The SSM/I wind speed parameter estimates the ocean wind speed by sensing the roughness of the ocean's surface caused by the surface wind. It does not give wind direction. It is valid for ocean areas 50 km (25 nm) or a greater distance away from the nearest coastline. Unless there is precipitation, the accuracy of the wind speed parameter is 2 m/s (4 knots) or better. The presence of rain between the satellite and the ocean surface obscures the satellite's view of the ocean and can degrade the accuracy of wind speed estimates by large amounts. In this document, rain- flagged areas are "blacked-out" to avoid misinterpretation by the user. Wind speeds are not reliable for speeds greater than about 20 m/s (40 kt). In the examples here, high wind speeds, like precipitation contamination and coastal contamination, are blacked out. Thus, the SSM/I wind speed parameter cannot be used to estimate wind speeds in especially intense storm systems or tropical cyclones. In these storms, rain contamination and high winds often combine to render the SSM/I wind parameter nearly useless. The surface wind speed represents the wind speed 19.5 meters above the sea surface. The surface wind speed is based on the roughening of the sea surface caused by the wind. The higher the wind, the more the sea surface roughening, and the higher the brightness temperature sensed by the SSM/I channels. This translates into a higher computed wind.

The SSM/I integrated water vapor parameter (IWV) shows the water vapor contained in a vertically integrated column above the surface of the ocean. It is measured in kilograms per meter squared. It is valid for ocean areas only. It strongly represents conditions near the surface of the ocean. In other words, high values of IWV mean that the boundary layer is moist, as in the tropics. Low values indicate that the boundary layer is dry, as near the poles. It is very different from the geostationary water vapor image, shown in the same composite. The geostationary image represents conditions in the mid troposphere. It doesn't "see" down to surface moisture conditions, as does the SSM/I. This difference explains the different appearances of the two vapor products.

The IWV product does not give the height of the different layers of atmospheric moisture. Thus, it is not a sounder. It can only estimate a top-to-bottom value.

For more information on GMS-5 data, please visit the following web sites:

GMS Realtime Data

JMA Brief Description

WMO Satellite Page

NASA Description

The Integrated Water Vapor (IWV) product is useful to assess how moist the lowest layers of the atmosphere are. It often correlates well with the Sea Surface Temperature (SST). Where the SST is high, the IWV is often high, suggesting a warm humid environment. Where the SST is low, the IWV is often low, suggesting cool, often dry conditions.

The IWV is often a useful indicator of atmospheric fronts. A strong gradient often indicates a frontal boundary. High values equator-ward of the front indicate warm, moist air. Low values pole-ward of the front indicate cooler, dryer, postfrontal air.

Perhaps the biggest problem is that the SSM/I can only provide wind speed, not wind direction. This can make the viewing of SSM/I winds confusing for some forecasters. Another spacecraft sensor, called a "scatterometer", is truly an active microwave (radar) system. It can show both wind speed and direction.

The disadvantages of the Integrated Water Vapor (IWV) display are several:

1. It only gives one value representing moisture conditions in the entire troposphere; it does not give moisture by level.

2. It is not available over land or ice.

3. It does not give relative humidity. In other words, it cannot assess how close a given air mass is to saturation, i.e. clouds.

The wind speed image in the lower left is not useful off Japan because of rain contamination off the coast (black). The integrated water vapor (IWV) image shows a distinct gradient between cool, low-dewpoint air to the north and warm, high-dewpoint air to the south. This gradient marks the location of the polar front.

To the right of the 4-panel is the GMS infrared satellite picture overlaid with NOGAPS surface wind barbs. Some of the strongest winds are off Japan (25 knots). Due to the rain flags in the SSM/I winds speeds, it is impossible to double-check the accuracy of the NOGAPS. However, see the examples below where rain contamination is less of a problem.

Notice how the frontal position marked by the IWV corresponds to a wind shift in the NOGAPS wind field. Cool air to the north has been advected over Japan by north to northwest winds. To the south, the northeast trades have advected in much drier air.

Note that the two vapor products on the 4-panel show two different "fronts". The GMS water vapor image (upper right) suggests that the axis of the jet stream is just east of Japan. This might be called the "upper front". The SSM/I IWV product (lower right) shows a gradient well to the south of Japan. This is the "surface front" and shows the demarcation between polar and tropical air masses at the surface.