Think of a snowflake. What happens when that snowflake hits the ground? Now, picture other snowflakes – millions of them – all hitting the ground and piling up on top of each other, crushing our first poor snowflake. Skiers love to talk (and dream) about “fresh powder.” But, what happens when the “powder” isn’t so fresh?
Those delicate, little snow crystals we imagine (or look at directly, if we click on links included in the text) undergo a transformation as soon as they hit the ground. Compression from the weight of the snow above, plus the occasional partial thaw and re-freeze cycle (when temperatures are in the right range), breaks up the snow flakes and converts the 6-pointed crystals into more circular grains of snow. As more and more snow accumulates on top, the air in between the individual snowflakes/grains (which is what helps make it a good insulator) gets squeezed out, making the snow more dense. If enough time passes and enough snow accumulates, individual snow grains can fuse together. These bonded snow grains are called “névé.” If this extra-dense snow can survive a whole summer without melting, then a second winter of this compaction and compression will squeeze out more air and fuse more snow grains, creating the more dense “firn.” After 20 or 30 years of this, what once was a collection of fragile snowflakes becomes a nearly solid mass of ice that we call a “glacier.” Glaciers can be made up of grains that are several inches in length.
But, you don’t need to hear me say it (or read me write it), you can watch a short video where a guy in a thick Scottish accent explains it. (Did you notice his first sentence was a lie? Snow is made of frozen water, so glaciers are made of frozen water, since they are made of snow. I think what he means is that glaciers aren’t formed the same way as a hockey rink, but the way he said it is technically incorrect.) At the end of the video, the narrator hints at why we are looking at glaciers today: glaciers have the power to grind down solid rock.
When a glacier forms on a non-level surface, gravity acts on it, pulling it down the slope. This mass of ice and friction from the motion acts like sandpaper on the underlying rock, converting the rock into a fine powder known as “glacial flour” or, simply, “rock flour.” In the spring and summer months, the meltwater from the glacier collects this glacial flour and transports it downstream, where it may be deposited on the river’s banks. During dry periods, it doesn’t take much wind to loft these fine particles of rock into the air, creating a unique type of dust storm that is not uncommon in Alaska. One that can be seen by satellites.
And, wouldn’t you know it, a significant event occurred at the end of October. Take a look at this VIIRS True Color image from 23 October 2016:
See the big plume of dust over the Gulf of Alaska? Here’s a zoomed in version:
That plume of dust is coming from the Copper River delta. The Copper River is fed by a number of glaciers in Wrangell-St. Elias National Park, plus a few in the Chugach Mountains so it is full of glacial sediment and rock flour (as evidenced by this photo). And, it’s amazingly full of salmon. (How do they see where they’re going when they head back to spawn? And, that water can’t be easy for them to breathe.)
Notice also that we have the perfect set-up for a glacial flour dust event on the Copper River. You can see a low-pressure circulation over the Gulf of Alaska in the above picture, plus we have a cold, Arctic high over the Interior shown in this analysis from the Weather Prediction Center. For those of you familiar with Alaska, note that temperatures were some 30 °F warmer during the last week in October in Cordova (on the coast) than they were in Glennallen (along the river ~150 miles inland). That cold, dense, high-pressure air over the interior of Alaska is going to seek out the warmer, less dense, low-pressure air over the ocean – on the other side of the mountains – and the easiest route to take is the Copper River valley. The air being funneled into that single valley creates high winds, which loft the glacial flour from the river banks into the atmosphere.
Now, depending on your preferences, you might think that the dust shows up better in the Natural Color RGB composite:
But, everyone should agree that the dust is even easier to see the following day:
You can also see a few more plumes start to show up to the southeast, closer to Yakutat.
Since Alaska is far enough north, we get more than one daytime overpass every day. Here’s the same scene on the very next orbit, about a 100 minutes later:
Notice that the dust plume appears darker. What is going on? This is a consequence of the fact that glacial flour, like many aerosol particles, has a tendency to preferentially scatter sunlight in the “forward” direction. At the time of the first orbit (21:01 UTC), both the sun and the dust plume are on the left side of the satellite. The sunlight scatters off the dust in the same (2-dimensional) direction it was traveling and hits the VIIRS detectors. In the second orbit (22:42 UTC), the dust plume is now to the right of the satellite, but the sun is to the left. In this case, forward scattering takes the sunlight off to the east, away from the VIIRS detectors. With less backward scattering, the plume appears darker. This has a bigger impact on the Natural Color imagery, because the Natural Color RGB uses longer wavelength channels where forward scattering is more prevalent. Here’s the True Color image from the second orbit:
The plume is a little darker than the first orbit, but not by as much as in the Natural Color imagery. Here are animations to show that:
There are many other interesting details you can see in these animations. For one, you can see turbid waters along the coast in the True Color images that move with the tides and currents. These features are absent in the Natural Color because the ocean is not as reflective at these longer wavelengths. You can also see the shadows cast by the mountains that move with the sun. Some of the mountains seem to change their appearance because VIIRS is viewing them from a different side.
The dust plumes were even more impressive on 25 October 2016, making this a three-day event. The same discussion applies:
Full disclosure, yours truly drove through a glacial flour dust storm along the Delta River on the north side of the Alaska Range back in 2015. Even though it was only about a mile wide, visibility was reduced to only a few hundred yards beyond the hood of my car. It felt as dangerous as driving through any fog. The dust event shown here was not a hazard to drivers, since it was out over the ocean, but it was a hazard to fisherman. Being in a boat near one of these river deltas means dealing with high winds and high waves. To forecasters, these dust plumes provide information about the wind on clear days (when cloud-track wind algorithms are no help), which is useful in a state with very few surface observing sites to take advantage of.
The last remaining issue for the day is one of terminology. You see, “glacial flour dust storm” is a mouthful, and acronyms aren’t always the best solution. (GFDS, anyone?) “Haboob” covers desert dust. “SAL” or “bruma seca” covers Saharan dust specifically. So, what should we call these dust events? Something along the lines of “rock flour”, only more proactive! And, Dusty McDustface is right out!
Q: When a tree falls in the forest and nobody is around to hear it, does it make a sound?
That’s an easy question to answer. It’s not a 3000-year-old philosophical conundrum with no answer. Sound is simply a pressure wave moving through some medium (e.g. air, or the ground). A tree falling in the forest will create a pressure wave whether or not there is someone there to listen to it. It pushes against the air, for one. And it smacks into the ground (or other trees), for two. These will happen no matter who is around. As long as that tree doesn’t fall over in the vacuum of space (where there is nothing to transmit the sound waves and nothing to crash into), that tree will make “a sound”. (There are also sounds that humans cannot hear. Think of a dog whistle. Does that sound not exist because a human can’t hear it?)
What if it’s not a tree? What if it’s 120 million metric tons of rock falling onto a glacier? Does that make a sound? To quote a former governor, “You betcha!” It even causes a 2.9 magnitude earthquake!
That’s right! On 28 June 2016, a massive landslide occurred in southeast Alaska. It was picked up on seismometers all over Alaska. And, a pilot who regularly flies over Glacier Bay National Park saw the aftermath:
If you didn’t read the articles from the previous links, here’s one with more (and updated) information. And, according to this last article, rocks were still falling and still making sounds (“like fast flowing streams but ‘crunchier'”) four days later. That pile of fallen rocks is roughly 6.5 miles long and 1 mile wide. And, some of the rock was pushed at least 300 ft (~100 m) uphill on some of the neighboring mountain slopes.
Of course, who needs pilots with video cameras? All we need is a satellite instrument known as VIIRS to see it. (That, and a couple of cloud-free days.) First, lets take a look at an ultra-high-resolution Landsat image (that I stole from the National Park Service website and annotated):
Of course, you’ll want to click on that image to see it at full resolution. The names I’ve added to the image are the names of the major (and a few minor) glaciers in the park. The one to take note of is Lamplugh. Study it’s location, then see if you can find it in this VIIRS True Color image from 9 June 2016:
Anything? No? Well, how about in this image from 7 July 2016:
I see it! If you don’t, try this “Before/After” image overlay, by dragging your mouse from side to side:
That dark gray area in the image from 7 July 2016 that the arrow is pointing to is the Lamplugh Glacier landslide! If the “Before/After” overlay doesn’t work, try refreshing the page, or look at this animated GIF:
Of course, with True Color images, it can be hard to tell what is cloud and what is snow (or glacier) and with VIIRS you’re limited to 750 m resolution. We can take care of those issues with the high-resolution (375 m) Natural Color images:
Make sure you click on it to see the full resolution. If you want to really zoom in, here is the high-resolution visible channel (I-1) imagery of the event:
You don’t even need an arrow to point it out. Plus, if you look closely, I think you can even see some of the dust coming from the slide.
That’s what 120 million metric tons of rock falling off the side of a mountain looks like, according to VIIRS!
Take a second to think about what would happen if Florida was hit by four hurricanes in one month.
Would the news media get talking heads from both sides to argue whether or not global warming is real by yelling at each other until they have to cut to a commercial? Would Jim Cantore lose his mind and say “I don’t need to keep standing out here in this stuff- I quit!”? Would we all lose our minds? Would our economy collapse? (1: yes. 2: every man has his breaking point. 3: maybe not “all”. 4: everybody panic! AHHH!)
It doesn’t have to just be Florida. It could be four tropical cyclones making landfall anywhere in the CONUS (and, maybe, Hawaii) in a 1-month period. The impact would be massive. But, what about Alaska?
Of course, Alaska doesn’t get “tropical cyclones” – it’s too far from the tropics. But, Alaska does get monster storms that are just as strong that may be the remnants of tropical cyclones that undergo “extratropical transition“. Or, they may be mid-latitude cyclones or “Polar lows” that undergo rapid cyclogenesis. When they are as strong as a hurricane, forecasters call them “hurricance force” (HF) lows. And guess what? Alaska has been hit by four HF lows in a 1-month period (12 December 2015 – 6 January 2016).
With very-many HF lows, some of which were ultra-strong, we might call them VHF or UHF lows. (Although, we must be careful not to confuse them with the old VHF and UHF TV channels, or the Weird Al movie.) In that case, let’s just refer to them as HF, shall we?
The first of these HF storms was a doozy – tying the record for lowest pressure ever in the North Pacific along with the remnants of Typhoon Nuri. Peak winds with system reached 122 mph (106 kt; 196 k hr-1; 54 m s-1) in Adak, which is equivalent to a Category 2 hurricane!
Since Alaska is far enough north, polar orbiting satellites like Suomi-NPP provide more than 2 overpasses per day. Here’s an animation from the VIIRS Day/Night Band, one of the instruments on Suomi-NPP:
It’s almost like a geostationary satellite! (Not quite, as I’ll show later.) This is the view you get with just 4 images per day. (The further north you go, the more passes you get. The Interior of Alaska gets 6-8 passes, while the North Pole itself gets all 15.) Seeing the system wrap up into a symmetric circulation would be a thing of beauty, if it weren’t so destructive. Keep in mind that places like Adak are remote enough as it is. When a storm like this comes along, they are completely isolated from the rest of Alaska!
Here’s the same animation for the high-resolution longwave infrared (IR) band (I-5, 11.5 µm):
I’ve mentioned Himawari before on this blog. Well, Himawari’s field of view includes the Aleutian Islands. Would you like to see how this storm evolved with 10 minute temporal resolution? Of course you would.
Here is CIRA’s Himawari Geocolor product for this storm:
Here is a loop of the full disk RGB Airmass product applied to Himawari. Look for the storm moving northeast from Japan and then rapidly wrapping up near the edge of the Earth. This is an example of something you can’t do with VIIRS, because VIIRS does not have any detectors sensitive to the 6-7 µm water vapor absorption band, which is one of the components of the RGB Airmass product. The RGB Airmass and Geocolor products are very popular with forecasters, but they’re too complicated to go into here. You can read up on the RGB Airmass product here, or visit my collegue D. Bikos’ blog to find out more about this storm and these products.
You might be asking how we know what the central pressure was in this storm. After all, there aren’t many weather observation sites in this part of the world. The truth is that it was estimated (in the same way the remnants of Typhoon Nuri were estimated) using the methodology outlined in this paper. I’d recommend reading that paper, since it’s how places like the Ocean Prediction Center at the National Weather Service estimate mid-latitude storm intensity when there are no surface observations. I’ll be using their terminology for the rest of this discussion.
Less than 1 week after the first HF storm hit the Aleutians, a second one hit. Unfortunately, this storm underwent rapid intensification in the ~12 hour period where there were no VIIRS passes. Here’s what Storm #2 looked like in the longwave IR according to Himawari. And here’s what it looked like at full maturity according to VIIRS:
Notice that this storm is much more elongated than the first one. Winds with this one were only in the 60-80 mph range, making it a weak Category 1 HF low.
Storm #3 hit southwest Alaska just before New Year’s, right at the same time the Midwest was flooding. This one brought 90 mph winds, making it a strong Category 1 HF low. This one is bit difficult to identify in the Day/Night Band. I mean, how many different swirls can you see in this image?
(NOTE: This was the only storm of the 4 to happen when there was moonlight available to the DNB, which is why the clouds appear so bright. The rest of the storms were illuminated by the sun during the short days and by airglow during the long nights.) The one to focus on is the one of the three big swirls closest to the center of the image (just above and right of center). It shows up a little better in the IR:
The colder (brighter/colored) cloud tops are the clue that this is the strongest storm, since all three have similar brightness (reflectivity) in the Day/Night Band. If you look close, you’ll also notice that this storm was peaking in intensity (reaching mature stage) right as it was making landfall along the southwest coast of Alaska.
Storm #4 hit the Aleutians on 6-7 January 2016 (one week later), and was another symmetric/circular circulation. This storm brought winds of 94 mph (2 mph short of Category 2!) The Ocean Prediction Center made this animation of its development as seen by the Himawari RGB Airmass product. Or, if you prefer the Geocolor view, here’s Storm #4 reaching mature stage. But, this is a VIIRS blog. So, what did VIIRS see? The same storm at higher spatial resolution and lower temporal resolution:
This storm elongated as it filled in and then retrograded to the west over Siberia. There aren’t many hurricanes that do that after heading northeast!
So, there you have it: 4 HF lows hitting Alaska in less than 1 month, with no reports of fatalities (that I could find) and only some structural damage. Think that would happen in Florida?
Think back to St. Patrick’s Day. Do you remember what you were doing? Hopefully you were wearing something green. And, hopefully, you didn’t leave anything green in the gutter behind the bar (e.g. undigested lunch or beverages or a mixture of the two). If you did, we don’t want to hear about it. It’s unpleasant enough that you had to read that and have that image in your mind. Apologies if you are eating.
If your mind was lucid enough that night, or the following night, did you remember to look up to the northern sky? Or, right above you, if you live far enough north? (Swap “north” for “south” if you live in the Southern Hemisphere. Everything is backwards there.) Was it a clear night?
If you answered “no” to the first two questions and “yes” to the third question, you missed out on an opportunity to see something green in the sky – one of the great atmospheric wonders of the world: the aurora. If you answered “yes” then “no”, tough luck. The lower atmosphere does not always cooperate with the upper atmosphere. If you answered “yes” on everything and still didn’t see the aurora, then you need to move closer to your nearest magnetic pole. Or, away from light pollution. (Although, truth be told, it is possible to live too far north or south to see the aurora. But, not many people live there. Those who do rarely have to worry about light pollution.)
If you forgot to look up at the night sky on 17-18 March 2015, you have no excuse. The media was hyping the heck out of it. That link is just one example of media predictions of the aurora being visible as far south as Dallas and Atlanta. While I couldn’t find any photographic evidence that that actually happened, there were people as far south as Ohio, Pennsylvania and New Jersey that saw the aurora. In the other hemisphere – the backwards, upside-down one – the aurora was seen as far north as Australia and New Zealand, which is a relatively rare occurrence for them. And there are no shortage of pictures and videos if you want proof: pictures, more pictures, even more pictures, video and pictures, video, and a couple more short videos here, here and here.
Now, we already know that VIIRS can see the aurora. We’ve covered both the aurora borealis and aurora australis before. This time, we’ll take a look at both at the same time – not literally, of course! – since the Day/Night Band viewed the aurora (borealis and australis) on every orbit for an entire 24 hour period, during which time it covered every part of the Earth. So, follow along as VIIRS circled the globe in every sense of the word during this event.
First, we start with the aurora australis over the South Pacific, south of Pitcairn Island, at 10:15 UTC on 17 March 2015. We then proceed westward, ending over the South Pacific, south of Easter Island at 08:16 UTC on 18 March 2015. Click on each image in the gallery to see the medium resolution version. Above each of those images is a link containing the dimensions of the high resolution version. Click on that to see the full resolution.
Notice how much variability there is in the spatial extent and shape of the aurora from one orbit to the next. Everything is represented, from diffuse splotches to well-defined ribbons (which are technical terms, of course, wink, wink). You can see just how close the aurora was to being directly over Australia and New Zealand. And, if you looked at the high resolution versions of all the images (which are very large), you might have seen this:
Just below center, the aurora is illuminating gravity waves forced by Heard Island. The aurora is also directly overhead of it’s “twin”, “Desolation Island” (aka Îles Kerguelen, upper-right corner right at the edge of the swath), although it looks too cloudy for the scientists and penguins living there to see it. (How many more Remote Islands can I mention that I’ve featured before?)
Now, I’m a sucker for animations, so I thought I’d combine all of these images into one and here it is (you can click on it to see the full-resolution version):
Here, it is easier to notice that the aurora is much further north (away from the South Pole) near Australia and New Zealand and further south (closer to the pole) near South America. This is proof that the geomagnetic pole does not coincide with the geographic pole. This also puts the southern tips of Chile and Argentina at a disadvantage when it comes to seeing the aurora, compared to Australia and New Zealand.
Now, repeat everything for the aurora borealis – beginning over central Canada (07:57 UTC 17 March 2015) and ending there ~24 hours later (07:40 UTC 18 March 2015):
Basically, if you were anywhere in Siberia where there were no clouds, you could have seen the aurora. (For those who are not impressed, Siberia is a big area.) Did you see the aurora directly over North Dakota? (I showed a video of that above.) Did you notice it was mostly south of Anchorage, Alaska? (Typically, it’s over Fairbanks.) It was pretty close to Moscow and Scotland, also. But, what about the sightings in Ohio, New Jersey, and Germany? It doesn’t look like the aurora was close to those places…
For one, the aurora doesn’t have to be overhead to see it. Depending on the circumstances (e.g. auroral activity, atmospheric visibility, light pollution, etc.), you can be 5 degrees or more of latitude away and it will be visible. Second, these are single snapshots of an aurora that is constantly moving. (We already know the aurora can move pretty fast.) It may have been closer to these places when VIIRS wasn’t there to see it.
Lastly, here’s an animation of the above images, moving in the proper clockwise direction, unlike in that backwards, upside-down hemisphere:
If you want to know more about what causes the aurora, watch this video. If you want to know why auroras appear in different colors, read this. If you want to know why aboriginal Australians viewed the aurora as an omen of fire, blood, death and punishment, and why various Native American tribes viewed the aurora as dancing spirits that were happy, well, you have a lot more reading to do: link, link and link.