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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| SSM/I 4-panel Composite |
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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 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. |
| Fair Weather--Light Winds | NOGAPS Surface Winds over IR |
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| The SSM/I windspeed image (lower left image in 4-panel on left) shows low wind speeds over the Atlantic off the East Coast. The blacked-out band shows a area of rain associated with a cold front: rain contaminates SSM/I wind speed retrievals. Notice that the GOES/NOGAPS image on the right confirms the light windspeeds over the region. The Integrated Water Vapor (IWV) image (lower right in 4-panel) shows the front as the gradient between moist air (yellow) and dry, post-frontal air (blue). | |
| Sheltering by Cape Hatteras | NOGAPS Surface Winds over IR |
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| Nearly two days after the previous study time above, a cold front is
pushing through the East Coast and into the Atlantic. The GOES/NOGAPS
image overlaid with surface winds (right side) show significant south to
southeast marine winds ahead of this system. The SSM/I wind speed image
(lower left in 4-panel composite on left) also shows elevated wind
speeds. However, the SSM/I wind speed shows a important mesoscale event
not captured by the NOGAPS winds. Notice that the SSM/I wind speed
product shows a dramatic gradient between stronger winds (yellows) south
of about Cape Hatteras and much weaker winds (blues) to the north. The
lower winds are apparently a leeside effect created by the Cape protruding
into the southerly flow. The IWV product (lower right in the 4-panel) shows the approaching moisture of the frontal system in yellows and reds. The GOES water vapor image (upper right in the 4-panel) also shows elevated water vapor values ahead of the front. However, it shows vapor at upper levels, not near the surface like the SSM/I product. | |
| Frontal system moving offshore | NOGAPS Surface Winds over IR |
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| Twenty-four hours after the previous example, the frontal system has fully pushed into the Atlantic. The SSM/I wind speed product (lower left in 4- panel) reveals that wind speeds are not particularly strong with the system except to the northern part of the image to the east of the frontal system. Here, red shades indicate elevated speeds. Notice that rain- contaminated wind speeds are blacked-out in the image. The IWV (lower right in 4-panel) image shows the front distinctly with light blue shades indicating prefrontal air ahead of the front, yellow/red shades indicating the frontal air mass, and dark blue shades indicating the postfrontal air mass. | |
Author: Tom Lee Last Updated: Tue Dec 3 07:16:35 2002 Produced by: The Composer (Ver: 1.1.1 ) |
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