ASCAT NRT Data Processing and Distribution at NOAA/NESDIS Paul S. Chang, Zorana Jelenak, Seubson Soisuvarn, Qi Zhu Gene Legg and Jeff Augenbaum National Oceanic and Atmospheric Administration (NOAA) National Environmental Satellite, Data and Information Service (NESDIS) Abstract Working collaboratively with EUMETSAT and KNMI, NOAA has established the infrastructure to acquire, process, and distribute ASCAT products from the EUMETSAT METOP satellites. NOAA commenced NRT ASCAT processing in a pre-operational mode in February 2007. ASCAT wind products at two different resolutions, 50 and 25km, are produced and distributed in near real-time (NRT) to NOAA s operational users. An overview of the NOAA/NESDIS NRT processing and distribution of ASCAT wind products are presented here. 1 INTRODUCTION NOAA and EUMETSAT have a partnership to cooperate in providing meteorological data from their polar-orbiting satellites. As part of the Initial Joint Polar-orbiting Satellite System (IJPS), NOAA and EUMETSAT collect and exchange environmental data and distribute it to users. EUMETSAT operates the METOP satellite series, while NOAA operates its Polar-orbiting Operational Environmental Satellites (POES) which will be succeeded by the National Polar-orbiting Operational Environmental Satellite System (NPOESS). The METOP satellite series consists of three spacecraft, and are designed to provide operational data until 2020. On October 19, 2006 EUMETSAT launched Europe's first polar-orbiting operational meteorological satellite system, METOP-A. Near real time ocean surface wind field measurements from METOP-A are being provided by the Advanced SCATterometer (ASCAT). ASCAT is an active microwave instrument that transmits pulses at 5.255GHz (C-band) frequency and measures backscattered power from the ocean surface. ASCAT builds upon the scatterometer heritage gained from the European Space Agency s ERS research satellites launched in the 1990 s. The changes in the near surface wind field modify the sea surface roughness which in turn modifies backscattered power which is used to infer the wind speed and direction at the ocean surface. NOAA is participating in the ASCAT calibration and validation (cal/val), and is processing and distributing ASCAT data in near real-time (NRT) for NOAA's operational users. The NRT ASCAT data flow at NOAA commenced in February 2007. In this paper, we describe the design, implementation, and operations of the ASCAT NRT processing system at NOAA/NESDIS. Specifically, we will describe the process flow, including the communications network, processing hardware and software, as well as the monitoring system used operationally at NOAA/NESDIS. 2 NOAA/NESDIS NEAR-REAL TIME ASCAT PROCESSING SUITE 2.1 Communication Network The NRT ASCAT data flow from the METOP satellite to the NOAA processing system is summarized on Figure 1. NOAA receives three flavors of L1B data, which are the non-averaged normalized radar backscatter (σ 0 ) measurements (the full resolution data set), the σ0 s averaged over 50km, and the σ0 s averaged over 25km. These are provided in 3 minute increment granules, to NOAA's server in
Darmstatd, Germany by EUMETSET within 135 minutes of observation. The NOAA ASCAT Ingest server then pushes the data to the operational processing system at Suitland and the parallel research processing system set up at NOAA s Center for Satellite Applications and Research (StAR) office. Figure 1 ASCAT data flow from EUMETSAT to NOAA 2.2 Hardware System Configuration The NOAA ASCAT NRT processing systems consists of a SUN SunFire V490 server for the parallel research system and a set of IBM p5/570 systems for operational processing and data distribution. The two IBM systems allow one machine to function as the primary processing server and the other functions as a backup processing server such that failure of a successful push to the primary server automatically results in a push to the backup server.
Figure 2 ASCAT IT infrastructure architecture 3 NEAR-REAL TIME DATA INGESTION AND PROCESSING The NRT ASCAT data flow at NOAA commenced in February 2007. To monitor the data distribution the NOAA processing system checks the processing at several levels. The time stamps indicating data ingest time, processing starting point and end point are saved and used to produce daily and monthly data distribution statistics. A mission requirement for NOAA is to make the ACAT BUFR products available to end users within 180 minutes of observation. Figure 3 shows the histogram of the mean monthly L1B file delays. The mean monthly latency of 60 min was achieved just one month after the initial NOAA NRT ASCAT data ingestion started. Figure 3 Histogram of mean monthly L1B 50km files delay
Figure 4 Percentage of L1B files arriving at NOAA within 100 and 120min from oldest measurement in the file A PERL script is run monthly to extract the statistics of any data missing so that any gaps in data can be identified and corrective action can be taken as necessary. The missing file statistics are shown on Figure 3. In the beginning months (Feb-Apr 2007) a high percentage of L1B files were missing for all L1B levels. In March, the percentage of missing files dropped below 5%, however it has consistently been hovering at that level. It has been noted that data gaps are occurring mainly in ASCAT ascending passes, and they are most frequent over northern Pacific Ocean Figure 6 (a). These data gaps are very apparent on global ocean wind maps produced on NOAA web Figure 6 (b). Figure 5 Percentage of L1B (50km) missing files between February and August 2007
(a) (b) Figure 6 Histogram (a) of L1B missing files for ASCAT ascending passes and (b) daily global ocean wind map showing gaps in data 4 FROM MEASUREMENTS TO WINDS 4.1 ASCAT Measurements Geometry ASCAT is an active microwave instrument that transmits pulses at 5.255GHz (C-band) frequency and measures power backscattered from the ocean surface. The ASCAT instrument employs two sets of three, three meter long, stick fan-beam antennae oriented at 45º, 90º and 135º with respect to the satellite track. This measurement geometry results in 2x550km swaths, one on each side of the satellite measurement track with 720km nadir gap. For each swath, the ocean surface is
simultaneously observed at three different directions by fore, mid- and aft-antennae. One triplet of averaged backscatter measurements is used to provide wind vector retrievals. 4.2 Wind Retrieval Algorithm Near surface ocean wind vector fields are retrieved from Level1b files, using the KNMI ASCAT processing software [1] that relates measured backscatter to near surface ocean wind field via a geophysical model function (GMF). Due to the harmonic nature of the GMF with respect to wind direction, the resulting solution is not unique with at least two possible ambiguities found for each WVC. They have generally similar wind speeds but differ 180 in direction. The directional ambiguity 2D-VAR ambiguity removal scheme [3,4] is utilized in the ASCAT processing. ASCAT processing software has been modified at NOAA/NESDIS to ingest NCEP/GFS forecast fields for ambiguity removal initialization. In the last processing step, sea ice screening will be applied, based on the scatterometer data itself and on sea ice history information. 4.3 Wind Vector Products The NOAA NRT ASCAT wind processing produces Level 2 (wind vector) data in two different formats: BUFR format for easy input into numerical weather prediction model suites and Native ASCAT orbital files in binary format (PFS). These files are available to users upon request. Separate files are produced for both the low (50km) and high (25km) resolution wind vector products. Besides the orbital BUFR and PFS files, reduced ASCAT-lite binary products are extracted from the full orbital files. These files contain only the selected wind vector solutions along with selected quality flags, the date/time stamp, and the location information for easier input into AWIPS (BUFR format) and N-AWIPS (binary format) systems. The NOAA ASCAT Level 2 products are also produced in NRT on the parallel research system at NOAA's StAR office for a further product development and validation. The graphical products displaying the global ocean wind maps, tropical storm centered wind vectors, and the global wind ambiguities are displayed on StAR s Ocean Winds Team web page. This site is updated on an hourly basis with any newly available data and can be found at: http://manati.orbit.nesdis.noaa.gov/ascat/.
5 CONCLUSIONS An overview of the NOAA/NESDIS NRT processing and distribution of ASCAT wind products has been presented. ASCAT L1B measurements have been recieved in NRT at NOAA since February 2007, and ASCAT L2 wind products have been produced on a pre-operational basis. During the first three months the data flow was unstable, however, it stabilized in March and the percentage of files missing has dropped below 5%. Data ingestion and processing latencies are well below the 180 minute target threshold. Two wind data products are being generated at a 50 and 25 km spatial resolution. These wind products are posted at http://manati.orbit.nesdis.noaa.gov/ascat, and are also being made available to OPC and TPOC via NAWIPS system starting in June 2007. 6 REFERENCES ASCAT Wind Product User Manual, available on http://www.knmi.nl/scatterometer/publications/pdf/ascat_product_manual.pdf de Vries, J., Stoffelen, A. and Beysens, J., Ambiguity Removal and Product Monitoring for SeaWinds, NWP SAF report NWPSAF_KN_TR_001, available on http://www.knmi.nl/scatterometer/publications/ Hersbach, H., Stoffelen, A. and de Haan, S., CMOD5 - An improved geophysical model function for ERS C-band scatterometry, J. Geophys. Res., accepted, 2007. Stoffelen, Ad, Siebren de Haan, Yves Quilfen, and Harald Schyberg, ERS Scatterometer Ambiguity Removal Comparison, OSI SAF report, 2000, available on http://www.knmi.nl/scatterometer/publications/