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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| Track Map of TD 05, Eastern Pacific, 1997 | Track Map of Danielle, Atlantic, 1998 |
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| The 85 GHz HW image can show important detail over weak tropical cyclones. It enhances the relatively warm end of the 85 GHz scale. Shown above are track maps for two storms analyzed in this section: Tropical Depression 05(1997) in the eastern Pacific and Hurricane Danielle in the Atlantic. | |
| Link to Paul McCrone's Tutorial on SSM/I, AFWA/Metsat
Applications: The 85 GHz H (W) image is created by stretching the higher brightness temperatures (Tb) from 220 to 285 K. It is intended for use over weak storms to enhance low-level circulations that might otherwise go undetected. Thus, it is most useful toward the start and end of the lifecycle of a storm when deep convection is often absent. It enhances the detail over water clouds, water vapor, and warm precipitation (no ice phase) bands. These features will appear with higher Tb (around 270 K) than surrounding cloud-free ocean areas (averaging around 250 K). Deep convection appears in Tb lower than about 255 K. Low-level cloud features generally appear as blue against a green background. |
| The 85 GHz W image shows circulations comprised of low-level clouds, water vapor, or warm rain. Thus, it can be used to trace cyclonic circulations that are often the only tracers of weak tropical cyclones. It can be used to infer the center of circulation when an eye is absent. It is especially useful at night because infrared images often fail to show weak circulations adequately. It is useful at all times of day when cirrus overcasts obscure the storm environment. |
| The 85 GHz W image is not designed to show the lowest Tb associated with storm convection. Use the 85 GHz, PCT, or color composite images for this purpose. The lowest temperature displayed on this product is 220 K. Another drawback is that cloud-free ocean backgrounds outside of the storm environment sometimes have low Tb which match the depressed Tb of storm convection. This can lead users to misidentify the sea surface for convection and visa vera. To avoid confusion in these instances, use the PCT or color composite images. Also, check the infrared image to ensure that low 85 GHz Tb appear in regions of cold cloudiness. |
| Dark blue areas trace low clouds of tropical depression on 85 GHz H (W) enhancement | Regular enhancement fails to |
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| The above images are of Tropical Depression 05 off Mexico in 1997. The 85GHz W enhancement on the left shows the storm center as dark blue gray shades against a green background in the center of the SSM/I image. The dark blue represents low clouds with some possible light rain (see rainrate example of same image below). Deep convection, especially in aband to the south of the center of the storm, appears in yellows, reds, and blacks. On the right is the unenhanced 85 GHz H enhancement which also shows the storm center. The lack of contrast makes it more difficult to interpret than the "W" enhancement. On this unenhanced image cloudwater appears as somewhat darker blue against a light blue cloud-free background. | |
| 85 GHz H (W) Enhancement sees circulations missed by other satellite sensors | GOES Infrared fails to see low-level circulation |
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| The image on the left is again the 85 GHz W image of Tropical Cyclone 05 off Mexico in 1997. To the right is the corresponding GOES infrared image. Notice that the low cloud details of the storm center can barely be seen if at all. This effect occurs for two reasons. First, low clouds have little thermal contrast with the ocean background. Thus, they tend to blend in and "disappear" on images. Second, the tropical atmosphere absorbs infrared energy, often partially obscuring the detail of near- surface features like low clouds. Notice that the deep clouds shown by the infrared image (for example the band in the southern portion of the image) corresponds well with depressed 85 Ghz Tb (reds, yellows, and blacks) shown in the 85 GHz H (W) image on the left. | |
| Dark Blue Circulation is low clouds, | Rainrate image |
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| The image on the left is again the 85 GHz W enhancement as shown above. The SSM/I rainrate image appears on the right. Notice that heavy precipitation, shown by blues and yellows, appears in the convective band across the southern part of the rainrate image. This heavy precipitation correlates well with depressed Tb in the 85 GHz W enhancement (image on left). Light rain rates (image on right) appear to the north of this band. However, these rain rates cover only a portion of the area corresponding to the dark blue cyclonic low cloud circulation on the 85GHz W image. This suggests that this area of low clouds is associated with light or no precipitation. Since the infrared image just above showed Tb higher than 0 C in this region, we can infer that any precipitation in this region is "warm" rain, i.e., no ice phase precipitation. | |
| 85 GHz H (W) image shows low cloud | IR image shows cirrus tops -- no low-level detail |
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| The 85 GHz H (W) enhancement on the left shows Danielle as a minimal
hurricane. Convection is limited shown by reds with embedded black. The
W enhancement also shows a large area of dark blues, especially south of
the convective center. These blue areas represent low cloud bands which
are a part of the low-level cyclonic circulation. Land, such as the
islands to the south, also appear as blue. To the northwest, red
(relatively low Tb) indicates a low-moisture region. Care must be taken
not to misinterpret these dry regions as convection which is also
characterized by low Tb. The infrared image on the right shows little evidence of the low-levelcirculation shown by the 85 GHz H (W) image. | |
| 85 GHz H (W) image of Danielle | Rainrate Image from the same SSM/I pass |
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| The 85 GHz H (W) image is the same as above (left). The rainrate image to the right shows precipitation rate within Danielle. Notice the heaviest precipitation falls from convection (low Tb) as seen from the 85GHz H (W) image on the left. Lighter precipitation falls from the lowclouds (blue areas from the image on the left). Thus, precipitation is produced from both convective clouds (ice phase processes) and shallow clouds (warm precipitation--no ice processes). | |
| 85 GHz H (W) image of Danielle | 85 GHz Color Composite |
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| The 85 GHz H (W) image (left) is replicated from the above examples. Itis compared with the 85 GHz color composite image (right). The 85 GHz H(W) image is ambiguous because both storm convection and the dry area inthe northwest portion of the image are red. The color composite image eliminates this ambiguity: storm convection is red, but the dry region to the northwest appears dark gray. The color composite image shows regions of heavy water vapor with embedded low clouds as blue-green. Blotches of red within the blue green clouds mark deep convection. This represents the moist storm environment. Islands also appear blue-green with red blotches representing thunderstorms. | |
Author: Tom Lee Last Updated: Wed Dec 18 09:18:08 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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