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 kt) 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 a 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 the GOES satellite, see:

Menzel and Purdom BAMS Article

NOAA NESDIS

University of Wisconsin CIMMS

University of Wisconsin SSEC

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.

There are several disadvantages concerning the Integrated Water Vapor (IPW) display.

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 integrated water vapor (IWV) product (lower right of 4-panel) shows a uniform blue color off the California coast, suggesting an even distribution of low- level water vapor. The gradient between blue and orange off Baja California on the IWV image suggests a frontal boundary between dry air to the north and much moister air to the south. Notice that the GOES-10 water vapor image (upper right of 4-panel) shows high clouds over the frontal boundary off Baja California. The presence of clouds means that no conclusions about moisture are possible. However, off Southern California in cloud-free air, the water vapor image shows an upper-level moisture feature (green). This feature is being shaped by the winds aloft. Notice that this feature does not appear on IWV panel. The IWV shows primarily surface moisture, not moisture aloft like the GOES water vapor product.

The IWV product (lower right panel of 4-panel) again shows a uniform distribution of moisture off southern California. In contrast, the GOES water vapor image (upper right panel of 4-panel) shows a water vapor "wave" aloft off southern California. The GOES water vapor image shows this feature because it is sensitive to moisture aloft, unlike the IWV product, which is mainly sensitive to variations in surface moisture. The NOGAPS winds (right panel) shows winds at the 300 mb level superimposed on the GOES infrared image. These winds indicate shear aloft over the Southern California region. This shear corresponds to the "wave" shown by the GOES water vapor image.

The IWV product (lower right in 4-panel) shows uniform low- level moisture conditions, except for an elevated region of moisture off the Central California coast (green). This band appears to be the remnants of a weak frontal system moving through the region. Notice that the upper- level distribution of moisture (shown by the GOES water vapor image upper right of 4-panel) shows a very different distribution of moisture features. This difference is explained by the difference between the two products. The SSM/I IWV shows moisture variations near the surface, while the GOES water vapor product shows conditions aloft.