NRL Monterey, Marine Meteorology Division
|SSM/I Surface Wind Speed and Integrated Water Vapor Composite|
|This composite of SSM/I wind speed, SSM/I integrated water vapor, and GMS
infrared and water vapor gives valuable information about the marine
operating environment. The information coming from two different
satellite sensors gives users an enhanced perspective that is impossible
to get from a single satellite alone. The SSM/I products give information
about the near-surface wind field and moisture. The GMS products give a
broader view. See Background, Advantages, and Limits below for more
information. The Special Sensor Microwave Imager (SSM/I) is a passive
microwave imager aboard the DMSP satellite series. It has seven different
channels measuring the earth's radiation field at different frequencies.
Scientists have combined the brightness temperatures, which represent
radiation measured at these frequencies, in algorithms to produce
geophysical parameters, such as surface wind speed and integrated water
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:
|Link to Paul McCrone's Tutorial on SSM/I, AFWA/Metsat Applications: Tutorial |
|The SSM/I wind speed parameter allows observation of fine-scale detail in
the surface wind structure. It is particularly useful in regions where
ship reports are lacking. In particular, it can show detail that global
models (like NOGAPS) cannot resolve. It can be used to check the accuracy
of mesoscale models (like COAMPS). It is especially useful in the
observation of topographically forced winds, such as gap winds in coastal
regions. When multiple SSM/I's are operating on multiple DMSP satellites,
it is possible to observe changes in the wind field from one product to
the next. |
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.
|The SSM/I wind speed has several disadvantages. It is only good for
speeds up to about 40 knots. It is not useful within about 50 km of
coastlines. It will not work where precipitation is falling. In the
displays shown here, all these problem areas are blacked out, so that only
valid values are shown. However, with the current algorithm, there is an
additional problem. In areas of heavy cloudiness or drizzle, the wind
speeds may be overestimates. Unfortunately, these areas are sometimes not
blacked out. Future algorithms promise to correct this problem. |
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.
|Rain-contaminated SSM/I Wind Speeds||NOGAPS Surface Winds over IR|
|The SSM/I composite at the right shows January conditions centered on
Japan in the western Pacific. The upper left panel shows the infrared
image. Cloud streets to the east of Japan indicate wind flow of cold air
onto warmer Pacific waters. The GMS water vapor image in the upper right
gives little information about surface conditions because the 6.7- micron
channel is sensitive to moisture in the middle troposphere. |
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.
|Gap Wind off Japan||NOGAPS Surface Winds over IR|
|The composite on the left shows a very strong gap wind event between the
Japanese islands of Hokkaido and Honshu. This is shown by the cloud
streets on the infrared image (upper left) and especially the SSM/I wind
speed image (lower left). Since rain contamination is limited on the wind
speed image, it is possible to observe that wind speeds nearing 40 knots
occur at the mouth of the island gap. The NOGAPS overlay of surface wind
on infrared shows wind speeds of only 25 knots in this region. This
discrepancy occurs because NOGAPS cannot resolve the narrow gap wind shown
by the SSM/I. |
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.
|Light winds return off Japan||NOGAPS Surface Winds over IR|
|The final 4-panel composite was imaged 11 hours after the 4-panel image above. Note that the gap wind is now completely gone, suggesting that these events can arise and dissipate very quickly. The NOGAPS overlay image to the right also suggests greatly reduced winds.|
Author: Tom Lee
Last Updated: Tue Dec 10 16:33:36 2002
Produced by: The Composer (Ver: 1.1.2 )