Ice, Ice, Baby

A winter storm moved through the Northeast U.S. over the weekend of 19-20 January 2019. This Nor’easter was a tricky one to forecast. Temperatures near the coast were expected to be near (or above) freezing. Temperatures inland were expected to be much colder. Liquid-equivalent precipitation, at least according to the GFS, was predicted to be in the 1-3 inch (25-75 mm) range the day before. This could easily convert to 1-2 feet (30-60 cm) of snow. The question on everyone’s mind: who gets the rain, who gets the snow, and who gets the “wintry mix”? The fates of ~40 million people hang in the balance. This is one of the situations that meteorologists live for!

The difference between 71°F and 74°F is virtually meaningless. The difference between 31°F and 34°F (with heavy precipitation, at least) is the difference between closing schools or staying open. It’s the difference between bringing out the plows or keeping them in the garage; paying overtime for power crews to keep the electricity flowing or just another work day; shutting down public transportation or life as usual.

Of course, the obvious follow-up question is: what is the “wintry mix” going to be? Rain mixed with snow? Sleet? Freezing rain? It doesn’t take much to change from one to the other, but there can be a big difference on the resulting impacts based on what ultimately falls from the sky.

So, what happened? Here’s an article that does a good job of explaining it. And, here are PDF files of the storm reports from National Weather Service Forecast Offices in Albany, Boston (actually in Norton, MA) and New York City (actually in Upton, NY). The synopsis: some places received ~1.5 inches (~38 mm) of rain, some places received ~11 inches (~30 cm) of snow and some places were coated in up to 0.6 inches (15 mm) of ice.

Of particular relevance here are the locations that received the ice. If you took the locations listed in the storm reports that had more than 0.1 inches (2.5 mm) of ice (at least the ones in Connecticut) and plotted them on a map, they match up quite well with this map of power outages that came from the article I linked to:

Map of power outages in Connecticut as a result of an ice storm (19-20 January 2019)

Map of power outages in Connecticut as a result of an ice storm (19-20 January 2019). Image courtesy Eversource/NBC Connecticut.

Now, compare that map with this VIIRS image from 22 January 2019 (after the clouds cleared out):

VIIRS channel I-3 image from NOAA-20, 17:09 UTC 22 January 2019

VIIRS channel I-3 image from NOAA-20, 17:09 UTC 22 January 2019

As always, you can click on the image to bring up the full resolution version. This is the high-resolution imagery band, I-3, centered at 1.6 µm from NOAA-20. Notice that very dark band stretching from northern New Jersey into northern Rhode Island? That’s where the greatest accumulation of ice was. Notice how well it matches up with the known power outages across Connecticut!

The ice-covered region appears dark at 1.6 µm because ice is very absorbing at this wavelength and, hence, it’s not very reflective. And, since it is cold, it doesn’t emit radiation at this wavelength either (at least, not in any significant amount). This is especially true for pure ice, as was observed here (particularly the second image), since there aren’t any impurities in the ice to reflect radiation back to the satellite. The absorbing nature of snow and ice compared with the reflective nature of liquid clouds is what earned this channel the nickname “Snow/Ice Band” (PDF).

At shorter wavelengths (less than ~ 1 µm), ice and snow are reflective. (Note how a coating of ice makes everything sparkle in the sunlight.) This makes it nearly impossible to tell where the ice accumulation was in True Color images:

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 from NOAA-20, 17:09 UTC 22 January 2019

VIIRS True Color RGB composite of channels M-3, M-4 and M-5 from NOAA-20, 17:09 UTC 22 January 2019

The Natural Color RGB (which the National Weather Service forecasters know as the Day Land Cloud RGB (PDF file)) includes the 1.6 µm band, which is what makes it useful for discriminating clouds from snow and ice. And, as expected, the region of ice accumulation does show up (although it is tempered by the highly reflective nature of snow and ice in the visible and “veggie” bands that make up the other components of the RGB):

VIIRS Natural Color RGB composite of channels, I-1, I-2 and I-3 from NOAA-20 (17:09 UTC, 22 January 2019)

VIIRS Natural Color RGB composite of channels, I-1, I-2 and I-3 from NOAA-20 (17:09 UTC, 22 January 2019)

Another RGB composite popular with forecasters is the Day Snow/Fog RGB (PDF file), where blue is related to the brightness temperature difference between 10.7 µm and 3.9 µm, green is the 1.6 µm reflectance, and red is the reflectance at 0.86 µm (the “veggie” band). This shows the region of ice even more clearly than the Natural Color RGB:

VIIRS Day Snow/Fog RGB composite of channels (I-5 - I-4), I-3 and I-2 from NOAA-20 (17:09 UTC, 22 January 2019)

VIIRS Day Snow/Fog RGB composite of channels (I-5 minus I-4), I-3 and I-2 from NOAA-20 (17:09 UTC, 22 January 2019)

Breaking things up into the individual components, we can see how the ice transitions from being reflective in the visible and near-infrared (near-IR) to absorbing in the shortwave-IR:

VIIRS high-resolution visible channel, I-1, from NOAA-20 (17:09 UTC, 22 January 2019)

VIIRS high-resolution visible channel, I-1 (0.64 µm), from NOAA-20 (17:09 UTC, 22 January 2019)

VIIRS high-resolution "veggie" channel, I-2, from NOAA-20 (17:09 UTC, 22 January 2019)

VIIRS high-resolution “veggie” channel, I-2 (0.86 µm), from NOAA-20 (17:09 UTC, 22 January 2019)

VIIRS channel M-8 from NOAA-20 (17:09 UTC, 22 January 2019)

VIIRS channel M-8 (1.24 µm) from NOAA-20 (17:09 UTC, 22 January 2019)

VIIRS channel M-11  from NOAA-20 (17:09 UTC, 22 January 2019)

VIIRS channel M-11 (2.25 µm) from NOAA-20 (17:09 UTC, 22 January 2019)

Of course, the 1.6 µm image was already shown, so I didn’t bother to repeat it. If you squint, you can even see a hint of the ice signature at 1.38 µm, the “Cirrus Band“, where most of the surface signal is blocked by water vapor absorption in the atmosphere:

VIIRS "cirrus" channel, M-9, from NOAA-20 (17:09 UTC 22 January 2019)

VIIRS “cirrus” channel, M-9 (1.38 µm), from NOAA-20 (17:09 UTC 22 January 2019)

If the ice had accumulated in southern New Jersey or Pennsylvania, though, it would not have shown up in this channel, since the air was too moist at this time to see all the way down to the surface. But, you can compare this image with the previous images to see why they call it the “cirrus band”, since the cirrus does show up much more clearly here.

So, mark this down as another use for VIIRS: detecting areas impacted by ice storms. And remember, even though ice storms may have a certain beauty, they are also dangerous. And, not just for the obvious reasons. This storm in particular came complete with ice missiles. So, for the love of everyone else on the road, scrape your car clean of ice before risking your life out there!

The Arctic, Saharan-like Gulf Coast

Today, we’re going to take a look at another less-covered VIIRS channel on this blog: M-9, also known as the “cirrus band”. (Disambiguation: if you’re looking for the electronic musical group “Cirrus (band)“, you’re in the wrong place.) We don’t use M-9 on this blog much because it doesn’t often provide amazing images. But, it is used for a lot of practical applications, so it is worth knowing about. We are also going to say “Hello!” to Suomi-NPP’s baby brother, NOAA-20, and welcome a new VIIRS instrument in space!

Unlike M-8, the “cirrus band” (PDF) is on nearly all of the new geostationary satellites (except Himawari). It’s also on MODIS, Landsat, and several other polar-orbiting satellite imagers. The “cirrus band” is unique in that it is highly sensitive to water vapor, but is located in the near-IR (1.38 µm) where emission from the Earth is minimal. (So, contrary to popular belief, VIIRS does have a water vapor channel. It just doesn’t behave like the typical mid-wave IR water vapor channels most people are used to.)

Electromagnetic radiation at 1.38 µm is absorbed by water vapor. But, the Earth and its atmosphere are too cold to emit much at this wavelength. (Thankfully, or we would have all melted by now.) Of course, the sun is hot enough. This means the 1.38 µm radiation coming from the sun is absorbed by water vapor in our atmosphere, and the only* radiation making its way back to VIIRS is what is reflected off of clouds above the water vapor. This makes channels centered at 1.38 µm particularly useful at identifying thin cirrus that would otherwise blend in with the background on other channels. Hence, the name “cirrus band”. (* Of course, reflection off of high clouds is not the only source, as we shall see. That’s the reason for this blog post.)

So, high clouds are white and the background is black – this is the assumption when looking at VIIRS’s cirrus band (unless you’re using a funky color table). But, take a look at this image that S-NPP VIIRS took on 17 January 2018:

S-NPP VIIRS channel M-9 ("cirrus band") image from 18:34 UTC, 17 January 2018

S-NPP VIIRS channel M-9 (“cirrus band”) image from 18:34 UTC, 17 January 2018

On my monitor, viewing angle makes a big difference as to how bright the features appear. If you are viewing this on a laptop or tablet, your screen is much easier to adjust that than my Jumbotron if it’s hard to see. You can also move your head around and see if anyone else looks at you funny. (This is also a good way to test out a TV in the store before you buy it. Will people sitting off to the side get the same view as someone directly in front of the TV? You might want to know that if hosting a party for the big game this weekend.)

Let’s zoom in on the area in question:

S-NPP VIIRS channel M-9 image from 18:34 UTC, 17 January 2018

S-NPP VIIRS channel M-9 image from 18:34 UTC, 17 January 2018

And, give the image maximum contrast:

S-NPP VIIRS channel M-9 image displayed with maximum contrast (18:34 UTC, 17 January 2018)

S-NPP VIIRS channel M-9 image displayed with maximum contrast (18:34 UTC, 17 January 2018)

There is a feature in that image that looks awfully like the coastline of the Gulf of Mexico stretching from Louisiana to the Florida panhandle. It sure looks like you can see the Mississippi River, the Tennessee River, and all the “lakes” in eastern Texas. But, I thought water vapor was supposed to absorb all the radiation before it made it to the surface! And, this is Louisiana we’re talking about. The entire coastal region of the state is a big swamp – I mean a collection of bayous. So, there should be plenty of water vapor around.

One would expect to see all the way to the surface in high-altitude arid areas, like the Bolivian Altiplano and the upper elevations of the Atacama Desert. And, you do:

S-NPP VIIRS channel M-9 image from 18:32 UTC, 1 June 2017

S-NPP VIIRS channel M-9 image from 18:32 UTC, 1 June 2017.

But, one does not expect to see the surface of Louisiana at 1.38 µm, since it is so close to sea level and it is one of the most humid parts of the United States. Maybe something is wrong with S-NPP VIIRS? Let’s look at our new baby, NOAA-20 VIIRS:

NOAA-20 VIIRS channel M-9 image (19:25 UTC, 17 January 2018)

NOAA-20 VIIRS channel M-9 image (19:25 UTC, 17 January 2018)

And, once again, with maximum contrast:

NOAA-20 VIIRS channel M-9 image displayed with maximum contrast (19:25 UTC, 17 January 2017)

NOAA-20 VIIRS channel M-9 image displayed with maximum contrast (19:25 UTC, 17 January 2017)

Note that NOAA-20 was launched back in November 2017, and is still undergoing post-launch testing and checkout, so it has not been declared operational just yet. But, this is a good test for the new VIIRS. It can see the same surface features S-NPP did 50 minutes earlier. And, it means that both instruments are working. So, why can we see all the way to the surface of Louisiana in the “cirrus band”? Because, the atmosphere was incredibly dry.

Here’s the sounding from Slidell, LA (on the other side of Lake Pontchartrain from New Orleans) on 12 UTC 17 January 2018. Notice the precipitable water value (“PWAT”) is 2.47, which is reported on soundings in mm. That’s just less than 0.1 inches. The nearby soundings taken at Shreveport and Lake Charles reported PWATs of 2.45 mm and 2.77 mm, respectively. Normal for this time of year is about 7 times greater! (Note that he corrected his typo.)

To put this into perspective, this was drier than the Sahara Desert was a few days later:

NOAA-20 VIIRS channel M-9 image displayed with maximum contrast (12:40 UTC, 22 January 2018)

NOAA-20 VIIRS channel M-9 image displayed with maximum contrast (12:40 UTC, 22 January 2018)

Notice you can’t see the surface of the Sahara, indicating there was more water vapor in the air over the desert than there was over Louisiana. The only thing you can see are the cirrus clouds and other clouds that made it to the upper atmosphere. This is more typical of the “cirrus band”.

Now, back to Louisiana: the dry, Arctic airmass resulted in a number of record low temperatures. Plus, this was accompanied by snow, as seen by both S-NPP and NOAA-20:

S-NPP VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (18:34 UTC 17 January 2018)

S-NPP VIIRS True Color RGB composite of channels M-3, M-4 and M-5 (18:34 UTC 17 January 2018)

NOAA-20 VIIRS True Color composite of channels M-3, M-4 and M-5 (19:25 UTC, 17 January 2018)

NOAA-20 VIIRS True Color composite of channels M-3, M-4 and M-5 (19:25 UTC, 17 January 2018)

Snow was reported all the way to Gulf Coast, and you can see evidence of it in the images around Houston, TX, which is pretty rare. But, wait! Why didn’t we see snow in the M-9 “cirrus band” images? Because snow is not very reflective at 1.38 µm, and it blends in with the background. To show what an interesting winter it has been, here’s a map put out by the Weather Prediction Center from 18 January 2018, showing estimated total snowfall accumulations for this winter (so far). Note that an area of Mississippi and Louisiana has had approximately the same amount of snow as most of Iowa and southern Wisconsin (and even here in Northern Colorado!). All 48 contiguous United States have received measurable snowfall!

Fun fact: you can open one of the True Color images in a new browser tab, and the other image in this tab and toggle back and forth between them. This allows you to see the clouds move, and the edges of the snowfield melt. If you have eagle eyes, you can also see that the S-NPP image is sharper on the east side of the image (close to its nadir), while the NOAA-20 image is sharper on the west side (close to its nadir). The satellites are both in the same orbit, but on opposite sides of the Earth. Since the Earth is constantly rotating underneath them, and the VIIRS swath is designed to fill all the gaps at the Equator (unlike MODIS), their ground tracks at low and mid-latitudes are separated by half the width of a VIIRS swath. Nadir for one VIIRS is near the edge of the swath of the other VIIRS. (But, not at high latitudes.) The distance from Tallahassee, Florida to Houston, Texas is a pretty good rule of thumb for the spatial distance between the two satellites when they’re over the United States. Fifty minutes is a good rule of thumb for the temporal distance between them (and this is true all over the globe).

So, for once, Louisiana was colder than the Arctic (Ocean, at least) and drier than the Sahara Desert!