Breaking waves aren’t just for the ocean

When thinking of the ocean, we visualize the beautiful waves. Wind in the lower atmosphere moves over the water below, creating waves that build and break along the shore.

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Photo courtesy of the Bureau of Ocean Energy Management

This is a familiar example of fluids (both the ocean and the atmosphere) with two different densities moving relative to one another, with the waves being created along the interface between these two fluid layers. This isn’t the only place, though, where these breaking waves can be seen. Take for example this recent picture from Jan Todo Russell over Seattle, Washington. Notice the similarity to the breaking ocean wave?

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Jan Todo Russell, Seattle, Washington, 26 Mar 2015

This wave pattern in the clouds, similar to that on the ocean surface, is due to Kelvin-Helmholtz instability, named after Lord Kelvin and Hermann von Helmholtz. In order to understand this instability, consider two layers with different densities on top of each other. The lighter (less dense) layer sits atop the heavier layer (think of the way less dense oil sits atop water). Not only do these layers have different densities, but they are also moving at different speeds. This leads to what is called “velocity shear” across the interface between the two fluids.

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Because of the higher velocities in the layer above, a small disturbance can form along the interface of the two fluids. The higher velocity air can, in a sense, grab the lower velocity/denser air below, allowing these disturbances to grow and eventually breaking over like an ocean wave crashing ashore. This ultimate causes turbulent mixing of the two layers, which is a more stable scenario than the separated layers previously observed.

This instability has been studied many times in computer-based and laboratory experiments. Equations have been derived to describe this instability and to determine the necessary conditions (the thresholds) for this instability to occur. Applying these equations in numerical simulations on computers allows for these waves to be produced and studied, such as in this example from the Hrenya Research Group at the University of Colorado, who describe this instability as “An aesthetically pleasing unstable behavior seen in traditional fluids.”

KHI_videoIn addition, lab experiments have been set up where this shear zone is established between two layers of different densities. By tilting these fluids, the upper-most, light fluid flows faster, eventually leading to this KH instability and creating the waves, such as in this video from the Department of Applied Mathematics and Theoretical Physics (DAMTP) of the University of Cambridge.

So what does this have to do with the atmosphere? Well, you can have vertical layers in the atmosphere with different densities, where the upper layer has faster wind speeds. This can lead to disturbances atop cloud layers, that lead to this instability and create Kelvin-Helmholtz wave clouds in the sky. Here are a few more dramatic examples from our Community Cloud Atlas.

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Brenda Dolan, Colorado

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Scott Ellis, Colorado

This instability isn’t unique to our atmosphere and oceans. This instability is out of this world with this wave pattern viewed along the edges of the magnetospheres of planets, including our own Earth and Mercury (see below), as well as in the vicinity of the Red Spot of Jupiter! Nature is amazing!

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Great Red Spot on Jupiter (courtesy of NASA)

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A figure from the published 2013 article in Nature Communications titled Dawn–dusk asymmetry in the Kelvin–Helmholtz instability at Mercury (Jan Paral and Robert Rankin): doi:10.1038/ncomms2676. The colors show the ion density of Mercury’s magnetosphere, where you can see the KH waves along the edge due to the solar wind.

Another day of storms in the central U.S.

In our last blog post (https://communitycloudatlas.wordpress.com/2015/04/03/kicking-off-a-stormy-u-s-spring/), we shared some photos from the stormy start of the severe weather season in the central U.S. (24 March 2015). Large cumulonimbus grew over portions of Oklahoma, Arkansas, and Missouri, producing large hail in some locations. On the next day (25 March 2015), another round of severe weather would bring the first reports of tornadoes for the year.

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There has already been an excellent summary created for this event that describes the atmospheric conditions and storm timeline: http://www.ustornadoes.com/2015/03/27/the-science-behind-the-oklahoma-and-arkansas-tornadoes-of-march-25-2015/

Earlier in the day, before the storms formed, mammatus clouds were observed over the National Weather Center in Norman, Oklahoma. Dena Grose shared with us her excellent photo, showing these bulbous clouds that can form when the air is much drier below the cloud deck.

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Dena Grose, Norman, Oklahoma, 25 March 2015 (2 PM CDT)

While there were severe storms later in the day, these mammatus were not associated with any storms. This is confirmed by looking at the corresponding radar image from this time.

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As time went on, a cold front provided the necessary lift to produce storms later in the evening. Matt Wing shared with us a picture of mammatus clouds, this time over Tulsa just prior to when a tornado warning was issued. In this case, the mammatus were indeed associated with severe storms.

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Matt Wing, Tulsa, OK, 25 Mar 2015

Post-storm damage surveys indicated several tornadoes that moved through the Tulsa area. The strongest tornado was an EF-2 reported in nearby Sand Springs. Here’s a summary of the damage survey from the National Weather Service.

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While Oklahoma experienced the worst of these storms, this unsettled weather provided beautiful views of turbulent skies over nearby regions. Shauna West sent us this picture from Pittsburg, Kansas.

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Shauna West, Pittsburg, Kansas, 25 Mar 2015

Further south, over Arlington, Texas, Whitney Coker Terrell shared her turbulent view beneath a storm that same evening.

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Whitney Coker Terrell, Arlington, Texas, 25 Mar 2015

Thanks for all the beautiful pictures!

Kicking off a stormy U.S. spring

Spring in the northern hemisphere means severe weather for much of the U.S. While this year was off to a slow start in terms of tornado reports, there were several days of severe weather reported last week. torgraphDuring this time of year, the necessary ingredients for severe storms come together in the central and southern part of the U.S. These ingredients include warm moist air from the Gulf of Mexico, which are commonly separated from the warm dry air from the southwest U.S. by what we call a dry line. This boundary can be seen on radar as a thin line and can be identified on surface observations by looking at where the warm moist air from the south/southeast is separated from the warm dry air coming from the southwest. Here is an example of the dry line from March 24, 2015; the first day of active weather last week. You can see the faint blue line in the radar image as well as a computer-generated yellow line on the surface map that indicates the dry line. Notice that temperatures are similar (in the 70s and 80s) on either side of the line, while to the west of the line, dewpoint temperatures (a measure of the amount of moisture in the air) are in the 20s and 30s, while 60+ degree Fahrenheit dewpoints to the east of the line indicate moist air. Dryline_Radar_Sfc_24Mar2015 Surface air ahead of the dry line may be warm and moist, but cooler, drier air above that usually comes from the west, forming what is called a “cap.” This means that the warm air is limited in how far it can lift so something needs to push the air upward above that cap so it can reach its level of condensation. At that point, it can tap into the energy available and grow into an impressive thunderstorm. Balloons are launched twice a day, sometimes more if severe weather is expected. These balloons measure temperature, moisture, wind, and pressure. Here is an example of the data from one of these “soundings” from southwest Missouri during a time before the dry line passed through. sgf_2015032419_annotatedThe lift that’s needed to break through this cap can come from the dry line. That’s why you typically see storms developing along this boundary. Storms that develop along the boundary can displace the air above it, creating what are called gravity waves. This is similar to the way that ripples disperse from the spot on the water surface where you throw a rock. ripples In the sky, the air wants to go up, but if the air is stable (as it is out ahead of the dry line in certain layers of the atmosphere), gravity will pull the air back toward the ground, creating ripples in the sky. Where the air is rising, assuming it’s moist enough, clouds will form. On this day that we’re discussing (March 24, 2015), Karl Kischel noticed some of these gravity wave clouds over Cuba, Missouri at around 3:30 PM CDT.

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Karl Kischel, Cuba, Missouri, 24 Mar 2015, 330 PM (CDT)

Because Karl gave us the exact time and location, we were able to go back and look at the corresponding satellite imagery, where you can clearly see the extent of these clouds. The infrared satellite image gives us a sense of the temperature of the cloud tops, where the warmer colors mean warm temperatures and therefore at lower levels. Notice how these wave clouds are lower in the atmosphere than the deep thunderstorms that create the waves downstream. satellite_visir_ict_201503242045UTC_annotated Storms continue to fire off this dry line as it moved eastward. Radar imagery shows this line of storms along the boundary, extending from Missouri down into Oklahoma and Arkansas. cent_plains_201503242200cent_plains_201503250100 Matt Wing was with friends in Huntsville, Arkansas and captured an incredible view of one of these storms. This picture showcases the characteristic anvil of the beautiful cumulonimbus cloud.

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Matt Wing, Huntsville, Arkansas, 24 Mar 2015

The bubbly characteristic of the middle of the storm indicates turrets of upward motion. The upward motion in these storms can be strong enough to support increasingly larger ice that can fall as hail. Indeed, storm reports on this day showed hail with diameters reaching 1-2″. stormrpts_20150324.gif As the sun was setting, David Holland was in Oklahoma City, looking at distant storms to the east.

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David Holland, Oklahoma City, 24 Mar 2015

He captured the beautiful cumulonimbus in the distance, with a curious section of cloud above the anvil. We suspect this is an overshooting top that has eroded with time. An overshooting top is a cloud directly above the updraft that penetrates through the stable layer where the anvil is seen. When the sun is shining at a low angle (like at sunset), the visible satellite can pick up on these overshooting tops, as is pointed out in this image. satellite_vis_ict_201503250000_annotated

Storms rolling through the Mid-Atlantic

After a relatively cool summer, hot humid conditions have prevailed in the Mid-Atlantic and Northeast of the U.S. in the past week. Today, in particular, featured hot, humid conditions, providing fuel for storms. A system passed over the region, providing the necessary lift to get these storms going. The result was a long line of storms moving through Maryland, producing heavy rain, lightning, and damaging winds. Multiple people shared their view of the storm, both during and after, which is displayed on this map showing their corresponding location for each picture.

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Overlay of pictures on map of MD. Times vary.

The two pictures in western MD were taken following the passage of the storm. The radar image below roughly corresponds to times of these pictures. You can see a north-south oriented line of reds, indicating heavy rain, that had just passed through.

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Radar image from 4:46 EDT, showing line of storms passing through Washington County, MD

Let’s take a closer look at the pictures from this time. While the overall sky remained covered in cloud, a key feature in both of these photos are the lower level clouds hanging beneath. These are referred to as fractus and form in the moist environment that remains after the rain has passed.

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Photo courtesy of Nancy Kirkpatrick in Cascade, MD (2 Sept 2014, after storm passed) showing fractus clouds.

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Photo after the storm passed showing lower residual clouds in Smithsburg, MD (Debbie Rowe, 2 Sept 2014)

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Another view from Debbie Rowe of the back edge of the storm.

This line of storm continued its path across Maryland throughout the evening. The radar image below shows that hours later, it maintained a linear structure on the other side of the state. Also notice storms developing over northern VA at this time; a focus for the next picture.

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Radar image from 6:43 EDT

This new cluster of storms, forming in a similar environment as the earlier round, later went on to also organize into a linear structure. In this later radar image, you can see this structure, the reds indicating heavy rain, and a line of lighter blues out ahead. This latter feature is what we call an outflow boundary, and indicates strong, cold winds moving out ahead of the storm, lifting up warm, moist air ahead of it along with any dust, insects, etc. in its path. (Check out our previous post to learn more about this outflow.)

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Radar image from 7:27 EDT. White arrow shows location of outflow boundary.

Notice at this time, the leading edge of this storm is moving over Washington, D.C. Jen Horneman was driving just outside the city at this time and snapped this photo out the window (the third picture in the map above). A close-up view reveals a much lower cloud base as the warm moist air being lifted along the outflow cools and condenses to form this menacing, low cloud. Strong winds were expected with this feature, followed by heavy rain.

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Photo courtesy of Jen Horneman near Arlington, VA

After these storms moved through, in addition to the heavy rain indicated by the oranges and reds in the radar image, damaging winds were associated with these storms. Many reports indicated large trees down throughout the state.

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Severe weather reports from the Storm Prediction Center. Blues indicate severe winds.

Were you affected by these storms? Feel free to share with us any pictures or stories you may have!

Shelf clouds, mammatus, gust fronts, oh my!

 

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Shelf cloud courtesy of Kevin Sheely (Warrenville, Illinois, 8/25/2014)

What an ominous sight heading your way! Kevin Sheely captured this incredible shot of a shelf cloud near Warrenville, Illinois. On this day, numerous storms moved into and formed within Illinois.

To understand what creates this shelf-like appearance, we first need to understand that the rain that is falling from the core of the storm cools the atmosphere. This cold air is denser than the surrounding air so it sinks to the ground and then spreads outward from where the rain is falling. This cold air spreading outward is referred to as “outflow” and the leading edge of this rapidly outward moving air is called an outflow boundary or gust front. This boundary (like other weather fronts) separates air of different temperatures/densities: in this case, the cold air from the storm and the warm air surrounding it.

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Same picture as above from Kevin Sheely with the cold outflow shown as blue arrow spreading outward and the warm moist air being lifted out ahead shown in red.

The warm, moist air ahead of it is less dense so it is lifted up and over the spreading cold air. This air is lifted and cooled, to the point where it condenses and forms the shelf cloud extending out ahead of the main storm along this outflow.

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Diagram showing shelf cloud formation along gust front

Notice in that diagram that the warm air out ahead is being lifted into relatively stable air above. This leads to the layered characteristics of the shelf cloud as it extends outward instead of continuing to grow upward like the parent storm.

This rapidly expanding air is often responsible for strong, potentially damaging winds at the surface. As these storms, with their shelf clouds, passed through Illinois on the 25th, downed trees were left in their path. These storm reports from the Storm Prediction Center show a cluster of blues in northern IL, indicating strong winds.

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So while the shelf clouds themselves aren’t dangerous, if you see them coming, you can expect strong, cool gusty winds shortly after, followed by heavy rain and possibly even hail.

Luckily these outflow boundaries can also be detected by radar to help warn of this impending gustiness. As the cold air lifts up the warm air ahead of it, it’s also lifting up dust, insects, birds, etc. These can be detected by radar, but the power return is much less than the heavy rain falling. So while the heavy rain in these storms appear orange and red on these radar images, you can see the “fine lines” of blue ahead of these storms indicating the location of the gust front. Notice in these series of radar images that storms produce these boundaries, which then go on to produce new storms that then produce their own gust fronts. And so the cycle continued on this day…

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Series of radar images from the Chicago National Weather Service radar (KLOT) showing numerous storms and their outflow boundaries.

 

We’ve learned that these shelf clouds are seen at the leading edge of the storm, indicating cold gusty winds to come, but what about behind the storm? Well, storms can only reach a certain height in the atmosphere, at which level they spread out horizontally. This is often seen as an anvil. The air below the anvil is typically drier and therefore sometimes mammatus clouds can form underneath. These bulbous beauties were seen on this day in other parts of Illinois by Bill Morris.

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Mammatus over Grundy County, IL courtesy of Bill Morris (8/25/2014)

And that’s not all! Moving ahead to the next day (8/26/2014), another round of storms moved through Illinois, allowing for another opportunity to photograph shelf clouds. This picture was submitted to us by Melissa Godbee. Even though this is looking at the storm from the side compared to the head-on view shown by Kevin, can you still see the resemblance?

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Shelf cloud (Melissa Godbee, Illinois, 8/26/2014)

Have you seen these ominous, yet beautiful, clouds where you live?

Pileus capping cumulus in MD

 

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Pileus atop cumulus congestus, Richard Barnhill (eastern MD, 12 Aug 2014)

This beautiful picture was taken by Richard Barnhill in eastern MD on 12 August 2014. The sun is highlighting the tops of growing cumulus congestus clouds, which are capped by another cloud, called a pileus cloud. Pileus is Latin for “cap” and resembles lenticular clouds that are also highlighted in this atlas. Lenticular clouds form when moist stable air encounters a mountain barrier, whereas these pileus clouds form when moist stable air is disrupted by the growing cumulus cloud below.

Strong updrafts occur within these growing cumulus clouds, defined by their well-defined edges and tufted appearance. If the air above is moist and relatively stable, but is forced upward by this strong upward motion from the cloud below, it can cool to its dewpoint, leading to condensation and the formation of this pileus cloud. This happens quite rapidly and the pileus cloud does not last long as typically the cumulus cloud beneath continues to grow through it.

Here is a schematic we created to try to simply explain this process:

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Schematic showing formation of pileus (courtesy of the Community Cloud Atlas admins)

The following series of images shows another example of a pileus cloud, where the cumulus below quickly produced this cap cloud and then grow through it to form a mature cumulonimbus.

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Example of cumulus growing through the pileus cloud

Here is yet another great example of a pileus cloud, sent to us from North Carolina back in May. Notice how clearly it sits atop the cumulus congestus cloud, resembling a cap cloud hugging the top of a mountain barrier.

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Pileus atop growing cumulus, Blake Smith (North Carolina, 18 May 2014)

The term “pileus” isn’t unique to clouds, however. Another example of this “cap” feature is given for the tops of mushrooms, such as shown in this diagram below. It’s great to see the commonalities in nature!

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“Pileus” term applied to mushrooms

 

Given that pileus are quite the fleeting phenomena, we are curious: have you seen a pileus? We would love to see your examples!

Pyrocumulonimbus cloud from Canada wildfire

Pyrocumulonimbus in Wood Buffalo National Park, Canada (Photo by Mike Smith at 1400 MDT on 5 Aug 2014)

Pyrocumulonimbus in Wood Buffalo National Park, Canada (Photo by Mike Smith at 1400 MDT on 5 Aug 2014)

Pyrocumulonimbus in Wood Buffalo National Park, Canada (Photo by Mike Smith at 1400 MDT on 5 Aug 2014)

Pyrocumulonimbus in Wood Buffalo National Park, Canada (Photo by Mike Smith at 1400 MDT on 5 Aug 2014)

For those of us in the U.S., we’ve been hearing a lot about the wildfires burning in the west, from California up through eastern Washington. But areas of Canada have also seen a devastating fire season. In fact, it has been one of their worst in recorded history with 354 fires so far this year in the Northwest Territories, burning over 2.8 million hectares (Source: Northwest Territories Environment and Natural Resources, Forest Management Division).

Mike Smith is a meteorologist with the Yukon Wildland Fire Management and has been offering on-site support for wildfires. He took these pictures on August 5 of a fire that started in the northwest portion of Wood Buffalo National Park and grew rapidly during August 3-5. To get ourselves oriented, here is a map showing where this park is located.

Location of Wood Buffalo National Park

Location of Wood Buffalo National Park

This particular fire, given the identifier 14WB-025, is one of many in this park on that day. This map from the Forest Fire Management Program shows the location of this fire in the northwest corner of the park, near Buffalo Lake; one noted as being managed.

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To put this fire in context of other fires in the greater region, this map, provided by the Canadian Wildland Fire Information System, shows fire “hotspots.” These are satellite pixels that indicate burning vegetation and may indicate one fire or several hot spots within a larger fire. You can see the high concentration of these hot spots in the Northwest Territories on the day the picture was taken, and this may even be underestimating hotspots if there was cloud cover involved.

Fire "hotspots" derived from satellite infrared data. Courtesy of the Canadian Wildfire Information System.

Fire “hotspots” derived from satellite infrared data. Courtesy of the Canadian Wildfire Information System.

Speaking of clouds, which is the purpose of this post, the heat from the fire rises rapidly into the atmosphere and cools, leading to condensation of water vapor to form cloud droplets. This heat typically produces even more intense upward motion than just the sun heating the ground on a warm summer day, and the ash/smoke in the air gives the clouds a darker appearance and can serve as nuclei for water droplets and ice crystals to form on. These storms can become so intense that they can produce rain and lightning; therefore technically being called “pyrocumulonimbus” clouds.

Luckily in this case, rainfall from this and other storms moving through helped to stall the growth as fire crews were protecting structures in a small community near the fire. Mike and the fire crews have been working hard to protect lives and property and deserve a huge thanks.

As if this pyrocumulonimbus cloud wasn’t impressive enough from the ground, the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite passed over this location around that same time (20:20 UTC or 14:20 MDT), offering an incredible perspective of this cloud from above. Wow!

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Aqua/MODIS, 5 Aug 2014, 20:20 UTC