Regional And Mesoscale Meteorology Team
Daily Satellite Discussion
Friday April 3, 1998
1. It appeared light gray and very difficult to see in the visible (see image above). However, upon close inspection of a zoomed visible loop (not shown), various lakes (which appear dark) become obscured by the dust. Clouds and snow appear white in the visible. Therefore, it was not a cloud.
2. GOES-8 was brighter than GOES-9 in the visible channel (not shown) due to forward scattering near the time of sunset. Haze and dust produce this signature.
3. The 10.7 micron temperature of the feature was substantially cooler than its surroundings. If it were haze, the temperature contrast would be much less (see figure below).
Looking at the 10.7 micron imagery (Figure 2), the dust cloud in north-central TX appears cool (10 C, blue) relative to the surrounding ground (20+ C, yellow). Sustained winds at the surface in northern-Texas were westerly at 30 kts, and gusts were near 40 kts.
Possible reasons why the dust signature in 10.7 microns is cooler than the surrounding ground:
1) The dust cloud is optically thick such that the radiation reaching the satellite is emitted from the dust alone. If the dust mixes with the cooler air near the top of the boundary layer, the dust at that level will also be cool.
2) The 10.7 micron radiation emitted from the ground is scattered as it passes through the dust cloud, reducing the emitted radiation reaching the satellite.
3) The dust is optically thick for short wave radiation, but less so for long wave radiation. With less short wave radiation reaching the ground, the ground cools. Cool ground emits less at 10.7 micron than the warmer ground not covered by the dust cloud.
Eric R. Hilgendorf