Don’t Eat Orange Snow

Posted on April 24, 2018 by Curtis_Seaman

Roughly one month ago, social media (and, later, more conventional media) outlets were inundated with numerous reports of orange snow in eastern Europe and western Asia – reports like this one, this one and this one. Of course, it wouldn’t really be a hit with the media unless someone could claim it was “apocalyptic”. And of course, the apocalypse didn’t happen. It was simply Saharan dust picked up by high winds from an intense mid-latitude cyclone and deposited far away. We’ve seen this before with VIIRS.

These reports focused on Sochi, Russia, home of the 2014 Winter Olympics. Unfortunately, every time I looked for it in VIIRS imagery, it was cloudy in Sochi. But, the plume of Saharan dust that caused this event was clearly visible over the Mediterranean:

NOAA-20 VIIRS true color composite of channels M-3, M-4 and M-5 (10:03 UTC 25 March 2018)

This image came from our new NOAA-20 VIIRS, which, at this point, is not operational and undergoing additional testing. If you look closer, you might also notice smoke or smog over Poland in the image above (upper left corner). If you really zoom in (click on the image to get to the full resolution version), you may notice a brownish tint to the snow along the north shore of the Black Sea – where the BBC report I linked to listed additional sightings of orange snow. But, the dust-covered snow shows up more clearly in this “before and after” image courtesy of S-NPP VIIRS and the @NOAASatellites twitter account:

“Before” and “After” S-NPP VIIRS true color images from 22 March 2018 (left) and 25 March 2018 (right).

(As an aside: differences in technique used to produce these true color images are likely larger than the differences between S-NPP VIIRS and NOAA-20 VIIRS, so don’t read too much into the fact that the dust-on-snow appears more clearly in the @NOAASatellites image than in my own.)

But, dust-on-snow is not limited to areas within a few thousand kilometers of the Sahara Desert. (It is limited to areas within 40,000 km of the Sahara [in the horizontal dimension, at least], since that is roughly the circumference of the Earth – and assuming you ignore dust storms on Mars.) Dust on snow can happen anywhere you have snow within striking distance of a source of dust. Another example was captured by a new Landsat-like micro-satellite, Venµs, and its non-microsat predecessor, Sentinel-2B, Landsat’s European cousin. A more dramatic example happened last week right here in Colorado. Here is a VIIRS true color image of Colorado from S-NPP VIIRS, taken on 14 April 2018:

S-NPP VIIRS true color composite of channels M-3, M-4 and M-5 (19:45 UTC 14 April 2018)

Here are similar images from NOAA-20 and S-NPP from 18 April 2018:

NOAA-20 VIIRS true color composite of channels M-3, M-4 and M-5 (19:20 UTC 18 April 2018)

S-NPP VIIRS true color composite of channels M-3, M-4 and M-5 (20:11 UTC 18 April 2018)

The trick is to compare these two images with the image from 14 April. The other trick is to know where you’re supposed to be looking. (Hint: we’re looking at the Sangre de Cristo mountains in southern Colorado.) Here’s a “before” and “after” image overlay trick I’ve used before. (You may have to refresh the page before it will work.) Both of these images are the S-NPP VIIRS ones, for simplicity:

If you slide the bar left to right, you should notice the snow is more brown in the mountains just right of center in the 18 April image. There are other areas where the snow melted between the two images, plus a couple of small clouds that add to the differences. Of course, this is only 750 m resolution. We get a better view with the 375m-resolution visible channel, I-1:

We lose the color information, of course, since we are looking at a single channel, but it is obvious the snow became less reflective in the 18 April image. And, we can prove that this was a result of dust. Here are the visible, true color, Dust RGB, “Blue Light Dust” and DEBRA Dust images from S-NPP on 17 April 2018, courtesy Steve M.:

S-NPP VIIRS true color composite of channels M-3, M-4 and M-5 (20:26 UTC 17 April 2018)

S-NPP VIIRS Dust RGB image (20:26 UTC 17 April 2018)

S-NPP VIIRS Blue Light Dust image (20:26 UTC 17 April 2018)

S-NPP VIIRS DEBRA Dust image (20:26 UTC 17 April 2018)

If you are unfamiliar with them, we’ve looked at the Dust RGB, Blue Light Dust and DEBRA before, here and here. As seen in the above images, this was not a difficult to detect dust case. Even Landsat-8 captured this event, which is surprising given the narrow swath and 16-day orbit repeat cycle. (Sure, it’s higher resolution than VIIRS, but will it be overhead when you need it?)

So now we get to why dust-on-snow is important. There is a growing body of research (e.g. this paper) that shows dust-on-snow has a big impact on water resources in places like the Rocky Mountains. You see, dirty snow is less reflective than clean snow. That means it absorbs more solar radiation. This, in turn, means it heats up and melts faster, leading to earlier spring run-off. The end result is less water later in the season, which opens the door to wildfires and more severe droughts. This article that, coincidentally, was published as I was writing this, sums things up nicely. It is so important, the Center for Snow and Avalanche Studies has formed CODOS: the Colorado Dust on Snow Program, whose purpose is to monitor dust on snow and provide weekly updates.

As for why you shouldn’t eat orange snow, that should be obvious. You shouldn’t eat any snow that isn’t pure white (and even that might be risky). But, feel free to eat colorful ice, as long as you know where it came from.

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.

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.

JPSS (SNPP and NOAA-20)

VIIRS Imagery and Visualization Team Blog

Tag Archives: colorado

Don’t Eat Orange Snow

A Wild Week of Wildfires

The Hewlett Fire