This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Friday, January 18, 2013

Atmospheric hydraulic jumps--rare beasts...

The photo in this post was taken on the morning of May 8, 2008,  by an instrument filming in visible wavelengths on the GOES-12 satellite. An animated sequence and the source of this discussion can be found here. The feature of interest is the long straight line that runs from left to right approximately through the center of the image (more specifically, from WSW to ENE).  This line extends from the thumb of Michigan on the left, across lower Michigan, through Toronto, and along the northern edge of Lake Ontario. This image (and the movie that was available to the meteorologists at the time) caused great marveling and wondering--what caused that long straight boundary? What caused the cloud-free region? What causes the more subtle straight lines in the clear region?

There is not enough topography in the region for this pattern to have been caused by topographic variations. It doesn't appear to be determined by land-water variations. It is clearly more moist on the north side than the south (clear, dry air).  During the night before this image was taken, a cold front had moved through causing widespread rain. The boundary appeared after the storm passed.  The consensus was that this was an atmospheric hydraulic jump formed when a thin layer of faster northern air discharged into a zone of lower velocity air to the south. The thiner fast layer (presumably laminar flow) transitions through the hydraulic jump into a deeper layer of slower air, presumably more turbulent).

Fortunately, data on the temperature and humidity structure in the atmosphere was available from rawinsonde for three stations near this phenomenon. This data showed that there was a temperature inversion (colder air over warm moist air) at low levels, and that the cold air (at the 900 hPa level--I'm not going to try to convert that to an actual altitude, but I think that it's a few kilometers). When a fluid goes through a hydraulic jump, it typically loses its laminar characteristics and becomes turbulent.  The turbulence, in this case, would mix in cold dry air that would "dry out" the moister air, producing the clear air downstream of the hydraulic jump. You can also see some fine linear clouds running from WSE to ENE in the clear area. The satellite geeks interpreted these as a manifestation of an undular bore--a gentle form of another hydraulic jump in which there is not a sharp single change of flow conditions, but a gentle oscillating one, often with multiple crests and troughs.

If anyone has photos of other atmospheric jumps, I'd love to collect them and post them to the blog--there are very few out there, and some are not available for use because of copyright limitations.

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