Tag Archives: split window difference
By now, you probably know the drill: a little bit of discussion about a particular subject, throw in a few pop culture references, maybe a video or two, then get to the good stuff – high quality VIIRS imagery. Then, maybe add some follow-up discussion to emphasize how VIIRS can be used to detect, monitor, or improve our understanding of the subject in question. Not today.
You see, VIIRS is constantly taking high quality images of the Earth (except during orbital maneuvers or rare glitches). There isn’t enough time in a day to show them all, or go into a detailed discussion as to their relevance. And, nobody likes to read that much anyway. So, as we busily prepare for the upcoming holidays, we’re going to skip the in-depth discussion and get right to the good stuff.
Here then is a sample of interesting images taken by VIIRS over the years that weren’t featured on their own dedicated blog posts. Keep in mind that they represent the variety of topics that VIIRS can shed some light on. Many of these images represent topics that have already been discussed in great detail in previous posts on this blog. Others haven’t. It is important to keep in mind… See, I’m starting to write too much, which I said I wasn’t going to do. I’ll shut up now.
Without further ado, here’s a VIIRS Natural Color image showing a lake-effect snow event that produced a significant amount of the fluffy, white stuff back in November 2014:
As always, click on the image to bring up the full resolution version. Did you notice all the cloud streets? How about the fact that the most vigorous cloud streets have a cyan color, indicating that they are topped with ice crystals? The whitish clouds are topped with liquid water and… Oops. I’m starting to discuss things in too much detail, which I wasn’t going to do today. Let’s move on.
Here’s another Natural Color RGB image using the high-resolution imagery bands showing a variety of cloud streets and wave clouds over the North Island of New Zealand:
Here’s a Natural Color RGB image showing a total solar eclipse over Scandinavia in 2015:
Here’s a VIIRS True Color image and split-window difference (M-15 – M-16) image showing volcanic ash from the eruption of the volcano Sangeang Api in Indonesia in May 2014:
Here’s a VIIRS True Color image showing algae and blowing dust over the northern end of the Caspian Sea (plus an almost-bone-dry Aral Sea):
Here is a high-resolution infrared (I-5) image showing a very strong temperature gradient in the Pacific Ocean, off the coast of Hokkaido (Japan):
The green-to-red transition just southeast of Hokkaido represents a sea surface temperature change of about 10 K (18 °F) over a distance of 3-5 pixels (1-2 km). This is in a location that the high-resolution Natural Color RGB shows to be ice- and cloud-free:
Here’s a high-resolution infrared (I-5) image showing hurricanes Madeline and Lester headed toward Hawaii from earlier this year:
Here are the Fire Temperature RGB (daytime) and Day/Night Band (nighttime) images of a massive collection of wildfires over central Siberia in September 2016:
Here is a 5-orbit composite of VIIRS Day/Night Band images showing the aurora borealis over Canada (August 2016):
Here is a view of central Europe at night from the Day/Night Band:
And, finally, for no reason at all, here’s is a picture of Spain wearing a Santa hat (or sleeping cap) made out of clouds:
There you have it. A baker’s ten examples showing a small sample of what VIIRS can do. No doubt it will be taking more interesting images over the next two weeks, since it doesn’t stop working over the holidays – even if you and I do.
Copahue, the Stinky Volcano
On the border between Chile and Argentina sits the volcano Copahue. (If you say it out loud, it is pronounced “CO-pa-hway”.) In the local Mapuche language, copahue means “sulfur water”. This name was given to the volcano as the most active crater contains a highly acidic lake full of sulfur. An eruption in 1992 filled the area with “a strong sulfur smell.” Later eruptions have involved “pyroclastic sulfur” (molten hot sulfur ash) and highly acidic mudflows. That doesn’t sound very pleasant.
Right before Christmas, Copahue was at it again. It erupted on 22 December 2012, sending a cloud of sulfur ash into the atmosphere, and MODIS got there first. VIIRS got there 4 hours later and took this image:
This is a “true color” image just like the MODIS one in the link. Make sure you click on the image, then on the “3200×2304” link below the banner to see it in full resolution. Then see if you can spot the volcanic ash cloud from Copahue. I’ll give you a hint: it’s the only cloud that appears brownish-gray.
If you still can’t see it, here’s a zoomed-in image with a yellow arrow to help you out:
Now compare the ash cloud in the VIIRS image with the ash cloud in the MODIS image from 4 hours earlier. (This is easier to do if you can locate in the VIIRS image the lakes marked as “Embalse los Barreales” in the MODIS image.) There’s a lot less ash in the VIIRS image, right?
Not so fast. As the ash dispersed, the plume thinned out, making it harder to see against the brown background surface. But, that doesn’t mean that it’s not there. Here’s the “split window difference” image from VIIRS at the same time:
That whole black plume is volcanic ash detected by the split window difference. The yellow arrow points to Copahue and the ash plume that is visible in the true color image. The red arrow points to the ash plume that is not visible in the true color image, yet is detected by this simple channel difference (M-15 minus M-16). A victory for the split window technique!
It was also a victory for the EUMETSAT Dust RGB, which didn’t work for the 100-year-old ash cloud over Alaska. Here’s what that RGB composite looks like when applied to VIIRS:
It is interesting that the ash plume right over Copahue is tough to detect in this RGB composite because it is red, just like a lot of the other clouds. As the plume thins out away from the volcano, its color changes to a variety of pastels of pink and blue, and even appears to extend out over the Atlantic Ocean. Where clouds and ash coexist near the coast of Argentina, pixels show up orange and yellow and green (click to the high-resolution image to see that).
Why does the plume appear to extend into the Atlantic Ocean in the EUMETSAT Dust RGB, and not in the split window difference? It is due to the fact that the Dust RGB uses channel M-14 (8.55 µm), which is sensitive to absorption by sulfur dioxide (SO2) gas. The split window difference is better at detecting sulfuric ash particles, which may have mostly settled out of the atmosphere before reaching the Atlantic coast. There are likely still some ash particles in the plume, though – just not enough to show up easily in the split window difference. Detection of SO2 gas plumes has been used to infer the presence of volcanic ash.
Being able to see the location of the volcanic ash very important to pilots. Aircraft engines don’t work that well when they are sucking in particles of liquified sulfur and other abrasive and corrosive materials spit out by stinky volcanoes like Copahue.
The Case of the 100-year-old Ash Cloud
Lost in all the commotion caused by Hurricane Sandy, a curious event occurred on the other side of the country on 30 October 2012. A cloud of ash obscured the skies of Kodiak Island, Alaska, diverting flights in the region and forcing the people of Kodiak to stay inside or wear masks. Alaska has quite a few volcanoes, so this may not be a big thing to them except, this was no ordinary volcanic eruption: it was the leftovers of a volcanic eruption from 100 years ago!
The volcano that came to be known as Novarupta erupted on 6 June 1912. It was one of the largest volcanic eruptions of recorded history. It was 10 times more powerful than Mt. St. Helens with 100 times more ash. The explosion was heard more than 1100 km (700 miles) away in Juneau. The force of the eruption caused nearby Mt. Katmai to collapse on itself (10 km away). It formed the Valley of Ten Thousand Smokes and, most importantly for us, covered the surrounding land with 150 m (500 ft) of ash.
This pile of ash – still there today – can be lifted by a stiff breeze (or, more appropriately, “strong breeze” or higher on the Beaufort wind scale), and blown pretty high off the ground (4000 ft according to the news report). This isn’t the first time this has happened. MODIS observed the same thing back in 2003.
So, what did VIIRS see? Here’s the “true color” image, the RGB composite of channels M-03 (0.488 µm, blue), M-04 (0.555 µm, green) and M-05 (0.672 µm, red):
Be sure (as with all the images) to click on the image, then on the link below the banner to see it at full resolution. (The link contains the dimensions of the full size image.)
The ash cloud (blowing right over the center of Kodiak Island) is not as obvious in this image as it was in the MODIS image in the link above, although it is visible. To be fair, the plume was much more optically thick in 2003, and there were fewer clouds and less snow to confuse it with.
Here is the false color (“pseudo-true color” or “natural color”) image, the RGB composite of channels M-05 (0.672 µm, blue), M-07 (0.865 µm, green) and M-10 (1.61 µm, red):
Hmmm. Once again, the ash plume is visible but not particularly noticeable. Is there a way to highlight the ash plume to make it easier to see?
EUMETSAT (the European Organisation for the Exploitation of Meteorological Satellites) has defined an RGB composite for detecting dust. Their product, which was developed primarily to detect dust storms over the Saharan desert, uses channels that are present (or similar to ones that are present) on VIIRS. This means we can apply the dust product for VIIRS as the difference between M-16 and M-15 (red), the difference between M-15 and M-14 (green) and M-15 by itself (blue), all in units of brightness temperature. If you do that, and use the same color scaling they use, you get this image:
The arrow points to the source region of the ash plume. In this RGB composite, dust shows up as hot pink (magenta), but it’s barely visible here. The reason is that this dust product is primarily useful where there is a large temperature contrast between the dust plume and the background surface, which we don’t have here.
A more common way to detect volcanic ash is to use the “split-window difference”. The “split-window difference” is the difference in brightness temperature between a 10.7-11.0 µm channel and a 12.0 µm channel. This difference is useful because volcanic ash has a difference of opposite sign to most everything else. Here’s what the split window difference (M-15 – M-16) looks like for this case:
This image has been scaled so that the colors range from -1 K (black) to +7 K (white). The ash plume stands out a bit more here by being much darker than the background. The only problem is, it isn’t perfect. Large amounts of water vapor, optically thick clouds, desert surfaces and boundary layer temperature inversions can all produce a negative difference (just like volcanic ash does).
These problems can be overcome to a certain extent by combining the “split-window difference” with a Principal Component Image (PCI) analysis technique. (This technique is too complicated to describe here but, if you have access to AMS journals, check out these journal papers.) Now, the ash plume is the only thing that’s black:
Notice the smaller plume identified by the orange arrow. This plume is not easy to identify in any of the previous images. The PCI technique works well. But, we’re not going to stop there.
Remember the dust plumes off the Cape Verde islands? They produced a strong signal in the difference between M-12 (3.7 µm) and M-15 (10.7 µm) due to solar reflection. Does a 100-year-old ash plume produce a similarly strong signal? See for yourself:
It does produce a signal, but it’s not as bright as the surrounding clouds. The color scale here ranges from -2 K (black) to +90 K (white).
M-06 (0.746 µm) is highly sensitive to anything that reflects solar radiation in the atmosphere or on the surface, which we learned from Hurricane Isaac. Here’s what the M-06 image looks like:
“Big deal,” you say. “None of those are better than the PCI analysis.” That may be true, but watch what happens when we combine M-06, the M-12 – M-15 image and the split-window difference image in a single RGB composite:
In this composite, blue values represent the M-06 reflectance scaled from 0 to 1.6, green values represent the brightness temperature difference between M-12 and M-15 scaled from -2 K to +90 K, and red values represent the brightness temperature difference between M-15 and M-16 scaled from -1 K to +7 K.
From a theoretical perspective, this RGB composite does exactly what you want: make the thing you’re trying to detect the only thing that is a certain color. For example, the ash plumes are the only things in this image that are green. From a practical perspective, however, this RGB composite doesn’t work so well. It only works because the ash plume is over water (otherwise M-06 wouldn’t be very useful). It only works during the day, where M-06 is available and the difference between M-12 and M-15 is significant (no solar component to M-12 at night).
Plus, the rainbow of colors is difficult to make sense of: green ash; clouds ranging from light blue to purple to orange (a function of optical thickness, particle size, and phase); bright purple snow; dark purple vegetation; maroon water. It’s not exactly pleasing to the eye. In contrast, the PCI analysis technique that uses the split-window difference works day and night, over ocean and over land. And it isn’t confusing to look at. Maybe we should have stopped when we got to the PCI technique. But then, we wouldn’t have learned anything new.
Cape Verde Waves and Plumes
Cape Verde is an island nation off the west coast of Africa, located in the North Atlantic. The islands are a popular initiation point for tropical storms. The original capital of the 10-island archipelago was sacked twice by Sir Francis Drake, the same one who, in his later years, would fail to sack the villages along Lake Maracaibo in Venezuela due to Catatumbo lightning. That guy really got around, and I mean that literally: he circumnavigated the globe between 1577 and 1580, sacking nearly every village and boat he came across. But, this isn’t about Francis Drake – it’s about the Cape Verde islands and the amazing view of them captured by VIIRS.
Can you see the 10 major islands? One of them (Santa Luzia) is almost obscured by clouds. If you click on the image, you’ll see each of the major islands identified. Go ahead and click on it. It will help for later.
The image above was made from the RGB composite of VIIRS high-resolution imagery channels I-01, I-02 and I-03. While it technically is a false color image (uses reflectance at 0.64 µm [blue], 0.865 µm [green] and 1.61 µm [red]), it looks realistic in many situations, so that we refer to it as “pseudo-true color”. Snow and ice show up as an unrealistic blue, however, which is the main difference between it and a “true color” image. You might also notice a few more differences between the “pseudo-true color” image above and the “true color” image below.
The true color image uses moderate resolution channels M-3 (0.48 µm, blue), M-4 (0.55 µm, green) and M-5 (0.67 µm, red), which actually observe radiation in the blue, green and red portions of the visible spectrum. Apart from differences in resolution, the vegetation on the islands shows up a bit better in the “pseudo-true color” image. The islands just look brown in the true color image.
What is particularly interesting about these images are the visible effect that the islands have on the local atmosphere. Downwind (southwest, or to the lower left) of Sal, Boa Vista, and Maio, you can see singular cloud streets, much like the flow of water around a rock. In the photograph in that link, you can see how the water dips downward on both sides of the center line downstream of the rock, and upward in the middle (along the center line). The islands are acting like rocks in the atmosphere, causing upward motion behind them, and this lift was enough to form cloud streets. On either side of these cloud streets there is downward motion and, as a result, clear skies.
Downwind of São Nicolau, São Vicente and Santo Antão, the cloud streets highlight von Kármán vortices and vortex shedding, which you can see in more-controlled lab conditions here and here.
Many of the islands appear to be producing their own aerosol plumes (i.e. dust), and if you zoom in on the area between Boa Vista and Santiago, you can see gravity waves present in some of the plumes (highlighted by the arrows in the image below).
A common way to detect dust is the “split-window difference”: the difference in brightness temperature between the 11 µm channel and the 12 µm channel. On VIIRS, this means subtracting M-16 from M-15 which, when you do that, gives you this image:
The color scale goes from -0.16 K (black) to +4.0 K (white). For some reason, the dust or aerosol plumes don’t produce a strong signal here. It may be that the dust is too low in the atmosphere and the lack of temperature contrast with the surface prevents a strong signal. Maybe water vapor absorption effects in M16 are washing out the signal. Or, there could be some other explanation waiting to be discovered.
The plumes are highly reflective in the 3.7 µm channel (M-12), as are the clouds, which show up as warm spots in the image below (not as warm as the islands, however):
Here, just to throw you off, the color scale has been reversed so that dark colors mean higher values. The scale ranges from 295 K (white) to 330 K (black). When you take the difference of this image and the 10.6 µm brightness temperature (M-15), the clouds and aerosol plumes really show up, along with the gravity waves and vortices:
In this case, the M-12 brightness temperatures are always greater than the M-15 brightness temperatures (due to the combination of Earth’s emission and solar reflection in M-12 as opposed to just surface emission in M-15), so the scale varies from +5 K (black) to +30 K (white). Higher (brighter) values on this scale show off where the most solar reflection occurs at 3.7 µm – the liquid clouds and aerosol plumes.
There are much more sophisticated ways of identifying dust and aerosol plumes. To find out more, check out this article written by one of our resident experts, Steve Miller, who is currently working on applying dust detection algorithms to VIIRS.
If you are more interested in the von Kármán vortices, NASA has put together a great page that you can visit here. If you take the original image in this post, zoom out and rotate it a little bit, you can get a sense of just how far the vortices extend from their parent islands:
Coincidentally, this image has been cropped to a size that makes it suitable for use as a desktop wallpaper, should you happen to have a 16:9-ratio monitor and a desire to stare at this image all day. (You have to click on the image, then click on the “1920 x 1080” link below the header to get the full resolution image.)