Hurricane Isaac: Before, During and After

Posted on September 4, 2012 by Curtis_Seaman

While Hurricane Isaac (then a tropical storm) did not destroy Tampa, Florida as many people feared, it certainly left its mark on the Gulf Coast. With many locations from Florida to Louisiana receiving more than 12″ of rain, and levees unable to keep out the storm surge, flooding was (and still is) a major problem. Look at these aerial photos of Isaac’s aftermath in Louisiana. The Visible Infrared Imaging Radiometer Suite (VIIRS) aboard Suomi NPP saw that flooding, also.

But first, let’s look at the high resolution infrared (IR) window channel (I-05, 11.45 µm) which, at ~375 m resolution, is the highest-resolution IR window channel on a public weather satellite in space today. This image was taken when Isaac was still a tropical storm in the middle of the Gulf of Mexico:

VIIRS I-05 image of Tropical Storm Isaac, taken 18:50 UTC 27 August 2012

This image uses a new (to this blog, anyway) color scale, developed by our colleagues at CIMSS, that really highlights the structure of the clouds at the top of Isaac. The color scale is included in the image. For comparison, here’s the GOES Imager IR window channel (channel 4, 10.7 µm) image from roughly the same time:

GOES-13 Imager channel 4 image of Tropical Storm Isaac, taken 18:45 UTC 27 August 2012

GOES has ~4 km resolution in its IR channels. VIIRS provides amazing details of the structure of tropical cyclones that you just can’t get with current geostationary satellites.

The real story from Isaac, however, is the flooding. It’s hard to capture flooding from a visible and infrared imaging instrument, since flooding usually occurs when it’s cloudy. Clouds block the view of the surface when looking at visible and infrared wavelengths. But, large quantities of water that fail to evaporate or drain into the local rivers after a period of several days can be seen after the skies clear. That’s what happened with Isaac.

Here are before-Isaac and after-Isaac images of the southern tip of the Florida Peninsula. These are false color (“pseudo-true color”) composites of VIIRS channels I-01, I-02 and I-03. These images were taken on the afternoon overpasses of 23 August and 29 August 2012. Many cities on the east coast of Florida got 10-16 inches of rain (250-400 mm for those of you outside the U.S.). See if you can pick out the flooding.

False color RGB composite of VIIRS channels I-01, I-02 and I-03 taken before and after Tropical Storm Isaac (2012)

If you have been following this blog, you know that, in the “pseudo-true color” RGB composite, water shows up very dark – in most cases, almost black. That’s not always true, of course. You can see sun glint (particularly in the “before” image) that makes water a lighter color and shallow water (where visible radiation [i.e. channel I-01] is able to penetrate to the bottom) shows up as a vivid blue.

Now, notice the Everglades. Many areas of the Everglades, particularly on the east side, appear darker in the “after” image, because those swampy areas have a lot more water in them. Water has a lower reflectivity than vegetation or bare ground at these wavelengths.

The effect of water on the land surface shows up even better in the moderate resolution channel M-06 (0.75 µm). M-06 is a channel not shown before because it is perhaps the worst channel for producing interesting images. M-06 was designed to aid in ocean color retrievals and/or other uses that require atmospheric correction. The M-06 detectors saturate at a low radiance, so any radiation at 0.75 µm that reflects off of clouds, aerosols or the land surface easily show up. About the only things that have low reflectivity in M-06 are atmospheric gases and water surfaces without sun glint. Ocean color retrievals need a very clean atmosphere with no aerosols or clouds and no sun glint to work correctly. You also need to be able to identify what is or is not water, which is what makes M-06 useful for identifying flooding.

Here are the similar before-Isaac and after-Isaac images of Florida from M-06:

VIIRS channel M-06 images of southern Florida taken before and after Tropical Storm Isaac (2012)

Both the land and optically thick clouds saturate M-06, so this channel is useless at identifying clouds over land (except you can see some cloud shadows). Sun glint is saturating the pixels over the Gulf of Mexico in the “before” image, while it is mostly to the east of Florida in the Atlantic Ocean in the “after” image. In the “after image”, reflective cirrus clouds over the Gulf of Mexico show up that are not as easily visible in the RGB composite. Of primary importance here, however, is the dark appearance of the Everglades in the “after” image. All that flood water reduced the reflectivity of the land surface, making it appear darker. That means, if you know where the clouds (and, hence, the cloud shadows) are, it may be possible to use M-06 to identify large flooded areas.

Louisiana and the coast of Mississippi were the hardest hit by Isaac, and the flooding is easily visible here, too. In fact, the massive flooding is easier to see in the RGB composite in this region. Compare the “before” and “after” images, taken on 26 August 2012 and 1 September 2012:

False color RGB composites of VIIRS channels I-01, I-02 and I-03 of southeast Louisiana

To make it easier to see, here’s a quick animation of the before and after images. Watch the highlighted areas.

Animated GIF of false color RGB composites taken from VIIRS before and after Hurricane Isaac

After the passage of Hurricane Isaac, Lake Maurepas and Lake Pontchartrain almost appear to merge into one big lake! Other flooding is visible near Slidell, Bay St. Louis, Pascagoula Bay, and the heavily hit parishes of Plaquemines, St. Bernard, Lafourche and Terrebonne.

Thin cirrus clouds are visible in the “after” image, which limit the ability of M-06 to detect some of the flooding, but M-06 is still able to see the large areas of flooding highlighted in the animation above. M-06 also detects reflection off of the Twin Spans as well as the Lake Pontchartrain Causeway. And this is at ~750 m resolution!

VIIRS channel M-06 images of southeastern Louisiana taken before and after Hurricane Isaac (2012)

So, don’t try to do ocean color retrievals in pixels obscured by big bridges.

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.

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.

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.)

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!

JPSS (SNPP and NOAA-20)

VIIRS Imagery and Visualization Team Blog

Tag Archives: I-1

Hurricane Isaac: Before, During and After

Fires in Paradise

Fires near the “Coldest City on Earth”

Daniel, Emilia and Fabio, oh my!

Remote Islands, part II: Tristan da Cunha