INTERPRETATION
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Regional and Mesoscale Meteorology Team Cooperative Institute for Research in the Atmosphere (CIRA) Colorado State University Fort Collins, Colorado |
April 7, 1999
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Temperature Analysis of Tropical Weather Systems |
Figure 1
Figure 2
Figure 3
Figure 4
Note: AMSU temperature retrieval files are provided by Mitch Goldberg, NOAA/NESDIS Office of Research and Applications, Washington, DC. |
Figure 1 is a GOES IR image
of the South Pacific region on March 1, 1999. Two distinctive cyclonic
cloud patterns are seen, one circulating around 32S 157W and another one
at 33S 133W. Qualitatively their appearance is quite similar suggesting
they may be very similar with regard to size and intensity. However,
an analysis of the NOAA-15 AMSU temperature retrievals reveals that their
vertical thermal structures are quite different. The center of the easternmost
system is colder than its surroundings in the middle troposphere, while
the one further west has a mid-level warm core. The histories of
the two cyclones also have distinct differences. The western cyclone
is a weakening tropical cyclone tracking to the south. It originated
several days earlier near the Cook Islands around 15S 165W, was named Tropical
Cyclone Gita, and attained a maximum intensity of 50 knots (25 m/s).
The eastern cyclone is a subtropical low which never developed tropical
cyclone characteristics.
AMSU temperature retrievals are analyzed over the same area at approximately the same time in Figures 2-4. The AMSU senses the atmospheric temperature even in the presence of clouds, except when liquid water is present. In the upper troposphere, liquid water influences are absent and at middle levels they are usually negligible. Figure 2 shows the AMSU temperature analysis at a pressure level 570 hPa. At this middle tropospheric level, the subtropical low has a distinct cold core structure and the tropical cyclone shows a very weak warm core. At 350 hPa, shown in Figure 3, the subtropical low shows a weaker cold core, and the tropical cyclone has a warm anomaly with magnitude close to the one in Figure 2. Intense tropical cyclones at this level have shown AMSU measured warm anomalies of several degrees C, however the level of the maximum warm core is typically up at 250-300 hPa, where the anomaly is 10-15 degrees C, for intense hurricanes. Figure 4 is at 250 hPa shows a better defined warm anomaly with TC Gita, and at this level shows a warm core rather than a cold core with the subtropical low. If one assumes that the atmosphere is hydrostatic and in gradient wind balance, then the maximum cyclonic vorticity is located at the level where the cyclone is cold core below and warm core above. With tropical cyclones this is very near the surface, but with subtropical cyclones it is in the upper troposphere. Therefore, with AMSU temperature retrievals alone, in the tropics and subtropics, one can deduce the vertical structure of winds, as well as temperature, with individual weather systems. Mid-latitude (extratropical) cyclones have a more complex
structure. They are strongly influenced by the stratospheric component
of the
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