Interesting Weather Information

Wednesday, July 26, 2017

Radar Meteorology: VIL (Vertically Integrated Liquid) and How To/Not To Use It

What is VIL?

Vertically Integrated Liquid (VIL) is the total amount of rain that would fall if all the liquid water in a column inside a rain cloud (usually a thunderstorm) would be brought  to the surface. Basically just sum up (integrate) all the water in a column.

VIL is not the same as PW (precipitable water). PW is a measure of the total water vapor in a column that could be converted to rain.

The units for VIL  are kg/m-2 and may seem to be odd at first glance. Why are we are talking  about a 3D cloud volume but measuring water in kilograms per square meter? It makes a great deal of sense if you consider this:




  • A cube of water, siting on the ground 1 meter on a side (a cubic meter) weighs 1000 kg (2,200 pounds or a metric ton).
  • The depth of the water over the square meter of land covered is 1 meter or 39.37".
  • So a VIL of 1000  kg/m-2 is the same as 1 meter of rain or in English units 39.37".
  • VIL values do not reach 1000  kg/m-2 , this is just a reference value I use to explain the concept.


Now we can convert VIL values to inches of rain using this formula:


[VIL/1000] *39.37 = Inches of rain 

Units Check:

[[kg m-2/kg m-3] *[in m-1]] = inches

or the simplified version

VIL * 0.03937 = Inches of Rain

Example: VIL = 79.5 kg/m-2


79.5 kg/m-2 * 0.03937 = 3.13" of rain
We will re-visit this value shortly.

Liquid Water Does Not Float in the Atmosphere!


In fact neither do clouds. Sorry to burst your romantic bubble poetry fans, William Wordsworth was dead wrong when he penned the following:

"I wandered lonely as a cloud 
That floats on high o'er vales and hills, 
When all at once I saw a crowd, 
A host, of golden daffodils; 
Beside the lake, beneath the trees, 
Fluttering and dancing in the breeze." 

I Wandered Lonely As A Cloud
-William Wordsworth (1770 - 1850)

As a side note I do not think William Wordsworth would mind that I called him out, he also said:

Come forth into the light of things, let nature be your teacher. - William Wordsworth


OK! Enough English Romanticism, let's get back to radar meteorology.

Because raindrops, cloud drops, hailstones, and dust - all off which are found in a cloud - do not float in the atmosphere something must hold them up.

That something is the force of the updraft. Even in non-thunderstorm clouds cloud materials are held aloft by updrafts. The stronger the updraft the more liquid water than can remain aloft and the higher the VIL.

Strong thunderstorm updrafts also support large hail. 

But ... Does a large VIL value indicate large hail?


VIL - What Is It Good For?

For years VIL was touted as having great potential for forecasting hail size. Large VIL values meant a strong updraft and that's where large hail is found.

The truth is VIL is a very poor indicator of hail size for a number of reasons:

Problem #1

VIL is calculated using reflectivity and anything that has an effect on reflectivity will affect VIL. One example is that because VIL was intended to measure liquid water only - i.e. ice contributions to VIL are eliminated - reflectivity is capped at 56.5dBz. Any return greater that that is reduced back to 56.5dBz resulting in potential error when no ice is present but water content of an updraft is high.

Problem #2
VIL calculations show a raindrop-size bias. In the example both cubes have the same reflectivity (Z). The left cube has 381 mm3 of water while the right cube has 14.1 mm3 of water. But because the two volumes yield the same reflectivity, VIL calculated for the left cube is the same as VIL for the right cube.


Problem #3
A thunderstorm too close to the radar can be partially obscured by the "cone of silence". Because part of the updraft is not sampled, VIL may be underestimated.

Problem #4
The VIL of a thunderstorm far from the radar can be exaggerated or VIL can be underestimated  Because the alltitude of the radar beam increases as it travel.s away from the antenna, the beam may overshoot the storm and under estimate VIL or  pass right through a mid level hail core and overestimate VIL.

Problem #5
The upper reaches of the updraft core in tilted thunderstorms may extend out of the sampling  volumeinto a neighboring grid box resulting in an underestimate of VIL.

Problem #6
A fast moving storm may move so much during the time it takes for an entire volume scan to be completed by a NEXRAD Doppler Radar the updraft may move out of the sampled cell or grid volume resulting in an underestimate of VIL

Problem #7
The VIL equation assumes all reflectivity is from liquid water in the thunderstorm. When hail is present the value of VIL is too high because of the high reflectivity of hail, not because of high liquid water content. Even small hail is bigger than large rain drops. Hail is also solid and when water coated it backscatters much more energy than raindrops, increasing reflectivity and therefore VIL.

Problem #8
VIL is higher for wet hail than dry hail.  Both are higher than raindrops.

Problem #9
VIL has air mass and seasonal dependencies. 

The wet bulb is the temperature TO WHICH air can be cooled by evaporating water into a column of air.  The web bulb zero level is the altitude where the air can be cooled to freezing by evaporation and it represents the highest level in a storm where hail begins to melt on the way down assuming the air has reached equilibrium (RH 100%) through evaporative cooling. 

Below the wet bulb zero level the air remains warmer than freezing even when large numbers of rain drops evaporate and cool the air. So hail melts as it falls. 


When the wet bulb zero height is low (7000' for example)  the radar beam samples more hail or other frozen targets than when the wet bulb zero height is high (14,000' for example). 

VIL can be overestimated in the case of a low wet bulb zero air mass and therefore hail size is predicted to be too large. Just the opposite is true with a high wet bulb zero level. Of course changing distance from the radar is a complicating factor.

A very high wet bulb zero level means hail can melt for a long time on the way to the surface and be significantly smaller when it lands.

SO ....

VIL is only good as a crude, first estimate of hail or hail size.  When used with dualpol products VIL is more effective.


HOWEVER ....


VIL is a good indicator of updraft strength.


AND


A rapid decrease of VIL may indicate a collapsing thunderstorm and the onset of a wet microburst.


FINALLY -  VIL calculations are most accurate when a storm is a moderate distance from the radar, moving slowly, the updraft is vertical (not tilted) and the freezing level is high.


It just so happens that during the evening of Friday 21 JUL 2017, centered on 8:10PM EDT (00:10z 22 JUL 2017) just such a storm formed south of Cincinnati near Walton, KY.


Case Study: 

High VIL Thunderstorm 21-22 July 2017

The cell we are talking about formed just to the northwest of Walton, KY moving towards the southeast at 5 mph, according to @NWSILN's severe thunderstorm warning.

From  the KILN sounding 00Z 22 JUL 2017

Freezing Level 16,000'
Wet Bulb Zero Level 14,750'






Tilt 1 0.5° Beam Center Altitude at Walton 4054' Temperature 21.9°C




Tilt 2 0.8° Beam Center Altitude at Walton 6144' Temperature 17.8°C


Tilt 3 1.2° Beam Center Altitude at Walton 8480' Temperature 13.9°


Tilt 4 1.7° Beam Center Altitude at Walton 10792' Temperature 7.6°C














Monday, July 24, 2017

Tornado Warning 2017.07.07 by @NWSILN - No Tornado but a Good Warning Part III

PART III - 3D Views

3d view of the supercell from the southeast at 21:17:26z. All reflectivity values >+ 52.5 dBz in solid red represent the core of the storm and the heavy rain. Lower reflectivity values are translucent to show general storm structure and higher values are not visible. KILN Radar using GR2Analyst.

 The Bounded Weak Echo Region (BWER) is clearly visible. This is where there is little or no rain due to the warm, moist inflow and the intense, rapid updraft. The rain drops cannot fall through the updraft because it exceeds their terminal velocity. The BWER was formerly called the vault or echo free vault.

The inflow and updraft. The inflow not only transports warm, humid air into the storm but it imports rotation gained from the environment caused by wind shear. The mechanism is partly explained in Part II and more fully explained here:  http://stevehorstmeyer.blogspot.com/2013/03/thunderstorm-primer-part-6-supercell.html

I have now added NROT (normalized rotation we call it N-ROT) to the image. Normalized Rotation was originally developed at SPC and operationalized by Gibson Ridge Software in GR2Analyst. I have found it a remarkably accurate and useful tool in pinpointing tornadoes, even small leading edge spin-ups. This confirms the updraft is rotating.


The final overlay is Spectrum Width which can be roughly understood by calling it turbulence. Imagine a wind blowing in a constant direction and at a constant velocity - that is no gustiness - that ideal wind has a spectrum width value of ZERO. In other words it is not gusty. Now imagine a turbulent tornado or severe thunderstorm updraft environment. Winds are changing direction and gusting violently. In other words the environment is turbulent and the spectrum width is high.  Spectrum Width measures the spread or variation of wind speeds and directions. Above, red is very turbulent and notice how it is in the same location as the updraft and rotation.  Spectrum width is a useful tool when trying to locate tornadoes and severe thunderstorm winds and identifying strong updrafts.

The 3D volumetric time lapse shows the evolution of the supercell and how spectrum width and NROT coincide with the rotating updraft.

Thursday, July 13, 2017

Tornado Warning 2017.07.07 by @NWSILN - No Tornado but a Good Warning Part II

PART II:
An Outflow Boundary, An Inflow Jet, and Spin Up

In Part I of this multi-part post I showed you what radar and @NWSILN saw Friday evening 2017.07.07 that lead up to the tornado warning they issued at 5:18 PM EDT (21:18 UTC).

Now we want to look how the spin-up occurred.

The Past as a Key to the Future
 At 3:36 PM EDT (17:36 UTC) KILN Doppler Radar showed an outflow boundary north of the Ohio River stretching from southeast Ohio, westward into southeast Indiana south of Connersville. In the second image the boundary is just north of the line I drew. Notice how you can see it taper off south of Connersville.




The video shows the movement of the outflow. Over southern Ohio it is pushing south while to the west from Finneytown to south of Connersville it is stationary.



An outflow boundary (aka gust front) is a mini front. We mostly think of them as mini cold fronts, the leading edge of air, cooled by evaporation in the downdraft moving away from the thunderstorm.  It is that cool air blast you have often felt before the thunderstorm downpour arrives.

In my experience doing point nowcasting, outflow boundaries can become stationary or stop and turn around. Thunderstorms born from the interaction of inflow jets and outflow boundaries can be tough to deal with when rain threatens an outdoor event.

Outflow boundaries often cause thunderstorms system to strengthen by acting as a source of lift. Warm moist air flowing towards the thunderstorm glide up and over the cold air advancing away from the storm. 

Thunderstorms can be boosted from routine to severe as they encounter outflow boundaries.

Outflow boundaries can also help create rotation in the environment surrounding a thunderstorm. When air is transported into the thunderstorm  and tilted to nearly vertical in the updraft, the rotation gained when inflow interacts with an outflow boundary can add enough spin to the total and help create a tornado.

The images below should give you a basic idea of how it works.




The animation below shows the specifics for the tornado warning of 7July2017.


Finally the last two animations: Watch the shower develop south of Connersville, develop explosively as the inflow jet pushes it into the outflow, merge with the main cell and the velocity couplet (right  panel) form north of Oxford.


 Below: Following the couplet


So it looks like all the elements were there and radar indicated spinup, eventhough there was no tornado in my book this is a good warning.

NEXT: PART III - 3D Views of the storm.

Tuesday, July 11, 2017

Tornado Warning 2017.07.07 by @NWSILN - No Tornado but a Good Warning Part I

PART I:
What @NWSILN Saw on Radar that Lead to the Warning

The Grey Zone
In the grey zone of meteorological knowledge is that shadowy region between rotation in a thunderstorm and damage on the ground thousands of feet below when a tornado touches down.

Thanks to doppler radar we can clearly see rotation in thunderstorms. Thanks to intense research efforts we now know that the rotation for a tornado is mostly imported to the storm, drawn in with the inflow and tilted to nearly vertical by the updraft. As the inflow is stretched by upwards acceleration in the updraft the rotation rate increases. Spin-up is taking place.

Think of the ice skater.

VoilĂ ! We can now forecast and warn for tornadoes.

Not so fast.  Between cloud base and the ground, in the grey zone, something happens. What it is and how it works is masked by a fog created by an environment too dangerous to make observations, by the small scale of the funnel and by the capricious nature of tornado occurrence.

A Weak Link
We have a weak link in our cascade of knowledge between rotation in the storm and rotation in the environment leading to a tornado on the ground.

That weak link is one reason there are so many false alarms -  tornado warnings without a tornado.

Friday evening, 2017.07.07 was one of those false alarms. But not all false alarms are equal.

A number of tornado warnings have been issued over the years by @NWSILN that have made me cringe. The polite version of my initial comment would be, "Just what are they thinking?" You can probably guess what the gritty version contains.

The tornado warning of 2017.07.07 was not one of those. I was a good but an un-verified warning. Good, because all the elements were there for a tornado touchdown.

Radar Views from Friday Evening 2017.07.07
Take a look at KILN Super-Res Velocity at 5:25PM EDT (21:25 UTC) 2017.07.07. below.




Doppler radar can only measure the component of the wind directly towards or away from the radar which is along a radial like the long arrow.  It does not measure the true wind only the "radial" part of the wind.

At 5:25 PM EDT KILN saw a tight velocity couplet between Oxford and Germantown. Red indicates wind blowing away from the radar or positive velocity. Green indicates winds blowing towards the radar - negative velocity.

Here are some closeup screen grabs.

On the left is reflectivity - precipitation  rate, on the right radial velocity. Remember the true velocity is higher because radar is looking only at the wind component along the radar pulse radial path.





Velocity values peaked a bit higher about 10 minutes before these images.

This is a tight velocity couplet and could indicate enough spin-up for a tornado to touch down. In addition the couplet weakened and strengthened several times, indicating here was plenty of energy and rotation and the couplet was not yet dying. In this case I feel the warning was a good one.

Watch the video to see the velocity couplet form, move and dissipate.




Coming Soon:
Part II - The Meteorology of the Spin-up
Part III - 3D Radar Views





Monday, July 10, 2017

Why I Critisize @NWSILN

Over the years I have not hesitated to be very critical of the National Weather Service Forecast Office in Wilmington, OH.  I will continue to speak out when there is something the public needs to know. After all NWS works, indirectly, for you and you have the right to know how your tax dollars are being spent.

Most recently I discussed what seems to me to be an over-the-top approach to Areal Flood Advisories (AFA). The number issued by @NWSILN is way more, in fact 261% more through July 7, 2017 at 10:10 AM than any other NWSFO in the region. That is the subject of my previous blog post dated July 6, 2017.

A local Cincinnati TV meteorologist suggested that I just ignore the huge number of AFAs because they are not "life threatening". He scolded me for calling attention to @NWSILN and questioning the unusually large number of AFAs.

There is no benefit from his head-in-the sand approach to dealing with real issues that impact public safety. Frankly I was surprised at the naive suggestion by the meteorologist and his reluctance to use his authority to better inform the public.

He is correct that Areal Flood Advisories do not represent immediate life threatening situations. They are meant to call attention to potentially life-threatening situations and are therefore a "heads up".

However, this well meaning meteorologist seems to have forgotten about the "cry wolf effect".

Image Courtesy of: http://www.insurancejournal.com
The public is now barraged with a confusing assemblage  of advisories, watches and warnings leading to information overload.  False Alarm Rates are high  because of the very nature of weather events, the limited availability of real-time data and the fact that all of us involved in informing the public are human and we do not always get it right.

Include in the mix the "social media/app effect" and an individual may get a single warning multiple times.

After a while it all becomes blah-blah-blah.

My point is that modern communications technology may very well multiply the "cry wolf effect".

Two recent studies find that the "cry wolf effect" (links below) is real and false alarms reduce the number of people who take action during threatening weather. Both studies also find that the issue is complex and a small reduction in the FALSE ALARM RATE has little effect.

Reading between the lines: my thought is that large reductions in the FALSE ALARM RATE may reduce inaction by weather information users. But for the reasons above that may not be possible.

What the authors did find is a clarification statement like, "there is a 90% chance of flash flooding ..." decreases the number of people who ignore potentially life-saving weather information.

Before you go apoplectic, I know there are problems with probability statements and some other form of qualifier may work better.

Here's is another thought.

Instead of blasting the public, every time it looks like a funnel may touch down due to rotation seen by radar, with "tornado warning .... take cover immediately"; Why not qualify the warning.  with something like, "Tornado warning ... radar indicates a small funnel may touch down with winds possible to 90 mph... take cover immediately".

Meteorologists know the difference between the leading edge spin-ups and the monster funnels born from a strong mesocyclone. Isn't the public deserving of the whole truth?

Luckily, most tornadoes are F/EF0s and F/EF1s.  That's is important because in the county warning area under the responsibility of @NWSILN, since 1950 there have been no deaths - ZERO -  from F/EF0 and F/EF1 tornadoes. (Some sources report a single death in Grant Co. from an F1 but it is unconfirmed).

Current initial NWS tornado warnings treat a weak EF0 funnel as being equal to devastating, deadly, much stronger, longer-lived mesocyclone tornadoes. (Note this is not an @NWSILN policy but a policy set up for the entire NWS).

Here is an example of a "tornado"  that was 10 yards wide and on the ground for 20 yards (10 yards/20 yards - you read it correctly) on May 1, 2012 west of Lebanon, OH. Here is the link:

https://www.weather.gov/iln/20120501_WLebanon/


We all remember the EF4 tornado that struck Piner, KY and the EF3 that struck Moscow, OH on March 2, 2012.

The initial warnings for these killers treated them no differently than the 10/20 whirlwind linked to above.

I am really just trying to help viewers make an informed decision by calling @NWSILN out when I see something that is not in the public interest.

I hold nothing against any @NWSILN meteorologist. I am sure they arrive at work each day ready and willing to do the best they can.

 If I am willing to openly criticize @NWSILN, however, I have an obligation to point out events when they perform well.

Friday July 7, 2017 @NWSILN issued a tornado warning but there was no tornado. Nevertheless they performed well.  I explain that in my next blog post later this week.

____
____

Here are links to the two academic articles concerning the cry wolf effect. They are full of research jargon.

https://www.stat.berkeley.edu/~aldous/157/Papers/leclerk.pdf

https://www.utdallas.edu/~emk120030/crywolf_post.pdf



Here is where you can read a summary article meant for the general public:

http://www.insurancejournal.com/news/national/2015/01/28/355667.htm




Thursday, July 6, 2017

Why Does the NWS Forecast Office in Wilmington, OH Issue So Many Areal Flood Advisories?


Below is the number of Areal Flood Advisories issued by NWS Wilmington, OH per year since 2008. Notice the high False Alarm Rate and the great increase through the end of June 2017.  Why the increase? I do not know.






It seems the False Alarm Rate is independent of the number of Areal Flood Advisories issued. My interpretation of this is too high a reliance on automatic algorithms and too low a level of critical human thinking.




I received justified criticism because how do we know  NWS ILN is over issuing Areal Flood Advisories if we do not know how other NWS offices compare. Take a look below.



Then I received one more criticism of my claim - again justified AND a good example of how honest, professional discourse can move the understanding of an issue along.

The individual asked if all NWS County Warning and Forecast Areas (CWFA) were the same size.  They are not.

So the graph below normalizes the number of Areal Flood Advisories by area.

[(Advisories issued) / (CWFA area sq. mi.)]  x 10000 = Number Issued per 10,000 sq. mi



NWS ILN is so far out in front here that it makes me wonder if there is a problem with the data source.

But the other 19 offices are consistent with each other in the number of Areal Flood Advisories issued and with terrain considerations.

For example, Jackson, KY and Morristown, (Knoxville area), TN have a great deal of steep terrain which increases runoff and the chance of flooding.  Northern Indiana is flat and not so prone to flooding so you would expect the first two NWS offices to issue more Areal Flood Advisories per 10k sq. mi.than Northern Indiana.

 Notes:
An Areal Flood Advisory verifies if there is at least one Local Storm Report of flooding, no matter how minor, within or on the border of the defining polygon. If there is a Local Storm Report of flooding outside the polygon, no matter how close it does not count for verification.

Advisory Data: Iowa State University
Iowa Environmental Mesonet, VTEC Browser

NWS Office  CWFA Areal Coverage:
in  SQ MI from NWS Service Manual 10-507
NOV 10, 2009


Friday, June 16, 2017

Tornado Database Update from SPC

NOAA's Storm Prediction Center has updated the tornado database and all tornadoes from 1950 through 2016 are included.

Below are maps of all tornadoes, without comments I plotted from the data for the Cincinnati area.
All maps plotted using the free, open source MapWindows GIS program and U.S. Taxpayer data from NOAA's Storm Prediction Center.

Click on any map for a larger version.

National and Regional Maps



OHIO MAPS






KENTUCKY MAPS




 INDIANA MAPS






Thursday, May 25, 2017



So what does James Whitcomb Riley, the great poet and author from Indiana have to do with the storms of March 24, 2017?

Riley, known as the "Hoosier Poet" and the "Children's Poet" is also credited with the following witticism, “When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.”

Thus was born the "Duck Test", finding the simplest explanation for something you observe. Unfortunately the Duck Test often fails for complex systems like clouds associated with thunderstorms.

Let's re-phrase Riley's saying, When I see a cloud that moves like a tornado, touches the ground like a tornado and comes from a thunderstorm like a tornado, I call that cloud a tornado.

The problem is ... is that there are many low clouds that at first glance look like a tornado.

RAIN SHAFTS

Take a look at this photo from NOAA.




This is a rain shaft, an intense downpour from a thunderstorm in the Great Plains. We see rain shafts here too. Take a look at these 3 photos from FOX19 Viewers.

Mary Campbell - Rain Shaft

Kelly Sears - Rain Shaft

Photographer Not Known - Rain Shaft

 Notice there are no horizontal striations or other indications of rotation. Rain shafts are often reported as tornadoes but they do not rotate and are not tornadoes.

SCUD CLOUDS

How about this photo?
Scud clouds - Courtesy Washington Post.

Nope, not a tornado. It looks like one but there is no rotation, these are scud clouds.

Scud clouds can be very tornado-like. Clouds material gets caught up in the turbulence and down draft of a thunderstorm and can masquerade as a tornado.
 Here are three more scud cloud photos:

Courtesy: strange.clouds.org

Courtesy: vk.com


Courtesy: vk.com



This one was taken by FOX19 NOW viewer Kathleen Niece in Glencoe, KY.

Photo by: Kathleen Niece


WALL AND SHELF CLOUDS


This is a scary looking thunderstorm, but there is not a tornado here. The wall cloud is large and low and wall clouds can partially descend to the ground.

Now take a look at these photos, sent to my by viewer Shannon Cross from Warren Co. last evening (WED may 24, 2017). She saved them from the Facebook pages of people in the area around Franklin and Carlisle, OH in Warren Co. and wanted to know if this was a big tornado like some people claimed.






The answer is, "NO", these pictures, of the same storm, do not show a tornado.

Evidence:

  • 1 The rotation as shown on radar was very weak.
  • 2. There are no - zero - damage reports
  • 3. The photographs show us this is not a tornado
  • Photo A  - No rotation evident, ragged edge like scud
  • Photo B - Still no rotation evident, dark back side looks interesting
  • Photo C -  A better look at the back side - the descending cloud formation is hollow
  • Photo D - The air is descending at the back of the cloud, not rising like in a tornado - look at the curl!

Weird clouds like these arise from the interplay of warm moist air flowing up and into the storm and cool, drier air sinking and flowing out of the storm.

Take a look at this amazing cloud  photograph taken by Stephanie West in Hebron, KY on June 15, 2012.



This is a shelf cloud and it forms from the interplay of inflow and out flow as shown in the second version of her photograph. The cool outflow is more dense and the warm, moist inflow rises op and over it. Beneath the warm air clouds descend in the sinking air.

To show how different one cloud can look from different directions, here is the same cloud, at the same time from in front of it near CVG.


Now ... imagine tilting the top photograph - left side up - and it does not take much imagination to get to what was seen in Warren Co.

So ...
It does not move like a tornado (no rotation).
It does not touch the ground like a tornado (no debris and no damage).
It does not come from a thunderstorm like a tornado (descending air not rising.

So it is not a duck. Oops! I mean it is not a tornado. It is a formation of scud clouds.

Thank you James Whitcomb Riley

Steve Horstmeyer