AMSR-2 Sea Surface Wind Speed
Frequently Asked Questions about the GCOM AMSR-2 Sea Surface Wind Speed Product
1) What is this product?
The GCOM AMSR-2 Sea Surface Wind Speed product identifies wind speeds over large bodies of water.. The resolution of this product is ?? km. The product is created both day and night since it is only reliant on microwave imagery. It's values range from 0 to 30 m/s (which are translated to knots in locally in AWIPS). There is no data produced for areas within 50 km of land or sea ice areas. There is also no data produced in areas of sun glint, rain, or radio frequency interference.2 The footprint of the data (swath width) is 1.450 km. The satellite orbits at 700 km above the earth.
This product is useful when trying to identify the wind speed over the ocean surface. This can be helpful when trying to verify model output of ocean surface wind speeds.
2) How often do I receive this data?
This data is available over the same area of interest twice a day. That time is at roughly 1:30am and 1:30pm local time, give or take (this is the time the satellite passes over the equator in either an ascending or descending orbit, not the exact time of every granule).
3) How do I display this product in AWIPS-II?
To display this product in AWIPS-II, go to the "Satellite" tab of the CAVE menu, then select "GCOM-W1." From there, select the "Sea Surface Wind Speed" product under the "AMSR-2" sub-heading.
4) How do I interpret the color maps associated with this product?
Color tables have not yet been created for this product.
5) What other imagery/products might I use in conjunction with this product?
This product can be used in conjunction with wind speeds produced by models. Doing so can give confidence to specific model output by comparing the two products and give confidence in which model is accurately capturing the state of the atmosphere. It can also be useful to compare this product with the synthetic aperture winds products from the RADARSAT and Sentinel satellites. If they are in relative agreement, greater confidence can be assigned to the AMSR-2 Sea Surface Wind Speed product.
6) How is this product created?
Microwave emission from the ocean depends on surface roughness. There are three mechanisms that are responsible for this variation in the sea surface emissivity. First, surface waves having long wavelengths, compared to the radiation wavelength, mix the horizontal and vertical polarization states and change the local incidence angle. The second mechanism is sea foam. This mixture of air and water increases in the emissivity for both polarizations. The third roughness effect is the diffraction of microwaves by surface waves that are small compared to the radiation wavelength. These three effects depend on wind speed, and contribute to the intensity of radiation coming from the pixel being observed by the antenna boresight.
To develop the sea surface wind speed (SSW) algorithm, we need to know how the characteristics of brightness temperature (Tb) vary with SSW. Although it is more desirable to understand the SSW affect on Tb for high frequencies (e.g. 36 GHz), the increased contamination by atmospheric effects makes it extremely difficult. For lower frequencies (e.g. 6 GHz), the increment in V-pol Tb for low wind speeds (≤ 6 m/s) is minimal and increases gradually with SSW above this speed. In contrast, the 6 GHz H-pol Tb increases even under weak wind conditions, and its increment is larger than that of the V-pol Tb for all wind speeds.
Further, an anisotropic feature depending on the relative wind direction (RWD), the angle made by the direction of AMSR2 viewing and the wind direction, is found for both polarizations. An RWD of 180° corresponds to an upwind direction; 90° and 270° correspond to a crosswind direction; and 0° (and 360°) correspond to a downwind direction. 6 GHz V-pol reaches a maximum in the upwind direction and a minimum in the downwind direction, while 6 GHz H-pol reaches a maximum in the crosswind direction and a minimum in both the up and downwind directions. We assume that Tb maintains similar characteristics at higher frequencies as well.
The ancillary data used in the Aerosol Detection product include:
- GFS Model WInds: In order to correct for wind direction effect, the modeled wind direction is required and can be obtained from the Global Forecast System (GFS) model.
1Jelenak, Zorana, Suleiman Alsweiss, Seubson Soisovarn, Jun Park, Pat Meyers, and Paul Chang. Algorithm Theoretical Basis Document: GCOM-W1/AMSR2 Day-1 EDR v.1.0. February 2014.