Wednesday, Apr. 10, 2019

Mikayla Jade The Voice Australia 2018 "Dancing on My Own" (1:36),  Maddison McNamara The Voice Australia 2018 "I Will Always Love You" (2:50), Lauren Bannon The Voice UK 2018 "Lean On" (1:49), Cody Frost The Voice UK 2016 "Lay All Your Love on Me" (1:52), Mennel Ibtissem The Voice France 2018 "Hallelujah" (2:34), Nargiz Zakirova The Voice Russia "Still Loving You" (3:38), Courtney Hadwin (Americas Got Talent) "Hard to Handle" (5:13)

We'll be using page 155, and page 156 from the ClassNotes today.

The first part of the Quiz #4 Study Guide is now available even though Quiz #4 is still two weeks away.  I would encourage you to review the material on Newton's 1st law of motion, forces, upper level and surface winds now.  Then use this new Optional Assignment to test your understanding (copies of the assignment will be available in class on Friday).  The Optional Assignment isn't due until Wednesday, Apr. 17.


 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 (http://gallery.mikeolbinski.com/thestormgallery).



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 (link was corrected after class) to automobile traffic.  Dust storms like this are sometimes called "haboobs".

There's lots of dust storm video on YouTube.  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 at about 1:20).  Here's another video of the same storm from a different location (South Mountain).  A July 2018 dust storm was captured on video by storm chaser Reed Timmer.  Time lapse video (00:30 to about 00:50 near the start of the video) shows the approaching dust cloud.  Notice the cloud forming near the top of the gust front.  The storm chasers drive into the dust cloud at about 2:20 and you can see how much the visibility can drop.  There are also several time lapse videos on Mike Olbinski's Time Lapse Gallery page (http://www.mikeolbinski.com/timelapse/#top).

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 (with the downed power lines remaining energized).  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 we'll 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 page 156 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 perhaps the 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 cloud.  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 (http://www.mikeolbinski.com/timelapse/) of a supercell thunderstorm in Texas from Mike Olbinski again.  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 higher up 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.

A similar kind of thing happens in the formation of a tornado cloud.

Weather radar and "hook echoes"

Thunderstorms with rotating updrafts and supercell thunderstorms often have a distinctive radar signature called a hook echo.  This is one of the ways that scientists are now able to better detect and warn of tornadic thunderstorms
We haven't discussed weather radar in this class.  In some ways a radar image of a thunderstorm is like an X-ray photograph of a human body.




An X-ray image of a person doesn't usually show the entire body, often just the bones and skeleton inside.


The radio signals emitted by radar pass through the cloud itself but are reflected by the much larger precipitation particles. The radar keeps track of how long it takes for the emitted signal to travel out to the cloud, be reflected, and return to the radar antenna.  The radar can use this to determine the distance to the storm.  It also knows the direction to the storm and can locate the storm on a map.  The intensity of the reflected signal (the echo) is often color coded.  Red means an intense reflected signal and lots of large precipitation particles.  The edge of the cloud isn't normally seen on the radar signal.  The amount and intensity of the precipitation is sometimes used in Tucson during the summer to issue a severe thunderstorm warning.

A Doppler radar (something we don't usually discuss in class) can detects small shifts in the frequency of the reflected radar signal caused by precipitation moving toward or away from the radar antenna.  This can be used to determine the direction and speed of the wind inside a thunderstorm (and sometimes in a tornado).
Below is an actual radar image with a prominent hook echo.  The hook is evidence of large scale rotation inside a thunderstorm and means the thunderstorm is capable of, and may already be, producing tornadoes.
 



This is the radar image of a thunderstorm that produced a very strong tornado that hit Oklahoma City in May 1999 ( http://www.spc.noaa.gov/faq/tornado/radscel.htm ).  The hook echo is visible near the lower left hand corner of the picture.  Winds in the tornado may have exceeded 300 MPH.  You can read more about this tornado here.  And here is some storm chase video of the tornado.

There were just a few minutes left at this point, so all of the material on Tornadoes Characteristics has been moved to the Friday, Apr. 12 notes.