Today's discussion looks at GOES-8 10.7 micrometer average radiance and maximum radiance
(minimum brightness count) composites comparing December 1996 with 1997. Figure 1 depicts
the minimum brightness count (maximum radiance or warmest temperature) for 1015
(left/night) and 2215 (right/day) UTC for 1-31 December in 1996 (top) and 1997 (bottom).
These images were derived by searching for the minimum brightness count at each pixel over
1-31 December for the particular hour. The derived images indicate that 1997 was warmer
than 1996 in many regions, both over land and ocean. The temperature difference (1997-1996)
of these minimum composites is shown in Figure 2, with blue indicating 1997 is colder
than 1996 by 1-2 degrees C, and red indicating 1997 is warmer than 1996 by 3-5 degrees C.
One feature of interest is the oval (significant difference) region in the Pacific Ocean
off the west coast of Costa Rica that is visible in the 1015 UTC image. Climatologically,
upwelling occurs in this region and causes cooler waters than those surrounding it.
The Tropical Marine Climatic Atlas (Sadler et al. 1987) shows the existence of cooler
waters in the same vicinity starting in December and lasting through February, the dry
months in Central America. The December 1997 imagery indicate that the cool pool has not
fully developed. Browse the Climate Prediction Center's (CPC) ENSO Diagnostic Advisory
for Sea Surface Temperature departures from normal for December 1997:
animation,
Our minimum brightness count product for the 10.7 imagery does not account for low level
moisture contamination and will not represent sea surface temperatures accurately. We
only have 2 years of data for comparison, but the trends seen in these derived products
correspond with those of CPC and suggest that December of 1997 was an abnormally warm year.
Figure 3 depicts the 10.7 micrometer average radiance for 1-31 December for 1996 (top) and 1997 (bottom) for 1015 (left) and 2215 (right) UTC. These images indicate that it was less cloudy and hence warmer and dryer in 1997 than in 1996. In fact, the averages for 1997 look very similar to the minimum brightness count composites, again indicating less cloudiness and hence warm and dry weather. For comparison, browse the Two-Week Old through Six-Week Old Global Climate Highlights (Covering December 1997) for the Southern Caribbean and Northwest South America discussing warmer and dryer weather over the region:
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Figure 1
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Figure 2
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A comparison of the remapped 10.7 um IR image with the remapped GOES-8 and GOES-9 composite 6.7 um (water vapor) image (right)
reveals that even in the coldest and probably driest (in the absolute sense) air masses, the water vapor channel does not indicate direct
sensing of the surface. The clouds are easier to discern in the 6.7 um image because the cold surface background is not seen and the
warmer brightness temperatures associated with the water vapor offer a better contrast with the colder cloud areas.
Finally, below is the 1215 UTC image with the surface temperatures overlayed on the 10.7 um IR image. Where the skies are clear,
notice how similar the surface temperatures and brightness temperatures are. For instance, the -37 C on the west coast of Hudson
Bay compares with -39 C brightness temperature. Similar comparisons were done for the area north of the approaching cloud
system and to the south of Hudson Bay and were found to be within 4 degrees C.
Brian Motta
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Figure 3
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