Interesting Weather Information

Friday, March 22, 2013

Thunderstorm Primer - Part 6 - The Supercell Thunderstorm

The long-track, steady state Supercell Thunderstorm is the definitive top dog in thunderstorm development. It is the 500 pound gorilla of the cumulonimbus world.

All thunderstorms are cumulonimbus clouds and whether they work alone (or discreetly) like ordinary summer afternoon cells and most supercells or together like the various forms of squall lines by definition they all have lightning and thunder and the cloud type is cumulonimbus.

A lone, single-cell thunderstorm, born of daytime heating on a humid summer afternoon may live 45 minutes and travel 10 miles. The long-track, steady state supercell can live 12 hours, travel 500 miles and all along the way wreak havoc and deadly destruction with large hail, flooding rains, gusty winds and the ultimate prize of the  storm chasers quest -  the tornado.

So what is the secret to living a long, destructive lifetime?

Part of the answer is the "steady state"  nature of the supercell.  That means the storm imports energy from the environment at about the same rate that it uses it.  As long as the balance persists the thunderstorm continues on its marathon journey across the landscape.

But there is more to this story.  You might wonder why a simple, solo, summer afternoon thunderstorm lives only 45 minutes while the supercell can persist for hours.

That some thunderstorms persisted for much longer than others was first brought to the world's attention in 1949 by Horace Byers and Roscoe Braham, Jr. They attributed the long life to wind shear in the environment that tilted the updraft causing rain to fall outside the updraft not down through the rising air.

This is were interaction between the supercell and its environment becomes important. Interactions between storms and the storm environment happen in many ways, but three types of interaction are particularly important.

For long-track, steady state supercells the environment provides high octane fuel in the form of warm moist air. The water vapor is imported into the updraft and lifted then condenses releasing heat that helps energize the storm.

The environment also supplies rotation that is pulled into the storm. If conditions are right a mesocyclone can "spin up" within the supercell giving birth to a tornado. I will cover this later in this post.

But it is the interaction of vertical wind speed shear in the environment with the storm that sets the stage for a cross country trek of several hundred miles.
Left: A "garden variety"  thunderstorm on a typical warm and humid summer
afternoon with little wind speed change with height means a vertical updraft.
Right: On a day with vertical wind speed shear the updraft tilts.
In a simple, single-cell thunderstorm (recall they are still called "air mass" thunderstorms but this is old and inaccurate terminology and are more accurately called single-cell or solo or ordinary thunderstorms) that forms in a low shear environment, like the one on the left in the image, the updraft is nearly vertical and the rain that condenses falls "down the chimney".  The updraft has to work against the falling rain and this limits how long the single-cell thunderstorm can live.

In an environment with vertical wind speed shear, right side of the diagram, the updraft tilts and the rain does not fall down through the updraft. The result is a storm with a longer lifetime.

If the shear is too great the storm will not form because the plume that becomes the updraft is sheared apart.  This is also an issue meteorologists face when forecasting hurricanes.

The supercell draws in warm, moist air from hundreds of square miles around it.  The energy that originally evaporated the water vapor is released back to the storm when it condenses and this conversion helps drive the storm for hours. When the warm, moist inflow is cut off from the the storm it decays.

Tilted updrafts, Echo Free Vaults and More

By the early 1960s tilted updrafts were recognized as an integral part of many thunderstorms, including what was called the  SR-Thunderstorm (Severe Right moving - of the average wind) by British Meteorologist Keith Browning. In 1964 he coined the term "supercell" for storms that had the characteristics that we know today as those long-track, long-lived thunderstorms that may produce big tornadoes. We will get back to Browning in a bit.

A Brief Digression: Why right moving of the average wind? 

You may run into an explanation for the right moving that says the storm is moving towards the more moist air to the south.  In most supercell situations the mean (average) wind direction is from the southwest. To the left (north and northwest) is the drier and cooler air. To the right (south and southeast) is the sticky, moist air from the Gulf of Mexico. However most often supercells are embedded in an expansive, moist air mass with little difference in moisture to the north of the cell.  

As supercells age and begin to weaken and the cool pool expands motion may curve towards a more southerly path towards the moist air there but  mature supercells follow a nearly straight path which is in a direction right of the mean wind. Look at the image below by Brian Tang of NCAR.

April 27, 2011. Path of the Supercell that caused the Tuscaloosa, AL tornado. 
Note the nearly perfect straight line.

I eye-balled the mean tropospheric wind direction so it is approximate. 

A similar explanation concerning the Coriolis Effect often surfaces.  But a simple coriolis based explanation would have to explain why the path is not a continuous curve to the right.  Coriolis does play a part but in a complex interplay with other forces in the very complicated life of a supercell.

So we are left with the two debated dynamic explanations listed below:
  1. Non-linear Pressure Perturbation Effect (aka vetical wind shear paradigm)
  2. Linear Pressure Perturbation Effect (aka helicity paradigm)
A complete explanation of the pressure perturbation effects involve some heady mathematics and a fairly deep understanding of atmospheric physics which is beyond the scope of this post. So I will not go into it here.

 In addition meteorologists still cannot agree which of the two pressure perturbation effects is more important so I will give a very generalized physical explanation and note that this explanation is incomplete.

Both mechanisms involve importing "tubes" or "helicies" (plural of helix) of environmental rotation and tilting them to the vertical which adds to the updraft rotation as shown below. And both involve increasing the pressure on the left side of the storm (rear flank downdraft or front flank downdraft side and decreasing pressure on the inflow (right side) in very complicated ways. The pressure perturbations happen through different pathways by the two dynamic mechanisms with the same result.

Environmental Rotation

The illustrations and captions explain the concept.
Left: Wind Speed Shear (change of velocity with altitude) creating horizontal rotation. Note the winds are all blowing in the same direction. Right: Wind Direction Shear (change of wind direction with altitude). The average wind is what ordinary thunderstorms follow. The direction of motion of complex supercells is modified by several mechanisms which raise the pressure on the left side of the storm in the down draft areas and lower the pressure on the right side of the storm in the updraft areas. This makes storm motion to the right of the red arrow in the diagram.

Tubes of rotating air can become a helix of rotating air when transported towards the supercell by inflow. They are tilted to the vertical by the updraft and add to the rotation of the updraft.
As the helices accelerate into the updraft they are stretched because the strongest updraft is at mid levels in the storm and the rotation rate increases as the helices narrow. Think of an ice skater pulling his or her arms towards the body. As long as this happens and it can for several hours the dynamic mechanisms listed can operate to increase pressure on the left of the storm and decrease pressure on the right of the storm.

Storm Movement Put Simply

Both dynamic mechanisms and the distribution of rainfall along with the growth and/or decay of the cold pool work together to dictate the fate of a supercell.

Put simply all the factors mentioned and likely others work to lower the pressure in the updraft on the right side of the storm and raise the pressure on the left side of the storm (rear flank downdraft and forward flank downdraft) causing the storm to move to the right of the mean wind. Rising air means lower pressure, sinking air means higher pressure.

Because the storm is steady-state (using energy at the same rate it imports it) and because the updraft is tilted and the rain falls outside the updraft the typical supercell travels in a nearly straight line to the right of the mean wind and can do this for hours until the steady-state balance is upset.

Now Back to Browning and the early 1960s.

Browning stopped his radar from rotating and scanned vertically getting the Range-Height Indicator (RHI) views below. Range means distance from the radar and height is self-explanatory. These radar reflectivity profiles were the start of our modern understanding of supercells.
Radar reflectivity profiles presented by Keith Browning in 1963 of two thunderstorms with
characteristics that  we know of today as supercells. Of course it is much more complicated than
the simple radar profiles above. Note the large amount of vertical exaggeration about 6.5x.

The radar images above are from Browning's research and by today's standards  very primitive, but then a technological marvel. He was able to see the heavy downpour of the supercell. The front edge of it was called the "wall". The echo free vault , now called the WER (weak echo region) or the BWER (bounded weak echo region) identifies the warm moist inflow and updraft.

You may ask why the updraft looks vertical and not tilted.  Remember this is a 2D slice of the storm and the tile occurs in 3D so it is not likely that a given slice will see the tilt/

Using today's technology here is what the BWER of the supercell that spawned the  Crittenden-Piner-Fiskburg-Morning View EF4 on March 2, 2012 looked like in plan view (map view ) also called the PPI Display (Plan Position Indicator). The white arrow shows the location of the cross section below.

Each panel shows the PPI display at different antenna tilts from 1.0° to 1.4° to 1.9° above horizontal. The weak echo region is bounded by higher reflectivity.

The cross section runs from SSW to NNE as shown by the white arrow in each map view. The BWER is also called the vault and is located where the inflow of warm, moist air joins the updraft as shown in the image below.

Compare this cross section to the 50-year old images from Browning above and it looks the same. Some of the differences you may see are because each storm is an individual and the vertical exaggeration differs.

Some Additional Information On How Radar Works

Fifty years ago Browning stopped the rotation of his radar and scanned vertically to get the RHI views above. Brownings RHI views are a 2D slice through the supercell.

Modern NEXRAD Doppler radar does a volume scan. It scans the lowest level, approximately 0.5° above horizontal for  360° then tilts to a higher elevation. It does this until the last tilt is 19.6° above horizontal. It starts again at the lowest level.

By interpolating we can get a complete 3D look or a slice look like the image above. The video below explains it. Use reload on your browser if you see an ad.

A Volumetric View

Using the advanced 3D rendering technology in the software GRLevel2 Analyst, you can see a 3D view of the Piner storm. I will have a number of 3D views when we get to tornadoes.

Volumetric view of the supercell that caused the EF4 Crittenden-Piner-Fiskburg-Morning View tornado. This is what radar sees and will not look exactly like what you would see from this vantage point. Notice the dome, or overshooting top indicating the top of the updraft.

Same view as above with the lower reflectivity levels stripped awav. For the technically minded only reflectivities of 50dBz or greater are shown. This reveals the heavy precipitation core of the storm in red. Note on the ground is the PPI view of radar. See the image below for an annotated view of this image.

Same image as above with annotations to explain what you see. Notice how the inflow rises into the BWER.

The Big Picture

Supercells are very complex systems and no two thunderstorms looking exactly alike but there are many identifiable characteristics that help the meteorologist diagnose the severity of a storm and what type of severe weather is most likely.

The image below summarizes a typical Classic Supercell.

The photos below are examples of some of the features in the diagram above.

Anvils and Overshooting Tops from The International Space Station

A thunderstorm grows until it bumps its head into the stratosphere at the tropopause. Because the stratosphere is a very stable layer the cloud cannot proceed upwards and it flattens out in a wide layer of clouds called the anvil, named that because in many cases from the ground it looks like a blacksmith's anvil. The higher winds at that level blow the cloud material down wind. The anvil can extend hundreds of miles downwind.
Blacksmith's Anvils. Courtesy:

The first two images show the anvil also extending a bit upwind. This is called a back-sheared anvil and happens in many thunderstorms bcause so much air is rising to the tropopause. Notice how there is a distinct edge to the anvils in the photographs.  The air that rises violently in the thunderstorm must come back down and in many systems it descends gently all around the thunderstorm creating the distinct edge of the anvil.

The image below shows 5 thunderstorms with merging anvils in 3 of the storms. Notice how clear it is around the storms and note the deep shadows under the anvils.
All three images courtesy NASA, from the International Space Station.

Anvils from the Ground

From the ground it is easy to see why the thunderstorm anvil was compared to the tool of the blacksmith. The first two are classic anvils and the third from the famous Chaparral Thunderstorm of April 3, 2004 that dropped 2" hail on Chapparral, NM.

The three images above coutesy, NOAA.

A developing thunderstorm off the coast of Texas near Houston. Note the developing anvil and the location of the updraft as indicated by the overshooting top. Also note the falling rain about 1/3 of the way up on the right. It is evaporating long before it hits the surface of the Gulf of Mexico. The rain is falling outside of the tilted updraft. This storm is pre-supercell and may or may not have developed that far. Thye annotated image is below.
Photograph by Steve Horstmeyer.

Mammatus Clouds - Underside of the Anvil

Cumulonimbus mammatus (most frequently just called mammatus and rarely mammatocumulus) is a cloud form found on the underside of an cumulonimbus anvil. The name is derived from the latin word "mamma" or "udder". They resemble cows's udders.

Mammatus indicate extreme turbulence - rapidly rising and sinking air juxtaposed in a small area. There are a number of mechanisms proposed for the formation of mammatus and the jury is still out. In situations like this there is often more than one mechanism responsible.

Courtesy, NOAA

Rainshafts are frequently mistaken for tornadoes and in fact rainshafts can hide tornadoes in some supercells. A tornado hidden by rain is called a "rain wrapped" tornado. More on this topic when we get to tornadoes.

Top: an Oklahoma rainshaft with torrential rain. Below: A Doppler on Wheels looks at a
rainshaft in the distance. Both photographs courtesy, NOAA.

Shelf Clouds
Shelf cloud, Hebron, KY June 15, 2010 by Stephanie West.

Orange arrow - warm moist air flowing into the thunderstorm, above and lifted by the
denser rain cooled air flowing out of the thunderstorm. Dark blue arrows- warm air
 mixes with the cool air below, condenses and falls through the cool air. Light blue arrow -
the cool outflow from the thunderstorm. Hebron, KY, June 15, 2012 by Stephanie West
from the Kroeger store near CVG.

Same shelf cloud as above from below as it arrives traveling to the right.
Same shelf cloud as in photos above but looking at it from the front near CVG. Image below is
enhanced for easier recognition.

An enhanced view of the shelf cloud from near CVG in the photos above. The shelf cloud has been lightened in  for illustration purposes. This is t he same shelf cloud as the two images above but from the front as it approaches.

A High Plains LP supercell shelf cloud.  More on LP and other types of supercells below.

Supercells - Classic, LP and HP

Supercells and all thunderstorms are cumulonimbus clouds. In meteorological shorthand written "Cb".  Supercells come in three flavors, Classic, LP (light or low precipitation) and HP (heavy or high precipitation).

The difference between them is essentially due to the amount of low and middle level moisture in the atmosphere and that is most often a function of geography.

Low Precipitation Supercells are found in the high Great Plains to the east of the Rocky Mountains where moisture is often scant resulting in less cloud volume than Classic or HP Supercells.. With a fairly dry atmosphere visibility is unrestricted and cloud development associated with and around the supercell is minimal resulting is a dramatic looking skyscapes. The photograph above is one example. 

The photo below is a remarkable example of a LP Supercell. Supercells like this are sometimes called "barber pole" LP Supercells.  Because cloud development around the rotating updraft is limited in LP Supercells the outer edges of the rotating updraft can often be seen as clouds moving in an upward spiral.

July 8, 2009, Beth Allan. Photographer's Description: An amazing barber pole LP supercell spins over the northern high plains of South Dakota. Although this never dropped a tornado, it tried hard for a little while and looked great while doing it.
I have annotated the image below the original showing features typical of LP Supercells

For more photographs by this remarkable photographer on a variety of topics check out the link:

The small shelf cloud is due to the fairly dry atmosphere which leads to less condensation and less cloud volume than in a more moist atmosphere. Rain falls outside the cloud because of smaller cloud volume. In a Classic or HP Supercell the rain would be hidden within the cloud until it emerges below cloud base.

LP Supercell Characteristics:
  1. Strong updraft because of low precipitation loading.
  2. Large hail because of strong updraft.
  3. High cloud base because of low moisture content of atmosphere.
  4. Strong straight-line winds because of strong evaporative cooling.
  5. Fewer and weaker tornadoes than other types of supercells because of high cloud base, weak or absent rear flank downdraft and forward flank downdraft because of less precipitation.
  6. Highly visible tornadoes because of  low moisture content of atmosphere.

04.08.2013 Work on this post continues.

Wednesday, March 20, 2013

Thunderstorm Primer - Part 5 - Mesoscale Convective Systems - The Mesoscale Convective Complex (MCC)

In this post I am writing about the Mesoscale Convective Complex or MCC. In Parts 2 - 4 of  The Thunderstorm Primer I covered linear mesoscale convective features, the squall line, the bow echo and the line echo wave pattern. Now  we talk about a cluster, a glob, a blob or a collection but not a random hodgepodge of thunderstorms.

The image below is an MCC from June 20, 2007 at 6:15 UTC (2:15 AM EDT) over the southern Great Plains. It shows the typical shape and characteristics of an MCC.

The definition of a mesoscale convective complex stipulates that the cloud top area with temperatures <= -32°C (-25°F) is 100,000 km²  (38,610 mi²) or greater. Alternately the area of cloud tops with temperatures <= -52 °C is 50,000 km² (19,305mi²).  Either of these conditions must persist for 6 hours or longer.  Its eccentricity (the ratio of the length of the minor axis to the major axis) is greater than or equal to 0.7 at maximum extent. If the eccentricity = 1 the area is a circle. This prevents overlap between MCCs and Linear MCSs.

MCCs most often form north of an west-to-east or northwest-to-southeast oriented warm front and follow the winds aloft along the front heading generally eastward. Near the end of the MCC's life many abruptly turn right (southward) and weaken quickly. This makes for a very difficult forecast situation because there is no warning that the turn will happen until it starts. 

In the High Plains "orogenic" MCCs often form late in the day along the slopes of the Rocky Mountains. When a strong low-level jet carries moist air northward during the heat of the day thunderstorms grow above the east slopes of the Rockies.  As they grow then begin to work cooperatively and become a unified system.

The thunderstorms become a cluster of cooperating cells and as the sun sets the MCC moves east, away from the mountains. The MCC moves all night and usually dissipates around dawn.  The remaining cool pool of air and associated left-over circulation from the MCC often serve to generate new thunderstorms later in the day. The cold pool is formed by evaporative cooling of the some of the heavy rain.

MCC May 12, 2010 13:15 UTC (9:15 AM EDT) Below is a radar loop for the same MCC

Surface map with a warm front from a low in the Panhandle of Texas 
to the Atlantic Ocean,  May 12, 2010 13z. The warm front acts as 
"railroad tracks"for the MCC. Plotted and analyzed with 
Digital Atmosphere,

Strong, broad flow of tropical moisture laden air northward to the warm front 
which is feeding the MCC. This is the 850 hPa level, roughly a mile up 
May 12, 2010, 12Z.

Next stop in The Thunderstorm Primer - the nadir of thunderstorm development - Supercells.

Tuesday, March 19, 2013

Thunderstorm Primer - Part 4 - Linear Mesoscale Convective Systems - The Line Echo Wave Pattern

A Line Echo Wave Pattern  (or LEWP - pronounced loop) is a Linear MCS (mesoscale convective system) that is composed of more than one bow echo. See my previous post for more on Bow Echoes.

Bow echoes can merge to form a LEWP, or a relatively straight squall line can develop the bow shaped bulges as shown in the schematic. The lows that develop initially start because of the mid-level Rear Inflow Jet (RIJ).  As the lows strengthen and heavy rains drive cool downdrafts, a very complicated process is taking place, and the net effect is that the RIJs strengthen. In turn the lows strengthen more.

At the apex where the southern end of one bow meets the northern end of another bow a strong, small-scale low pressure system can form leading to enough spin for isolated tornadoes. Supercells can form within LEWPs but are fairly rare. Most of the tornadoes associated with a LEWP are leading edge vortices and form near the low pressure systems, but not exclusively.

 Bow Echoes and LEWPs can become derechos, large, long lived squall lines which come in two varieties: serial derechos and progressive derechos. More on this in a later post.

In the next  Thunderstorm Primer post is a big blob or cluster of thunderstorms called a mesoscale convective complex.

Monday, March 18, 2013

Thunderstorm Primer - Part 3 - Linear Mesoscale Convective Systems - Bow Echoes

A Bow Echo is pretty much what it sounds like it is - a line of thunderstorms shaped like a hunter's bow. Bow Echoes range from small, covering a few counties and lasting for a couple of hours to very large covering several states and lasting up to 24 hours.  The big ones are often considered to be a special classification called "derechos". The word derecho (pronounced DAY- ray - cho) is Spanish for "straight" referring to the damaging straight-line winds that often accompany derechos. This post will cover bow lines later after the Thunderstorm Primer is complete I will post  a detailed description of derechos.

Bow Echo over Eastern Oklahoma (index map lower right) may 24, 2003 10:48 GMT.

Ted Fujita, the F-Scale guy, was the first to describe in detail how this type of Linear MCS evolves. The schematic below is based on Fujita's original from 1978.

A large cumulonimbus develops and evolves  because of the strong rear inflow jet (RIJ). As the line moves to the right on the diagram then central region bows in response to the mid-tropospheric RIJ. The north end of the line often develops into a mesoscale low and there may be enough turning in the northern bookend vortex to spawn a tornado. The rotation of the southern bookend vortex is opposite the rotation of Earth (i.e. opposite the coriolis effect) and does not develop.

The northern end  and the southern end are subject to rotation due to their location north and south of the Rear Inflow Jet. At the southern end the rotation is opposite to the coriolis effect and does not develop. The rotation at the northern end is enhanced by the coriolis effect effect and the vortex strengthens often leading to a coma shaped radar echo with the wide part with the northern vortex. You can see this on the radar image above.

The vortices are called "Bookend Vortices" and there is enough spin that tornadoes frequently develop in association with the northern vortex.

Because the inflow jet is dry, evaporation at the rear of the storm cools it, makes it more dense and then it can descend towards the ground in a long line of straight line winds along the leading edge of the line. Wind damage is widespread when this happens.

Leading edge vortices (still called tornadoes almost everywhere) can develop as the inflow interacts with the straight line winds moving down out of and ahead of the line.

The video above is from a derecho, essentially a long-lived, large bow echo that moved through the Cincinnati area during the evening of June 29, 2012.  I will have much more on derechos in a post after the conclusion of Thunderstorm Primer.

Sunday, March 17, 2013

Thunderstorm Primer - Part 2 - Linear Mesoscale Convective Systems - Squall Lines

During the first half of the 20th Century lines of thunderstorms were generally called squall lines and the thunderstorms in the line were classified as frontal thunderstorms.  In some instances the term squall line was synonymous with the term cold front. But in those years the modern concepts of cold and warm fronts were still not universally recognized.  The first organization in the U.S. to analyze fronts on weather maps was the United States Navy and that was not until the 1930s.

The situation as we understand it today  is much more complicated. So we now speak of single-cell thunderstorms (formerly called air mass thunderstorms), which can grow into mesoscale convective systems as storms grow and merge. Or the linear mesoscale convective system can develop in a relatively straight or gently curved line (squall line), a warped line (bow line or LEWP) or a round to elliptical cluster of storms (mesoscale convective complex).

Scales of Atmospheric Motion

This is a topic with too many details for this short post but I have created a webpage (primarily for my meteorology students at The College of Mount St. Joseph) covering scales of atmospheric motion and here is  the link:

More Information on the Sales of Atmospheric Motion - CLICK

What is important for understanding this discussion is the mesoscale is smaller than synoptic scale and larger than both storm scale and micro scale. Synoptic scale is the scale of a national weather map with fronts, highs and lows. Storm scale is the size of individual cumulonimbus and microscale can be the size of the dust devil on a baseball field or leaves whirling around the corner of a building on a blustery autumn day.

So when we talk about a mesoscale convective system we mean a middle sized system, made of multiple thunderstorms that share a source of lift (in addition to differential surface heating most of the time). The storms may or may not work cooperatively and may or may not compete for resources. Most of the time the storms compete for low-level moist air  which is high-octane fuel for convective systems. Often new storms form and old ones dissipate as the mesoscale convective system propagates, moves and evolves.

Liner Mesoscale Convective Systems - Squall Lines

The video shows a squall line developing, ahead of a cold front on June 14, 2012.

The individual cells share a source of lift, in this case the front and surface convergence ahead of the front along with the heating of the day. Warm moist air to the south and southeast is pulled into the thunderstorms, after all they are low pressure systems at the storm scale, and when the vapor condenses tremendous amounts of heat are released to the environment and drive the storm beyond what the initial energy sources would.

In summary, the sources of energy for this system are:
1. differential surface heating
2. surface convergence
3. frontal lift
4. condensation of water vapor in the updraft.

Below are two additional examples of squall lines or as we call them now linear mesoscale convective systems.

Linear MCS June 6, 2008
In this exc
Linear MCS just to the west of Cincinnati May 25, 2011.

Tornado Watch #370 2011  a PDS Watch (Particularlly Dangerous Situation)  along tje Linear MCS on May 25, 2011. Note the date on the image says 2011 0526/0134 UTC which is 8:34 PM CDT May 25.

Linear MCS Systems rarely spawn large rotating supercells leading to large tornadoes. Most of the time given sufficient instability when a tornado is spawned by a Linear MCS like this it is a leading edge vortex and usually rated as EF0 to EF2 (occasionally EF3) on the Enhanced Fujita Scale. This is discussed in a later post.

The Linear MCS of May 25, 2011 caused 5 small tornadoes in southern Indiana, two were rated as EF2 and three rated as EF1.

The situation gets more complicated when Linear MCSs interact with their environments in complex ways and give birth to bow line echoes or  line echo wave patterns (or LEWPs - pronounced loops). Both are squall lines, but squall lines with complications. The bow line is covered in the next post and the LEWP follows.

I end this post as I did the previous one. What's in a name? The term squall line evokes a more ominous  tactile experience  than the term Linear MCS. The term squall line can refer to any line of thunderstorms. A Linear MCS on the other hand is a category that includes squall lines, bow echoes and  line echo wave patterns (LEWPs).

Friday, March 15, 2013

Thunderstorm Primer - Part 1: Storms Formerly Known As Air Mass Thunderstorms

As I mentioned in my previous post, the Introduction to this Thunderstorm Primer, we probably should not call the simplest of thunderstorms "Air Mass" thunderstorms because they really are not. This is an old, overused concept and it should be retired to the "Of Historical Interest" section of your meteorology lexicon.

Meteorologists originally thought that thunderstorms were with the air mass type or frontal type. Air mass thunderstorms were thought to pop up at random on a hot, humid summer afternoon and the sole source of energy is differential surface heating. In fact thunderstorms are never random. We may not know precisely why one develops where it does but it is not random.

The lifetime of a simple or ordinary or single-cell or garden-variety thunderstorm - all are better than the old term air mass thunderstorm) is illustrated below and ranges generally from 15 to 45 minutes. Because the updraft is vertical and the heavy thunderstorm rains fall against the updraft  the lifetime of s single-cell thunderstorm is limited to a short duration. One meteorologist said it is like pouring water down a chimney.

The life cycle of a typical single-cell thunderstorm.

Because the storm does not last very long it has little time to interact with its environment. Remember this point because we come back to storm-environment interaction in the next post.

In Summary:
Simple Single-Cell Thunderstorms
· are primarily convective but other sources of lift can contribute
· have an updraft that is typically vertical so rising air has to push against the rainfall
· mostly have a lifetime of 15 – 45 minutes
· can be severe (with 1” hail and winds >=58 mph) but mostly storms are below severe limits
· are individual cells but several can grow together to form a MCS (mesoscale convective system).

Frontal type thunderstorms were thought to develop in lines along fronts. Differential surface heating along  lift along the front supplied energy to the storm. Lines of thunderstorms are now classified as a type of mesoscale convective systems. This will be covered in the next post.

So, what's in a name! The term air mass thunderstorm still hangs around even though it is obsolete. A better name is isolated or single-cell or simple or garden-variety thunderstorm. Naming issues will be visited again when we distinguish between mesocyclone tornadoes and leading edge vortices.

Thursday, March 14, 2013

Thunderstorm Primer - Introduction

My Thunderstorm Primer is a series of posts that will discuss thunderstorms both severe and non-severe along with the associated phenomena of lightning, mesocyclone tornadoes, leading edge vortices and hail in 10 separate posts in addition to this introduction. Here they are:

Introduction: Convective Storms - Background Information
Part 1. (So Called) Air Mass Thunderstorms
Part 2. Mesoscale Convective Systems - Squall Lines
Part 3. Mesoscale Convective Systems - Bow Echoes
Part 4. Mesoscale Convective Systems - The Line Echo Wave Pattern (LEWP)
Part 5. Mesoscale Convective Systems - The Mesoscale Convective Complex (MCC)
Part 6. The Supercell Thunderstorm

Some Terminology
In meteorology convection is the process of transporting heat vertically or nearly so by the movement of a mass of air. The transport of heat horizontally by the  movement of a mass of air is termed advection. So when warm air is arriving from Texas a meteorologist would say "warm advection' is occurring.

Convection is possible because of differential heating of the surface.  Even though TV meteorologists often just use the term "surface heating", the deep convectionthat causes thunderstorms requires the sun's energy be absorbed un-equally by  neighboring surfaces.

A large park will absorb less solar radiation than and adjacent shopping mall with a mega-parking lot. In turn the air above the shopping mall will become warmer and less dense. The air over the park will be cooler and more dense than the air over the mall.The cooler, denser air is pulled by gravity more than the warmer, less dense air. The cooler air then undercuts the warmer forcing it to rise. That's convection.

Before we proceed ....

Fighting Bad Science by Being Persnickety  - Steve's Meteorological Pet Peeve #1 
 Seemingly, poetically, magically clouds appear to float above our heads. But clouds do not float and warm air does not rise. We like to say they do, but sorry they do not because there is no magic in science.

To be fair sometimes saying clouds float is just a shortcut for saying "the clouds moved across the sky held aloft by a gentle updraft". You see it is easier to say they float. They do not.

Nothing moves in this physical world of ours without a force to do the work. Isaac Newton told us this in 1687. Warm air does not rise and clouds do not seemingly defy the force of gravity without forces. Warm air is forced to rise as cooler, denser air, pulled more by gravity than neighboring warm air undercuts it. That updraft keeps the heavier-than-air clouds aloft.

Back to the post...

A convective storm is one driven by unequal heating of the surface. The world is complex and it is not quite  that simple because rarely is there a single source of lift driving storms. In addition storms interact with their environments in many ways and to many degrees.

We can use these two factors, the sources of lift and the amount and type of interaction wit hte storm's environment to explain the forms and severity of the storms we see.

Next, Part 1 of my Thunderstorm Primer discusses what we probably should not call Air Mass Thunderstorms.

Saturday, March 9, 2013

Severe Storms Rock Stars

Long before the insightful investigations of Ted Fujita led him to develop the F-scale for estimating tornado wind velocities and even longer before scientists, storm adventurers, severe weather rock stars Howie Bluestein and Josh Wurman could see the intricate dance of small vorticies as they merged to form large tornadoes using a doppler-on-wheels (DOW - see videos below) -  there was John Parker Finley.

Tetsuya Theodore "Ted" Fujita in his University if Chicago laboratory.
Howard Bluestein
Josh Wurman and a DOW (Doppler on Wheels)

The middle video shows convergence (red) and vorticity (i.e. spin or rotation outlined in black) . Notice the voticity being imported into the main rotation. Top: Doppler on Wheels view of a tornado. Bottom: Rotation being pulled into a tornado, pay close attention to the left panel. All courtesy

Finley was a rock star severe storm investigator long before rock-and-roll's seminal instrument the electric guitar was possible. He dug deep and demanded perfection. He developed a storm-spotter network and compiled a comprehensive history of tornado touchdowns for the United States - the first tornado climatology in the world.

John Park Finley's map of tornado touchdowns in the United States. from 1760 - 1885. Notice how the heart of  Tornado Alley, present day Oklahoma, then Indian Territory, lacks tornado reports. This problem still plagues researchers. We have to adjust for population density before we take the reports seriously. The lack of tornadoes where the Appalachian Mountains are is a combination of both low population density and real lower tornado touchdown occurence.

Finley's contour map of tornado occurrence and again the Indian Territory lacks reports, but if you take into account what we know today you can see where Tornado Alley is.

Finley advocated insurance against tornado damage, offered advice on staying safe and presented plans for what he called a "tornado cave". In fact Finley along with the Burlington Insurance Company in Iowa sponsored a contest for the best design of a tornado cave. The contest was won by architect John R. Church of Rochester, NY out of a total of 122 entries. 

The prize was $200 which in 2013 dollars is $4250.

Finley also gave advice on tornado safety after reviewing many tornado touchdown sites. Take a look at the diagram below.

In addition he tried to understand how tornadoes work.

John Park Finley, a severe storms rock star long before that title made any sense.