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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| This composite of SSM/I wind speed, SSM/I integrated water vapor, and GOES
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 GOES 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
vapor. 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
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| Link to Paul McCrone's Tutorial on SSM/I, AFWA/Metsat Applications: Tutorial
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| 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 (25 nm)
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 "scatterometer", is truly an active microwave (radar) system. It can show both wind speed and direction. The disadvantages of the Integrated Water Vapor (IPW) 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. |
| Higher winds north of Hawaii | SSM/I & GOES Rain Composite |
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| This mid-winter situation shows a nearly stationary front northwest of the
Hawaiian Islands. In the SSM/I windspeed product (lower left in 4-panel)
and SSM/I precipitation product (right in 2-panel) a thin band of
precipitation is associated with this front. In the wind speed product,
this front separates relatively light winds to the south over the Hawaiian
Islands (blues) from stronger winds northwest of the front (oranges and
yellows). This image illustrates the capability of the windspeed product
to capture narrow windspeed transition zones. The IWV product (lower right on 4-panel) shows the front to the northwest of the islands as the gradient between moist air to the south (light blue) and dry air to the north (dark blue). This gradient represents conditions near the surface. The GOES water vapor product (upper left) does not show this gradient for two reasons. First, it shows water vapor effects aloft and not near the surface like the SSM/I IWV does; and second, clouds contaminate the GOES vapor product unlike in the SSM/I IWV. Beneath this text description are GOES infrared images overlaid with NOGAPS data. The frontal zone to the northwest of the islands is sandwiched in between two high-pressure zones (left overlay). The surface wind overlay (right) shows an increase of wind to the northwest of this zone but does not capture the detail of the SSM/I wind speed product above. | |
| NOGAPS Surface Pressure over IR | NOGAPS Surface Winds over IR |
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| Lee Sheltering by Big Island | SSM/I and GOES Rain Composite |
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| The NOGAPS overlays beneath this text show a typical winter trade wind
regime over the Hawaiian Islands. The SSM/I wind speed product (lower
left on 4-panel above) also shows the trade wind regime but with important
detail. For example, to the west of the big island of Hawaii there is a
small blue spot. This represents a lee sheltering effect cast by the
island's terrain. These lee zones of reduced winds are similarly
positioned on about half of the wind speed images of the Hawaiian Islands.
South of the islands, blacked-out areas on the windspeed image are areas
of rain-contamination. The rain rate in these areas can be seen on the 2-
panel image on the right. The IWV product (lower right on 4-panel) shows a strong gradient of surface moisture just south of the Hawaiian Islands. The GOES water vapor image shows a gradient in about the same place; however, the image gives information about upper level moisture, not surface moisture. | |
| NOGAPS Surface Pressure over IR | NOGAPS Surface Winds over IR |
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Author: Tom Lee Last Updated: Tue Dec 10 16:43:31 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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