Tag Archives: clouds

The nice (and dedicated) people of N-ICE

Imagine this scenario: you’re stuck on a boat in the Arctic Ocean in the middle of the night. The winds are howling, the air is frigid, and the boat you’re in is completely encased in ice. Step off the boat and your face is constantly sand-blasted by tiny ice particles. Blink at the wrong time and your eyes freeze shut. The ice may crack under your feet (or between you and the boat)  – without notice – leaving icy water between you and the only warm place for hundreds of kilometers. Have to swim for it? Look out for jellyfish. Decide to stay on your crumbling patch of ice? I hear polar bears can get pretty hungry. Death awaits every misstep and every wrong turn. Cowering in the boat? Internet access is limited, there are no re-runs of Friends to keep you entertained, and the shuffleboard court is outside. (Actually, it’s worse than that: there is no shuffleboard court!)

Now imagine this: you actually wanted to be there!

Most people would say, “That’s crazy! I would never do that!” But, for the scientists and crew aboard the research vessel (RV) Lance, it is a unique opportunity to further our understanding of the Arctic and its role in the Earth’s climate system.

You see, we are nearing the mid-point of the N-ICE 2015 field experiment, which is taking place from 1 January to 1 August 2015. The idea behind the experiment is to take a boat, freeze it in the Arctic ice sheet, and constantly monitor the environment around the boat for about six months. A group of scientists work in six-week shifts where they monitor everything from the weather to local biology. Of course, the primary objective is to see what happens with the ice itself.

One of our very own researchers at CIRA (and one of the world’s leading experts on snow) was on board during the first leg of the experiment.  So, what is a snow expert doing on a ship whose primary purpose is to study ice?

Here’s the lowdown. There are two types of ice that concern Arctic researchers: “young” and “multi-year”. As the name implies, multi-year ice is ice that survives the summer and lasts for more than one year. Young ice does not reach its first birthday – it melts over the summer. Arctic researchers have been finding out that, not only is the Arctic ice sheet shrinking, it’s lost most of its multi-year ice, which is being replaced by young ice.

Multi-year ice is thicker, more resilient and tends to be brighter (more reflective), while young ice is thinner, darker (less reflective of sunlight), and less resilient. The less sunlight that is reflected, the more sunlight is absorbed into the system and this leads to warming, which melts more ice (and is a positive feedback). The less ice there is, the more open ocean there is, and open water is a lot less reflective than ice, which leads to more absorption of sunlight, more warming, more melting, etc.

The thinner “young ice” breaks up more easily due to wind and waves. This creates more leads of open water. The water, being much warmer than the air above it, pumps heat and moisture into the atmosphere, creating more clouds and snow – just like lake-effect or sea-effect snow. And, while most people have a hard time believing it, snow is a good insulator. Snow on top of the ice will create a blanket that protects the ice from the really cold air above. This reduces the rate at which the ice thickens up, keeping the ice thinner, and we have another positive feedback.

That’s just one of the things being studied on the 2015 Norwegian Young Sea Ice Cruise. Of course, I wouldn’t be mentioning any of this unless VIIRS could provide information to help out with the mission.

Go back to the N-ICE 2015 website. Notice the sliding bar/calendar on the bottom of the map. You can use that to follow the progress of the ship. Or, you can use the VIIRS Day/Night Band.

At the time of this writing, the Lance is docked in Longyearbyen, the largest town on the island of Spitsbergen in Norway. (Spitsbergen is part of the Svalbard archipelago, which has a direct connection to VIIRS. Svalbard has a receiving station used by NOAA that collects and distributes data from nearly all of their polar-orbiting satellites.) Longyearbyen is where the RV Lance and Norwegian icebreaker KV Svalbard departed for the Arctic back in mid-January. KV Svalbard escorted the RV Lance into the ice sheet, then returned to Longyearbyen while the Lance froze itself into the ice. See if you can see that in this loop of VIIRS Day/Night Band images from 12 – 17 January 2015:

Animation of VIIRS Day/Night Band images from 12-17 January 2015

Animation of VIIRS Day/Night Band images from 12-17 January 2015. These images cover the area of the N-ICE field experiment, north of Svalbard.

Notice how the one bright light follows a lead in the ice until it stops. Then the light appears to split in two, with one light source heading back the way it came and the other stuck in the ice. That is the start of N-ICE 2015!  The KV Svalbard did its duty. If you look closely, there are also some other boats hanging out in the open water near the edge of the ice sheet during this time.

If you suspect there are jumps in the images you’re right. VIIRS passes over this area every day 6-8 times between 00 and 12 UTC, with no overpasses for the next 12 hours.

Toward the end of January you can see how the RV Lance drifted to the west along with the ice:

Animation of VIIRS Day/Night Band images from 23-30 January 2015

Animation of VIIRS Day/Night Band images from 23-30 January 2015. These images cover the area of the N-ICE field experiment, north of Svalbard.

This was all according to plan. But, then, in February, the winds shifted and helped the ice spit the boat back out towards the open water:

Animation of VIIRS Day/Night Band images from 8-15 February 2015

Animation of VIIRS Day/Night Band images from 8-15 February 2015. These images show the area of the N-ICE field experiment, north of Svalbard.

After this, the RV Lance needed help from the KV Svalbard to be repositioned in the ice sheet near where it started a month earlier. Otherwise, all the instruments they placed in the ice would no longer be in the ice – they’d be at the bottom of the ocean as the ice sheet broke up all around them.

If you want to know why the ship seems to disappear and reappear every day, you can thank the sun. You see, the first few weeks of the experiment took place during the long polar night. But, by mid-February, twilight began to encroach on the domain during the afternoons. This was enough light to drown out the light from the ship. (Sunrise occurred in early March.)

Another thing to notice with these last two animations: the cloud streets that form over the open water near Svalbard. The direction these cloud streets move gives a pretty good indicator of where the ice is going to go, since both the clouds and icebergs are being pushed and pulled by the same wind.

It’s fascinating to watch the movement of the ice over the first 6 weeks of the field experiment. To save on file size and downloading time, the animation below only uses one image per day (between 10 and 11 UTC). Here’s 6 weeks of images in 5 seconds:

Animation of VIIRS Day/Night Band images from 11 January to 28 February 2015

Animation of VIIRS Day/Night Band images from 11 January to 28 February 2015. These images show the area of the N-ICE field experiment, north of Svalbard.

And you probably thought of sea ice as being relatively static.

Once again, we lose sight of the RV Lance because of afternoon twilight in mid-February, so we can’t see it or the KV Svalbard after that. And note that there’s a lot less open water near Svalbard by the end of the period.

What if we didn’t have the Day/Night Band? You wouldn’t be able to see the ships at all, that’s for sure! Plus, this area was under darkness (no direct sunlight) for this six week period, so none of the other visible wavelength channels will work.  That leaves us with the infrared (IR), which looks like this:

Animation of VIIRS IR (M-15) images from 11 January to 28 February 2015

Animation of VIIRS IR (M-15) images from 11 January to 28 February 2015. These images cover the area of the N-ICE field experiment, north of Svalbard.

Note that clouds appear to have a greater impact on the detection of ice (and distinction between ice and clouds) in the IR. When it’s relatively cloud-free, there is enough of a temperature contrast between the open water and ice to see the icebergs but, pretty much any cloud will obscure the ice. So, why doesn’t the Day/Night Band have this problem?

That has to do with the optical properties of clouds at visible and IR wavelengths. Most of these clouds are optically thick in the IR and optically thin in the visible. The Day/Night Band can see through these clouds (most of them, anyway) while channels like M-15 (10.7 µm) shown here, can’t. We’ve seen more extreme examples of this before.

In the rapidly changing Arctic, it is nice to know that there are a few dedicated individuals who risk frostbite, hypothermia and polar bears to provide valuable information on how the ice impacts the environment both locally and globally. Me: I’ll just stick to analyzing satellite data from my nice, comfortable office, thank you.

By the way, the N-ICE field experiment has it’s own blog, and pictures and other snippets of information about the people and progress of the mission are regularly posted to Instagram, Facebook and Twitter.

Camouflage Clouds

The natural world is full of examples of animals that have evolved camouflage. Check out this list and see how many of the animals you can find. Another example that I find particularly interesting is the Potoo bird. Some animals, like the Potoo, use camouflage to hide from predators, while others, like the Polar Bear, are predators who use camouflage to hide from their prey and make it easier to sneak up on them. Clouds also use camouflage (or at least it seems that way) to hide from weather satellites. Are they predators trying to hunt down and destroy innocent weather forecasts? Are they hiding because they fear some atmospheric phenomenon will find them and glaciate them? It’s tough to tell what goes on in the mind of a cloud, since it isn’t alive and has no brain.

Did you click on the first link above and take the test? If so, you are now aware of the skills you’ll need to detect clouds in the Arctic.

Let’s start with an infrared (IR) image of Alaska taken by VIIRS at 23:29 UTC on 3 February 2014:

VIIRS IR image (I-5), taken at 23:29 UTC 3 February 2014

VIIRS IR image (I-5), taken at 23:29 UTC 3 February 2014

The question is: where are all the clouds?

Colors correspond to the color table in the lower right corner of the image. IR images typically use color tables like this one to highlight the structures of cold cloud tops. And given the long Arctic winter nights, IR images like this are typically all that are available. The problem that arises is that low clouds, like fog and stratus, have a brightness temperature similar to the background surface, making them hard to spot. Sometimes temperature inversions exist and the low clouds in the image are warmer than the background surface. Cloud-free valleys and the ice in the Arctic Ocean may be colder than the clouds you’re trying to see, so you can’t always use temperature or temperature differences to detect clouds.

Now, we’ll get some help for this case, since 23:29 UTC is 2:30 in the afternoon (for most of Alaska), so there is some sunlight. This allows us to compare the IR image above with visible-wavelength images. Of course, that doesn’t always help, since clouds can be camouflaged at many different wavelengths. Here’s the VIIRS “True Color” RGB composite (a composite of M-3, M-4 and M-5, which are at blue, green and red portions of the visible spectrum, respectively):

VIIRS "True Color" RGB composite of  M-3, M-4 and M-5, taken 23:29 UTC 3 February 2014

VIIRS “True Color” RGB composite of M-3, M-4 and M-5, taken 23:29 UTC 3 February 2014

Many of the clouds here are still camouflaged because the clouds and snow and ice all appear white. The clouds that are easy to spot in the IR image (e.g. over the Gulf of Alaska) are similarly easy to spot in the True Color image. But, what about the clouds that are still hiding?

For now, we’re going to focus on three interesting regions that contain camouflage clouds: the Tanana River valley near Tok, the Arctic Ocean north of Russia, and the northern tip of the Yukon Territory. Keen-eyed observers may already be able to spot the clouds I’m referring to by noticing cloud shadows or by remembering where forests or mountains are located that are now obscured. (Although, the clouds in the northwest Yukon Territory are really difficult to see because of saturation issues at the terminator.) The three areas I’m referring to are highlighted below:

VIIRS IR image (I-5), taken 23:29 UTC 3 February 2014

VIIRS IR image (I-5), taken 23:29 UTC 3 February 2014. The areas of interest discussed in the text are highlighted.

Clouds are not so easy to see in these three areas, are they? (Remember, you can click on any image to see the high-resolution version.)

The Day/Night Band (and its Near Constant Contrast counterpart) show the clouds in these areas a bit better than the True Color image (and certainly better than the IR image). Here we show the Near Constant Contrast (NCC) image, so we’re not impacted by the presence of the day/night terminator:

VIIRS NCC image, taken 23:29 UTC 3 February 2014

VIIRS NCC image, taken 23:29 UTC 3 February 2014.

The clouds over Tok (lower right oval) are bit difficult to see, but you should be able to see the shadow they cast.

The clouds over northern Yukon Territory (upper right oval) are interesting for a couple of reasons: they obscure the terrain (the easiest way to tell those are clouds); they hug the surface so they aren’t casting any shadows; and the cloud on the northwest side of the oval is much warmer than the cloud on the southeast side of the oval even though they look similar at visible wavelengths (compare the visible images with the IR images).

The left oval over the Arctic Ocean shows the big difference in opacity between looking at a cloud in the IR versus the visible wavelengths.  The IR image shows an opaque, slightly darker (i.e. warmer) shape barely discernible from the background ocean and ice. The NCC image shows a semi-transparent cloud (also slightly darker [i.e. less reflective] than the background ice) with a lot of structure due to gravity waves. Underneath the cloud feature, you can clearly see where the icebergs and open water are located. Try doing that with the IR image.

The shortwave (or what the JPSS program office calls “midwave”) IR image (I-4) is not the most intuitive to interpret, but it also shows these camouflage clouds (some better than others):

VIIRS shortwave IR image (I-4), taken 23:29 UTC 3 February 2014

VIIRS shortwave IR image (I-4), taken 23:29 UTC 3 February 2014

The I-4 band is centered at 3.74 µm, a wavelength where reflection of solar radiation and the Earth’s emission both play an important role in what you are seeing. In the color table used here (best at highlighting wildfires and volcanic eruptions), highly reflective objects and warm objects show up darker. Ice clouds, snow and sea ice are all poorly reflective and cold, so they appear brighter. Liquid clouds are highly reflective, which makes the clouds over Tok easily visible.

The Yukon clouds are still pretty camouflaged because, even though they are liquid, they are colder, and don’t have as big a contrast with the background surface. As mentioned before, the clouds to the northwest in the oval are darker (warmer) than the clouds in the southeast part of the oval.

The Arctic Ocean clouds are interesting here. The reflective component reveals the gravity waves, but the emissive component obscures the ice and open water below. These clouds are not only camouflaged in certain wavelengths, they also act as camouflage for the ice below!

This example shows that not all clouds are easy to see with individual channels – even when looking at two or three different wavelengths. But, it does show that the visible-wavelength information provided by the NCC image is quite a bit different from the IR information that is typically used. And, even though this was a daytime scene, all the stuff I wrote still applies at night (except you lose the reflective component to the shortwave IR imagery).

Finally, let’s look at another RGB composite, what EUMETSAT calls “Natural Color”:

VIIRS "Natural Color" composite of I-1, I-2 and I-3, taken 23:29 UTC 3 February 2014

VIIRS “Natural Color” composite of I-1, I-2 and I-3, taken 23:29 UTC 3 February 2014. This image has been cropped relative to the other images to reduce the file size.

This composite uses bands I-1 (0.67 µm), I-2 (0.86 µm) and I-3 (1.61 µm) as the blue, green and red components, respectively. Snow, ice and ice clouds appear the bluish color known as cyan because they are highly reflective in I-1 and I-2, but poorly reflective in I-3. Liquid clouds appear white, to dirty white, to a grayish, pale cyan color depending on particle size and reflectivity. Vegetation is very green. Unlike the True Color RGB composite, the low liquid clouds in all three ovals are easier to see here because now they are a significantly different color than their respective backgrounds. Plus, the Arctic Ocean clouds are still transparent enough to show the ice and open water below.

The Natural Color composite may be the best way to detect low liquid clouds in this region, but it’s only available when the sun is above the horizon. The Day/Night Band (or NCC) is a useful stand-in, when it’s not available.

It just goes to show: the clouds may try to hide, but VIIRS can always find them!