Sea-effect Snow

Posted on January 13, 2015 by Curtis_Seaman

Take a look at this image:

Photo credit: İskender Şengör via Severe Weather Europe on Facebook

Is this picture from A) the Keweenaw Peninsula of Michigan in 1978? B) Orchard Park, New York in November 2014 (aka “Snowvember”)? or C) İnebolu, Turkey from just last week?

If you pay attention to details, you will have noticed that I credited İskender Şengör with the picture and properly surmised that the answer is C. If you don’t pay attention to details, get off my blog! The details are where all the interesting stuff happens! You’d never be able to identify small fires or calculate the speed of an aurora or explain the unknown without paying attention to details.

If you follow the weather (or social media), you probably know about lake-effect snow. (Who can forget Snowvember?) But, have you heard of sea-effect snow?

Areas downwind of the Great Lakes get a lot more snow than areas upwind of the Lakes. I was going to explain why in great detail, but this guy saved me a lot of time and effort. (I have since been notified that much of the material in that last link was lifted from a VISIT Training Session put together by our very own Dan B. You can watch and listen to that training session here.) The physical processes that cause lake-effect snow are not limited to the Great Lakes, however. Anywhere you have a large body of relatively warm water (meaning it doesn’t freeze over) with episodes of very cold winds in the winter you get lake-effect or sea-effect snow.

When you think of the great snowbelts of the world, you probably don’t think of Turkey – but you should! Arctic air outbreaks associated with strong northerly winds blowing across the Black Sea can generate snow at the same rate as Snowvember or Snowpocalypse or Snowmageddon or any other silly name that the media can come up with that has “snow” in it (Snowbruary, Snowtergate aka Frozen-Watergate, Snowlloween, Martin Luther Snow Day, Snowco de Mayo, Snowth of July… Just remember, I coined all of these phrases if you hear them later). Plus, the Pontic Mountains provide a greater upslope enhancement than the Tug Hill Plateau in Upstate New York.

One such Arctic outbreak occurred from 7-9 January 2015, resulting in the picture above. Parts of Turkey received 2 meters (!) of snow (78 inches to Americans) in a 2-3 day period, as if you couldn’t tell from that picture or this one.

From satellites, sea-effect snow looks just like lake-effect snow. (Duh! It’s the same physical process!) Here’s a VIIRS “True Color” image of the lake-effect snow event that took place last week on the Great Lakes:

VIIRS “True Color” RGB composite, taken 19:24 UTC 7 January 2015.

Wait – that’s no good! We need to be able to distinguish the snow from the clouds. Let’s try that again with the “Natural Color” RGB composite:

VIIRS “Natural Color” RGB composite, taken 19:24 UTC 7 January 2015.

That’s better. Notice how the clouds are formed right over the lakes and how the clouds organize themselves into bands called “cloud streets“. The same features are visible in the sea-effect snow event over Turkey (from one day later):

VIIRS “Natural Color” RGB composite, taken 10:36 UTC 8 January 2015.

Look at how much of Turkey is covered by snow! (Most of that snow cover is from the low pressure system that passed over Turkey a couple days before the sea-effect snow machine kicked in.) And – *cough* attention to details *cough* – you can even see snow over Greece and more sea-effect snow on Crete. There’s also snow down in Syria, Lebanon and Israel (Israel is off the bottom of the image), which is bad news for Syrian refugees.The heavy snow has shut down thousands of roads, closed schools and businesses, and was even the source of a political scandal.

But, on the plus side, the Arctic outbreak in the Middle East brings a unique opportunity to see palm trees covered in snow. And, how often do you get to see the deserts of Saudi Arabia covered in snow? (EUMETSAT has provided more satellite images of this event at their Image Library.)

Take another look at that image over the Black Sea. See how the biggest snow band extends south (and curving to the southeast) from the southern tip of the Crimean Peninsula? That is an example of how topography impacts these snow events. Due to differences in friction, surface winds are slightly more backed over land than over water, therefore areas of enhanced surface convergence exist downwind of peninsulas. The snow bands are more intense in these regions of enhanced convergence. There are also bigger than normal snow bands downwind of the easternmost and westernmost tips of Crimea, and extending south from every major point along the west coast of the Black Sea. This is not a coincidence. Land-sea (or land-lake) interactions explain this. Go back and listen to the VISIT training session for more information.

Sea-effect snow affects other parts of the globe as well. It’s why the western half of Honshu (the big island of Japan) and Hokkaido are called “Snow Country“. Japan was also hit with a major sea-effect snowstorm last week and, of course, VIIRS caught it:

VIIRS “Natural Color” RGB composite, taken 03:48 UTC 8 January 2015.

See the clear skies over Korea and the cloud streets that formed over the Sea of Japan? Classic sea-effect clouds. You can even see snow all along the west coast of Honshu in between the breaks in the clouds. Topographic impacts are once again visible. Notice the intense snow band extending southeast from the southern tip of Hokkaido/northern tip of Honshu similar to the super-strength snow band off of Crimea. And there’s another one downwind of the straits between Kyushu and Shikoku. Another detail in this image you should have noticed is the impact that Jeju Island has on the winds and clouds. Those are classic von Kármán vortices which we have discussed before.

Fortunately, 8 January 2015 was near a full moon, so the Day/Night Band was able to capture a great image of these von Kármán vortices:

VIIRS Day/Night Band image, taken 18:09 UTC 7 January 2015.

So, to the people of the Great Lakes: Remember you’re not alone. There are people in Turkey and Japan who know what you go through every winter.

UPDATE #1: While I was aware (and now you are aware) that sea-effect snow can impact Cape Cod, it was brought to my attention that there is a sea-effect snow event going on there today (13 January 2015). Here’s what VIIRS saw:

VIIRS “Natural Color” RGB composite, taken 17:29 UTC 13 January 2015.

According to sources at the National Weather Service, some places have received 2-3 cm (~ 1 inch) of snow in a four-hour period. It’s not the same as shoveling off your roof in snow up to your neck, but it’s something!

Fires in Paradise

Posted on August 16, 2012 by Curtis_Seaman

Sometimes, it seems like the whole world is on fire. Siberia. The western United States (which has been burning for some time). And now, the Canary Islands. The Spanish islands have been under a drought, as has much of Spain. (As an indication of how dry it has been, one fire in mainland Spain was started by someone flicking a cigarette butt out of their car window in a traffic jam – a fire that ultimately led to two deaths.) Back in July, fires got started on Tenerife – a major resort destination – and earlier this month, fires began on La Palma and La Gomera. At least two firefighters have already died battling these fires.

For your reference, here is a VIIRS “true color” image (M-3 [0.488 µm], M-4 [0.555 µm], M-5 [0.672 µm]) of the Canary Islands, with the major islands labelled:

VIIRS true color RGB composite of channels M-3, M-4 and M-5, taken 14:01 UTC 5 August 2012

If you look closely at this image, from 5 August 2012, you can see smoke plumes coming off of La Palma and La Gomera. You can also see what looks like a von Kármán vortex street downwind of La Palma. That’s the west coast of Africa in the lower-right corner of the image.

As discussed previously, the true color RGB composite is better for viewing the smoke plume, but you can’t actually see the fire directly. So, here’s the M-5 (0.672 µm), M-7 (1.61 µm) and M-11 (2.25 µm) composite from the same time:

VIIRS RGB composite of channels M-5, M-7 and M-11, taken 14:01 UTC 5 August 2012

It’s easy to see where the fires are actively burning with this composite. Let’s zoom in to make it even more obvious:

VIIRS false color RGB composite of channels M-5, M-7 and M-11, taken 14:01 UTC 5 August 2012

All the bright red pixels indicate where the fire is actively burning. You can also see the burn scar on Tenerife (not as easily as in Siberia) where the M-5, M-7, M-11 RGB composite shows the fire was back in July:

VIIRS false color RGB composite of channels M-5, M-7 and M-11, taken 14:38 UTC 18 July 2012

La Gomera has been the hardest hit island, where thousands of people had to be evacuated, and approximately 10% of Garajonay National Park has burned. Garajonay National Park is home to one of the last remaining laurisilva forests, which has been around for 11 million years. That lush vegetation burned hot, and channel I-04 (3.7 µm) reached saturation as that area went up in flames:

VIIRS channel I-04 image of fires in the Canary Islands, taken 14:01 UTC 5 August 2012

The two white pixels on La Gomera are where I-04 reached saturation and “fold-over” due to the heat from the fire. M-13 (4.0 µm), which is a dual-gain band designed to not saturate, reached a brightness temperature of 451 K over La Gomera, compared with a saturation brightness temperature of 367 K for channel I-04.

The fires also showed up in the Day/Night Band that night:

VIIRS Day/Night Band image of the Canary Islands, taken 02:25 UTC 6 August 2012

The red arrows point out the fires on La Palma and La Gomera. The fire on La Gomera covers a significant percentage of the island. The yellow arrow points to Lanzarote, which, for some reason, is not part of IDL’s map. On the night this image was taken, the moon was approximately 84% full, so you can see a number of clouds as well the city lights from the major resort areas of the Canary Islands. The biggest visible city in Africa is El Aaiún, the disputed capital of Western Sahara.

Finally, here’s the “pseudo-true color” composite of VIIRS channels I-01 (0.64 µm), I-02 (0.87 µm) and I-03 (1.61 µm) from 13:42 UTC 6 August 2012. This is a full granule at the native resolution of the Imagery bands with no re-mapping, showing the rich detail of VIIRS high-resolution imagery, including more interesting cloud vortices:

VIIRS false color RGB composite of channels I-01, I-02 and I-03, taken 13:42 UTC 6 August 2012

Make sure to click on the image, then on the “6400×1536” link to see it in its full glory.

Cape Verde Waves and Plumes

Posted on June 8, 2012 by Curtis_Seaman

Cape Verde is an island nation off the west coast of Africa, located in the North Atlantic. The islands are a popular initiation point for tropical storms. The original capital of the 10-island archipelago was sacked twice by Sir Francis Drake, the same one who, in his later years, would fail to sack the villages along Lake Maracaibo in Venezuela due to Catatumbo lightning. That guy really got around, and I mean that literally: he circumnavigated the globe between 1577 and 1580, sacking nearly every village and boat he came across. But, this isn’t about Francis Drake – it’s about the Cape Verde islands and the amazing view of them captured by VIIRS.

False color RGB composite of VIIRS channels I-1, I-2 and I-3 taken 14:41 UTC 6 June 2012

False color RGB composite of VIIRS channels I-1, I-2 and I-3 taken 14:41 UTC 5 June 2012

Can you see the 10 major islands? One of them (Santa Luzia) is almost obscured by clouds. If you click on the image, you’ll see each of the major islands identified. Go ahead and click on it. It will help for later.

The image above was made from the RGB composite of VIIRS high-resolution imagery channels I-01, I-02 and I-03. While it technically is a false color image (uses reflectance at 0.64 µm [blue], 0.865 µm [green] and 1.61 µm [red]), it looks realistic in many situations, so that we refer to it as “pseudo-true color”. Snow and ice show up as an unrealistic blue, however, which is the main difference between it and a “true color” image. You might also notice a few more differences between the “pseudo-true color” image above and the “true color” image below.

True color RGB composite of VIIRS channels M-3, M-4 and M-5 taken 14:41 UTC 6 June 2012

True color RGB composite of VIIRS channels M-3, M-4 and M-5 taken 14:41 UTC 5 June 2012

The true color image uses moderate resolution channels M-3 (0.48 µm, blue), M-4 (0.55 µm, green) and M-5 (0.67 µm, red), which actually observe radiation in the blue, green and red portions of the visible spectrum. Apart from differences in resolution, the vegetation on the islands shows up a bit better in the “pseudo-true color” image. The islands just look brown in the true color image.

What is particularly interesting about these images are the visible effect that the islands have on the local atmosphere. Downwind (southwest, or to the lower left) of Sal, Boa Vista, and Maio, you can see singular cloud streets, much like the flow of water around a rock. In the photograph in that link, you can see how the water dips downward on both sides of the center line downstream of the rock, and upward in the middle (along the center line). The islands are acting like rocks in the atmosphere, causing upward motion behind them, and this lift was enough to form cloud streets. On either side of these cloud streets there is downward motion and, as a result, clear skies.

Downwind of São Nicolau, São Vicente and Santo Antão, the cloud streets highlight von Kármán vortices and vortex shedding, which you can see in more-controlled lab conditions here and here.

Many of the islands appear to be producing their own aerosol plumes (i.e. dust), and if you zoom in on the area between Boa Vista and Santiago, you can see gravity waves present in some of the plumes (highlighted by the arrows in the image below).

False color RGB composite of VIIRS channels I-1, I-2 and I-3 taken 14:41 UTC 5 June 2012

A common way to detect dust is the “split-window difference”: the difference in brightness temperature between the 11 µm channel and the 12 µm channel. On VIIRS, this means subtracting M-16 from M-15 which, when you do that, gives you this image:

Split-window difference from VIIRS (M15 minus M16) from 14:41 UTC 5 June 2012

The color scale goes from -0.16 K (black) to +4.0 K (white). For some reason, the dust or aerosol plumes don’t produce a strong signal here. It may be that the dust is too low in the atmosphere and the lack of temperature contrast with the surface prevents a strong signal. Maybe water vapor absorption effects in M16 are washing out the signal. Or, there could be some other explanation waiting to be discovered.

The plumes are highly reflective in the 3.7 µm channel (M-12), as are the clouds, which show up as warm spots in the image below (not as warm as the islands, however):

Moderate resolution 3.7 µm image (M-12) from VIIRS, taken 14:14 UTC 5 June 2012

Moderate resolution 3.7 µm image (M-12) from VIIRS, taken 14:41 UTC 5 June 2012

Here, just to throw you off, the color scale has been reversed so that dark colors mean higher values. The scale ranges from 295 K (white) to 330 K (black). When you take the difference of this image and the 10.6 µm brightness temperature (M-15), the clouds and aerosol plumes really show up, along with the gravity waves and vortices:

Brightness temperature difference between VIIRS channels M-12 and M-15 from 14:14 UTC 5 June 2012

Brightness temperature difference between VIIRS channels M-12 and M-15 from 14:41 UTC 5 June 2012

In this case, the M-12 brightness temperatures are always greater than the M-15 brightness temperatures (due to the combination of Earth’s emission and solar reflection in M-12 as opposed to just surface emission in M-15), so the scale varies from +5 K (black) to +30 K (white). Higher (brighter) values on this scale show off where the most solar reflection occurs at 3.7 µm – the liquid clouds and aerosol plumes.

There are much more sophisticated ways of identifying dust and aerosol plumes. To find out more, check out this article written by one of our resident experts, Steve Miller, who is currently working on applying dust detection algorithms to VIIRS.

If you are more interested in the von Kármán vortices, NASA has put together a great page that you can visit here. If you take the original image in this post, zoom out and rotate it a little bit, you can get a sense of just how far the vortices extend from their parent islands:

False color RGB composite of VIIRS channels I-1, I-2 and I-3 taken 14:41 UTC 5 June 2012. This image has been rotated from the previous images to highlight the length of the vortex streets.

Coincidentally, this image has been cropped to a size that makes it suitable for use as a desktop wallpaper, should you happen to have a 16:9-ratio monitor and a desire to stare at this image all day. (You have to click on the image, then click on the “1920 x 1080” link below the header to get the full resolution image.)

JPSS (SNPP and NOAA-20)

VIIRS Imagery and Visualization Team Blog

Tag Archives: vortex street

Sea-effect Snow

Fires in Paradise

Cape Verde Waves and Plumes