Remote Islands VI: Return to Gough

Posted on February 4, 2020 by Curtis_Seaman

You youngins are not old enough to remember, but we took a look at Gough Island before. Well, not directly, but as part of the British territory of Saint Helena, Ascension and Tristan da Cunha eight years ago. We also did a special feature on Saint Helena and Ascension four years ago. So, why are we re-visiting a group of tiny islands in the middle of the South Atlantic Ocean for a third time? Because of the great view that VIIRS provided earlier this month, and because Gough Island is an interesting place.

For starters, it rhymes with “scoff” and not with “dough” despite the spelling. So now you know. It is also home to one of the more unique jobs in meteorology. It has no permanent residents, but every year a group of 5-10 people are brought in to run the weather station on it for the South African Weather Service and study the biology of the island for the South African National Antarctic Programme (SANAP) even though it is a British island. (At least one member of the team has to be a doctor, since there are no hospitals within 400 km and boats only stop by a couple of times per year.) From the pictures and video, it certainly looks like unique place to spend a year.

Now, on to the interesting satellite imagery. We begin our visit to Gough Island with a loop from Meteosat-11, and its imager, SEVIRI (PDF document):

Note that Meteosat data was provided to NOAA by EUMETSAT and the video above shows their “Enhanced” Natural Color RGB. I can also take this opportunity to promote the fact that we are now allowed to share Meteosat imagery on our ultra-popular website, SLIDER, which is where the above loop came from.

Credits and advertising out of the way, did you see Gough Island? If not, you could try viewing the video in full-screen mode. Or, it might help if I zoomed in on the area, like this:

Meteosat-11 “Enhanced” Natural Color RGB (07-18 UTC, 5 January 2020)

The southernmost green dot is Gough Island. The other green dots are Tristan da Cunha, Inaccessible Island, and Nightingale Island. What caught my attention was two things: it’s rare to get such a clear view of these islands and the waves produced by Gough Island clearly impact clouds that never even passed over the island. Of course, having come from SEVIRI, this loop is limited to 3 km resolution (since the HRV band isn’t part of this RGB, and doesn’t normally cover this part of the world).

What if we had 375 m resolution? What would that look like? Well, on VIIRS, it looks like this:

NOAA-20 VIIRS Natural Color RGB composite of channels I-01, I-2 and I-3 (14:38 UTC 5 January 2020)

Click on the image to view the full resolution. It’s worth it.

It should be noted that I haven’t applied the same “enhanced” version of the Natural Color RGB that removes the cyan color of ice clouds and snow. Another difference is something that you don’t see in the SEVIRI loop: sun glint. That’s because Meteosat-11 isn’t viewing the scene from the same angle as VIIRS.

Look closely downwind (or leeward) of Gough Island and you’ll see from the sun glint that the island is producing waves not only in the atmosphere, but on the surface of the ocean:

Same image as above, only zoomed in on Gough Island

Of course, if you clicked on the sun glint link, you saw a more extreme example of this, and if you bothered to read the article, you also saw the explanation (written much more succinctly and accurately than I could without plagiarism).

That was only the NOAA-20 view. We also have the Suomi-NPP view, which covered this area before and after NOAA-20. Here are all three views combined:

Animation of VIIRS Natural Color RGB images (5 January 2020)

You have to click on the image above to see the animation play. Now you can see the motion of the clouds, yet the waves are nearly stationary. That’s because they are “tied” to the island that is producing them. This is an example of trapped lee waves. And pilots beware: as this case shows, these waves are present even where there are no clouds to reveal them.

What is perhaps more interesting is that the waves in the ocean show up in the mid-wave infrared (IR) thanks to the sun glint:

S-NPP VIIRS I-04 image (13:46 UTC, 5 January 2020)

This is I-04, the 375 m resolution channel at 3.7 µm, from the first S-NPP overpass (13:46 UTC, 5 January 2020). See the waves on the lee of both Gough Island and Tristan da Cunha? (Tristan da Cunha’s waves aren’t apparent in the clouds. Since these are trapped lee waves, they are occurring below the height of the cirrus clouds to the northwest.) Now, let’s animate the three overpasses:

Animation of VIIRS I-04 images (5 January 2020)

The impact of sun glint on the these images, especially the middle one (NOAA-20) is obvious. The last image from S-NPP (15:29 UTC) has no sun glint, so these waves are much harder to spot.

Now check out the high-resolution longwave IR (LWIR) band, I-05 (11.4 µm):

Animation of VIIRS I-05 images (5 January 2020)

Pay attention to the change in scaling as revealed by the color table. Three things stand out: with this combination of scaling and color table, you can see structure in the sea surface temperature, the waves downwind of Gough are still visible in the ocean even in the LWIR, and “limb cooling” is something to watch out for.

More detail on the items of note: the sea surface temperature (SST) structure is easier to spot in I-05 because it is not impacted by sun glint. This is because the Earth emits significantly more radiation in the LWIR than what it receives from the sun. In the midwave-IR, the contribution from the sun is significant (as these images show). The waves are still visible in I-05 because the winds on the downward portion of the wave are hitting the ocean surface and modifying the exchange of heat between the atmosphere and the ocean, leading to waves of warmer and cooler SST. And, third, “limb cooling” is the name given to the fact that, at high satellite viewing angles, the path length of the radiation through the atmosphere increases, and more radiation comes from higher up where temperatures are colder. (More on limb cooling may be found on slides 19-21 here.) Look to the clear sky areas on the left edge of the swath on the first I-05 image and compare it to the middle image. Then do the same for the right edge of the swath on the last image. The limb cooling effect is readily apparent.

There’s one more interesting thing from this same scene. Look at the True Color images from these three overpasses:

Animation of VIIRS True Color RGB images (5 January 2020)

See any variations in the color of the ocean not related to sun glint? That is phytoplankton, a source of life and death in the ocean. In fact, Gough Island’s location, where warmer sub-tropical water mingles with colder mid-latitude water is what makes it such a great nesting site for birds. The fish eat the phytoplankton and the birds eat the fish. Unfortunately, stowaway mice brought to Gough Island by accident are eating the birds.

All that interesting science from one tiny island in the middle of the South Atlantic Ocean.

Remote Islands V: St. Helena and Ascension

Posted on May 2, 2016 by Curtis_Seaman

You may have missed it in the news, but history was made last week:

A plane landed! Wow!

But, that’s not any old plane – that’s the first commercial airliner to land on St. Helena Island, which just completed the construction of their very first airport. That means there may be no more commercial sailing to this tiny island.

People prone to seasickness may be cheering the news. People afraid of flying might not. Did you notice it took three attempts to land that plane in the video above? The first pass was getting everything all lined up with no intention of landing. The landing gear wasn’t even down. The second – which looked like a roller coaster – was waived off due to the heavy crosswinds. The third time was the charm. However, it was such a shaky first landing, they’ve postponed the official opening of the airport.

So, where is St. Helena (pronounced Ha-LEEN-a), anyway? And why should I care?

Well, to answer the first question, it’s somewhere in this image:

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (12:45 UTC 26 April 2016).

Did you find it? To help you with your bearings, Africa is just outside this VIIRS swath on the right side of the image. Two hints: click on the image to bring up the full resolution version. St. Helena is just northwest of the center of the image. It’s the only island in the image not covered by clouds. Fun fact: every island within this VIIRS swath is part of the British Overseas Territory of St. Helena, Ascension and Tristan da Cunha. We already looked more closely at Tristan da Cunha, so let’s take a look at the other two.

We can get a higher resolution look if we use the I-band Natural Color RGB composite:

VIIRS Natural Color RGB composite of channels I-01, I-02 and I-03 (12:45 UTC 26 April 2016).

Notice the island appears green in the center, surrounded by a ring of brown – just the way it looks on a really high resolution satellite image. VIIRS has the resolution to pick this out!

As for why you should care, I don’t know if I can answer that. If your first thought is to ask that question, you probably don’t care. But, there are a few interesting things to note about St. Helena (besides its new airport):

– It was once an important stopping point for ships sailing from Europe to India in search of spices. At least, until the Suez Canal opened.

– It later became a prison, housing those who fought against the British government and lost, including Napoleon Bonaparte, Dinuzulu, King of the Zulu Nation, and POWs from the Boer War.

– Along with Ascension Island, St. Helena helped inspire the modern environmental movement. And it was here that the first large scale experiments in weather modification took place. (Not counting rain dances, of course.)

After witnessing the effect of deforestation on the island in the late-1700s and early-1800s, it was believed that re-foresting would help keep moisture on the island, which would lead to more clouds and more rainfall. Ascension Island, which was essentially a barren wasteland when first discovered, was also planted with trees, creating it’s Green Mountain, which is clearly visible on very high resolution satellites.

Speaking of Ascension Island – where is that located? In the first image above, showing most of the Southern Atlantic, Ascension is near the upper left corner. It’s hard to see because it is covered by clouds. Just follow the 8 °S latitude line in from the left edge of the image.

Here it is at high resolution during a clear day:

VIIRS Natural Color RGB composite of channels I-01, I-02, and I-03 (14:03 UTC 20 April 2016).

If you look closely, you’ll see that there is a small cloud or two right over Green Mountain, so maybe the efforts of the early environmentalists paid off!

For completeness, Tristan da Cunha is in the lower left of the True Color image I posted at the top. While it is covered by clouds, you can tell it’s there because it is creating its own waves. Here it is on the next orbit, where it is closer to satellite nadir:

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (15:24 UTC 26 April 2016).

If I’ve inspired you to visit these islands, ask the government to give me a commission. But, seriously, don’t forget to say “Hi!” to Jonathan. Or see the many other plants and animals that are found nowhere else on Earth.

UPDATE (16 October 2017): Reuters has reported that the airport is now officially open to commercial flights (only a year and half after I wrote the original blog post)!

Abafado Bruma Seca

Posted on August 2, 2013 by Curtis_Seaman

Hopefully, Google Translate didn’t steer me wrong on the meaning of “abafado”. “Bruma seca” is a term used by Portuguese and Spanish speakers that literally translates to “dry mist”. It is typically used to refer to thick haze or the brownish air caused by dust and, more specifically, to the Saharan Air Layer (scroll down a bit on this Weather Underground blog post for nice description of what that is).

We’re speaking Portuguese today because we are re-visiting Cape Verde, an island nation where people speak Portuguese. (Actually, many people speak a creole version called Kriolu kabuverdianu that has Western African elements added to the Portuguese.) Last time we visited Cape Verde, the islands were creating interesting waves and plumes in the atmosphere. This time, Cape Verde is buried under a plume – a plume of Saharan air that is so thick, you can barely see the islands:

VIIRS "True Color" RGB composite of channels M-03, M-04 and M-05, taken 15:07 UTC 30 July 2013

I had to plot the map boundaries on the image just to see where the islands are. Otherwise, they would be lost in a sea of brown dust. Also, without the map, it’s difficult to find the shoreline of western Africa because the dust looks just like the Sahara Desert where it came from.

This image is (and the images to follow are) a “True Color” RGB composite. (As always, click on the picture, then on the “2442×1920” link below the banner to see the full resolution image.) Unlike many previous true color images shown on this blog, these have been “Rayleigh corrected.” This means the impact of Rayleigh scattering by the molecules in the atmosphere has been removed. The reason for doing this is that it makes the surface easier to see and it better represents what people normally see when looking out of the window on an airplane. Dust particles, on the other hand, are Mie scatterers at visible wavelengths (refer back to that last link) so they still show up. In fact, this is one of the strengths of the True Color composite: it is quite sensitive to particulate matter in the atmosphere like smoke, smog, haze and dust.

The image above was taken on 30 July 2013, one day after the dust really started to be pushed off the African coast. It is not clear if the people of Cape Verde were forced indoors by this dust since I wasn’t able to find any news reports on it. The western edge of the dust plume (between 28° and 29° W longitude) almost looks like it is casting a shadow, which would indicate the dust is lofted pretty high in the troposphere in this image.

This dust plume pushed across the Atlantic Ocean over the following days. VIIRS passed over Cape Verde on 31 July 2013 (14:48 UTC) and captured this image:

VIIRS "True Color" RGB composite of channels M-03, M-04 and M-05, taken 14:48 UTC 31 July 2013

Here, the dust plume extends from one side of the swath to the other – over 3000 km. On the very next orbit (16:29 UTC 31 July 2013), the plume can be seen on four consecutive data granules, extending almost to the middle of the swath. (The satellite covers a distance of over 2000 km over four granules.)

VIIRS "True Color" RGB composite of channels M-03, M-04 and M-05, taken 16:29 UTC 31 July 2013

Hold on. What’s that strip of white-colored stuff extending north-northwest from 50° W longitude label? Some kind of white dust? That happens to be in a straight line? Nope. It’s what is called “sun glint” and it’s the same basic phenomenon as the glare you see looking out over a body of water without polarized sunglasses. The dust is all the brown stuff on the right side of the image. That’s South America and the Lesser Antilles on the left side of the image.

If you click to the full resolution version of the image above, you may find that the image doesn’t seem very big considering it is made of four granules. (Its pixel size is 1600×1536. In contrast, the image above that is only two granules, yet is 3200×1536 in size.) That’s because I had to reduce the resolution of the data in order to plot it all without running out of memory on my computer. VIIRS has twice the resolution of what is shown in the latter image. (And this high resolution requires a lot of computing power to display!)

On 1 August 2013, the plume pushed even closer to the Lesser Antilles (although they are off the left side of this image).

VIIRS "True Color" composite of channels M-03, M-4 and M-05, taken 16:10 UTC 1 August 2013

Again, the resolution has been degraded by a factor of two. It is interesting to note that one granule covers an area of the Earth about 3040 x 570 km in size (1.7 million sq km, or 669,000 sq mi), so four granules is about 6.9 million km². That’s 2.6 million square miles. In comparison, the size of the lower 48 states is about 3.1 million square miles (3.7 million square miles if you add on Alaska and Hawaii). Now notice that the dust covers most of the last image. If you add on the area of the dust plume that stretches all the way back to Africa, you are talking about an area well over the size of the United States! By the time it arrives in the Caribbean, that dust better learn to speak Antillean Creole. It is a long way from Cape Verde.

So, what does all of this mean? It is often claimed that the presence of Saharan dust layers is bad for hurricane formation. Evidence for that claim is provided here and here. However, there are also scientists who refute that claim, which you can read about here. Scientists at the U.S. Geological Survey (USGS) have found that Saharan dust may be harmful to people and to coral reefs. According to this article in Nature, the dust is beneficial for the Amazon rainforest.

This event was also discussed on the Weather Channel. Compare his visible images to mine, which use only one color of the visible spectrum to my three color images. So, whether Saharan dust is good or bad, I think we can all agree that VIIRS is good!

UPDATE (5 August 2013): Remember the “split window difference”? It was mentioned the last time we visited Cape Verde. Here’s is a split window difference product produced at CIMSS that highlights the plume as it traveled across the Atlantic. This loop starts on 29 July and ends on 2 August 2013 and is made of data collected by the geostationary satellite MSG-3.

UPDATE (19 August 2013): Here’s another animation of the dust plume, made using observations from the Ozone Mapping and Profiler Suite (OMPS), one of the new instruments aboard Suomi NPP alongside VIIRS. (Actually, it’s on the opposite end of the satellite from VIIRS, so it’s not literally alongside VIIRS, but you get the idea.)

JPSS (SNPP and NOAA-20)

VIIRS Imagery and Visualization Team Blog

Tag Archives: Atlantic

Remote Islands VI: Return to Gough

Remote Islands V: St. Helena and Ascension

Abafado Bruma Seca