book06remsens - Weather Observation and Analysis John...

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Weather Observation and Analysis John Nielsen-Gammon Course Notes These course notes are copyrighted. If you are presently registered for ATMO 251 at Texas A&M University, permission is hereby granted to download and print these course notes for your personal use. If you are not registered for ATMO 251, you may view these course notes, but you may not download or print them without the permission of the author. Redistribution of these course notes, whether done freely or for profit, is explicitly prohibited without the written permission of the author. Chapter 6. REMOTE SENSING 6.1 Satellite Thermal Sounders The Earth is a planet covered by a blanket of warm gas. Except for when there are clouds, that blanket is nearly transparent to sunlight, which is how the ground heats up during the day. The Earth and its atmosphere are not hot enough to glow themselves, at least not that we can see. They do emit radiation depending on their temperature; it turns out to be mostly in the range of wavelengths we call infrared. One neat thing about the atmosphere is that, in the infrared range, different gases absorb and emit radiation at different wavelengths. (If a gas can absorb at a particular wavelength, it can emit there too.) So a satellite sensor out in space that detects infrared radiation might see a glowing ball of gas, if it’s tuned to a wavelength that a gas in the atmosphere absorbs and emits, or it might see radiation coming all the way from the Earth’s surface, if it’s tuned to a wavelength that no gas in the atmosphere touches. ATMO 251 Chapter 8 page 1 of 23
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Think about those two wavelengths for a moment. Suppose you could adjust the satellite to change its detection wavelength gradually from the atmosphere-opaque one to the atmosphere-transparent one. As the wavelength changes, most of the radiation detected by the satellite will be coming from farther down in the atmosphere, until eventually it starts seeing some of the Earth’s surface and, still later, all of it. At this point, we’re still in the “who cares” portion of this exposition, as in “Who cares what the radiation is coming from?” The answer to that question involves two very key facts about electromagnetic radiation. The first fact is that the intensity of emitted radiation depends directly on the temperature of whatever object (or gas) emitted the radiation. I bet you can see why that might be useful, since the temperature of the Earth and its atmosphere is kinda relevant to the ATMO 251 Chapter 8 page 2 of 23
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weather. But there’s a potential problem with using that information to convert satellite measurements to temperature measurements: what if there’s something between the emissions source and the satellite receiver that’s blocking part or all of the radiation? In that case, wouldn’t you underestimate the temperature? Well, you would, except for the second key fact: anything that
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This note was uploaded on 04/02/2008 for the course ATMO 251-501/50 taught by Professor Alcorn during the Fall '07 term at Texas A&M.

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book06remsens - Weather Observation and Analysis John...

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