and http earth esa int pcs ers scatt articles 1 5 Literature 1 Figa J and A Stoffelen 2000 On the Assimilation of Ku Band Scatterometer Winds for Weather Analysis and Forecasting IEEE Transactions on Geoscience and Remote Sensing special issue on Emerging Scatterometer Applications 38 4 pp 1893 1902 2 Graham R D Anderson A Hollingsworth and H B ttger Evaluation of ERS 1 wind extraction and ambiguity removal algorithms meteorological and statistical evaluation ECMWF report ECMWF Reading England 1989 3 Mastenbroek Kees Wind Wave Interaction thesis at the Delft University of Technology Delft the Netherlands 12 December 1996 4 Stoffelen Ad A Generic Approach for Assimilating Scatterometer Ob
26. ughness as depicted in figure 2 Ref 2 In return backscatter measurements can be used to determine the wind speed and wind direction in a Wind Vector Cell WVC A schematic illustration of the processing is given in figure 3 After defining the wind output and motivating the Geophysical Model Function that is used the algorithms developed at KNMI are described Incident Reflected Figure 2 Schematic representation of microwave Wave ee wave scattering and reflection at a oS smooth a rough b and very e rough c ocean surface As the roughness increases more microwave power is returned towards the direction of the microwave source Smooth Surface Sy Back scattered component Slightly rough surface Rough surface 3 1 Wind definition A scatterometer measurement relates to the ocean surface roughness see figure 2 while the scatterometer product is represented by the wind at 10m height over a WVC It is important to realize that in the approach followed here the radar backscatter measurement o is related to the wind at 10 meter height above the ocean surface simply because such measurements are widely available for validation This means that any effect that relates to the mean wind vector at 10 meter height is incorporated in the backscatter to wind EARS ERS Product User Manual version 1 6 November 2008 8 KNMI publicatie 212f relationship As such air stability the appearance of surface slicks a
27. y Control rejections the mean residual or the wind speed bias with respect to the NWP background is above certain threshold values then the monitoring event flag is raised since the product is suspicious The threshold values are based on evaluation of the product statistics over a long period Ref 10 Because of the granular nature of the EARS product where files containing only a few minutes of data are generated and disseminated in real time this mechanism raises problems if there are not enough data in one pass for a statistically valid check The multi step monitoring check is sensitive to noise which is larger when mean values are calculated over fewer WVCs If accordingly the thresholds are set high many bad products will pass the check On the other hand if the threshold values are set too low too many false alarms will be raised This problem can be solved probably by evaluating not only the data of the last processed pass but by evaluating the last 60 or 70 minutes of data present although originating from typically 4 or 5 passes of 15 minutes In case of instrument degradation or other problems the monitoring event flag will be raised with some delay inherent to the discontinuous nature of the data stream Sensitivity tests will provide guidance on this issue It is planned to implement the monitoring event flag in a future version of the EARS product running on ASCAT data Since the quality of backscatter measurements is somewhat
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