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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| Hurricane Blas near maximum intensity | Track map of Hurricane Blas |
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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 here
(85 GHz, 85 GHz Weak, PCT, Color Composite), 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 Hurricane Blas. To the right is a track of Blas, the storm used as an example below. | |
| Link to Paul McCrone's Tutorial on SSM/I, AFWA/Metsat
Applications: 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. Users wanting more information about SSM/I and tropical cyclone applications are strongly urged to read: Spencer, R.W., H.M. Goodman, and R.E. Hood, Precipitation retrieval over land and ocean with SSM/I:Identification and characteristics of the scattering signal. J. Atmos. Ocean. Technol., 6, 254-273, 1989. |
| 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 beused to follow intensity changes from one storm to another. Near the start and end of storm life cycles, 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 sometimes 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 85GHz 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 overland, interpretation of the product is more limited. It can still be usedto 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 blend in with the land background. Therefore, over land, any inferences about storm structure must come from observing the convection aloft (red). |
| Blas forms in Color Composite Image | 85 GHz H Comparsion -- Low clouds hard to detect |
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| The color composite image on the left shows Blas in the development stage off Mexico. Red marks deep convection on the east side of the storm. Bright blue spiral bands indicate low clouds on the western side of the storm. The Mexican mainland is bright blue with redmarking thunderstorms. On the right side the 85 GHz image shows the convection in green with embedded red. The low-level spiral bands are rather difficult to detect, appearing in a darker blue than the ocean background. | |
| Blas develops in 85 GHz Color Composite | Infrared Image with eye partially covered by cirrus |
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| The color composite image on the left shows a developing eye mostly enclosed by convection (red). Low-level cloud bands (light blue)form a cyclonic circulation that extends well away from the storm center. Note that the developing eye in the corresponding infrared image (right) is smaller than in the composite image on the left. This suggests that eye is partially obscured by thin cirrus. | |
| 85 GHz Color Composite shows well-defined eye | Infrared Image shows only indistinct eye |
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| The color composite image early on the 24th shows a completely-enclosed eye and a well-defined convective circulation (image on left). The infrared image valid for the same time (right) shows the eye less distincly. Note that much of the infrared image is covered withcirrus (white color). Cirrus is transparent in the SSM/I and contributes no effect on the color composite image. | |
| Color Composite shows Blas | Corresponding Infrared |
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| The color composite image above (left side) shows an intense hurricane with two outer convective rainbands (red) in addition to the eye wall. The infrared image (right) shows considerable cirrus which hides much of the lower rainband structure. | |
| Color Composite shows Blas starting to weaken | Corresponding Infrared |
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| By June 26, Blas has started to weaken, as shown bythe color composite image (left side). The area of convection (red) has decreased and no longer completely encircles the storm.The storm is moving northwest into an area of lower sea surface temperatures and has less water vapor available to it. The reduced moisture is shown in the extreme northwest corner of the swath by the black color. | |
| Color Composite shows waning convection (red), | Corresponding Infrared |
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| Two days after the previous examples, the color composite image (left) shows continuing weakening of Blas. It has now moved into an area of lower sea surface temperatures and less water vapor. The dark gray coloration outside the cyclonic storm circulation represents dry subtropical air, which is enveloping the storm. The storm still has a well-developed low-level circulation (bright blue-green), but the convection aloft (red) has decreased. The infrared image (right) also shows the diminishing storm. | |
| Color Composite suggests that Blas | Corresponding Infrared shows little trace of storm |
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| Thirty-six hours after the previous images, the color composite image (left) shows practically no convection (red) remaining. The storm circulation is aparent only in the low levels as shown by the bright blue- green. The dark gray coloration to the north and south of the storm indicates a low-moisture subtropical air mass that cannot support a tropical cyclone. The infrared image (right) shows a shearing effect. The upper clouds are drifting to the northeast in the upper-level flow. The low-level circulation is moving toward the northwest and can barely be discerned on the image. This image pair illustrates the usefulness of the color composite product to detect low-level circulations not captured wellby the infrared image. The final color composite image shows the low- level storm circulation (blue-green) merging into subtropical stratocumulus to the west. The absence of red color indicates that no upper-level convection is present. No independent surface reports are available. However, it is doubtful that this mass of clouds is producing more than drizzle. The infrared image (right) shows very little trace of low-level circulation. A few remnants of high clouds appear to the distant northeast. | |
Author: Tom Lee Last Updated: Tue Dec 17 15:25:19 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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