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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| The SSM/I wind speed parameter, which gives oceanic surface wind speed but
not direction, has the most utility away from heavy clouds and
precipitation. Thus, its application within tropical cyclones islimited.
The main utility of this parameter is outside most of the heavy clouds and
precipitation and on the fringes of the storm where wind speeds can be
successfully observed. On the left above is an example of a wind speed image for Typhoon Dan,which will be discussed in detail below. To the right above is a trackmap of Dan. Ocean winds generate capillary waves and foam, both of whichact to roughen the surface at micro-wavelengths and increase its emissivity. This increase can be correlated with surface wind speed. Under rain-free conditions, this emissivity increase can be detected usingparticularly the 37 GHz data, and converting it to wind speeds. Other channels are used to minimize cloud and water vapor effects. The current operational SSM/I algorithm is a version of the Goodberlet et al. (1989), which uses 19V, 22V, 37V and 37H data. Accuracy is on the order of 2-5 m/s(5-10 kt), varying with atmospheric contamination. The presence of rainor thick clouds can significantly degrade the accuracy of the wind speedestimates. In storms with high wind gradients, the wind speed values willbe underestimated. Use of the SSM/I wind speed parameter near tropical cyclones should be considered very experimental. In particular, since thewinds are not valid for winds greater than 40 kt, this data set can giveno information about hurricane-force winds. Even when the storm windspeeds are within the observable 0 - 40 kt range, precipitation contamination is a frequent problem. It is most useful to use the windspeeds to observe the outer periphery of storms where rain and heavy clouds are absent. Goodberlet, M. A., Swift, C. T. and Wilkerson, J. C., Remote sensing ofocean surface winds with the Special Sensor Microwave/Imager, J. Geophys.Res., 94, 14,574-14,555, 1989. | |
| Link to Paul McCrone's Tutorial on SSM/I, AFWA/Metsat Applications:Tutorial
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| The SSM/I wind speed product is of sufficient accuracy that it has been assimilated into NOGAPS with documented model improvement. Over the open ocean and away from major storms, it can give detailed information about trough lines, frontal locations and calm regions. It can show elevated wind speeds on the outer edges of a storm, even if it cannot see the rain- contaminated speeds near the center. It is useful to view the wind speed parameter in conjunction with visible and infrared satellite pictures, or surface wind products produced by numerical models. |
| The SSM/I wind speed parameter is of limited use over tropical cyclones!
NRL Monterey posts this product for experimental purposes only. There are
several sources of contamination, and users should proceed with extreme
caution! It cannot be used for wind speeds >20 m s-1 (40 kt). It isvalid
for ocean areas >50 km (25 nm) away from the nearest coastline. Itis not
valid in areas flagged as containing precipitation. On the displays shown
here, coastal areas and rain-flagged areas are filled with infrared or
visible imagery. Just adjacent to these contaminated areas, a fringe of
high wind speed values will sometimes appear. While these may represent
valid wind speeds, they should sometimes be disregarded as erroneous.
These biased areas can may be identified by looking at the corresponding
infrared image to locate regions of heavy clouds, which probably indicate
contamination. The SSM/I wind speed product does not give wind direction; that is givenby radar scatterometers. Thus, it is sometimes difficult to understand a wind speed image unless the forecaster also refers to surface wind charts or satellite pictures of the same region. |
| 1. SSM/I wind speed over Tropical Depression (future Typhoon Dan) | 2. SSM/I rain rate over Tropical Depression |
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| The SSM/I wind speeds in the vicinity of Dan are low, mostly much less than 20 knots. Rain flagged-areas substitute infrared imagery from the corresponding GMS satellite pass. Also, infrared imagery appears over the entire area that is outside the SSM/I pass region (yellow). The rainrate image (Fig. 2) shows that the precipitation in Dan is quite light. Notice the rain-flagged regions for the wind speed image (no wind speed retrievals shown) are larger than the areas of active rain shown by therain rate product. This is because even heavy cloudiness just at the verge of raining is enough to contaminate the wind speed and is therefore not included in the wind speed image. | |
| 3. SSM/I wind speed image, | 4. Corresponding SSM/I rain rate image |
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| The rain rate image (Fig. 4) shows much more precipitation over the central core of the storm than the rain rate image (Fig. 2) about 14 hours earlier. However, the wind speed image (Fig. 3) shows little increase in winds near the outside of the storm over this period. | |
| 5. Dan approaches China | 6. SSM/I Rainrate |
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| Most of the wind speed retrievals within Typhoon Dan (Fig. 5) are contaminated and therefore filled in with corresponding infrared imagery. This means that typhoon and gale-force winds cannot be observed directly. The rain rate image (Fig. 6) shows that many of the contaminated wind speed regions are also precipitating. The most useful wind speeds appear on the southeast side of the storm where speeds up to 25 knots can be observed (yellow). | |
| 7. Landfall of Dan | 8. Relatively light rain over China |
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| Most of the wind retrievals over Dan are contaminated either by rain or coast/land (Fig. 7). Thus, only slow winds are observed to the southwest (up to 20 knots). Rain is falling associated with the weakening storm (Fig. 8). | |
Author: Tom Lee Last Updated: Tue Dec 17 15:28:16 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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