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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| Wind Speed Image of Hurricane Gert | Track Map of Gert |
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| Ocean winds generate capillary waves and foam, both of which act 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 using 37 GHz data in
particular, and converted to wind speeds. Other channels are used to
minimize cloud and water-vapor effects. Unlike the SSM/I which is also
used to produce wind speed images, the TRMM sensor has an additional 10.7
GHz channel which enables superior retrievals of wind speeds in regions
of heavy cloudness. Accuracy is of the order of 2 m s-1 (4 knots) for
the precipitation-screened retrievals we present here. Accuracy is less
in contaminated regions which we do not show (see reference below). Use
of the TRMM wind speed parameter near tropical cyclones should still be
somewhat experimental. In particular, the winds are not accurate for
speeds greater than about 30 knots (with decreasing accuracy the winds can
be used up to about 40 knots). Thus, this data set can give little
direct information about storm-force and especially hurricane-force winds.
Connor, Laurence N, Paul S. Chang, Determination of ocean surface wind speeds from the TRMM Microwave Imager. In Review, IEEE Transactions on Geoscience and Remote Sensing For more information on passive microwave radiometry, please visit the following web site:
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| The TRMM wind speed images can be helpful in estimating the radius of gale-force winds. However, it cannot consistently be used to observe gale- force winds directly because the parameter has been properly validated only up to 30 knots. However, the images can reveal regions where gale- force winds are not occurring. Thus, overly-broad spatial estimates of gale-force winds can be reduced significantly in many cases. In addition, the images gives a detailed and revealing view of the horizontal wind profile at the outer regions of a tropical storm. |
| The TRMM wind speed parameter is of limited use over tropical cyclones!
There are several sources of contamination, and users should proceed with
caution! It has not been validated for speeds >15 m s-1 (30 knots). From
30 to about 40 knots the winds may have some accuracy, but no validation
has been performed. It is valid for open ocean areas only and not over
land or near coastlines. The displays shown below substitute visible or
infrared data for precipitation contamination and over coastlines and
land. One of the prime limitations is that rain and heavy cloudiness contaminate the wind retrievals. Since the strongest winds are nearly always in regions of heavy rain, this means that hurricane force winds are never observed. The TRMM wind algorithm outputs only wind speed, not wind direction. This can make the interpretation of images difficult. Where there is precipitation or heavy cloudiness, we substitute visible or infrared imagery on the image. Thus, the centers of storms nearly always have visible or infrared data, not wind retrievals. However, the algorithm does not successfully find all the contaminated regions, which appear as small colored patches. Users should be alert not to misinterpret these areas as valid wind speed retrievals. Fortunately, these regions are small and usually easy to spot. The following are some guidelines for distinguishing valid wind retrievals from the few contaminated regions. 1. In broken fields of convection, common in tropical depressions, contaminated regions are common as broken, colored dots surrounding precipitating convection. 2. Valid wind speed areas are spatially smooth; contaminated values are spotty. Valid wind speeds will form somewhat concentric rings, especially around more intense storms. Contaminated values have a random appearance. 3. Contaminated regions will have a high bias compared to surrounding valid speeds. The "bad" values often exceed 30 knots, the high limit of validated TRMM wind speeds. Low wind speeds, less than about 20 knots, are much less likely to be contaminated. Thus, this rule of thumb: trust low speeds, question high speeds. |
| TRMM Wind speeds in Tropical Depression (Gert) | Corresponding TRMM Rain rate |
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| The TRMM wind speed product (left side) shows wind speeds in color and rain contamination as IR cloud imagery. Outside of the TRMM pass is IR data from the GOES-8 satellite. The image shows future Hurricane Gert as a evolving tropical depression in the eastern Atlantic. The greens shades indicating winds up to about 20 knots are believable. The yellows and especially reds are less believable since they have a spotty appearance and appear just adjacent to convective rain bands (for comparison, see the TRMM rain rate image on the right). This example illustrates that it is risky to use the TRMM wind speeds over weak, disorganized storms. | |
| TRMM Wind speed image shows speeds faster north of the Storm | Corresponding TRMM Rain rate image |
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| The highest wind speeds on the TRMM image (left side) are yellow indicating wind of 20 to 25 knots. These winds are believable because they cover a broad area and do not lie immediately adjacent to heavy rain bands (see rain rate image on right side). Although no gale-force winds are observable on the TRMM image, it would make sense to guess that gale- force winds extend further to the north of the storm than to the east where the available TRMM wind speeds are lower. | |
| TRMM Wind speed Image shows Gale force winds on West Side | Corresponding TRMM rain rate image |
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| On the west side of Gert winds speeds increase approaching the core of the storm. The interior is rain-flagged precluding the view of the analyst of faster winds. However, the red shades on the west side indicate maximum winds of about 30 to 35 knots. Since these red winds are relatively uniform and are generally away from rain (see rain rate image on right side), they can be used to fix the radius of gale-force winds. On the east side of Gert, however, the fast (red) wind speeds should be viewed with suspicion. The one small area appears to be biased by a nearby convective band. | |
| Gale force wind speeds well south of Gert | Little Precipitation in TRMM swath |
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| Since there is relatively little precipitation in the TRMM swath, the wind speed retrievals appear very reliable. Gale-force extend about 120 nm south of Gert. This examples shows that even when a TRMM swath passes well away from a tropical cyclone, useful information can be derived. | |
Author: Tom Lee Last Updated: Tue Dec 17 15:28:53 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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