SATELLITE 
INTERPRETATION 
DISCUSSION
 
NOAANESDIS
 Regional and Mesoscale Meteorology Team
Cooperative Institute for Research in the Atmosphere (CIRA)
 Colorado State University  Fort Collins, Colorado

August 20, 1999
 A West Texas Dryline as Seen from the GOES-8 12.0 µm Channel

Figure 1
 
 
 
 
 


Figure 2

 The GOES 12.0 µm channel (channel 5) is sensitive to low level atmospheric water vapor, and therefore can be useful in monitoring the position of the dryline. Figure 1 is a visible GOES-8 image over Texas at 2145 UTC 20 May 1999.  It shows clear skies over western Texas and a cumulus field over central and eastern Texas.  The plotted surface observations (2200 UTC) indicate the existence of a dryline in west central Texas, but because of the large distances between stations, a good estimate of the dryline's position cannot be made.  The visible imagery is also of little use in accurately locating the dryline in this case. Click on images to enlarge figures

Figure 2 is a 12.0 µm image, also taken at 2145 UTC. This image clearly shows the position of the dryline (denoted by black arrows) as the border between the milky grey of central and east Texas, and the charcoal colored regions of west Texas.  Also seen is a cold front in northwestern Texas (white arrow), behind which lay relatively cool, moist air.  This air mass also appears milky grey.  The color bar at the bottom of the image shows that the milky grey color corresponds to a lower brightness temperature than the charcoal grey. 
That during the afternoon hours the air mass west of the dryline results in higher brightness temperatures at 12.0 µm than the air east of the dryline can be understood by looking at Figures 3 and 4, and again noting that 12.0 µm is sensitive to lower atmospheric water vapor.  Figure 3 shows a typical afternoon atmospheric temperature and moisture profile west of the dryline.  Because there is little water vapor, the 12.0 µm signal results primarily from the surface skin temperature, which is hot.  The afternoon atmospheric profile east of the dryline typically resembles that shown in Figure 4.  Here there is abundant low level water vapor, so the signal received at 12.0 µm is a combination of the surface skin temperature, which is not as hot as that west of the dryline, and the low level water vapor, which is emitting at a cooler temperature than the ground, as can be seen from the positive lapse rate below the inversion.  The resultant brightness temperature east of the dryline is therefore less than west of the dryline. 
 
 		          

Figure 3


Figure 4


 

Figure 5

Figure 6

Figure 7

 
During the nighttime, however, the brightness temperature gradient across the dryline reverses, as seen in Figure 5, the 0225 UTC 12.0 µm image of 21 May 1999.  Here the darker (warmer) signal is to the east of the dryline, and the cooler (lighter) signal comes from west of the dryline.  Again, examining typical nighttime temperature and moisture profiles west and east of the dryline provides the explanation. 
Figure 6 shows a typical nighttime sounding from west of the dryline.  The surface skin temperature drops rapidly and dominates the 12.0 µm brightness temperature, because there is still a lack of low level moisture.  The result is a cool signal.  East of the dryline (Figure 7), the surface skin temperature decreases more slowly, and is higher than that west of the dryline.  In addition, the low level water vapor now lies in a surface inversion, that is, it is warmer than the surface skin temperature.  The combination of the ground and the low level moisture is a 12.0 µm signal which is now warmer than the signal west of the dryline.

The attached loop shows the movement of the dryline from 2145 UTC to 0515 UTC.  Note that because the 12.0 µm brightness temperature west of the dryline drops from greater than, to less than that east of the dryline, there necessarily exists a period of time when the brightness temperature difference across the dryline vanishes, and the position of the dryline cannot be determined using the 12.0 µm channel alone.

The 10.7 µm channel may also be used for this purpose, but it has less moisture attenuation, so the contrast between dry and moist air masses will not be as great.

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