What is Ocean Wind?
Wind is the movement of air and is caused by the difference in atmospheric pressure between high and low pressure systems. Over land, we use instruments called anemometers to measure the surface wind speed and wind direction. These anemometers exist in high-density for many areas and lower-density in less populated regions. But over the oceans, measurement of surface wind characteristics is far more limited, and is primarily obtained from anemometers located at small island weather stations, on ships, and on buoys floating in the ocean. Since the ocean regions are so large, especially the Pacific Ocean, knowledge of the wind characteristics over this vast space is important to weather forecasting, ocean navigation, and climate study. Enter the benefit of satellite wind measurement. One of the first approaches was to use visible images to study cloud motion and indirectly determine wind speed and direction. This method is still in use today.
Microwave Measurement of Ocean Wind
Two types of microwave instruments measure ocean surface winds, the passive microwave radiometer and the active microwave scatterometer.
The radiometer measures ocean surface roughness which we correlate to wind speeds at 10 meters above the water's surface. We can only retrieve wind speeds from radiometers with the exception of the polarimetric radiometer, WindSat, that can measure both wind speed and wind direction. WindSat is the first satellite microwave polarimetric radiometer and was launched in 2003. A typical resolution for radiometer winds is about every 25 km over global oceans. A radiative transfer model and ocean emissivity model is needed to derive the wind speeds from ocean brightness temperatures. We can not obtain winds over land using MW radiometers. The consistent satellite MW radiometer wind speed data record began in 1987 with the launch of the DMSP F08 SSM/I.
The scatterometer is an active instrument and sends a signal to the Earth's surface which reflects off the ocean Bragg waves (these are wind generated surface ripples—capillary waves) on the surface of the larger scale ocean waves. The reflected energy measured by the scatterometer is translated using a geophysical model function into a 10 meter neutral wind speed and direction. Scatterometers typically operate at either C-band (~5GHz frequency) or Ku-band (~14 GHz frequency). With special processing techniques, we can obtain wind speeds and directions every 12 km over the oceans. Scatterometers can also be used to measure sea ice and land ice characteristics. The scatterometers that have been in operation longer than a brief period are ERS-2 (C-band), QuikScat (Ku-band), and ASCAT (C-band). RSS scientists are working on merging these long-term wind vectors into a product for climate study.
It is surprizing how well these two independent and different measurements agree. In 2003, both a radiometer (AMSR) and scatterometer (SeaWinds) were flown on the Midori-2 satellite. Wind speeds from these two instruments showed excellent agreement in rain-free conditions.
RSS Wind Data Products
1-DEGREE, MONTHLY Wind Speed
We have merged the wind speed measurements from the many radiometers in operation since 1987, including SSM/I, SSMIS, and WindSat. These data were all processed in a consistent manner using our radiative transfer model and careful instrument intercalibration. The wind speeds from these instruments are used to create a Mean Wind Speed product that is best for use in climate study. This 1-degree, monthly gridded product is further described in the document Merged Monthly 1-degree Wind Speeds. This wind product is available in netCDF format from the RSS ftp server and from the NASA GHRC. We have browse images of mean winds, wind speed anomalies, 12 monthly climatology maps (constructed using 1988 to 2007 data), and a global trend map.
The Cross-Calibrated Multi-Platform (CCMP) gridded surface vector winds are produced using RSS V7 radiometer winds, QuikSCAT and ASCAT scattereometer winds, quality-checked moored buoy winds, and the ECWMF ERA-Interim Reanalysis model wind field as a background wind. A 4-dimensional variational analysis (VAM) is used to produce the fully populated nearly-global wind fields from the input data. As such, the CCMP is considered to be a Level-3 ocean vector wind analysis product consisting of four maps daily of a 0.25 degree gridded vector wind field. This product is an update and extension of the original V1.1 CCMP product. RSS has transitioned the CCMP processing code to run using our most up-to-date satellite data observations. All methodology remains the same as that used in the original CCMP product and most of the CCMP processing code is unchanged, with only minor alterations to compensate for the different operating systems and compilers. The CCMP V2.0 data are fully described in the online document and support documentation referenced.
Individual Radiometer and Scatterometer Gridded Binary Data Files
In addition, wind speed is one of the measurements in the radiometer binary gridded data files and is also available along with wind direction in the scatterometer data files. For detailed product information and data access, see the Missions page for each instrument and quick buttons for accessing the data.
|Instrument||Period of Operation||Version|
|SSM/I, SSMIS||1987 - present||V7|
|TMI||1997 - 2015||V7.1|
|AMSR-2||2012 - present||V7.2|
|AMSR-E||2002 - 2011||V7|
|GMI||2014 - present||V8.1|
Wind speed and direction (polarimetric radiometer and scatterometers):
|Instrument||Period of Operation||Version|
|WindSat||2003 - present||V7.0.1|
|QuikScat||1999 - 2009||V4|
|SeaWinds||2003: Apr - Oct||V3a|
|ASCAT||2006 - present||V1.2|
Related Data Products
The RSS data are used to create merged wind products by other researchers. These are a few examples:
The original V1.1 Cross-Calibrated Multi-Platform Ocean Winds (CCMP) data set is produced by Atlas et al. The 4x daily data files for 1988 to 2011 are available from the JPL PO.DAAC. The winds are produced using a 4 dimensional variational analysis to derive wind speeds and directions using RSS wind speeds, in situ measurements and ECMWF model data as input.
The Blended Sea Winds data set is produced by researchers at NOAA NCDC. This product uses all RSS radiometer wind speeds and an optimal-interpolation processing to merge the winds into a 4x daily product.