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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| Convective Cloud Top |
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| Cloud top height gives the heights of convective and other upper-level
clouds in a color-coded format. It is based on a combination of NOGAPS
and geostationary satellite data. It is especially useful for analysis of
thunderstorms and other weather systems that produce high cloud tops. To
determine cloud top height, we first take the infrared brightness
temperature from each pixel of the geostationary satellite image. The
temperature at this pixel is compared to a NOGAPS vertical temperature
profile that is valid for the same time as the image. Interpolation fills
in between missing NOGAPS levels. When a temperature match is found, that
NOGAPS level is assumed to be the cloud top height. For example, if the
brightness temperature is -40 C, the NOGAPS temperature profile is
searched for -40 C. If the height of the NOGAPS -40 C temperature is
30,000 feet, then that level becomes the cloud top at that pixel in the
cloud top image. For clear skies or cloud tops below 15,000 feet, the cloud top image shows black, meaning no cloud top is determined. The product omits low clouds because of the difficulty in determining their heights correctly. The absence of clouds at low levels should not be taken to mean that clouds do not exist there. The accuracy of the product depends on various factors including the optical depth of the cloud in question, i.e. the thickness of the cloud. But it should generally be assumed to be about plus or minus 5000 feet. |
| The cloud top image is similar in appearance to an IR image, but has different information content. An IR image shows cloud height only indirectly: colder pixels usually represent higher clouds. On an IR image a thunderstorm looks "cold" (or bright white using a black and white color table), and one can assume that the cloud tops of the storm are indeed "high" in a qualitative sense. However, the cloud top product gives the height explicitly. It is especially useful for aviators on long flights. It can be used to estimate the tops of convection that pilots will likely encounter en-route. |
| The cloud top product cannot give cloud tops for clouds lower than 15,000
feet. This is because the assumptions of the product often fail below
this altitude. The "temperature matching" technique, which assumes a
roughly linear decrease of temperature with height, fails at low-levels
where inversions are common. Second, land and sea surfaces often have
nearly the same satellite temperature as low clouds, especially at night.
Thus, the product may type relatively warm land pixels as low-level
clouds. To avoid these problems, our algorithm simply does not allow
clouds below 15,000 feet. The user should not believe, however, that the
absence of cloud tops at low levels in the product means that there are no
clouds there. The second problem concerns thin clouds. Thin clouds may have warmer brightness temperatures than the actual physical temperature of the clouds. This effect occurs because the satellite is "seeing through" the thin clouds to warmer clouds or the warmer surface below. Thus, a thin cloud tends to have heights that are too low, because the temperature matching technique matches them with a NOGAPS temperature that is warmer (lower height) than the physical cloud temperature. |
| Cloud Top Image for the Western Pacific | Cross Section of Cloud Tops from Tokyo to Hawaii |
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| The image on the left shows the cloud top image, color-coded over a range of cloud tops. In mid-latitudes the cloud tops are mostly blue and purple, indicating heights of 15-25 thousand feet. In the southwest portion of the image within the tropics, the convective cloud tops are 50- 55 thousand feet. The image on the right shows a cross section of cloud tops from Tokyo (left) to Hawaii (right). The cross section shows three separate, major regions of clouds, corresponding to three regions of cloudiness along this track, shown in the image on the left. | |
| Cloud Top Image for the Western Pacific | Infrared Image (basis of Cloudtop Image) |
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| The image on the left is the same cloud top image as above. On the right is the infrared image that it is based upon. The infrared image shows temperature only, and colder temperatures (whiter whites) indirectly show higher cloud tops. The advantage of the cloud top image is that it shows cloud tops directly. Notice that some of the cloud systems have "purple" ring around the outside. This purple is often thin cirrus. The product can show thin cirrus with unrealistically low heights. See limitations above. Also, note that the cloud top product shows no low clouds below 15,000 feet. See limitations above. | |
Author: Tom Lee Last Updated: Tue Dec 10 16:34:29 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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