Head in a Cloud

troposphere and stratosphere meet blogosphere

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October 24th, 2007 by Sean Davis · 1 Comment

Since early October, I’ve been participating in the Aqua Validation and Intercomparison Experiment, Aquavit, at the AIDA chamber located at the Forschungszentrum (“Research Center”) Karlsruhe. Aquavit is an experiment with the purpose of testing and intercomparing instruments that measure water vapor in the atmosphere, especially those that measure in the extremely dry stratosphere. Below is a description of the experiment, and I have posted some pictures here.

Compared to the atmosphere at the surface of the Earth, which can have water vapor concentrations that constitute upwards to ~2% of the atmosphere — or around 20,000 parts per million by volume (ppmv or ppm) — the stratosphere is extremely dry, having only a few ppm. In other words, water vapor varies by around 4 orders of magnitude in the Earth’s atmosphere. This large variability throughout the atmosphere, as well as the low values in the stratosphere, have partly contributed to difficulties in measuring the water vapor amounts in the upper atmosphere.

As it turns out, water vapor is a very important component of the Earth’s atmosphere, and stratospheric water vapor, even though there is very little of it, is also important. I don’t have time to go into the details, but there is a fair bit of literature on why upper tropospheric and stratospheric water is important. One reason that immediately comes to mind is that the amount of water vapor in the stratosphere partially controls the ability of polar stratospheric clouds (PSCs) to form. These PSCs are the staging ground for the polar ozone hole destruction chemistry.

So stratospheric water vapor measurements are important, but it turns out that there have been some historical discrepancies between aircraft-based, balloon-based, and satellite-based measurements. This issue has been recognized for some time, and was the subject of a report by the SPARC group in 2000.

These discrepancies have motivated the water vapor community to come together and put all of the instruments at the same place at the same time to try and understand how the instruments perform in controlled conditions. Thus, the Aquavit experiment in the AIDA chamber. The AIDA chamber is a temperature-controlled chamber (around -90 C to room temperature) that can produce controlled water vapor mixing ratios as low as 0.3 ppm.

I don’t have time to go into the details of the experiments we’ve been doing here, but basically the referees of the experiment have been creating controlled (and unknown to us) amounts of water vapor, which the various instruments (there’s around 15 of them) sample. The participants then submit their data, and a blind intercomparison (meaning the instrument teams don’t know the “true” value) is made. In a few months, after everyones data is finalized, the data will be compared by the referees. The results should be very interesting, and will hopefully illuminate some of the discrepancies that have plagued this field for years. If the results of the comparison indicate that everybody’s instrument measures properly in the lab, then differences between the instruments must be related to different conditions present “in the field” versus in the laboratory.

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1 response so far ↓

  • 1 Don // Oct 24, 2007 at 5:04 pm

    Mother feels she is very “enlightened” now…..I’m confused as hell……oh well, that’s my normal state, isn’t it?

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