Friday, Apr. 15, 2016

Wilsen "House on a Hill" (5:26), "Sirens" (4:12), Sarah Jaffe "Clementine" (4:59)

Air mass thunderstorm life cycle
The events leading up to the initiation of a summer air mass thunderstorm are summarized in the figure below (p. 151 in the ClassNotes).    It takes some effort and often a good part of the day before a thunderstorm forms.  The air must be lifted to just above the level of free convection (the dotted line at middle left in the picture).  Once air is lifted above the level of free convection it finds itself warmer and less dense that the air around it and floats upward on its own.  I've tried to show this with colors below.  Cool colors below the level of free convection because the air in the lifted parcel is colder and denser than its surroundings.  Warm colors above the dotted line indicate parcel air that is warmer and less dense than the surroundings.  Once the parcel is lifted above the level of free convection it becomes buoyant; this is the moment at which the air mass thunderstorm begins. 




Once an air mass thunderstorm gets above the level of free convection it goes through a 3-stage life cycle




In the first stage you would only find updrafts inside the cloud (that's all you need to know about this stage, you don't even need to remember the name of the stage).

Once precipitation has formed and grown to a certain size, it will begin to fall and drag air downward with it.  This is the beginning of the mature stage where you find both an updraft and a downdraft inside the cloud.  The falling precipitation will also pull in dry air from outside the thunderstorm (this is called entrainment).  Precipitation will mix with this drier air and evaporate.  The evaporation will strengthen the downdraft (the evaporation cools the air and makes it denser).  The thunderstorm is strongest in the mature stage.  This is when the heaviest rain, hail, strongest winds, and most of the lightning occur.

Eventually the downdraft spreads horizontally throughout the inside of the cloud and begins to interfere with the updraft.  This marks the beginning of the end for this thunderstorm. 


The downdraft eventually fills the interior of the cloud.  In this dissipating stage you would only find weak downdrafts throughout the cloud.

Note how the winds from one thunderstorm can cause a region of convergence on one side of the original storm and can lead to the development of new storms.  Preexisting winds refers to winds that were blowing before the thunderstorm formed.  Convergence between the preexisting winds and the thunderstorm downdraft winds creates rising air that can initiate a new thunderstorm.

Here's a sketch of 4 thunderstorm clouds, what information could you add to each picture.




You should be able to say something about the first three.  The 4th cloud might be a bit of a puzzle.  You'll find the answer at the end of today's notes.
Gust fronts and the dust storms (haboobs) they can produce
The picture below shows some of the features at the base of a thunderstorm.



 The cold downdraft air spilling out of a thunderstorm hits the ground and begins to move outward from underneath the thunderstorm.  The leading edge of this outward moving air is called a gust front.  You can think of it as a dust front because the gust front winds often stir up a lot of dust here in the desert southwest (see below).  There are several very nice examples in Mike Olbinski's Storm Gallery.



The gust front in this picture (taken near Winslow, Az) is moving from the right to the left.  Visibility in the dust cloud can drop to near zero which makes this a serious hazard to automobile traffic.  Dust storms like this are sometimes called "haboobs".


There's lots of video on YouTube of an impressive dust storm a few summers ago.  Here's an example from Gilbert Arizona (July 5, 2011).  You can see day literally turn to night when the dust cloud is overhead (start about 1:20).  Here's another video of the same storm from a different location (South Mountain).  There are also several time lapse videos on Mike Olbinski's Time Lapse Gallery page.

Here's a video from a summer 2012 dust storm captured from the front window of a vehicle that drove through the storm.  Check the last minute or two of the video where visibility drops to near zero (about 9:00 minutes into the video).  Officials recommend that you drive off the highway under conditions like this, turn off your lights, and take your foot off the brake so that your brake lights are not on (otherwise someone might follow your lights thinking you're still on the highway and run into you from behind).

Microbursts


Thunderstorm downdraft winds can be a serious hazard whether they stir up dust or not.

A narrow intense thunderstorm downdraft is called a microburst.  At the ground microburst winds will sometimes reach 100 MPH or more (over a limited area).  As we will see most tornadoes have winds of 100 MPH or less.  Microburst winds can damage homes (especially mobile homes that aren't tied to the ground), uproot trees, and seem to blow over a line of electric power poles at some point every summer in Tucson.  Wind damage from a microburst is often incorrectly attributed to a tornado.

Microbursts are a serious threat to aircraft especially when they are close to the ground during landing or takeoff.  Microburst associated wind shear was largely responsible for the crash of Delta Airlines Flight 191 while landing at the Dallas Fort Worth airport on Aug. 2, 1985 (caution some of the links at the end of the article contain audio of actual cockpit communications).


Falling rain could warn of a wet microburst (see photo below). In other cases, dangerous dry microburst winds might be invisible (the virga, evaporating rain, will cool the air, make the air more dense, and strengthen the downdraft winds).






Here are a couple of microburst videos.  The first video was taken in the heavy rain and strong winds under a thunderstorm in the microburst.  You'll see a power pole snapped in half by the microburst winds at about 2:26 into the video.  We'll look at portions of a 2nd longer video in class.  It was taken in or near San Tan, Arizona.  The microburst doesn't look too impressive at the start of the footage but the storm winds soon get pretty violent (at about the 3:15 point in the video) and winds were strong enough to break the trunks of at least one tree (6:45 into the video).  It is probably more common to see recently planted trees uprooted.

Severe thunderstorms, wind shear, mesocyclones, and wall clouds

Next I wanted to look at some of the conditions that can lead to severe thunderstorm formation and some of the characteristics of these storms.  Severe thunderstorms last longer, grow bigger, and become stronger than ordinary air mass thunderstorms.  They can also produce tornadoes.


Severe storms are more likely to form when there is vertical wind shear (the picture above is on p. 154a in the ClassNotes).  Wind shear (Point 1) is changing wind direction and/or wind speed with distance.  In the case shown above, the wind speed is increasing with increasing altitude, this is vertical wind shear.

A thunderstorm that forms in this kind of an environment will move at an average of the speeds at the top and bottom of the cloud (Point 2).  The thunderstorm will move to the right more rapidly than the air at the ground which is where the updraft begins.  Rising air that is situated at the front bottom edge of the thunderstorm will find itself at the back edge of the storm when it reaches the top of the cloud. 

This produces a tilted updraft (Point 3).  The downdraft is situated at the back of the ground.  The updraft is continually moving to the right and staying away from the downdraft.  The updraft and downdraft coexist and do not "get in each others way."  If you remember in air mass thunderstorms, the downdraft gets in the way of the updraft and leads to dissipation of the storm.

Sometimes the tilted updraft will begin to rotate.  A rotating updraft is called a mesocyclone (Point 4).  Meso refers to medium size (thunderstorm size) and cyclone means winds spinning around low pressure (tornadoes are sometimes called cyclones).  Low pressure in the core of the mesocyclone creates an inward pointing pressure gradient force needed to keep the updraft winds spinning in circular path.

The cloud that extends below the cloud base and surrounds the mesocyclone is called a wall cloud (Point 5).  The largest and strongest tornadoes will generally come from the wall cloud. 

Note (Point 6) that a tilted updraft also provides a way of keeping growing hailstones inside the cloud.  Hailstones get carried up toward the top of the cloud where they begin to fall.  But they then fall back into the strong core of the updraft and get carried back up toward the top of the cloud.

Here is a link to an exceptional time lapse video of a supercell thunderstorm in Texas (from Mike Olbinski).  Find the Supercell near Booker, Tx frame near the bottom of the page.  In time lapse you can see the rotation of the wall cloud.  If you watch closely you'll see another interesting feature: moisture from air in the downdraft that reaches the ground is drawn into the thunderstorm updraft (starting at about 0:38 in the video up until the end of the first segment of video).  As the downdraft air moves upward cloud begins to form. 

It is worth trying to understand why the wall cloud surrounds the mesocyclone and why it extends below the rest of the cloud.

Clouds form when air rises, expands, and cools as shown above at left.  The rising air expands because it is moving into lower pressure surroundings at higher altitude.  Only when the air has risen high enough, moved into low enough pressure, expanded and cooled enough will a cloud form.  Just for the purposes of illustration we'll assume that once air has traveled from the ground to 900 mb pressure it will have expanded and cooled enough for a cloud to form.

Air in the center of the rotating updraft has a little lower pressure than the air surrounding it at the same altitude.  I've assumed that the pressure in the middle of the mesocyclone at cloud base altitude is 890 mb.  In this part of the picture 900 mb pressure is found a little bit closer to the ground.  Thus air that rises into the rotating updraft doesn't have to go as high before it encounters 900 mb pressure and has expanded and cooled enough to form a cloud.

Here's the answer to the question about thunderstorm features embedded in today's notes: