12 Nov

There Is More Than One Way to Start a Tornado

Of the 2011 Southern tornado outbreak,  April 27th particularly stands out. A total of 199 tornadoes occurred in a 24-hour period leading to 316 fatalities.  That not a misprint.  199 tornadoes.  What makes the event meteorologically interesting is that tornadoes came from  three rounds of weather, each with unique characteristics. Any good, Southern, armchair meteorologist knows the basics of a supercell leading to a tornado. But tornadoes can originate also from quasi-linear convective systems, a only partially understood and complex process. The early morning and midday tornadoes of April 27th arose from just such systems.

From Knup et al. 2014

From Knup et al. 2014.  QLCS vs. supercells in Alabama on April 27th, 2011

When thunderstorms become organized and active at larger scale, the overall complex is referred to as a mesocscale convective system (MCS). When these MCSs approximate something near linear, typically at the leading edge of a cold front, they are referred to as a quasi-linear convective system (QLCS). You may know this better as a squall line. As many a Southerner knows, the squall line contains heavy rains, hail, frequent lighting, and strong winds. Basically your typical Southern spring day.

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QLCS over Arkansas

A QLCS can also produce a tornado.  The whole process is very complex and only partially understood.  The cold front lifts the warm air ahead of it forcibly forming the rain line. The rain cools the air causing the air to sink, called a cold pool, which produces strong winds. These winds rushing out causing the squall line to bow.

Ahead of the storm the cold and dense winds force the warmer air to loft. As these winds “empty” the space behind the bow, a low-pressure area is created.  This low-pressure area is filled in by drier air above the storm. This movement continues to accelerates the whole process.  A rear-inflow jet (RIJ) forms caused by the  elevated area of low pressure caused by a tilted updraft over top of the cold  pool.

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The bookend circulation at the tips of the bow echo is caused by differences air density due to temperature and pressure, i.e. cold dense air sinks and warm light air rises.  This vertical air movement causes horizontal rotation. Imagine taking a pool noodle, turn it on its axis, and bend it into an arch.

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The circulation occurring at the northern part of the bow is amplified due to Coriolis effects.  Interestingly despite the significant rotation,  not all QLCS tornadoes are produced in bookend vortices.  In fact, most  form in smaller-scale vortices at the leading edge of a QLCS.  

bowfujitIn the South a significant number of tornadoes can develop from QLCSs. In one study nearly 55% of the tornadoes in Mississippi and Tennessee over a 5-year period developed from QLCSs.  QLCS tornadoes are unlike supercell tornadoes.  They both form and dissipate quickly and initiate below the radar detection heights. This combinations of factors make it difficult to warn people of a QLCS tornado. Typically by the time the warning is issued… the tornado is already gone. The fact that more QLCS tornadoes occur during the late night/early morning hours make this lack of warning even more concerning.

Screen Shot 2015-11-11 at 6.03.30 PMLuckily, QLCSs do not often form larger tornadoes. Rarely, however, QLCS spun tornadoes can reach EF2-3. This is exactly what happened on the April 27, 2011 (see figure above from Knupp et al 2014). The first 76 tornadoes of the day developed from a strong MCS that developed in Arkansas, grew stronger in Mississippi, and evolved into a QLCS in Alabama in the early morning. In the mid-day a second QLCS developed, producing 7 weak tornadoes. The earlier QLCS tornadoes caused multiple local power outages across Alabama. This reduced the possibility of warnings, as electricity is needed for sirens, radio, and television. The afternoon saw the development of supercells that spawned the largest tornadoes of the April 27th outbreak. Many people never received the warning.

 

 

14 Apr

The difficulty of predicting tornadoes

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Legendary blues singer, Alger “Texas” Alexander was born in Jewett, Texas, north of Houston in 1900. In 1934, Alger composed and sang with his big, deep voice the Frost Texas Tornado Blues. Just four years earlier a F4 tornado ripped through the town of Frost just south of Dallas. Sixty were killed and many more never found. The entire town was leveled and a mass funeral occurred on the porch of the only house left standing.

I was sitting looking: way out across the world

Said the wind had things switching: almost in a twirl

Says I been a good fellow: just good as I can be

Says it’s Lord have mercy: Lord have mercy on meOkeh_Record_Label

Mmm: mmm

Says I been a good fellow: just as good as a man could be

Some lost their baby: was blowing for two three miles around

When they come to their right mind: they come on back to town

Said rooster was crowing cows was lowing: never heard such a noise before

Does it seem like hell was broke out: in this place below 

frost_tornado_photo_12Although we still sing the blues about tornadoes, much has changed since the 1930’s with regard to tornadoes. We have better home construction and much, much better warnings now.  However, knowing exactly when and where  a tornado will occur…well that is a bit more challenging.

Let me clarify.

With ever-increasing precision we can predict, and warn, of conditions that are likely to produce a tornado. The ingredients—wind shear, instability, heat and moisture, and forcing mechanism—are mesoscale phenomena that can be modeled, quantified, and even predicted. Roger Edwards, lead forecaster at the Storm Prediction Center described the intricate process that it takes to predict a thunderstorm and tornado outbreak days to a week in advance. “It takes a combination of skill, luck, and a team of people. The key is trying to predict a phenomenon not just temperature and rain.” Data of different types—wind, temperature, moisture—at scales ranging from the globe to North America to a particular county in Oklahoma must all be integrated. The data itself comes from satellite, aircraft observations, weather balloons, weather stations, and radar. All of this data must be assimilated and ran through multiple modes that would fry your home computer and probably your neighbor’s on top of that. Then the computers spit out an answer. Of course that answer might be garbage. “Let’s not forget the human element to see that models are going awry. You need to conceptualize in three dimensions what is going on in atmosphere. We still analyze charts by hand which causes us to slow down and think with pencils and paper in hand. Our experience is vital. “

But these ingredients form a supercell, not a tornado itself, and not all supercells produce tornadoes.   “The atmosphere has a way of getting the four together in ways with minor differences to either create a large EF5 tornado or a just some rain. We don’t know when and where these ingredients form in just the right way,” states Edwards. Indeed, 70% of the time a tornado warning is issued no tornado actually forms. It’s important to understand this error would be much worse if it were reversed. In other words, if 70% of the time a tornado occurred no tornado warning was issued. That is the major advance I’m talking about. 

Scientists still have little idea what causes tornadogensis particularly those microscale phenomena, those minor winds, temperature, and pressure differences that occur in areas less than a mile to less than football field that trigger the whole shoot and caboodle. Measuring, analyzing, and modelling this is the cutting edge of tornado research.  In the last several weeks, I have been speaking with the biggest names in tornado researchers from Virginia to Oklahoma to Illinois. I asked everyone what is the biggest question in tornado science. Every one of them responded that the holy grail, my word not theirs, is the when, where, and how of which storm will make a tornado.

Charles Doswell is a meteorologist who although did not originate the concept of supercell, that’s thanks to a Brit named Keith Browning, did with Les Lemon improve Browning’s idea that gave us the modern conceptual model of supercells and how they form. That paper from 1979 is one of the most cited papers in meteorology and tornado science. I’ll leave with Charles’s words from our recent conversation. “We are still struggling with the area of tornadogenesis. Apparently you need every single one of the details…and very high confidence in them too.” Perhaps it’s fitting that Doswell and “Texas” Alexander share more in common that a fascination with tornadoes. Doswell is too a lover of the blues, perhaps fitting for a tornado scientist, and used to host a blues radio show.

 

24 Mar

The Making of a Tornado

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A siren goes off in the distance. It’s not stopping and the anxiety I feel rises with every second it continues. That sound, a warning, signals conditions are “right” for a tornado. Even though that siren just started, the ingredients that brought us to this moment were mixed much earlier.

Building a Supercell 

Everything begins in the upper parts of the troposphere, which roughly spans the lowest 6 miles (10 km) of our atmosphere and contains all of the earth’s weather. Thermal winds, as the name implies, were caused by differences in temperature. However, contrary to the name a thermal wind is not really a wind. There’s an old saying, “You can’t fly a kite in thermal wind.” Technically, it’s the difference in wind between two levels of the atmosphere. Colder air is denser than warmer air. This creates differences in thickness values, the vertical distance between two pressure levels. Think of a sandwich under a brick. The differences in the thickness values from cold to warm create a pressure gradient generating the speedier winds

Thermal wind applied B. Geerts and E. Linacre http://www-das.uwyo.edu/~geerts/cwx/notes/chap12/thermal_wind.html

Thermal wind applied B. Geerts and E. Linacre http://www-das.uwyo.edu/~geerts/cwx/notes/chap12/thermal_wind.html

Winds in the upper troposphere are now moving faster than wind closer to the ground. This creates vertical wind shear, more simply put is a change in wind speed or wind direction with height. Much like a paddle wheel, this wind shear generates horizontal rotation. This is the rotation of the tornado in its very infancy but much more is needed for birth. 

National Weather Service http://www.srh.noaa.gov/jetstream/tstorms/windshear.htm

National Weather Service http://www.srh.noaa.gov/jetstream/tstorms/windshear.htm

The second ingredient-rotating updraft

For the next step, this horizontal rotation needs to become vertical. In the middle troposphere, westerly winds are transporting a cool, dry air mass over the warm moist air coming from Gulf of Mexico. The overlap of these two air masses creates instability. The hot air wants to rise because it’s less dense. The energy of the rising air can be measured as convective available potential energy (CAPE), the total amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere. Higher CAPE values indicate more energy. This updraft can tilt the horizontal rotation into vertical rotation. 

However, a cap of warmer air prevents this. This layer of air originated in the desert southwest. Some of this Southwestern air gets lofted. The warm dry air eventually rides over the warm moist air from the Gulf of Mexico. Now warm, moist air from the Gulf is beneath the warm, dry air from the desert Southwest. The cap is stable and prevents the updrafts from penetrating very high into the atmosphere, but as the day progresses, the conditions change. By mid to late afternoon, during peak heating, the rising air from the surface layer of air is warmer than the cap. The cap is broken and air can now ascend several miles into the sky. A thunderstorm with a rotating updraft, a supercell, has now developed. 

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Supercell.svgTurning a Supercell into a Torndao

Not all supercells produce tornadoes. Indeed, 70% of the time a tornado warning is issued no tornado actually forms.  The when, where, and how of which storm will make tornado is one of the biggest questions in tornado science. Scientists hypothesize the key to tornadogenesis is a downdraft occurring on the backside of the storm, the rear-flank downdraft (RFD). This downdraft forms as some of the falling precipitation wraps around the updraft producing a characteristic hook echo on radar. The RFD brings rotation from aloft to the ground. The RFD can even generate additional horizontal rotation that can be titled into the vertical rotation. This results in a region of broad rotation at the surface, but a tornado is defined as a “violently rotating column of air in the contact with the ground.” As the air from the RFD converges beneath the updraft, it can be ingested by the updraft, amplifying its rotation exponentially. Think of the age-old physics example of a figure skater bringing his or her arms closer to increase the speed of his or her spin. The air is more likely to enter the updraft if it is relatively warm, since warmer air is less dense than cold air. With the third and final ingredient the warning has become reality.

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 Special thanks to Gabe Garfield (@WxGabe), research meteorologist for the National Weather Service,  and Jeff Frame (@VORTEXJeff), Assistant Professor of Atmospheric Sciences at University of Illinois, for both teaching me about tornadoes but providing feedback on this post.

Featured image above from Daniel Rodriguez on Flickr.   El Reno EF-5 Tornado taken from just north of Banner Rd and 15th Street.