The Quick Scatterometer (a SeaWinds instrument placed in orbit quickly) was launched in June 1999 and operated until November 2009. We refer to this instrument as QuikScat (or QSCAT) to distinguish it from the nearly identical SeaWinds scatterometer on Midori-2 (ADEOS-II), which operated briefly from December, 2002 to October 2003. The primary mission of these SeaWinds scatterometers was to measure winds near the ocean surface. They are also useful for some land and sea ice applications. The SeaWinds instruments are the third in a series of NASA scatterometers that operate at Ku-band (i.e., a frequency near 14 GHz). The first Ku-Band scatterometer was flown on SeaSat in 1978. Eighteen years later, NSCAT was launched on Japan's Midori-I (ADEOS-I) spacecraft in August 1996. The Europeans also fly satellite scatterometers, which operate at C-band (approx. 5 GHz). Two of these, named ASCAT, are currently operating.
SeaWinds scatterometers are essentially radars that transmit microwave pulses down to the Earth's surface and then measure the power that is returned back to the instrument. This "backscattered" power is related to surface roughness. For water surfaces, the surface roughness is highly correlated with the near-surface wind speed and direction. Hence, wind speed and direction at a height of 10 meters over the ocean surface are retrieved from measurements of the scatterometer's backscattered power.
Over many years, we developed and improved a Ku-band Geophysical Model Function (GMF). The current version is called Ku-2011. We use it to derive Wind Speed and Direction at 10-meter height over ice-free oceans from backscattered scatterometer signals. Our latest version of QuikScat data, V4, was released on April 28th, 2011 after a complete reprocessing using this new GMF. The goal of this activity was to improve wind speed retrievals, with special attention devoted to high winds.
Accurate observations of high winds over the oceans are important for analyses of phenomena that range from local (storms, cyclones) to global scales (air-sea exchanges, ocean circulation). However, satellite retrievals of high winds are challenging for two reasons. First, they are often contaminated by the presence of rain and it is difficult to separate the wind and rain signals in the retrievals. Second, validation data of winds greater than 20 m/s are scarce and therefore limit our understanding of the accuracy of the satellite-retrieved high winds.
QuikScat data underwent a final reprocessing after the instrument stopped functioning. We used our improved Ku-2011 GMF to produce the Version-4 (V4) data products. The GMF is used to derive Wind Speed and Direction at 10-meter height over ice-free oceans from backscattered scatterometer signals. Our goal is to provide high quality scatterometer wind speed retrievals for use in climate study.
When the methodology for QuikScat wind retrieval was developed at Remote Sensing Systems (Wentz and Smith, JGR 2001), validation data containing high winds were extremely limited. Extrapolations and assumptions were made for winds greater than 20 m/s. In the 10+ years since the launch of QuikScat, more validation data became available and it was easier to assess the quality of our Ku-2011 high winds.
The newly developed Ku-2011 GMF (Ricciardulli and Wentz, manuscript in preparation) was developed using WindSat data as a calibration target. RSS has recently made advancements in polarimetric wind vector retrievals. Meissner and Wentz (IEEE TGARS, 2009) developed a new algorithm for WindSat winds which is valid even in rain and was trained in storm conditions using the NOAA Hurricane Research Division (HRD) dataset. The new QuikScat model function, Ku-2011, uses 7 years of observed radar backscatter ratios (sigma_0) collocated with rain-free WindSat wind retrievals. The WindSat retrievals are believed to be accurate for winds up to at least 30 m/s. Additionally, WindSat is able to accurately detect rain, and is used to discard QuikScat sigma_0 in the proximity of rain when developing the scatterometer GMF.
Note that the QuikScat Ku-2011 (v4) wind speeds have been calibrated to rain-free WindSat data. Rain impacts scatterometer retrievals, on average resulting in a positive bias at low wind speeds (due to signal backscatter by rain drops) and a negative bias at high wind speeds (due to atmospheric attenuation of the signal). The wind direction is less affected by rain, except at high rain rates (above 8 mm/hr). QuikScat wind retrievals displayed on this website and provided in the data files on our ftp server exist in both rain-free and rainy areas. When possible, we provide a rain-rate estimate from one of our radiometers in addition to a stand-alone scatterometer rain flag. We advise Users to rely on both the scatterometer rain flag and the radiometer rain information within the file to remove rain-contaminated data from data processing. See the notes within the read routines provided for more information on how to access and apply the rain flags.
For the case of QuikScat, we use 4 satellite microwave radiometers (F13 SSMI, F14 SSMI, F15 SSMI, and TMI) to determine if rain is present at the location of the QuikScat observation. In addition, all available SSMIs are used to detect sea ice. Using the SSMI daily observations of sea ice, the scatterometer observations can be properly flagged so that reliable wind vectors can be obtained immediately next to the marginal ice zone. For the case of SeaWinds on Midori-2, the AMSR microwave radiometer on the same platform is used for rain and sea ice detection.
Changes one should expect to see in the v4 QuikScat data include:
- Decreased wind speeds for winds greater than 20 m/s
- Improved wind directions at very low (< 5 m/s) and high wind speeds (> 15 m/s)
- Better agreement between radiometer and scatterometer winds in typically high wind speed regions
The data file format remains the same for both the QuikScat V4 and SeaWinds V3a data. The data file folder name is "bmaps_v04" and the file version designation has changed to "v4." All read routines (Fortran, IDL, MatLab and Python) will work on QuikScat v4 data as well as SeaWinds v3a data. More detailed information about the Ku-2011 GMF and QuikScat reprocessing can be found in Reprocessed wind vectors QuikScat V04 with Ku-2011 Geophysical Model Function.
RSS Scatterometer Data Products
We release both QuikScat (version-4) and SeaWinds on Midori-2 (version-3a) data products. For both instruments, two types of products are available: 1) swath products which contain the data in one swath per file organized by swath rows perpendicular to satellite path or 2) gridded binary data files similar in many ways to those provided for RSS radiometer data.
|Surface Wind Speed||
Wind speed 10 meters above the water surface, derived from surface roughness (wind stress), roughly equivalent to an 8 - 10 minute mean surface wind.
|Surface Wind Direction||
The angle of air movement (oceanographic convention).
Scatterometer data is less accurate in rain. Data products include a scatterometer derived rain flag, and co-located radiometer columnar rain rates.
Gridded Binary Files
We produce daily and time averaged (3-day, weekly, monthly) gridded data files by mapping the scatterometer orbital data to a 0.25 deg longitude by 0.25 deg latitude Earth grid.
Folders and file names follow these conventions:
|Time||Directory Path||File Name|
Where [year], [month], "yyyy", "mm", "dd", and "v" stand for:
|[year]||year folder||y2002, y2003 etc.|
|[month]||month folder||m01 (Jan), m02 (Feb) etc.|
|yyyy||year||2002, 2003 etc.|
|mm||month||01 (Jan), 02 (Feb) etc.|
|dd||day||01, 02, ... 31|
Note that 3-day and weekly files are named for the day they end on (including that 3rd or 7th day).
Data are encoded in single byte values. Each data file contains a sequence of byte maps, or bmaps, each representing the Earth at quarter degree resolution: 1440 x 720 bytes.
Daily files are byte arrays of size 1440 x 720 x 4 x 2 (longitude, latitude, parameter, orbit segment (ascending or descending passes)). The 4 parameters are: UTC Time of Observation, Ocean Surface Wind Speed, Ocean Surface Wind Direction, and a Rain Flag / Collocated Radiometer Rain combination value (see below). Two maps exist for each parameter: one of ascending orbit segments (local morning passes) and the other of descending orbit segments (local evening passes).
|1440||longitude||0 to 360|
|720||latitude||-90 to 90|
|4||parameter||UTC time, wind speed, wind direction, rain info|
|2||orbital segment||ascending passes, descending passes|
Thus, daily files contain 8 global maps:
[ascending time, ascending speed, ascending direction, ascending rain info, descending time, descending speed, descending direction, descending rain info]
Time averaged files (3-day, weekly, monthly) are byte arrays of size 1440 x 720 x 3 (longitude, latitude, parameter). The 3 parameters are: Ocean Surface Wind Speed, Ocean Surface Wind Direction, and a Rain Flag / Collocated Radiometer Rain Rate combination value (see below).
|1440||longitude||0 to 360|
|720||latitude||-90 to 90|
|3||parameter||wind speed, wind direction, rain info|
Time averaged files contain 3 global maps: [wind speed, wind direction, rain info]
For averaged scatterometer data, wind speeds are scalar averaged, while wind directions are vector averaged. Thus, if daily observations record strong winds blowing in opposing directions, the scalar speed average will reflect the high average speed, and the vector direction average will point out the prevailing direction.
A time composite grid cell will contain data if a minimum number of observations exist:
|Averaging Time||Potential # observations||Typical # observations||Minimum # observations|
Data on daily maps are overwritten at both the high latitudes where successive orbits cross and at the "seam" or region where the last orbit of the day overlaps the first orbit of the day.
The center of the first cell of the 1440 column and 720 row map is at 0.125 E longitude and -89.875 latitude. The center of the second cell is 0.375 E longitude, -89.875 latitude.
The data values fall between 0 and 255. Specific values have been reserved:
|0 to 250 =||valid geophysical data|
|251 =||not used for scatterometers|
|252 =||not used for scatterometers|
|253 =||scatterometer observations exist, but are bad|
|254 =||no scatterometer observations|
|255 =||land mass|
The data values between 0 and 250 need to be scaled to obtain meaningful geophysical data. To scale the data:
|TIME:||either multiply by||6.0||to get||0 to 1440 minute of day UTC|
|or multiply by||0.1||to get||0.0 to 24.0 hour of day UTC|
|Wind Speed||multiply by||0.2||to get||0.0 to 50.0 m/sec|
|Wind Direction||multiply by||1.5||to get||0.0 to 360.0 degrees|
|Rain Flag||extract first bit||to get||0 = no rain; 1 = rain|
|Radiometer Rain||extract bits 3 to 8||(x/2)-0.5||to get||0 to 31 km*mm/hr|
Wind directions are provided in oceanographic convention. This means:
Winds blowing towards the North: 0° (or 360°)
Winds blowing towards the East: 90°
Winds blowing towards the South: 180°
Winds blowing towards the West: 270°
The Rain byte contains 3 pieces of information. Use bit extraction to obtain the following:
|scatterometer rain flag
|0 = no rain
1 = rain
|collocated radiometer flag
|0 = no radiometer data within 60 minutes
1 = radiometer data within 60 minutes
|radiometer columnar rain rate
(bits 3 - 8)
|0 = no rain
1 = rain in adjacent cells
2 thru 63 = value/2 -0.5 rain rate in km*mm/hr)
The files are stored in zipped form. If your programming environment does not read compressed files directly, use any GZIP compatible tool to unzip files before reading.
We provide Fortran, IDL, Matlab and Python read routines via FTP at ftp://ftp.remss.com/qscat/scatterometer_bmap_support
The QuikScat and SeaWinds data file formats are identical. The read routines work for both QuikScat and SeaWinds datasets.
There are gaps within these data. Missing data generally affect Daily and 3-Day products, but can also reduce the number of observations in Weekly and Monthly averages.
When browsing imagery, the navigation may skip dates with no data, or you may see a blank map stating that no data are available for that time.
Binary data files for dates with completely missing data are not produced; they will be absent from our FTP server.
Data gaps are generally due to missing data upstream from our processing facility, such as the instrument being turned off. Occasionally, there are delays in obtaining and/or processing recently recorded data; beyond several weeks, it is unlikely that missing data will become available.
Official information on missing QuikScat and SeaWinds data can be found at:
Rain is a well known problem affecting scatterometers. It tends to result in erroneous cross track vectors and/or unrealistically high speeds.
Here is an example:
Note the rain contaminated data in the image. The scatterometer derived rain flag is used to draw the arrows grey instead of black. Collocated radiometer rain rates are also available in the data files. Scientists should use the rain flag and radiometer rain rates to help remove rain effects from the data files when doing research.
Regional Ice and Land Problems
Undetected winter sea ice affects the Sea of Asov (northern Black Sea) and the Northern Caspian Sea. The shrinking Aral Sea is affected year round by land exposure. Note that all of these areas are rain flagged.
Each daily, weekly, or monthly scatterometer image in our browse data section shows the wind speed and direction for a specific geographical region.
The daily browse images display the ascending and descending satellite passes separately. The approximate UTC time of each pass is labeled near the bottom of the image. The date of the data displayed is the UTC date when the data was collected (See the Map dates and Times section of the FAQs). The observation times of ascending and descending pass segments are interleaved throughout the day. When browsing daily pass segments with the Previous and Next buttons, approximately half of the Earth will be browsed in temporal order; the other half will not be browsed in temporal order.
A scale of 10 meter ocean surface wind speeds is located on each image and extends between zero and 45 m/s. Land regions are colored gray. Areas where scatterometer data are not available are black. For the daily maps, the black color includes areas where the satellite did not pass over and no data was collected, areas where data was collected but it was determined to be bad, coastal areas, and regions containing sea ice.
The browse images are produced from the same gridded data files available on our FTP server.
Swath Data Files
In addition to the gridded daily and time-averaged files, we release swath files only for scatterometer data. These files contain the measurements for one orbit of the satellite around the Earth. The data are organized by observation cells that are perpendicular to the direction the satellite travels. This makes the data grid at an angle to the standard lat/lon grid used in our other products.
Orbital scatterometer data are publicly available via FTP at: ftp://ftp.remss.com/qscat/qscat_wind_vectors_v04/ and ftp://ftp.remss.com/seawinds/seawinds_wind_vectors/
The files are stored in directories based on orbit number (00000to09999, 01000to01999, 02000to02999, etc.). The file names have the form:
where RRRRR is the five digit orbit number.
The orbital data file format is described at: ftp://ftp.ssmi.com/qscat/readme_scatterometer.txt
The columns of the above files represent:
|Orbit Number||Number of good WVC rows||Equatorial Crossing Date (UTC)||Equatorial Crossing Time (UTC)||Equatorial Crossing Longitude||
Read routines are provided in Fortran, IDL, and Matlab at: ftp://ftp.remss.com/qscat/scatterometer_orbit_support/
The QuikScat and SeaWinds swath file formats are identical. The read routines work for both QuikScat and SeaWinds datasets.
Verification data to help ensure you are reading the data file correctly is at: ftp://ftp.remss.com/qscat/scatterometer_orbit_support/readme.txt
Mears, C.A., D.K.Smith, and F.J.Wentz, 1999, Development of a Rain Flag for QuikScat, technical report number 121999, Remote Sensing Systems, Santa Rosa, CA, 13 pp.
Ricciardulli, L. and F.J.Wentz, 2011, Reprocessed QuikSCAT (V04) Wind Vectors With Ku-2011 Geophysical Model Function, technical report number 043011, Remote Sensing Systems, Santa Rosa, CA, 8pp.
Wentz, F.J. and D.K. Smith, 1999, A Model Function for the Ocean-Normalized Radar Cross Section at 14 GHz Derived From NSCAT Observations, Journal of Geophysical Research, 104(C5), 11499-11514.
Related Data Sets
The Ku-2011 GMF was provided to the Jet Propulsion Lab to be run on QuikScat data. The data available at the NASA PO.DAAC are very similar to those available at RSS. Different rain flagging and quality flagging are applied and a different gridding process is used to create the Level-3 wind products at JPL. The winds values themselves should contain only minor differences from the RSS data.
For more information and access to QuikScat data, see JPL PO.DAAC
QuikScat and SeaWinds data were produced by Remote Sensing Systems with thanks to the NASA Ocean Vector Winds Science Team for funding and support.
How To Cite
Production of this data set could not have happened without support from NASA. We need you to be sure to cite these data when used in your publications so that we can demonstrate the value of this data set to the scientific community. Please include the following statement in the acknowledgement section of your paper:
"QuikScat (or SeaWinds) data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Vector Winds Science Team. Data are available at www.remss.com. "
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