Chinese Super-Smog

Posted on February 15, 2013 by Curtis_Seaman

No, not a Super-Smörg, super smog. Smog that is so thick, you can taste it. The smog in many parts of eastern China has been so bad this winter, it is literally “off-the-charts“. Based on our Environmental Protection Agency‘s not-very-intuitive Air Quality Index (see pages 13-16, in particular) any value above 300 is hazardous to everyone’s health. The scale doesn’t even go above 500 because the expectation is that the air could never get that polluted. Applying this scale to the air in Beijing, the local U.S. Embassy reported an Air Quality Index value of 755 on 13 January 2013. Visibility has been reduced to 100 m at times. This video (from 31 January 2013) gives a vivid description of the problems of the smog:

If that wasn’t bad enough, here’s video from NBC News where Brian Williams reveals a factory was on fire for three hours before anyone noticed because the smog was so thick!

Did you happen to notice in the beginning of the NBC video that the “air pollution is so bad that the thick smog can now be seen from space”? Of course, the satellite image shown in that clip came from MODIS. (It must have friends in high places. That, or people get the MODIS images out on their blogs less than two weeks after the event occurred, unlike this blog.) Needless to say, VIIRS has seen the smog, too, and it is terrible.

For comparison purposes, here’s what a clean air day looks like over eastern China:

VIIRS "true color" RGB composite of channels M-03, M-04 and M-05, taken 05:21 UTC 28 September 2012

This is a “true color” composite taken 05:21 UTC 28 September 2012. (As always, click on the image, then on the “2040×1552” link below the banner to see the full resolution image.) There appears to be some air pollution in that image (look near 33° N latitude between 112° and 116° E longitude), but it’s not that noticeable.

Here’s what it looks like when Beijing is reporting record levels of air pollution (04:56 UTC 14 January 2013):

VIIRS true color RGB composite of channels M-03, M-04 and M-05, taken 04:56 UTC 14 January 2013

You may have heard of a “brown cloud of pollution“. Here the clouds actually appear brown thanks to all that pollution. Notice the area around Shijiazhuang – the most polluted city in China – and how brown those clouds are in comparison to the clouds on the left and right edges of the image. Then look south from Shijiazhuang to where everything south and west of the cloud bank has a dull gray color. That is all smog! It’s enough to make anyone with a respiratory condition want to cough up a lung just from seeing this.

Now, this is a complicated scene with clouds, snow, ice and smog. So, to clear things up (in a manner of speaking), here is the same image with everything labelled:

VIIRS true color RGB composite of channels M-03, M-04, and M-05, taken 04:56 UTC 14 January 2013

The gray smog can be seen around Beijing as well, but it pales in comparison to the rest of eastern China. Think about that! Replay the videos above and consider that might not have even been the worst smog in China at the time!

Too bad there are a lot of clouds over the area. What does it look like on a “clearer” day? (“Clearer”, of course, refers to the amount of clouds, not air pollution.) It looks worse! The image below was taken at 04:32 UTC on 26 January 2013:

VIIRS true color RGB composite of VIIRS channels M-03, M-04, and M-05, taken 04:32 UTC 26 January 2013

The area covered by smog rivals the area of South Korea, which is visible on the right side of the image. (One of the reports I linked to above put the figure at 1/7th of the land area of China covered by smog around this time, which is actually a lot bigger than South Korea!) I’m just counting the smog in the image that is thick enough to completely obscure the surface. There is likely smog that isn’t as obvious (and isn’t labelled) in that image. The snow between Shijiazhuang, Tianjin and Beijing is covered by smog that isn’t quite thick enough to totally obscure it. And the large area of snow south of Tianjin is likely covered with smog. (It sure is a lot dirtier in appearance than the snow near the top of the image.)

If you don’t believe my labels, the “pseudo-true color” or “natural color” RGB composite clearly identifies the low clouds (which usually appear a dirty, off-white color even without smog), ice clouds (pale cyan) and snow (vivid cyan):

VIIRS false color RGB composite of channels M-05, M-07 and M-10 (a.k.a. "natural color"), taken 04:32 UTC 26 January 2013

Notice the smog in this image. It is an unholy grayish-greenish color with a value near 70-105-93 in R-G-B color space. The “natural color” composite is made from channels M-05 (0.67 µm, blue), M-07 (0.87 µm, green) and M-10 (1.61 µm, red), which are longer wavelengths than their “true color” counterparts. Longer wavelengths mean reduced scattering by atmospheric aerosols, so the higher green value may be due to the strong surface vegetation signal in M-07 being able to penetrate through the smog. (Either that or the smog is composed of some chemical compound that has a higher reflectivity value in M-07 than in the other two channels.)

I’ve looked at the EUMETSAT Dust, Daytime Microphysics and Nighttime Microphysics/Fog RGBs, which you might think would show super-thick smog and they don’t. At least, it’s not obvious.

The EUMESAT Dust RGB applied to VIIRS, valid 04:32 UTC 26 January 2013

The Dust RGB above uses M-14 (8.55 µm), M-15 (10.7 µm) and M-16 (12.0 µm) and requires there to be a large temperature contrast between the dust (cool) and the background surface (hot). Smog almost always occurs when there is a temperature inversion (the air at the ground is colder than the air above) so the necessary temperature contrast won’t exist.

The Daytime Microphysics RGB shows the smoggy areas are a slightly different color than other cloud-free surfaces, but that color can be confused with other non-smoggy surfaces. The clouds really stand out, though:

The EUMETSAT Daytime Microphysics RGB applied to VIIRS, valid 04:32 UTC 26 January 2013

Perhaps, with a different scaling, the smog might stand out more.

The Nighttime Microphysics RGB from the night before (18:50 UTC 25 January 2013) is interesting. Notice the cloud identified by the letter “B” and the non-cloud next to it, “A”:

The EUMETSAT Nighttime Microphysics/Fog RGB applied to VIIRS, valid 18:50 UTC 25 January 2013

Now compare this with the Day/Night Band image from the same time:

VIIRS Day/Night Band image of eastern China, taken 18:50 UTC 25 January 2013

This was a day before full moon. Thanks to the moon, clouds, snow and smog are visible in addition to the city lights. Points “A” and “B” have nearly identical brightness in the Day/Night Band, but only “B” shows up as a cloud in the Nighttime Microphysics RGB. These lighter areas around “A” and “B” are partially obscuring city lights, indicating “B” is a cloud, while “A” is smog. (If either was snow, you’d be able to see the city lights more clearly. See the lighter area northwest of Beijing, which is snow.)

Nothing sees super-smog like the true color composite, but the Day/Night Band will see it as long as there is enough moonlight. Smog as optically thick as a cloud… *hacking cough* … Yuck!

End of Autumn in the Alps

Posted on December 17, 2012 by Curtis_Seaman

Much of the United States has had a below-average amount of snow this fall (and below-average precipitation for the whole year). Look at how little snow cover there was in the month of November. Parts of Europe, however, have seen snow. It’s nice to know that it’s falling somewhere. But, can you tell where?

Here is a visible image (0.6 µm) from Meteosat-9, taken 12 December 2012 (at 12:00 UTC):

Meteosat-9 visible image of central Europe, taken 12:00 UTC 12 December 2012. Image courtesy EUMETSAT.

And here’s the infrared image (10.8 µm) from the same time:

Meteosat-9 IR-window image of central Europe, taken 12:00 UTC 12 December 2012. Image courtesy EUMETSAT.

These are images provided by EUMETSAT. Can you tell where the snow is? Or what is snow and what is cloud?

Here’s a much higher resolution image from VIIRS (zoomed in the Alps), taken only 3 minutes later:

VIIRS visible image of central Europe, taken 12:03 UTC 12 December 2012

VIIRS visible image (channel I-01) of central Europe, taken 12:03 UTC 12 December 2012

Now is it easy to differentiate clouds from snow? Just changing the resolution doesn’t help that much.

This has long been a problem for satellites operating in visible to infrared wavelengths. Visible-wavelength channels detect clouds based on the fact that they are highly reflective (just like snow). Infrared (IR) channels are sensitive to the temperature of the objects they’re looking at, and detect clouds because they are usually cold (just like snow). So, it can be difficult to distinguish between the two. If you had a time lapse loop of images, you’d most likely see the clouds move, while the snow stays put (or disappears because it is melting). But, what if you only had one image? What if the clouds were anchored to the terrain and didn’t move? How would you detect snow in these cases?

EUMETSAT has developed several RGB composites to help identify snow. The Daytime Microphysics RGB (link goes to PowerPoint file) looks like this:

Meteosat-9 "Daytime Microphysics" RGB composite of central Europe, taken 12:00 UTC 12 December 2012. Image courtesy EUMETSAT.

Snow is hot pink (magenta), which shows up pretty well. Clouds are a multitude of colors based on type, particle size, optical thickness, and phase. That whole PowerPoint file linked above is designed to help you understand all the different colors.

The Daytime Microphysics RGB uses a reflectivity calculation for the 3.9 µm channel (the green channel of the RGB). Without bothering to do that calculation, I’ve replaced the reflectivity at 3.9 µm with the reflectivity at 2.25 µm (M-11) when applying this RGB product to VIIRS, and produced a similar result:

VIIRS "Daytime Microphysics" RGB composite of the Alps, taken 12:03 UTC 12 December 2012

Except for the wavelength difference of the green channel (and minor differences between the VIIRS channels and Meteosat channels), everything else is kept the same as the official product definition. Once again, the snow is pink, in sharp contrast to the clouds and the snow-free surfaces. We won’t bother to show the Nighttime Microphysics/Fog RGB (link goes to PowerPoint file) since this is a daytime scene.

EUMETSAT has also developed a Snow RGB (link goes to PowerPoint file):

Meteosat-9 "Snow" RGB composite of central Europe, taken 12:00 UTC 12 December 2012. Image courtesy EUMETSAT.

This also uses the reflectivity calculated for the 3.9 µm channel. Plus, it uses a gamma correction for the blue and green channels. Is it just me, or does snow show up better in the Daytime Microphysics RGB?

If you switch out the 3.9 µm for the 2.25 µm channel again and skip the gamma correction when creating this RGB composite for VIIRS, the snow stands out a lot more:

VIIRS "Snow" RGB (with modifications as explained in the text), taken 12:03 UTC 12 December 2012

Now you have snow ranging from pink to red with gray land areas, black water and pale blue to light pink clouds. This combination of channels makes snow identification easier than the official “Snow RGB”, I think.

All of this is well and good but, for my money, nothing beats what EUMETSAT calls the “natural color” RGB. I have referred to it as the “pseudo-true color“. Here’s the low-resolution EUMETSAT image:

Meteosat-9 "Natural Color" RGB of central Europe, taken 12:00 UTC 12 December 2012. Image courtesy EUMETSAT.

And the higher resolution VIIRS image:

VIIRS "Natural Color" RGB of central Europe, taken 12:03 UTC 12 December 2012

VIIRS "Natural Color" RGB composite of channels M-5, M-7 and M-10, taken 12:03 UTC 12 December 2012

The VIIRS image above uses the moderate resolution channels M-5, M-7 and M-10, although this RGB composite can be made with the high-resolution imagery channels I-01, I-02 and I-03, which basically have the same wavelengths and twice the horizontal resolution. Below is the highest resolution offered by VIIRS (cropped down slightly to reduce memory usage when plotting the data):

VIIRS "Natural Color" RGB composite of channels I-01, I-02 and I-03, taken 12:03 UTC 12 December 2012

Make sure to click on the image and then on the “2594×1955” link below the banner to see the image in full resolution.

This RGB composite is easier on the eyes and easier to understand. Snow has high reflectivity in M-5 (I-01) and M-7 (I-02) but low reflectivity in M-10 (I-03) so, when combined in the RGB image, it shows up as cyan. Liquid clouds have high reflectivity in all three channels so it shows up as white (or dirty, off-white). The only source of contention is that ice clouds, if they’re thick enough, will also show up as cyan.

Except for the cyan snow and ice, the “natural color” RGB is otherwise similar to a “true color” image. Vegetation shows up green, unlike the other RGB composites where it has been gray or purple or a very yellowish green. That makes it more intuitive for the average viewer. You don’t need to read an entire guide book to understand all the colors that you’re seeing.

Compare all of these RGB composites against the single channel images at the top of the page. They all make it easier to distinguish clouds from snow, although some work better than others. Now compare the VIIRS images with the Meteosat images. Which ones look better?

(To be fair, it’s not all Meteosat’s fault. The images provided by EUMETSAT are low-resolution JPG files [which is a lossy-compression format]. The VIIRS images shown here are loss-less PNG files, which are much larger files to have to store and they require more bandwidth to display.)

As a bonus (consider it your Christmas bonus), here are a few more high-resolution “natural color” images of snow and low clouds over the Alps. These are kept at a 4:3 width-to-height ratio and a 16:9 ratio, so they make ideal desktop wallpapers.

VIIRS "natural color" composite of channels I-01, I-02 and I-03, taken 12:29 UTC 14 November 2012. This is an ideal desktop wallpaper for 4:3 ratio monitors.

That was the 4:3 ratio image. Here’s the 16:9 ratio image:

VIIRS "natural color" composite of channels I-01, I-02 and I-03, taken 12:29 UTC 14 November 2012. This is an ideal desktop wallpaper for 16:9 ratio monitors.

Enjoy the snow (or be glad you don’t have to drive in it)!

Remote Islands, part III: Îles Kerguelen and Heard Island

Posted on November 9, 2012 by Curtis_Seaman

“At 10 o’clock the Captain was walking on deck and saw what he supposed to be an immense iceberg. … the atmosphere was hazy, and then a heavy snow squall came up which shut it out entirely from our view. Not long after the sun shone again, and I went up again and with the glass, tried to get an outline of it to sketch its form. The sun seemed so dazzling on the water, and the tops of the apparent icebergs covered with snow; the outline was very indistinct. We were all the time nearing the object and on looking again the Captain pronounced it to be land. The Island is not laid down on the chart, neither is it in the Epitome, so we are perhaps the discoverers, … I think it must be a twin to Desolation Island, it is certainly a frigid looking place.”

VIIRS false color composite of channels I-01, I-02 and I-03, taken 09:16 UTC 27 October 2012

The text above was the journal entry of Isabel Heard, wife of the American Captain John Heard, on 25 November 1853. The couple was en route from Boston, Massachusetts to Melbourne, Australia (a long time to spend in a boat) and the land they spotted became known as Heard Island. It should be noted that “Desolation Island” refers to Îles Kerguelen, which has its own unique story of discovery.

Kerguelen Island was discovered in 1772 by Yves-Joseph de Kerguelen de Trémarec, a French navigator commissioned by King Louis XV to discover the unknown continent in the Southern Hemisphere that he believed to be necessary to balance the globe. (Look at a globe or map of the world and notice that most of the land area is in the Northern Hemisphere.) Kerguelen himself never set foot on the island, but he told his king the island was inhabited and full of forests, fruits and untold riches. He called it “La France Australe” (Southern France). Captain Cook actually did land on the island a few years later and named it Desolation Island because it had none of that stuff, and King Louis XV imprisoned Kerguelen after his lie was discovered. Oops.

Îles Kerguelen, made up of the main island (Kerguelen to us, La Grande Terre to the French) and the many small surrounding islands are part of the French Southern and Antarctic Lands (Terres Australes et Antarctiques Françaises or TAAF). Heard Island is part of the Australian territory of Heard Island and McDonald Islands (HIMI).

These islands are in the “Roaring Forties” and “Furious Fifties”, the region of the Southern Ocean (southern Indian Ocean in this case) between 40 °S and 60 °S latitude. Get out your globe or world map once again and notice that there is very little land in this latitude range. This region is where strong, persistent westerly winds circle the globe. With no land in the way, there isn’t much to disturb this flow. The high winds almost always from the same direction create huge waves of 10 m (33 ft) or more. (Now imagine being John or Isabel Heard. Well, actually, if you suffer from sea-sickness you probably shouldn’t imagine it.) The cold winds flow over the relatively warmer waters of the ocean, forming persistent cloudiness. If you zoom in on the image above (click on the image, then on the “1893×1452” link below the banner for full resolution) you can see quite a bit of structure in the resulting “cloud streets“.

The persistent cloudiness makes Kerguelen and Heard Island a rare sight from any satellite. We can see them here because the flow is stable and the islands are producing the equivalent of a “rain shadow” on the clouds. (It’s tempting to call it a “cloud shadow” but, since clouds actually do cast shadows, it would just confuse people.) If we zoom in on Kerguelen, this shows up more clearly:

VIIRS false-color RGB composite of channels I-01, I-02 and I-03 taken 09:16 UTC 27 October 2012

Notice how all the clouds are piling up on the west (windward) side of Kerguelen, where the highest mountains, are located. (These mountains are covered with snow and glaciers, as the cyan color indicates.) Could that be the equivalent of a bow shock near 68 °E longitude where there is an apparent crack in the clouds? On the leeward side of the island, downwind of the mountains, the air is descending, which prevents clouds from forming. Kerguelen created a hole in the clouds by disrupting the flow.

Now, let’s zoom in on Heard Island:

VIIRS false-color RGB composite of channels I-01, I-02 and I-03 taken 09:16 UTC 27 October 2012

In addition to creating a hole in the clouds, Heard Island is creating all sorts of waves in the atmosphere. The ones you probably noticed first look like the wake created by a boat (and have the same basic cause). But, why do they start well out ahead of the island where the yellow arrow is pointing? Because those first waves are actually caused by the McDonald Islands (discovered by Capt. William McDonald in 1854). Even though the highest point on McDonald Island is only 186 m above mean sea level (610 ft), it’s enough to disrupt the flow.

The highest point on Heard Island is Mawson Peak at 2745 m (9006 ft), which is actually the highest elevation in Australia. It is part of Big Ben, an active volcano that last erupted in 2008. This peak is creating a series of lenticular clouds in the above image. A patch of cirrus clouds also exists downwind of Heard Island (the more cyan colored clouds), although it is not clear if these clouds were formed by the waves caused by Heard Island.

If you’re interested in visiting either of these islands, here are some other interesting facts: Kerguelen has a year-round population of ~100, almost all scientists. It has a permanent weather station and office maintained by Météo-France (France’s version of the National Weather Service), and the French version of NASA (CNES) has a station for launching rockets and monitoring satellites. Heard Island has no permanent residents. Every few years a scientific expedition sets out for the island to study the geology, biology, weather and climate of the island. The next one is planned for 2014 and is being called an “open source expedition”. There may still be time to join in if you’re looking for an adventure!

Greenland Eddies and Swirls

Posted on October 22, 2012 by Curtis_Seaman

Last time we visited Greenland, it was because VIIRS saw evidence of the rapid ice melt event in July 2012. We return to Greenland because of this visible image VIIRS captured on 18 October 2012:

VIIRS channel I-01 image taken 12:43 UTC 18 October 2012

This image was taken by the high-resolution visible channel, I-01 (0.64 µm), and was cropped down to reduce the file size. Greenland is in the upper-left corner of the image. The northwest corner of Iceland is visible in the lower-left corner of the image.

So, what’s with all the swirls off the coast of Greenland? Are they clouds swirled around by winds? Or some kind of sea serpent – perhaps a leviathan or a kraken? (Based on the descriptions, they would be big enough for VIIRS to see them.)

Sadly, for all you science fiction and fantasy fanatics, those swirls are just icebergs breaking up as they enter warmer water, the chunks of ice caught up in eddies in the East Greenland Current. This is easier to see when you look at the “true color” image below:

VIIRS "true color" RGB composite of channels M-3, M-4 and M-5, taken 12:43 UTC 18 October 2012

Make sure to click on the image, then on the “3200×1536” link below the banner to see the image at full resolution. Since the true color RGB composite is made from moderate resolution channels M-03 (0.488 µm, blue), M-04 (0.555 µm, green) and M-05 (0.672 µm, red), we can include more of the swath before we get into file size issues. That allows us to see the extent of the ice break-up along the Greenland coast.

There is a lot to notice in the true color image. The large icebergs at the top of the image breakup into smaller and smaller icebergs as they float down the east coast of Greenland, until they finally melt. These visible “swirls” (or “eddies” in oceanography terms) extend from 75 °N latitude down to 68 °N latitude where the ice disappears (melts).

The upper-right corner with missing data is on the night side of the “terminator” (the line separating night from day), where we lose the amount of visible radiation needed for these channels to detect stuff. (The Day/Night Band would still collect data, however, as it is much more sensitive to the low levels of visible radiation observed at night.) See how the ice and the high clouds appear to get a bit more pink as you move from west (left) to east (right)? It’s the same reason cirrus clouds often look pink at sunset. The sun is setting on the North Atlantic and more of the blue radiation from the sun is scattered by the atmosphere than red radiation. The red radiation that’s left is then reflected off the clouds (and ice and snow) toward the satellite.

Just to prove that the swirls are indeed ice and not clouds, here’s the “pseudo-true color” (a.k.a. “natural color”) RGB composite made from channels M-05 (0.672 µm, blue), M-07 (0.865 µm, green) and M-10 (1.61 µm, red):

VIIRS natural color image of channels M-05, M-07 and M-10, taken 12:43 UTC 18 October 2012

The deep blue color of the swirls in this RGB composite is indicative of ice, not clouds. These channels are not impacted by atmospheric scattering at any sun angle, though, so there is no change in the color of the clouds as you approach the terminator.

You may have also noticed the cloud streets downwind of the icebergs off the coast of Greenland. These clouds are formed in the same way as lake-effect clouds are in the Great Lakes. Cold, arctic air flowing south over the icebergs meets the relatively warm water of the open ocean. The moisture evaporating from the warmer waters condenses in the cold air and forms clouds.

How much warmer is that water? Here’s the high-resolution infrared (IR) image (I-05, 11.45 µm):

VIIRS channel I-05 image, taken 12:43 UTC 18 October 2012

At ~375 m resolution at nadir, this is the highest resolution available in the IR on a non-classified satellite today. Look at all the structure in the cloud-free areas of the ocean! Lots of little eddies show up in the IR that are invisible in the visible and near-IR channels shown previously. The only eddies visible in the true color and natural color images are the ones that had ice floating in them. Here we see they extend much further south than the ice.

The ice-free water that is not obscured by clouds is 10-15 K warmer than where the icebergs are found. The eddies are caused by the clash between the southward flowing, cold Eastern Greenland Current and the northbound, warm North Atlantic Drift (the tail end of the Gulf Stream), which are important in the global transport of energy. They are not ship-sinking whirlpools caused by any krakens in the area – at least VIIRS didn’t observe any.

UPDATE (February 2013): Below is another image of the eddies and swirls off the eastern coast of Greenland. This “natural color” image was taken 13:34 UTC 15 February 2013:

VIIRS false color RGB composite of channels M-05, M-07 and M-10, taken 13:34 UTC 15 February 2013. Image courtesy Don Hillger.

Since it is winter, the ice extends further south along the coast before it melts. Once again, there is a lot of structure visible in the edge of the ice, where the East Greenland Current and North Atlantic Drift interact. Another thing to notice is the shadows. At the top of the image just right of center is Scoresby Sound, which is completely frozen over. Given that the sun is pretty low in the sky over Greenland in the winter (if it rises at all, since most of Greenland is north of the Arctic Circle), the mountains south of the Sound cast some pretty long shadows on the ice. It’s possible to use the length of the shadows with the solar zenith angle to estimate the height of those mountains (although there are more accurate ways to determine a mountain’s elevation from satellite). VIIRS provides impressive detail, even from the moderate resolution bands.

VIIRS and the Greenland Ice Melt

Posted on August 2, 2012 by Curtis_Seaman

First, a preface: The purpose of this blog (and this blog post) is not to ignite some debate about global warming. This is about what one new satellite instrument has observed and the information it is providing to the scientific community.

With that out of the way, we can begin.

You may have heard on the news a story about the rapid ice melt that occurred in Greenland a couple weeks ago. Over a period of four days, the percentage of the surface of Greenland’s ice sheet that showed evidence that the ice was melting went from 40% to 97%. NASA’s Thomas Wagner does a good job explaining it in this interview. You’ll notice in the first link (from the Earth Times) that the rapid melt was first noticed by someone analyzing data from Oceansat-2. The ice melt was detected by its microwave scatterometer and was later confirmed by MODIS. Well, if MODIS can see this ice melt, surely VIIRS can see it, too. Let’s see.

First, let’s look at the false color RGB composite made from channels I-01 (0.64 µm, blue), I-02 (0.865 µm, green) and I-03 (1.61 µm, red). These images are comprised of 5 VIIRS granules stitched together and cropped slightly to get them in under the 15 MB limit for attachments to this blog. You really need to see them zoomed in to full resolution to see the kind of detail that the VIIRS bands provide. This isn’t even the full resolution of the satellite – these two images have been shrunk by a factor of 2 to get in under the file size limit, so it’s actually more like the resolution of the M-bands. (Click on the image, then click on the “2350 x 3372” link below the banner to see the full resolution image.)

Here’s what VIIRS saw on 8 July 2012, at 14:35 UTC:

False color RGB composite of VIIRS channels I-01, I-02 and I-03, taken 14:35 UTC 8 July 2012

And here’s what VIIRS saw five days later (14:42 UTC, 13 July 2012):

False color RGB composite of VIIRS channels I-01, I-02 and I-03, taken 14:42 UTC, 13 July 2012

First thing to notice is that the low liquid clouds over Greenland really stand out in this composite above the ice sheet. As discussed before, this is one of the advantages of this kind of RGB composite. The second thing to notice, which is easier to see in the 13 July image, is that Iceland is the island that’s green, and Greenland is the island that is almost entirely ice. (Those silly Vikings and their misnomers!)

What is relevant here, though, is more subtle. The ice sheet appears to be a significantly darker blue over much of Greenland on 13 July than it does on 8 July. Notice also in these composites that large bodies of liquid water appear black. Now, there’s a lot going on here.

Small, liquid droplets (which are nearly spherical) that make up many of the clouds in the scene are very good at reflecting the solar radiation at all three wavelengths (0.64 µm, 0.865 µm, and 1.61 µm). When you combine high (and nearly equal) levels of red, green and blue on a computer monitor, you get something close to white. This is why the liquid clouds appear whitish.

The small ice particles (found in some of the clouds in these two images) are very good at reflecting radiation at 0.64 µm and 0.865 µm, but not as good at reflecting radiation at 1.61 µm. That means, for this RGB composite, we have high levels of blue and green, but low levels of red. This gives the pale bluish color known as cyan. Snow and ice on the ground are even worse at reflecting radiation at 1.61 µm (they absorb it), so you have a more pure color of cyan. (Although, snow and ice do reflect more than water at this wavelength.)

Liquid water (not in tiny spherical droplets) is not a good reflector at any of these wavelengths. Therefore, the low (and nearly equal) levels of red, green and blue give you black. As snow and ice melt, the reflectivity changes at each of these wavelengths (as the ice becomes more water-like), so the cyan color becomes darker.

It should be said that the primary purpose of the 1.61 µm channel is to aid in snow and ice detection. VIIRS actually has two of these channels: I-03 and M-10. In fact, you can see the effect of the melting ice a bit easier when looking at this channel alone. Here are the M-10 images of Greenland from 8 July and 13 July 2012:

VIIRS channel M-10 reflectance image of Greenland, taken 14:35 UTC 8 July 2012

VIIRS M-10 reflectance image of Greenland, taken 14:42 UTC 13 July 2012

In the first image from 8 July 2012, you can see that the clouds stand out as being bright (highly reflective) and the area of still-frozen ice is visible (a medium to dark gray, meaning somewhat reflective) over the most of the center of Greenland. On 13 July 2012, Greenland shows up as black – just like the surrounding ocean – except for small patches of land along the coast that are not underneath the massive ice sheet (and the clouds, of course). It is particularly noticeable in south-central Greenland. This decrease in reflectivity at 1.61 µm over this period of time is due to the snow and ice becoming more water-like as it is melting. So VIIRS can say a thing or two about the ice melt event.

JPSS (SNPP and NOAA-20)

VIIRS Imagery and Visualization Team Blog

Tag Archives: ice and snow

Chinese Super-Smog

End of Autumn in the Alps

Remote Islands, part III: Îles Kerguelen and Heard Island

Greenland Eddies and Swirls

VIIRS and the Greenland Ice Melt