NOAA-NESDIS
Regional And Mesoscale Meteorology Team
Satellite Interpretation Discussion
September 11, 1998
The purpose of today's satellite interpretation discussion is to provide evidence that significant reflection of 3.9 µm solar radiation does occur at the top of some Central Plains thunderstorms.  Figures 1 and 2 display the 3.9 µm and 10.7 µm images at 2130 UTC, respectively.  A developing thunderstorm over north central Nebraska is designated as CLOUD 1 while the thunderstorm over east central South Dakota is identified as CLOUD 2.  The small boxes, superimposed on the storms tops, indicate where readings were taken for the 3.9 µm and 10.7 µm average brightness temperatures. 
 
Figure 1 
(3.9  µm image) 
Click picture to enlarge
Click to view 3.9 loop 
Figure 2 
(10.7 µm image)
Click picture to enlarge
Click to view 10.7 loop
 
A time series of the average brightness temperatures for CLOUD 1 and CLOUD 2 are shown in Figures 3 and 4, respectively. The average 3.9 µm brightness temperature in the small box on CLOUD 1 was slightly above 0 C at 2130 UTC while the average 10.7 µm brightness temperature was -50 C (Figure 3).  Inspection of the time series for this cloud shows that while the sun was setting, the average 3.9 µm brightness temperature decreased approximately 45 C from 2130 to 0130 UTC. Over the same period, the average 10.7 µm brightness temperatures remained relatively constant at -53 C. This suggests that the cooling observed at 3.9 µm was due to the reduction of reflected solar radiation from the storm top to the satellite while the sun was setting. 

A similar plot for CLOUD 2 is shown in Figure 4. The average 3.9 µm brightness temperature decreased 5C while the 10.7  µm temperature increased 9C. This suggests that reflection of solar radiation did not contribute significantly to the brightness temperature characteristic of CLOUD 2 over this time period. 
 
Figure 3 
(10.7 µm image)
Click picture to enlarge.
 Figure 4 
(3.9 µm image)
Click picture to enlarge. 

Further, CLOUD 1 was 27C warmer than CLOUD 2 at 2130 UTC when there average 10.7 µm brightness temperatures were the same for both clouds. This suggests that CLOUD 1 was reflecting significantly more solar radiation than CLOUD 2. At the same time the 3.9 µm and 10.7 µm ifference was 51C for CLOUD 1.  This difference was 20C larger than that for a marine stratocumulus layer west of California (whose 10.7 µm temperature was above 0C). This suggests that the ice layer 
comprising the top of CLOUD 1 was a better reflector of 3.9 µm solar radiation than cloud liquid water. 

Just how reliable is the 2130 UTC average 3.9 µm brightness temperature for CLOUD 1? Shown in Figure 5 is the relationship between brightness temperature and radiance. One can see that at 273K there is an insignificant error in the conversion. 
 
 



Figure 5 
(3.9 µm image) 
Click picture to enlarge


 










What is causing the significant reflection of 3.9 µm solar radiation from the top of CLOUD 1? Is there liquid water at cloud top? The 10.7 µm temperature of -50C suggests that there is none and it is unlikely that cloud liquid water could avoid contact with an ice particle while ascending the turbulent updraft within the developing thunderstorm represented by CLOUD 1.  It is the authors conclusion that there is something about the ice particles habit that is responsible for the large reflection. 
 
 



Back to top


 










 

We welcome your comments and discussion at ramsdis@comet.ucar.edu

Previous RAMMT Satellite Interpretation Discussions