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Author Topic: Clouds blocking passive microwave sensing?  (Read 3610 times)

Nightvid Cole

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Clouds blocking passive microwave sensing?
« on: July 06, 2013, 11:05:16 PM »
If you look closely at the DMI ice concentration animation, it looks like you can see clouds and weather systems going by and blocking the signal.

If you look closely at the MODIS visible images for the same date, the clouds seem to be in the same positions as the transient white streaks on the DMI concentration maps.

If so, how does this not defeat one of the supposed advantages of using microwave data as opposed to visible?

Pmt111500

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Re: Clouds blocking passive microwave sensing?
« Reply #1 on: July 19, 2013, 10:02:36 AM »
I've been using only Modis visible and 367 composites thus far, it would be great if the presence of ice could be detected through clouds. The trouble with clouds (and low level mist/fog) is becoming too much when the surface melt gets going. Then we have icy fog vs. icy water on top of slushy ice at nearly the same temperature (as ice is getting nearer melting point...) the only way these could be separated from the orbit could be a presence of dark solutes or insolubles in the water on top of the ice. I don't know if any instrument in orbit is sensitive enough to notice the difference even if there is some. Attaching a modifed channel 367 image where I presumed low level fog is present where Modis shows clear skies. The adjustment to the red scale has been done only on areas that are too bright to see the individual floes that might/should be present. (darkened the darkest reds on areas guessed have fog present.) This is a top right corner of r05c04.2013196. (thanks jdallen for finding a tile with reasonably clear skies, http://forum.arctic-sea-ice.net/index.php/topic,92.msg10053.html#msg10053 )
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Nightvid Cole

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Re: Clouds blocking passive microwave sensing?
« Reply #2 on: July 19, 2013, 05:00:01 PM »
Maybe a combination of data from more different wavelength bands will do the trick?

Clouds shouldn't be similar to slush in that case, because with clouds you are dealing with Rayleigh scattering (wavelengths ~ 4 - 10 mm, much larger than typical droplets ~ 10 μm) in addition to absorption. With slush there is mostly absorption and not too much scattering since the dielectric constant is similar between solid and liquid states.

Pmt111500

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Re: Clouds blocking passive microwave sensing?
« Reply #3 on: July 19, 2013, 05:30:14 PM »
Might do the trick, yes. I've a hunch some wavelenghts used normally on vegetation mapping might be useful here too but the signal/noise ratio may well be too high here. Also, the slush on top of the ice should be less saline than the ocean below, if that's of any help. If only there were planes flying across the arctic all the time under the clouds... no. Tricky business, this remote sensing.

ehmm, noise to signal,  of course. I mean, it might be that the signal of open sea is limited to a few measurement units (say, 1-10 of 255 (whole range)) so the first task would be to set a high pass filter (like setting the measurements above 10 to 10 f.e.) on some of the channels 8-15 in here http://modis.gsfc.nasa.gov/about/specifications.php
Anyway it looks like the max resolution is 1000m on these channels, but if there is a signal among those that spans intensity range somewhat (say 4 measurement units), this could be smoothed to 250m resolution. The same should be done with water vapor channel, maybe also with some aerosol channel. I don't know which though.
« Last Edit: July 20, 2013, 03:24:54 PM by Pmt111500 »
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SteveMDFP

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Re: Clouds blocking passive microwave sensing?
« Reply #4 on: July 19, 2013, 11:08:21 PM »
Dirigible drones, we need dirigible drones.
Really, put solar cells on top....could periodically use electrolysis to get hydrogen from seawater to keep buoyancy.  Put all the instruments in it that go on satellites.
Could do studies all summer long.  Re-fit over the winter.

LRC1962

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Re: Clouds blocking passive microwave sensing?
« Reply #5 on: September 12, 2013, 09:54:03 PM »
An idea already in use:
http://www.treehugger.com/aviation/giant-forever-flying-solar-powered-drones-replace-satellites.html
Images still on drawing block, but versions already being used.
Advantage of dirigible is that it can lift a heavier payload and can hover better.
Can not get up as high as plane can I do not believe (could be wrong about that).
Even non solar powered a drone version could stay up for a very long time. Could be a great use in Arctic as it can cover spots missed by satellites and if something is interesting can change altitude if clouds in the way. Big problem with them is that very difficult to control in bad weather.
Same would be said of those planes but can definitely get above any weather issues.
« Last Edit: September 12, 2013, 10:03:58 PM by LRC1962 »
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