Description of WindSat Data Products

Version Notes Introduction Geophysical Data Products Graphic Image Maps Gridded Binary Data Files Acknowledgement

Version Notes

New WindSat Version-7 Geophysical Products – Released November, 2011

We have updated the Remote Sensing Systems WindSat data using the Version-7 algorithm consistently applied to all microwave radiometer data processed at RSS. Please be sure to use data files with creation dates after September 2011. The changes to the geophysical ocean products include the following:

1. We removed some previously undetected RFI at medium resolution (18 Ghz) around North America.
2. We removed a large source of RFI off Spain from both the 10 and 6 Ghz channels. The products affected are the Wind_LF, Wind_AW, SST, and WDIR.
3. We removed several sources of RFI from ground based sources (oil platforms) that affected the lower 10 and 6 GHz resolution channels. This improved the wind_LF, wind_AW, and SST products.
4. We applied a more conservative land mask to the low resolution (10 GHz) channel which affected the Wind_LF product.
5. We removed additional RFI at 10 GHz and 6 GHz channels in the Indian Ocean around Madagascar, off Argentina, and off Chile. These regions will be missing SST, Wind_LF, and Wind_AW in the affected areas.

WindSat Geophysical Products include Sea Surface Temperature (SST); a variety of Surface Wind Speeds and Directions; Atmospheric Water Vapor; Cloud Liquid Water; and Rain Rates.

The earlier files offered from April 2011 to Sept 2011 were considered a preliminary data set and should no longer be used.

Introduction

This web site offers geophysical parameters derived from observations made by the WindSat Polarimetric Radiometer developed by the Naval Research Laboratory (NRL) and launched on January 6, 2003 aboard the Department of Defense Coriolis satellite. This radiometer is well-calibrated and contains the lower frequency channels required for SST retrievals. The radiometer operates in 5 discrete bands: 6.8, 10.7, 18.7, 23.8 and 37.0 GHz. The 10.7, 18.7 and 37.0 GHz bands are fully polarimetric whereas the 6.8 and 23.8 GHz bands have only dual polarization. The feedhorns of each frequency band trace out different arcs along the scan, therefore the Earth Incidence Angles (EIA) are different for each frequency band. Unlike previous radiometers, the WindSat sensor takes observations during both the forward and aft looking scans. This makes the WindSat geometry of the earth view swath quite different and significantly more complicated than the other passive microwave sensors. The RSS WindSat product is the only dataset available that uses both the fore and aft look directions which results in a wider swath and more complicated swath geometry visible in the provided maps.

Table 1 summarizes the channels, polarizations, EIAs and resolutions of the WindSat sensor:

Table 1

For a detailed description of the WindSat instrument, see the NRL Windsat Instrument Description.

Geophysical Data Products

Remote Sensing Systems performs a detailed processing of WindSat instrument data in two stages.

The first stage produces a near-real-time (NRT) product (identified by "rt" within the file name) which we make available as soon as possible. This is generally within 3 hours of when the data are recorded, except for occasional delays in receiving or processing data. Although suitable for many timely uses, and the differences may appear subtle, the NRT products are not intended to be archive quality. They will be deleted within several days and should not be archived.

"Final" data (identified by "v7" within the file name) are processed when we receive the NCEP FNL analysis. The NCEP wind directions are particularly useful for retrieving more accurate SSTs and wind speeds. The final "v7" products will continue to accumulate new swaths (half orbits) until the maps are full, generally within 2 days. Occasionally, data become available after more lengthy delays, and such data will be added to final "v7" products. Thus, final refers to the type of swath processing, and does not imply that a data file is finalized (as in, will not change). Newly available data swaths will be added to final "v7" products.

Browse imagery will display NRT ("rt") data while it is available, final data thereafter.

Several steps are involved in the WindSat data processing:

1. US Naval Research Lab provides RSS with Level-1A antenna temperatures also known as Temperature Data Records (TDR). We process the data into Level-2A calibrated top of the atmosphere brightness temperatures (TBs). The most important issues of this calibration procedure are discussed in the GMI ATBD.
2. The top of the atmosphere TB are then optimally interpolated onto a fixed 0.125 degree Earth grid, from which the various geophysical products are retrieved. The products available on our ftp site are then created by simple bin resampling to a 0.25 deg grid. Though each product is placed on this 0.25 deg grid, the effective resolution of the data varies. The following table gives an overview of the WindSat geophysical products in the order they appear in the binary files, and lists each product's effective spatial resolution:

Table 2

If any of the required channels for a geophysical parameter are not available, then the retrieval is not completed. For example, this could result in VAPOR retrievals for a given location, but not WSPD_LF or SST in the same location. SST retrievals are often missing at the center of the swaths because there are no 6.8 GHz observations. Wind direction is not available at the edge of the swaths because there are no polarimetric observations. This "missing data" appearance is different than for previous RSS radiometer products.

Two new geophysical products are part of the RSS V7 WindSat data release:

1. WDIR: Because of its polarimetric capabilities, WindSat can measure surface wind direction. WDIR is only provided if the wind speed exceeds 3 m/s and the rain rate is below 15 mm/hr. The wind directions in our product are in oceanographic convention.

2. WSPD_AW: We have made significant progress in improving the retrieval of wind speeds in the presence of rain. See Wind-Vector Retrievals Under Rain With Passive Satellite Microwave Radiometers, for more details on the algorithm used. If the 6.8 GHz channels are available together with all the other vertically and horizontally polarized channels, we can retrieve WindSat surface wind speed in both rain-free and rainy atmospheres. We combine various algorithms, including a global wind speed through rain algorithm that works under all rain conditions and an H-wind algorithm that has been specifically trained for tropical cyclones. The final all-weather wind speed, WSPD_AW, provided in our binary files is a smooth blend between the standard wind speed obtained in non-raining conditions WSPD_LF, the global wind speed through rain and the H-wind derived wind speed.
    

In addition to the all-weather wind speeds, we produce two standard rain-free radiometer wind products: WSPD_LF (low-frequency) and WSPD_MF (medium frequency).
 

The first, WSPD_LF is created using the frequency channels at 10.7 GHz and above (see Table 1) and is most similar to the AMSR-E wind. The second, WSPD_MF, uses frequency channels at 18.7 GHz and above and is most similar to the SSM/I wind. Each wind product has distinct advantages.
 

The WSPD_LF is less affected by the atmosphere and rain, but is affected by 10.7 GHz RFI and sun glitter effects. The WSPD_MF has a higher effective spatial resolution, is less affected by ice and land contamination, and only slightly affected by sun glitter effects, and RFI. The WSPD_MF are a little noisier than the WSPD_LF.

Daily and Time Composite Data

Gridded data are organized according to observation date. All dates and times are Coordinated Universal Time (UTC), also known as Greenwich Mean Time (GMT), Zulu Time (Z), Universal Time (UT), and World Time.

Data products include daily and time averaged geophysical data as follows:

Daily	orbital data mapped to 0.25 degree grid divided into 2 sets of maps based on ascending and descending passes when adjacent passes overlap at high latitudes and daily "seam", earlier data is chosen. (See note below)
3-Day	average of 3 days ending on and including file date
Weekly	average of 7 days ending on and including the Saturday file date
Monthly	average of all data within the calendar month

Note that all the geophysical parameters are gridded onto a 0.25 deg map (approximately 25 km grid). However, the effective spatial resolution for each geophysical parameter is determined by the frequency channels used in the retrieval as listed in Table 2 in Geophysical Data Products.

Missing Data

There are gaps within these data. Missing data generally affect only 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 and 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.

Graphic Image Maps

WindSat data comprises the period from launch in January 2003 until present. As with all the other RSS satellite products, the data are provided in gridded, binary format. Geophysical graphic images can be viewed on our web page and binary data files can be downloaded from our ftp server: ftp.ssmi.com/windsat. Daily, near real-time (NRT) data processing will be implemented in the near future. In addition to NRT data, we will also soon deliver 3-day, weekly and monthly averaged maps.

Each daily, 3-day, weekly and monthly graphic image displays one geophysical parameter: Sea Surface Temperature (SST), 10 meter Surface Wind Speed at low frequency resolution (WSPD_LF), 10 meter Surface Wind Speed at medium frequency resolution (WSPD_MF), Columnar Water Vapor (VAPOR), Cloud Liquid Water (CLOUD), Rain Rate (RAIN), 10 meter surface All-Weather Wind Speed (WSPD_AW), and Wind Directions (WDIR). The date of the data displayed is the UTC date when the data were collected. (See Map dates and Times for more information). The scale for each map is located next to the map for reference. Though the valid data range (minimum to maximum) is given in Table 3, the scale bars are set to visually enhance the data and may vary:

Table 3

White areas on the daily maps represent regions of likely sea ice contamination as determined by the WindSat field of view. In the time-averaged maps, ice is indicated when ice is present over 50% of the time, or when the number of times a cell is identified as containing ice is greater than the number of times the cell contains data. Land regions are colored gray. Those areas where WindSat data are not available are black. The black color includes areas where the satellite did not pass over and no data were collected, areas where data were collected but were determined to be bad, coastal areas, areas affected by sun glint on the SST and wind speed maps, areas near sea ice on the SST and wind speed maps, areas of rain on the SST and wind speed maps, areas of heavy rain on the water vapor map, areas of high wind speed (>20 m/s) on the SST map, and areas of RFI (radio frequency interference).

Gridded Binary Data Files

We currently produce Daily and Time-Averaged (3-day, weekly and monthly) binary data files The daily files consist of WindSat geophysical products mapped to a regular grid complete with data gaps between orbits. Two maps exist for each parameter, one of descending orbit segments (local morning passes) and the other of ascending orbit segments (local evening passes). Daily data files contain time maps consisting of the UTC observation time for each set of passes (descending and ascending). Time-Averaged data files do not contain any time information.

Each daily binary data file available from our ftp site consists of eighteen 0.25 x 0.25 degree grid (1440,720) byte maps. Nine descending (morning) maps are provided in the following order: Time (TIME), Sea Surface Temperature (SST), 10-meter Surface Wind Speed at Low Frequency (WSPD_LF), 10-meter Surface Wind Speed at Medium Frequency (WSPD_MF), Atmospheric Water Vapor (VAPOR), Cloud Liquid Water (CLOUD), Rain Rate (RAIN), All-Weather 10-meter Surface Wind speed (WSPD_AW), and Surface Wind Direction (WDIR). Nine ascending (evening) maps follow in the same order.

Daily, 3-day and monthly files are located in the directory /windsat/bmaps_v07/yYYYY/mMM. The respective file name conventions are wsat_yyyymmddv7.gz, wsat_yyyymmddv7_d3d.gz, and wsat_yyyymmv7.gz. Weekly files are located in the directory /windsat/bmaps_v07/weeks/, with the file name convention wsat_yyyymmddv7.gz

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:

The data values between 0 and 250 need to be scaled to obtain meaningful geophysical units. To scale the data, multiply by the scale factors (and add the offsets) listed in Table 3.

All binary data files have gzip compression to reduce size and decrease transfer time. They should be downloaded in binary mode.

Read Routines:

Binary file read routines and verification files are available on our ftp server in the windsat/support directory.

Note about overwriting adjacent satellite passes

Consecutive swaths of measurements from polar orbiting satellites partially overlap at high latitudes ( > 60 degrees) and at the beginning/end of the day. When producing daily maps in which swath data are placed on an Earth-grids we can either average observations from different swaths, choose earlier observations or overwrite early observations with later observations. For all RSS satellite products (SSM/I, SSMIS, AMSR-E, TMI, QuikSCAT) except WindSat, daily maps are created by overwriting earlier observations when the time difference between the observations is greater than 50 minutes. For WindSat we use the early observations and do not overwrite unless an observation is missing. This is done so that all geophysical parameters for a given 0.25-deg cell refer to the same observational time. WindSat is more complicated than other satellites because each frequency channel (and thus each geophysical parameter) has a distinct viewing geometry and therefore a slightly different time. If we were to overwrite early WindSat observations with later observations, it would cause large data gaps at high latitudes in the low resolution channels, especially at swath edges.

Acknowledgement

WindSat data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science MEaSUREs DISCOVER Project and the NASA Earth Science Physical Oceanography Program. RSS WindSat data are available at www.remss.com. We are grateful to Peter Gaiser at the Naval Research Laboratory in Washington, DC and Paul Chang at NOAA/NESDIS/STAR for providing us with WindSat TDR data.

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Last updated: August 06, 2012