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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| The 85 GHz Color Composite image gives a high-resolution view of the
structure of tropical cyclones. Of all the 85 GHz products we
show, the Color Composite provides the most consistent and least ambiguous
view. For non-quanitative applications, it is strongly
recommended for routine use. To the left is an example from an unnamed storm, later to become Typhoon Virgil. To the right is a track of Virgil in the western Pacific. The Tropical Rainfall Measuring Mission (TRMM) is a satellite that orbits the tropical areas of the earth with a mission to derive rainfall estimates for climate studies. However, it has also proved invaluable as a means to observe tropical cyclones in near realtime. Aboard the TRMM is the TRMM Microwave Imager (TMI) that was based on the SSM/I instrument. The key differences on TRMM are the addition of a pair of 10.7 GHz channels with horizontal and vertical polarizations. TRMM flies at about half the altitude of the SSM/I and therefore has a narrower swath width (759 km) and smaller footprint sizes. The footprint size for the widely used 85 GHz channels is roughly 6 kilometers. Thus, TRMM images have a "sharper" appearance than SSM/I images.The 85 GHz color combination image is created through a color combination process. Three different input images are used. The 85 GHz V image is cast in the blue color gun, displayed from 270 to 290 K. The 85 GHz H image is cast in the green color gun, displayed from 240 to 300 K. The PCT image (based on the difference of 85 GHz H and V) is cast in the red gun and displayed inversely (lowest value the brightest, highest value the darkest) from 220 to 310. The result is a product that takes advantage of the different strengths of the input products but does not suffer from their individual limitations. Deep convection appears in red, an effect contributed by the PCT image. Low-level clouds, water vapor and warm precipitation (no ice process) all appear as blue-green. Relatively cloud-free and dry atmospheres over ocean areas outside of the storm environment appear gray or black. For more information on passive microwave radiometry, please visit the following web site:
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| Unlike either of the 85 GHz images shown by this page, there is never a problem on this image distinguishing deep convection (red) from the sea surface (gray or black). It also shows low- level clouds, vapor, and warm rain (all three blue-green). It can be used to follow intensity changes from one storm to another. Near the start and end of storm lifecycles, the blue-green coloration will often dominate in the absence of convection. The blue- green regions will often trace a low-level cyclonic circulation. If the red areas increase in size and become more organized over time, it generally means a storm is becoming stronger. As storms get stronger, often the amount of blue-green will decrease, becoming obscured by the overlying storm convection (red). If red areas decrease in size or appear less organized over time, it generally means the storm is weakening. |
| The color composite product is a qualitative guide only. Unlike the 85 GHz and PCT products, the colors do not correspond to specific brightness temperatures (Tb). The product is tuned for tropical cyclones within the tropics over ocean regions. When storms become extratropical, the interpretation rules set down here may not apply. When storms move over land, interpretation of the product is more limited. It can still be used to identify deep convection (red) characterized by ice-phase precipitation aloft. However, it can no longer be used to identify low-level clouds, precipitation or water vapor. (These features have similar Tb as land and therefore blend in with the land background.) Therefore, over land any inferences about storm structure must come from observing the convection aloft (red). |
| 1A TRMM color product shows Center of Circulation | 1B Corresponding Visible Image |
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| The image on the left is the color image for Virgil in the develoment stage. Outside of the TRMM coverage area the image shows GMS visible. The juxtaposition of visible and the TRMM product allows spatial continuity across the TRMM pass edge. The TRMM product shows the low- level center of the storm in a bright cyan. A patch of red to the left (west) of the center indicates deep convection. | |
| 2A Virgil gaining strength -- Red indicates deep convection | 2B Corresponding Visible |
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| Six hours later the storm is more organized. The TRMM color image (left side) shows cyclonic convective bands in red on the east and west sides of the circulation. The bright cyan color in between the two convective bands is the low-level center of the storm. | |
| 3A Virgil near max Strength -- TRMM pass catches half the Storm | 3B Corresponding Visible |
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| The red shades (color product on left) near the center of the storm show deep rain bands. The storm eye can be seen faintly in this image, even though it does not appear through the cirrus shield on the visible image on the right. Physically, the red shades indicate precipitation scattering by large ice particles (snowflakes) above the freeezing level. Thus, quite literally the red shows a snowstorm aloft in a typhoon. The low-level circulation is shown by a cyan color. | |
| 4A Deep convection diminishing--exposing low-level center | 4B Corresponding Visible shows strong shear |
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| The visible image on the right shows significant "blowoff" toward the southwest, a sign of strong vertical shear. To the northeast on the visible image a portion of the low-cloud center is now exposed. On the left is the color product. Only a few small areas of deep convection remain (red). Bright cyan shows the low-level circulation. | |
| 5A Color Product same as directly above | 5B TRMM rainrate product |
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| The rainrate image shows (right side) shows TRMM rainrates overlayed on top of visible imagery. The diagonal lines indicate the extent of the TRMM coverage. The heaviest rain appears in the regions of deepest convection (red areas, image on left side). Lighter rains appear over the central portion of the storm. | |
Author: Tom Lee Last Updated: Mon Dec 9 16:55:42 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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