JPSS (SNPP and NOAA-20)

Fires in Paradise

Posted on August 16, 2012 by Curtis Seaman

Sometimes, it seems like the whole world is on fire. Siberia. The western United States (which has been burning for some time). And now, the Canary Islands. The Spanish islands have been under a drought, as has much of Spain. (As an indication of how dry it has been, one fire in mainland Spain was started by someone flicking a cigarette butt out of their car window in a traffic jam – a fire that ultimately led to two deaths.) Back in July, fires got started on Tenerife – a major resort destination – and earlier this month, fires began on La Palma and La Gomera. At least two firefighters have already died battling these fires.

For your reference, here is a VIIRS “true color” image (M-3 [0.488 µm], M-4 [0.555 µm], M-5 [0.672 µm]) of the Canary Islands, with the major islands labelled:

VIIRS true color RGB composite of channels M-3, M-4 and M-5, taken 14:01 UTC 5 August 2012

If you look closely at this image, from 5 August 2012, you can see smoke plumes coming off of La Palma and La Gomera. You can also see what looks like a von Kármán vortex street downwind of La Palma. That’s the west coast of Africa in the lower-right corner of the image.

As discussed previously, the true color RGB composite is better for viewing the smoke plume, but you can’t actually see the fire directly. So, here’s the M-5 (0.672 µm), M-7 (1.61 µm) and M-11 (2.25 µm) composite from the same time:

VIIRS RGB composite of channels M-5, M-7 and M-11, taken 14:01 UTC 5 August 2012

It’s easy to see where the fires are actively burning with this composite. Let’s zoom in to make it even more obvious:

VIIRS false color RGB composite of channels M-5, M-7 and M-11, taken 14:01 UTC 5 August 2012

All the bright red pixels indicate where the fire is actively burning. You can also see the burn scar on Tenerife (not as easily as in Siberia) where the M-5, M-7, M-11 RGB composite shows the fire was back in July:

VIIRS false color RGB composite of channels M-5, M-7 and M-11, taken 14:38 UTC 18 July 2012

La Gomera has been the hardest hit island, where thousands of people had to be evacuated, and approximately 10% of Garajonay National Park has burned. Garajonay National Park is home to one of the last remaining laurisilva forests, which has been around for 11 million years. That lush vegetation burned hot, and channel I-04 (3.7 µm) reached saturation as that area went up in flames:

VIIRS channel I-04 image of fires in the Canary Islands, taken 14:01 UTC 5 August 2012

The two white pixels on La Gomera are where I-04 reached saturation and “fold-over” due to the heat from the fire. M-13 (4.0 µm), which is a dual-gain band designed to not saturate, reached a brightness temperature of 451 K over La Gomera, compared with a saturation brightness temperature of 367 K for channel I-04.

The fires also showed up in the Day/Night Band that night:

VIIRS Day/Night Band image of the Canary Islands, taken 02:25 UTC 6 August 2012

The red arrows point out the fires on La Palma and La Gomera. The fire on La Gomera covers a significant percentage of the island. The yellow arrow points to Lanzarote, which, for some reason, is not part of IDL’s map. On the night this image was taken, the moon was approximately 84% full, so you can see a number of clouds as well the city lights from the major resort areas of the Canary Islands. The biggest visible city in Africa is El Aaiún, the disputed capital of Western Sahara.

Finally, here’s the “pseudo-true color” composite of VIIRS channels I-01 (0.64 µm), I-02 (0.87 µm) and I-03 (1.61 µm) from 13:42 UTC 6 August 2012. This is a full granule at the native resolution of the Imagery bands with no re-mapping, showing the rich detail of VIIRS high-resolution imagery, including more interesting cloud vortices:

VIIRS false color RGB composite of channels I-01, I-02 and I-03, taken 13:42 UTC 6 August 2012

Make sure to click on the image, then on the “6400×1536” link to see it in its full glory.

Posted in Uncategorized | Tagged Canary Islands, day/night band, fire, I-1, I-2, I-3, I-4, M-11, M-13, M-3, M-4, M-5, M-7, resolution, RGB composite, VIIRS, vortex street | Comments Off

Fires near the “Coldest City on Earth”

Posted on August 10, 2012 by Curtis Seaman

Raise your hand if you’ve only ever heard of Yakutsk because of the board game “Risk”. (If you raised your hand, you might want to look around and make sure that no-one saw you raise your hand for no reason.) Yakutsk is actually the capital city of the Sakha Republic (a.k.a. Yakutia), which, according to Wikipedia, is the largest sub-national governing body in the world (only slightly smaller than India in terms of land area). Over 260,000 people live in Yakutsk, which has been called the “Coldest City on Earth” (with 950,000 total in Yakutia) even though, according to this article, it doesn’t sound very pleasant in the winter (or summer, for that matter). In January, the average temperature is -42 °C (-45 °F), and it isn’t very far from Oymyakon, where the lowest temperature ever recorded in a permanently inhabited location was observed (-71.2 °C or -96.2 °F). In the summer, it can make it up to +35 °C (95 °F) and legends tell of reindeer dying from choking on all the insects that cloud the air.

This summer, large areas of Siberia (including Yakutia) have been on fire. Some pictures from MODIS have already been circulating around the internet (e.g. here and here). And someone beat me to posting VIIRS images already. To make it easier to judge the size of the fires that are visible in the VIIRS Day/Night Band (DNB) image in the last link, here is a close-up with latitude and longitude lines added:

VIIRS DNB image of fires in Siberia, taken 16:25 UTC 4 August 2012

At this latitude, longitude lines are ~55 km apart. The latitude lines are ~111 km apart. So, you can see that these fires cover quite a large area. Unfortunately, you can’t see Yakutsk, which is underneath the clouds (and possibly smoke) at about 62° N, 130° E.

For comparison, here is the M-13 (4.05 µm) image from the same time. The primary purpose of M-13 is to detect wildfires. Notice how all of the hot spots (black spots) line up with all of the light sources that the DNB saw:

VIIRS channel M-13 brightness temperature image taken 16:25 UTC 4 August 2012

The visible image from earlier that day showed just how much smoke was produced by all of these fires:

Visible image of fires in Siberia from VIIRS channel M-5, taken 02:38 UTC 4 August 2012

Except for a few clouds near the edges of the scene, that is pretty much all smoke.

A few days later, the burn areas were easily visible with many fires still active, although not producing nearly as much smoke. RGB composites can really highlight what is going on with these fires, so let’s look at a few.

You should already be familiar with the “true color” image (M-3, 0.488 µm [blue], M-4, 0.555 µm [green] and M-5, 0.672 µm [red]):

True color image from VIIRS channels M3, M4 and M5 of fires in Siberia, taken 03:22 UTC 7 August 2012

And the “pseudo-true color” image made by combining the first three I-bands (I-01, 0.64 µm [blue], I-02, 0.865 µm [green] and I-03, 1.61 µm [red]):

False color (or "pseudo-true color") image of fires in Siberia from VIIRS channels I-01, I-02 and I03, taken 03:22 UTC 7 August 2012

The “pseudo-true color” image may be referred to as “natural color” depending on who you talk to. It should be noted that these last two images were kept at the native resolution of VIIRS with no re-mapping or re-sizing the image. There is only cropping to keep the file sizes manageable.

As discussed before, the pseudo-true color composite has the advantage of easily distinguishing ice and snow from liquid clouds, and it is really sensitive to vegetation. Plus, scattering by molecules in the atmosphere is greatly reduced, so you don’t have to do any atmospheric correction to produce a nice image. There is also the advantage that it uses I-bands, which have twice the resolution of the M-bands. But, that advantage was almost always neutralized by the fact that the images would have to be compressed to create a reasonable file size so that it would fit on this blog. If you click on the images above, then on the full-resolution link below the banner, you can easily compare the true resolution between the M-band image and the I-band image.

You can see here that the burn scars (all the dark brown areas) show up really well in the pseudo-true color image. (Some of the lighter or reddish brown areas are mountain ranges.) You might also notice that the active fires are still producing smoke, which shows up a lot better in the true color image. Some of the burn scars cover an area close to 60 km across.

As luck would have it (or, more accurately, the planning ahead by the scientists and engineers who designed VIIRS), channels M-5 (0.672 µm), M-7 (0.865 µm) and M-10 (1.61 µm) are very similar to the first three I-bands, so we can easily produce an M-band “pseudo-true color” image:

"Pseudo-true color" composite of VIIRS channels M-5, M-7 and M-10 of fires in Siberia, taken 03:22 UTC 7 August 2012

For reference, the location of Yakutsk has been identified. Also, if you’re curious, the big river that curves from the left-middle of the image to the top-center is the Lena River. It is up to 10 km wide in parts, particularly north of Yakutsk. Its second largest tributary, the Aldan River, is also easily visible as it meanders through a lot of the burn areas.

If you replace M-10 with M-11 (2.25 µm) as the red channel, you get this image:

False color RGB composite of VIIRS channels M-5, M-7 and M-11, taken 03:22 UTC 7 August 2012

Here, the green is darker due to the lower reflectivity of the surface in M-11 compared with M-10. The advantage of this RGB composite it that, if you zoom in, you can actually see where the fires are still active, as those pixels show up bright red. (If the fire is hot enough, you’ll get red pixels in the “pseudo-true color” composite also, but M-11 is more responsive to heat from fires than M-10, so you can see lower temperature fires this way.) You can also see the faint bluish smoke plumes originating from the areas that are actively burning.

If you go in the other direction and use only the shortest wavelengths, the surface becomes difficult to see, but the smoke stands out more. Here is the RGB composite of M-1 (0.412 µm [blue]), M-2 (0.445 µm [green]) and M-3 (0.488 µm [red]):

False color RGB composite of VIIRS channels M-1, M-2 and M-3, taken 03:22 UTC 7 August 2012

Here, the wavelengths of these channels range from the violet to the blue portion of the visible spectrum. At these shorter wavelengths, scattering in the atmosphere becomes much more important and the solar radiation has a tough time making it all the way to the surface. All the smoke and haze increases the scattering, so it is difficult to pick out features on the surface. That same scattering, though, really highlights the smoke plumes, which are difficult to see in the other false color composites. Since the scattering by the stuff in this image doesn’t vary much between these three channels, you get an image without much color to it.

With much of Colorado and, really, much of the western U.S. having burned already this year, it’s easy to know what the people of Siberia are going through. Fortunately, none of the fires have really threatened any towns. And, another plus: I bet those clouds of mosquitoes don’t like the dry weather that has caused all of these fires.

Posted in Uncategorized | Tagged day/night band, false color, fire, I-1, I-2, I-3, M-1, M-10, M-11, M-2, M-3, M-4, M-5, M-7, resolution, RGB composite, Siberia, true color, VIIRS | Comments Off

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.

Posted in Uncategorized | Tagged false color, Greenland, I-3, ice and snow, M-10, RGB composite | Comments Off

Daniel, Emilia and Fabio, oh my!

Posted on July 18, 2012 by Curtis Seaman

It’s been a while since we last looked at some tropical cyclones with VIIRS. If you don’t keep up to date on tropical activity, you might not know there that have been a few. Granted, since Debby dumped a bunch of rain on Florida three weeks ago, the Atlantic basin has been pretty quiet. The East Pacific basin, however, has had one storm after another. The national media has largely ignored them since they have posed no threat to any landmasses. See this article from the L.A. times. Boring! Unless you can capture video of Jim Cantore struggling to stand upright, it isn’t a hurricane, right?

Wrong! First of all, eastern Pacific hurricanes affect some major shipping lanes. Second, and this is true of all hurricanes: they transport energy and moisture and help moderate the temperature imbalance between the tropics and mid-latitudes. They are important components of global energy transport.

In this post, we are going to compare the view of hurricanes provided by VIIRS against the view provided by GOES (specifically GOES-15). On 9 July 2012, there were two storms in the East Pacific: Daniel and Emilia.

Here is the GOES-15 view of Daniel followed by the VIIRS view of Daniel in their respective visible channels:

GOES-15 visible image (channel 1) of Hurricane Daniel, taken 22:45 UTC 9 July 2012. Image courtesy John Knaff.

VIIRS visible image (channel I-01) of Hurricane Daniel, taken 22:29 UTC 9 July 2012

Both images have the same latitude and longitude lines printed on them for reference and they both use the same color scales. If you zoom in, you’ll notice that the VIIRS image, with ~375 m resolution at nadir shows a bit more detail than the 1 km (1000 m) resolution GOES image. The additional detail provided by VIIRS really stands out in the infrared (IR) window channels, where GOES has 4 km resolution and VIIRS still has ~375 m resolution:

GOES-15 IR image (channel 4) of Hurricane Daniel, taken 22:30 UTC 9 July 2012

VIIRS IR image (channel I-05) of Hurricane Daniel, taken 22:29 UTC 9 July 2012

Now, it is worth noting that the high resolution IR image of VIIRS shown above comes from channel I-05, which is centered at 11.45 µm. The GOES image was produced from Imager channel 4, which is centered at 10.7 µm, so the two channels don’t exactly have the same spectral properties. VIIRS has a 10.7 µm IR channel as one of its moderate resolution bands (M-15). Here’s what that image looks like:

VIIRS IR image (channel M-15) of Hurricane Daniel, taken 22:29 UTC 9 July 2012

There isn’t a big difference between the two VIIRS channels, although you can see a bit more detail in the higher resolution (I-05) image.

On the previous orbit, VIIRS caught images of Hurricane Emilia, which was also in the view of GOES-15. Here’s how the images compare:

GOES-15 visible image (channel 1) of Hurricane Emilia, taken 21:00 UTC 9 July 2012. Image courtesy John Knaff.

VIIRS visible image (channel I-01) of Hurricane Emilia, taken 20:48 UTC 9 July 2012

GOES-15 IR image (channel 4) of Hurricane Emilia, taken 20:48 UTC 9 July 2012

VIIRS IR image (channel I-05) of Hurricane Emilia, taken 20:48 UTC 9 July 2012

In addition to the resolution differences, there is also a time difference of ~15 minutes between the VIIRS images and the GOES images. If you were to overlap these images, you would see that Emilia rotated a bit during that time. Emilia was not willing to hold the same pose for that long when having her picture taken. Once again, the M-15 image from VIIRS looks pretty similar to the I-05 image, so there’s no pressing need to show it.

Finally, let’s compare GOES-15 with VIIRS on Hurricane Fabio, which formed about a week after Daniel and Emilia were hurricanes.

GOES visible image (channel 1) of Hurricane Fabio, taken 20:30 UTC 15 July 2012

GOES-15 visible image (channel 1) of Hurricane Fabio, taken 20:30 UTC 15 July 2012. Image courtesy John Knaff.

VIIRS visible image (channel I-01) of Hurricane Fabio, taken 20:36 UTC 15 July 2012

GOES-15 IR image (channel 4) of Hurricane Fabio, taken 20:30 UTC 15 July 2012

VIIRS IR image (channel I-05) of Hurricane Fabio, taken 20:36 UTC 15 July 2012

The GOES and VIIRS images of Fabio were taken only 6 minutes apart, so there is less movement to impede the comparison.

In all three hurricanes, you can see a lot more structure to the VIIRS images in the both the visible and IR channels. It’s as if GOES represents a standard definition TV camera, and VIIRS represents a hi-def TV camera. All those wrinkles GOES is smoothing over are showing up in VIIRS. Daniel, Emilia and Fabio are going to need more makeup. (Or, they would if they weren’t already dead.)

Posted in Uncategorized | Tagged Daniel, Emilia, Fabio, GOES, I-1, I-5, M-15, Pacific, resolution, tropical storm, VIIRS | Comments Off

Remote Islands, part II: Tristan da Cunha

Posted on June 29, 2012 by Curtis Seaman

Are you tired of 100 °F heat? We sure are in Colorado. Denver tied an all-time record of five consecutive days of 100+ °F high temperatures this week (two of which had the all-time highest recorded temperature of 105 °F). Much of the country experienced record-breaking heat as well. What better place to escape the heat than to visit the Islands of Refreshment?

The islands were given the name by a group of four Americans who sailed there in 1810, intending to make it their own kingdom. Unfortunately, 75% of them died in a boating accident less than two years after they arrived. I suppose, if the fourth one died we never would have heard this story. To the rest of the world, the islands were and are known as Tristan da Cunha, named after Tristão da Cunha – the Portuguese explorer who first found them in 1506.

It’s hard to get more remote than Tristan da Cunha. The four main islands, Tristan da Cunha, Inaccessible Island, Nightingale Island and Gough Island are part of the British Overseas Territory of Saint Helena, Ascension and Tristan da Cunha. The only way to visit them is by boat from South Africa – which takes a week – and boats only come around once or twice a month. You also need to write a proposal to the Secretary of the Administrator outlining what you plan to do there in order to gain permission to visit. The permanent population of the islands is less than 300, and they’ve even developed their own version of English. Another interesting fact: they only acquired television in the last 10 years (according to Wikipedia).

So where is Tristan da Cunha? The small island territory is 2,816 km from the nearest continent (Africa) and 2,430 km from their administrative capital (St. Helena). Let’s see if you can find it in high-resolution visible (I-01, 0.64 µm) imagery from VIIRS:

Visible image (I-01) of Tristan da Cunha from VIIRS, taken 14:49 UTC 25 June 2012

Give up? I’ll make it easier and show the false color RGB composite (I-01, I-02 and I-03):

False color RGB composite of VIIRS channels I-01, I-02 and I-03 taken 14:49 UTC, 25 June 2012

Three of the islands are easy to pick out now, particularly if you click to get the full size image. (Click on the image, then click on the 1512×1226 link below the banner.) The fourth island is difficult to see as it is covered by clouds and ice and snow, which look like clouds.

Here they are, labelled:

False color RGB composite of VIIRS channels I-01, I-02 and I-03 taken 14:49 UTC, 25 June 2012

Nightingale Island, at 3.2 km², is only about 5×4 pixels in size! The volcano that makes up the main island, Queen Mary’s Peak, rises 6,765 ft. above sea level and is casting a “cloud shadow” (i.e. no clouds are seen immediately downwind, or northeast, of the island). There may even be a von Kármán vortex behind it. Gough Island is also casting a “cloud shadow”, although it is much smaller.

If you really zoom in, you can almost convince yourself that VIIRS can identify two much smaller islands off the northern tip of Nightingale Island, Middle Island and Stoltenhoff Island:

False color RGB composite of VIIRS channels I-01, I-02 and I-03 taken 14:49 UTC, 25 June 2012

Look for the two greenish pixels above Nightingale Island. These islands are both about 25 acres in size (0.1 km²).

While the only town, Edinburgh of the Seven Seas, is on Tristan da Cunha, there is also a year-round research facility on Gough Island. There are three meteorologist positions on the island, as it is an important weather station for South Africa and the United Kingdom. As a bonus, the record high temperature has never come close to 100 °F. So, if you’re really looking to get away from the heat (and everything else), Gough Island might be the place for you!

Posted in Uncategorized | Tagged false color, I-1, remote island, resolution, RGB composite, VIIRS | Comments Off

Wild Week of Wildfires, Part II

Posted on June 20, 2012 by Curtis Seaman

Last time on “Wild Week of Wildfires“, we looked at the Little Bear Fire and High Park Fire, two lightning-ignited fires burning out west that were so hot they caused saturation in the two 3.7 µm channels on VIIRS (I-04 and M-12). There was mention of the Duck Lake Fire, a lightning-ignited fire in northern Michigan, which VIIRS also saw, and I couldn’t resist showing some more images.

On 9 June 2012, the same day the High Park Fire exploded (figuratively speaking), the Duck Lake Fire finally reached 100% containment after burning over 21,000 acres. The next day (10 June 2012), Suomi NPP passed over the Upper Peninsula of Michigan, and it was actually a clear day. (This joke comes courtesy of 20+ years experience of living in Michigan.) Even with 100% containment, the hot spot of the fire was still clearly visible in VIIRS channel I-04 (3.7 µm) that afternoon:

Channel I-04 image of the Duck Lake Fire from VIIRS, taken 18:18 UTC 10 June 2012

The highest brightness temperature in the burn area in this channel at this time was ~331 K. As we saw before with the Lower North Fork Fire, the high resolution false color composite of channels I-01, I-02 and I-03 is useful in highlighting the burn area:

False color RGB composite of VIIRS channels I-01 (blue), I-02 (green) and I-03 (red), taken 18:18 UTC 10 June 2012

Notice the large, brown area that coincides with the hot spot in the I-04 image. The combination of wavelengths used in this composite (0.64 µm [blue], 0.865 µm [green] and 1.61 µm [red]) is quite sensitive to the amount (and health) of the vegetation.

You might have also noticed several other interesting features in the image that show up better when you zoom in:

False color composite of VIIRS channels I-01, I-02, and I-03 from 18:18 UTC 10 June 2012

The Upper Peninsula of Michigan was based on mining for most of its history, and several large mines and quarries still exist, which VIIRS can easily see.

If you didn’t know any better, you might confuse the iron mine southwest of Marquette, Michigan with a frozen lake, or miraculously un-melted snow leftover from winter, since that is just what snow and ice look like in this kind of RGB composite. Compare that with the true color view of the same area:

True color RGB composite of VIIRS channels M-3, M-4 and M-5, taken 18:18 UTC 10 June 2012

In this case, the iron mine stands out as a bright red. Why?

The true color composite uses wavelengths at 0.48 µm (blue), 0.55 µm (green) and 0.67 µm (red). The red channel in the true color composite is actually in the red portion of the visible spectrum. The blue channel in the false color composite (0.64 µm) is also in the red portion of the visible spectrum.

This example shows that the iron oxide (rust) produced at the iron mine is highly reflective in the red portion of the visible spectrum. That’s what gives it the characteristic rust color. Iron oxide is not nearly as reflective at shorter or longer wavelengths, so it shows up blue when red wavelengths are used as the blue channel (as in the false color composite) and red when they are used as the red channel (as in the true color composite).

Let this be a lesson to anyone who uses the false color composite as part of a snow and ice detection algorithm. Snow and ice are not the only things to show up that color. You may be looking at a really large iron mine.

Posted in Uncategorized | Tagged false color, fire, great lakes, I-4, michigan, RGB composite, true color, VIIRS | Comments Off

A Wild Week of Wildfires

Posted on June 14, 2012 by Curtis Seaman

The last few weeks have been filled with lightning-ignited wildfires across the United States. The County Line Fire, along the Florida-Georgia border was caused by lightning on 5 April 2012 and burned ~35,000 acres. The Whitewater-Baldy Complex (began 16 May 2012) – the largest wildfire in New Mexico history – started as two different fires (both caused by lightning) that merged together. It’s over 280,000 acres (that’s not a typo) and continues to burn (as of 13 June 2012). The Duck Lake Fire (began 24 May 2012) burned 21,000 acres of Michigan’s Upper Peninsula and was caused by lightning. The Little Bear Fire (began 4 June 2012), also in New Mexico, was caused by lightning and has burned ~37,000 acres. Much closer to home, the High Park Fire (began 9 June 2012) is already the largest wildfire in Larimer County history and the third largest fire in Colorado history. It has burned ~46,000 acres and I bet you can guess what caused it.

It’s not clear who is to blame here – there is a long list of suspects – but I bet it was Thor. Even though the U.S. is generally the domain of the Thunderbird, Thor has a mountain-crushing hammer called Mjöllnir, which makes him as good a suspect as any. He may have been in cahoots with Indra or Marduk who are the bringers of rain, and have been holding back on us. Look at how dry it has been across the majority of the country.

With all of these fires, it’s hard to know where to begin. We’re going to ignore the County Line Fire as it was put out over a month ago. We’re also going to ignore the Whitewater-Baldy Complex, as it is so big, it can be seen by GOES. (Kidding! We kid because we love.) Plus, it’s been done before. The VIIRS view of the High Park Fire has also been looked at by CIMSS, with an interesting comparison between VIIRS and MODIS.

What we are going to do is show off interesting features of some of these fires that haven’t been shown or discussed before (as far as we know). We begin with “saturation”. Both the High Park Fire and Little Bear Fire saturated the VIIRS 3.7 µm channels (I-04 and M-12):

Channel I-04 image of the Little Bear Fire from VIIRS taken 20:16 UTC 9 June 2012

Channel I-04 (3.7 µm) image of the Little Bear Fire from VIIRS taken 20:16 UTC 9 June 2012

Channel M-12 image of the Little Bear Fire from VIIRS taken 20:16 UTC 9 June 2012

Channel M-12 (3.7 µm) image of the Little Bear Fire from VIIRS taken 20:16 UTC 9 June 2012

Channel I-04 image of the High Park Fire from VIIRS taken 19:59 UTC 10 June 2012

Channel I-04 (3.7 µm) image of the High Park Fire from VIIRS taken 19:59 UTC 10 June 2012

Channel M-12 image of the High Park Fire from VIIRS taken 19:59 UTC 10 June 2012

Channel M-12 (3.7 µm) image of the High Park Fire from VIIRS taken 19:59 UTC 10 June 2012

The top two images are of the Little Bear Fire, which formed near the border of Lincoln and Otero counties in New Mexico. The bottom two images are of the High Park Fire in Larimer County, Colorado. For each fire, the high resolution 3.7 µm channel (I-04) is compared with the moderate resolution 3.7 µm channel (M-12). The colors range from white (cold) to black (hot). But, wait a minute! If white is cold, why are there white pixels mixed in with the black ones that indicate the hot spots? That’s because these channels are saturating and experiencing “fold-over”. The peak brightness temperatures these channels can measure is ~ 367 – 368 K. Anything warmer than that won’t be detected, so the channel is said to be saturated. When it really gets above that limit you can have “fold-over”, where not only are you not observing the higher, correct temperature, the detectors actually report a lower temperature or radiance. In these fires, the fold-over is resulting in brightness temperatures down to 203 K for M-12 and 208 K for I-04, which is about 90-100 K colder than even the area surrounding the fires!

Luckily, VIIRS has a 4.0 µm channel (M-13) that was designed to not saturate at the temperature of typical wildfires. Compare the hottest pixels in the M-13 images below with the fold-over pixels from M-12 and I-04 above:

Channel M-13 image of the Little Bear Fire from VIIRS taken 20:16 UTC 9 June 2012

Channel M-13 (4.0 µm) image of the Little Bear Fire from VIIRS taken 20:16 UTC 9 June 2012

Channel M-13 image of the High Park Fire from VIIRS taken 19:59 UTC 10 June 2012

Channel M-13 (4.0 µm) image of the High Park Fire from VIIRS taken 19:59 UTC 10 June 2012

The hottest pixel in M-13 reached a temperature of 588 K for the Little Bear Fire and 570 K for the High Park Fire – over 200 K warmer than the saturation points of M-12 and I-04!

These fires were so hot, they appeared in channels that don’t usually show a fire signal. Limiting our attention to the High Park Fire (which was almost literally in our back yard), here’s the I-05 (11.5 µm) image from 10 June 2012:

Channel I-05 image of the High Park Fire from VIIRS taken 19:59 UTC 10 June 2012

Channel I-05 (11.5 µm) image of the High Park Fire from VIIRS taken 19:59 UTC 10 June 2012

The highest temperature observed in I-05 was 380 K. Longer wavelength channels, such as in I-05 are less sensitive to sub-pixel hot spots than channels in the 3.7 – 4.0 µm range, so fires don’t often show up. For pixels to have a 380 K brightness temperature in I-05, it means that the average temperature over the entire pixel had to be above +100 °C – hot enough to boil water!

Fires don’t often show up at shorter wavelengths, either, because the amount of solar radiation usually dwarfs any signal from the Earth’s surface. But, the High Park Fire did reach saturation at 2.25 µm (M-11):

Channel M-11 image of the High Park Fire from VIIRS taken 19:59 UTC 10 June 2012

Channel M-11 (2.25 µm) image of the High Park Fire from VIIRS taken 19:59 UTC 10 June 2012

The color scale has been reversed so that it is more inline with visible imagery. The white pixels represent saturation in M-11 at a radiance of 38 W m^-2 µm^-1 sr^-1. The reflectance of these pixels saturated at a value of 1.6, which means that the amount of radiation detected in this channel was more than 1.6 times the amount you would expect to see if the surface was a perfect mirror reflecting all the solar radiation back to the satellite. Thus, the fire’s contribution to the total radiance was significant in this channel.

The contribution from the surface (i.e., the fire) was also visible in the 1.6 µm channel (M-10), but it isn’t exciting enough to show. One channel shorter down on VIIRS (M-9, 1.38 µm) and the signal disappears against the high reflectivity of the smoke plume.

It’s impossible to leave out the Day/Night Band, which shows just how large and how close the High Park Fire got to Fort Collins:

Day/Night Band image of the High Park Fire from VIIRS taken 09:58 UTC 11 June 2012. Image courtesy Dan Lindsey.

The smoke plume, while not exactly visible, is affecting the view of the east side of the fire and Fort Collins, making them appear more blurry than they would if the sky were completely clear. You can also see that, overnight on 11 June 2012, the fire covered an area larger than any of the cities visible in the image (except for Denver, which is mostly cropped off the bottom of the image).

Hopefully, Marduk will start doing his job and bring us some rain and these will be the last fires for a while.

Posted in Uncategorized | Tagged colorado, day/night band, fire, I-4, I-5, M-11, M-12, M-13, new mexico, VIIRS | Comments Off

Cape Verde Waves and Plumes

Posted on June 8, 2012 by Curtis Seaman

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.

False color RGB composite of VIIRS channels I-1, I-2 and I-3 taken 14:41 UTC 6 June 2012

False color RGB composite of VIIRS channels I-1, I-2 and I-3 taken 14:41 UTC 5 June 2012

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.

True color RGB composite of VIIRS channels M-3, M-4 and M-5 taken 14:41 UTC 6 June 2012

True color RGB composite of VIIRS channels M-3, M-4 and M-5 taken 14:41 UTC 5 June 2012

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).

False color RGB composite of VIIRS channels I-1, I-2 and I-3 taken 14:41 UTC 5 June 2012

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:

Split-window difference from VIIRS (M15 minus M16) from 14:41 UTC 5 June 2012

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):

Moderate resolution 3.7 µm image (M-12) from VIIRS, taken 14:14 UTC 5 June 2012

Moderate resolution 3.7 µm image (M-12) from VIIRS, taken 14:41 UTC 5 June 2012

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:

Brightness temperature difference between VIIRS channels M-12 and M-15 from 14:14 UTC 5 June 2012

Brightness temperature difference between VIIRS channels M-12 and M-15 from 14:41 UTC 5 June 2012

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:

False color RGB composite of VIIRS channels I-1, I-2 and I-3 taken 14:41 UTC 5 June 2012. This image has been rotated from the previous images to highlight the length of the vortex streets.

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.)

Posted in Uncategorized | Tagged Cape Verde, dust, false color, gravity waves, M-12, M-15, M-16, RGB composite, split window difference, true color, VIIRS, vortex street | Comments Off

Catatumbo Lightning in the Day/Night Band

Posted on May 29, 2012 by Curtis Seaman

You may have noticed that many of the recent posts have featured imagery from the VIIRS Day/Night Band (DNB). That’s because the nighttime imagery produced by the DNB is so awesome! The DNB has seen clouds at night, auroras, forest fires, oil and gas flares, and even volcanic eruptions. Many of the previous images shown have included high resolution views of city (and even small town) lights. This post shows another interesting facet of DNB imagery: lightning. More specifically, Catatumbo lightning.

For those of you who don’t know (and didn’t click on that last link), Catatumbo lightning is one of the world’s most frequent lightning displays, with thunderstorms forming over the Catatumbo River in Venezuela an average of 160 nights per year. The lightning displays last up to 9 hours, beginning shortly after dusk. The lightning is nearly continuous and so vivid and reliable that it has been called the “Lighthouse of Maracaibo” or the “Catatumbo Lighthouse”, as fisherman and sailors have historically used it as a navigation aid. It is said that the locals were saved from an invasion by Sir Francis Drake in 1595, as the invading navy could not covertly enter Lake Maracaibo at night due to all the bright lightning. There is even a campaign to make Catatumbo lightning a UNESCO world heritage site. The lightning is so prominent, the state of Zulia in Venezuela has included it in their flag and coat of arms. Two years ago, the storms suddenly stopped for several months, causing mass panic in the streets- I mean, on the river- I mean… um, actually the villagers in this video don’t seem to care all that much.

Earlier this month, when the moon was about 80% full, Suomi NPP passed over Lake Maracaibo at night and, sure enough, a thunderstorm was present right over the mouth of the Catatumbo River.

VIIRS I-05 image of thunderstorms near Lake Maracaibo, Venezuela taken 06:44 UTC 10 May 2012

This image, taken from the high resolution imagery IR-window channel (I-05, 11.45 µm) on 10 May 2012, shows the deep convection over Venezuela and Colombia. The largest thunderstorm near the center of the image formed along the shore of Lake Maracaibo, near the mouth of the Catatumbo River. Here’s what the DNB saw at the same time:

VIIRS Day/Night Band image of thunderstorms near Lake Maracaibo, Venezuela taken 06:44 UTC 10 May 2012

The bright, almost rectangular streaks in the image are lightning strikes. The red arrow points out a lightning strike from the Catatumbo storm – a “Catatumbo lightning” strike, if you will.

The blocky appearance of lightning is due to the fact that VIIRS is a scanning radiometer. As the instrument scans the swath of the Earth that it sees, a bright, transient flash (such as from lightning) will show up in the along-scan direction as an individual streak of light in each sensor. The DNB has 16 different sensors that scan the swath simultaneously, and since lightning typically stretches over a large enough area to be detected by all of them, you get 16 different streaks all lined up next to each other. By the time the sensors have rotated back around for the next scan, the lightning flash has ended, producing abrupt edges in the direction along the satellite track. Compare this with the DMSP Operational Linescan System, which produces much more “streaky” lightning.

In addition to the “Catatumbo lightning”, you can see several other lightning flashes in the two deepest thunderstorms over Colombia. These are far enough away from Lake Maracaibo that they probably don’t count as Catatumbo lightning.

Other interesting features can be seen in these images as well. The moon was bright enough to cast shadows in the DNB image, allowing for the detection of the overshooting tops. These match-up with the coldest brightness temperatures in the I-05 image (which show up as dark blue to pure white in this color scale). A few pixels in the largest storm over Colombia (the one with two visible lightning flashes) have managed to make it to pure white on the color scale, indicating temperatures below 190 K (-83 °C). The dark blue pixels indicate brightness temperatures between 196 and 190 K (-77 to -83 °C). Brrr.

Overshooting tops exist when the convection is so vigorous, it peaks out above the anvil of the storm and penetrates the stable layer above (which is usually the stratosphere in storms this deep). In addition to acting as an indicator for severe weather, overshooting tops are important for energy and chemical transport between the troposphere and stratosphere.

It’s also interesting to see what looks like thin cirrus over the Caribbean Sea near Panama (left center of the image) that show up in the infrared (I-05) image, but not in the DNB. Plus, a number of cold clouds over Venezuela would appear to be optically thick due to their low brightness temperatures in the infrared image (yellow starts at 245 K down to green at 214 K), but they are optically thin enough to see city lights below in the DNB image. Awesome!

Posted in Uncategorized | Tagged day/night band, DMSP, I-5, lightning, OLS, overshooting tops, resolution, thunderstorm, VIIRS | Comments Off

The Hewlett Fire

Posted on May 21, 2012 by Curtis Seaman

According to reports, a man camping along the Hewlett Gulch trail in Roosevelt National Forest on 14 May 2012 had his camping stove knocked over in a gust of wind. One week (and $2.9 million) later, the Hewlett Fire scorched more than 7600 acres before fire crews could gain the upper hand. At one point 80 homes were evacuated but, thankfully, none of them were damaged. The smoke plume could be seen as far away as Laramie, Wyoming. Less than 20 miles away from the Cooperative Institute for Research in the Atmosphere, our home, it certainly caught our attention.

VIIRS aboard Suomi NPP monitored the fire day and night. About an hour after the fire was first reported, VIIRS captured the hot spot in channel I-04 (3.7 µm):

Image of the Hewlett Fire from VIIRS channel I-04, 20:05 UTC 14 May 2012

In the above image, the warmest (darkest) pixel had a brightness temperature of 350 K. A simple RGB composite of channels I-01 (0.64 µm), I-02 (0.87 µm) and I-03 (1.61 µm), with no other manipulation, from the same time as the I-04 image above, produces a red spot right over the I-04 hot spot:

False color RGB composite of VIIRS channels I-01, I-02 and I-03, 20:05 UTC 14 May 2012

Perhaps more amazing (but less useful from a firefighting perspective) is that, if you look closely (and you know the geography of the area), you can make out the locations of the following highways: I-25, I-76 and I-80, plus the main Union Pacific railroad tracks that more-or-less parallel I-80 in southern Wyoming. The high resolution imagery bands on VIIRS have enough resolution to identify interstate highways!

Suomi NPP passed over the area that night (15 May 2012) and the Day/Night Band (DNB) captured the fire burning brightly:

Day/Night Band image of the Hewlett Fire, 08:25 UTC 15 May 2012. Image courtesy Dan Lindsey.

By the time of the 17 May 2012 nighttime overpass – two days later – the fire had grown significantly. With no clouds around, the DNB easily saw the Hewlett Fire, as it was the brightest thing in the area. The image below has been enhanced to make the nearby city lights easier to see relative to the fire.

Day/Night Band image of the Hewlett Fire, 09:26 UTC 17 May 2012

In the above image, lights from various cities have been identified. The red arrow indicates the Hewlett Fire, which was bright enough and large enough to be confused for a city. The yellow arrow indicates what might be oil and/or gas flares burning in rural Weld County, which you can also see in the 15 May 2012 DNB image. Weld County is home to a third of all the oil and gas wells in Colorado.

In this zoomed-in image, you can see that the light from the fire covered an area approximately one third the size of Fort Collins:

Zoomed Day/Night Band image of the Hewlett Fire, 09:26 UTC 17 May 2012. Image courtesy Dan Lindsey.

This image was taken before the burn area even reached its maximum size. At the same time, channel I-04 also saw this ring of fire (not to be confused with the “ring of fire” caused by the recent annular eclipse):

VIIRS channel I-04 image of the Hewlett Fire, 09:26 UTC 17 May 2012

Once again, darker colors indicate higher brightness temperatures. The peak temperature in channel I-04 at this time was 356 K.

Even though it caused no damage to homes or structures, it was a little too close for comfort for many people.

As a final note, our partners up the hill in the Department of Atmospheric Science have taken an interest in the Hewlett Fire. If you are interested in the non-satellite side of the research into this fire, research groups led by Professors Rutledge, Kreidenweis and Collett have collected radar observations and in situ aerosol samples of the smoke plume. Contact them for more information.

Posted in Uncategorized | Tagged colorado, day/night band, false color, fire, I-4, resolution, RGB composite, VIIRS | Comments Off

JPSS (SNPP and NOAA-20)

Fires in Paradise

Fires near the “Coldest City on Earth”

VIIRS and the Greenland Ice Melt

Daniel, Emilia and Fabio, oh my!

Remote Islands, part II: Tristan da Cunha

Wild Week of Wildfires, Part II

A Wild Week of Wildfires

Cape Verde Waves and Plumes

Catatumbo Lightning in the Day/Night Band

The Hewlett Fire

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