Dan's Wild Wild Science Journal

It sounds like something right of an episode of Doctor Who. High winds arrive suddenly, and disappear 4 hours later in a flash. That’s just what happened over North Alabama Sunday night. Sonic screw drivers, and a Tardis might help explain it, but I have a pretty good idea, just the same. I can promise that the Daleks had nothing to do with it. (Although I would not put it past the Cybermen).

So what did happen to cause roaring winds of up to 70 mph that knocked down trees and power lines across 5 counties? Simply put, mother nature through a rock in the pond!

Base Reflectivity 11:58 pm Sunday 12 April. From NOAA & Plym. St.U. archive data. High winds developed, on the back edge of the rain shield.

The heavy rain Sunday evening was the key, but you may have noticed that the winds followed, behind the rain. This is the big clue. It’s rare, but sometimes a large band of convection can act a barrier to the winds aloft.

As the wind flows over the thunderstorms, a wave develops in the atmosphere. Just like any wave, it has a high point and a low point. As the wind flows to the lowest point, it can reach the surface. This can bring the strong winds aloft, down to the ground.

John McLaughlin of KCCI TV in Des Moines Iowa captured a gravity wave event on his stations tower cams, a few years ago. Take a look. You can clearly see the waves.

There is growing interest in understanding these events, and in the operational forecasting of them.

Here is what we do know.

They tend to occur, when there is a stable layer of air near the ground, and unstable air aloft. Meteorologists call this elevated convection. IF, the depth of the stable layer is just right, then a wave can set up, that accelerates winds, in this stable layer, toward the surface.

Clues to an event include a jet stream pattern, with a trough to the west and a ridge to the east. A warm front, is usually located to the south of the area, affected by the gravity wave. A Wake low also develops behind the rain bands. The change in pressure, tends to track with the increase in winds.

So did we see that here Sunday night??
You better believe it.

Surface Map from 2:15 AM Monday 13 April 2009. The warm front is south of the Tennessee Valley. The dark red, is the cold cloud tops, associated with the rain bands.

Still, these conditions happen frequently, and they do not all bring 100 km/hr winds! Understanding, is the first step in forecasting them though.

You can find out more here.

For fellow nerds:

This is quite good.

It’s been a crazy Spring so far. It may not be over yet!

UPDATE: Dr. Tim Coleman is the expert on these events and has written a guest post about this event here. Check it out!

Further update on Monday April 20:
The local Metr. community is still discussing this event, and an analysis by the Huntsville NWS office has some interesting information. The winds peaked before the lowest pressures were recorded. This seems to indicate that this was primarily a wake low event, rather than a gravity wave. As I wrote above, these things are still poorly understood, and believe me, this is the kind of thing that gets we weather nerds very excited.

Later,
Dan

300 millibar chart for 00z Monday 13 April. (Jet stream winds for 7pm Sunday night. The spreading (diffluence) of the winds shows up clearly with a trough West, and ridge to the East of Alabama.

Pressure Trace (Red) and Average Wind Speed (Blue) from our Monte Sano Weather net station. Notice the winds increase as the pressure drops rapidly. The event begins around 10 PM, and ends by 3 AM.

The email tonight (2000+ in my in box, since last Friday), brought this stunning image, of the Marshall/Dekalb County tornado, on Friday. The tornado was snapped, as it crossed Lake Guntersville. Thanks to Martha Tellefsen, for taking such an incredible picture.

The tornado was on the ground for 28 miles, from 3:07pm, until around 3:40PM. It reached 900 meters wide at times.

Later,

Dan

Good Friday 2009 will long be remembered in Alabama and Tennessee. Huge hailstones, and violent tornadoes pounded the region. Only two deaths, and some 50 injuries, but a lot of destruction. It certainly could have been worse, and to families that lost loved ones and homes, it is a tragedy that will be remembered for a life time.

Many times, when severe storms hit, the only view of hail or tornadoes for most people, is on TV. Not this time. The news room was inundated with calls from viewers, watching large hail stones dent their car, and ruin their roofs. Local car dealers here have over 300 damaged cars.

Modern technology is still not good enough to give reliable advance warning of small EF 0 and EF 1 tornadoes. (For those reading from other countries, the U.S. rates tornadoes on the Enhanced Fujita scale. This scale runs from zero to five. Five is very bad.) The large killer tornadoes are forecasted much better now than in the past. I warned viewers Thursday night that we faced such a threat, and showed on a map, where the threat was highest. We made the warning the lead story in our newscast. This kind of detailed forecast was impossible 25 years ago. Science has learned a lot.

Everyone in the path of the tornadoes had several minutes of warning Friday. I would like to think that this explains the low numbers of deaths, and injuries.

Looks like "Cricket ball" size to me.

We have had hundreds of images of the hail and the storms, along with a bunch of questions about the event directed to the Meteorologist. Since that’s me, I spent the day refreshing my memory on the science of hail, and wind. What follows are some answers. I have also written to one of the top experts in the field of hail research, so another post is possible soon.

A lot of the hail on Friday was the size of golf balls, but many reports of tennis ball, and even softball sized hail were reported. Viewers sent me pictures to back up the reports. They sent a LOT of pictures. These images allowed forecasters like me to better predict the size of the hail, in areas further East.

The super-cell storms are constantly being measured, as the tracked across Alabama and Tennessee. One of these measurements is called VIL. It stands for Vertically Integrated Liquid. The VIL is an estimate of the total liquid water in column through the storm.

If you divide the VIL of a storm, by the height of the storm, you get a number called the VIL density. This number tends to be a good measure of hail size. It’s not perfect, and there are other factors that can affect how large the hailstones that reach the ground are. One number we look at is called the “Wet Bulb Zero” height. It tells us how close to the ground the hail stones will start to melt. Detailed info on VIL, and VIL density is available for my fellow nerds here.

When viewers start sending me pictures of golf ball sized hail, and I know the storm has a VIL density of say 4, then I can be quite certain that other storms that day with a vil density of 4 will have larger than golf ball sized hail. These images are very helpful indeed, but I worry that people will get hurt in their attempt to take photos of the hail. There was an injury Friday. A woman was hit in the head with a large hail stone, while running to a car, to get out of the storm.

Coins are a good way of communicating hail size. avoid using the term "marble size". I don't know how big your marbles are! You may have even lost them!

Some of the hail was clear ice, instead of opaque. This clear ice is likely related to the freezing rate, and the amount of supercooled water in the highest reaches of the super-cell thunderstorm. Slower freezing, and slower hail growth, gives clear ice. The amount of supercooled water around the hail stone seems to play a big roll in the determination of clear vs. opaque.

Another viewer asked about the jagged hailstones. These are likely hail stones that are made up of several chunks of ice that froze together. A large stone like this, can only be thrown upward, and held aloft, by an updraft of around 160 km/hr! Stones this size are rarely seen and when they are, a violent tornado is usually close by. The exception is Colorado, where stones this large can form with no tornado. So, that rule is mainly for areas East of the High Plains.

The Hail Friday was Quite Varied In Shape/Size

Several images of hail that was “egg” shaped were sent in. I am not certain how this happens. It may very well be related to the intense low pressure circulation inside the tornadic thunderstorm. Centrifugal force perhaps? You can read more about hail on the UCAR Fact Sheet.

Meteorologists call this the mesocyclone, and Doppler weather radars can detect the strength of the mesocyclone. The mesocyclone near the largest hail on Friday was very intense. So intense, that I was quite certain, a tornado was on the ground, and doing damage. Storms like these we tracked on radar Friday, are seen rarely. In my 30 years of forecasting, I have tracked storms like that on maybe 6 or 7 days.

Wall clouds are very popular with the public. Storm spotter courses show lots of pictures of them, and when we have strong storms, I get many photos from viewers of what looks to them like a wall cloud. I always tell our reporters in the field, that a wall cloud will look just like a wall cloud. If there is doubt, it probably is not one. Wall clouds are the parent cloud of a tornado, and a rotating wall cloud is a sign that you may be about to see a tornado. Best not be in a location ahead of one!

Wall clouds will form under the "rain free base" of a supercell.

Sometimes a pendant will slope down, from the wall cloud toward the rain shaft. This is called a tail cloud. Usually, rotation in the wall cloud is quite apparent.

The tornado that tracked through Marshall, Dekalb, and Jackson counties on Friday has been rated as an EF-3 on the enhanced Fujta scale. This means winds of over 150 mph. The maximum width of the path of damage is nearly 1KM!

The estimate of the tornado’s size, and strength is based on several factors, The width of the path, and it’s length. The maximum winds can be estimated by the damage to trees, and structures. It is important to realize that if a large tornado does no damage, then it cannot be given a rating.

Many times, people will insist that a tornado has been under rated because there are many destroyed homes. If these homes are mobile homes/trailers, then an EF2 tornado will completely destroy them. If there is no major damage to wooden homes, then the tornado rating is likely correct. An EF2 will also usually destroy a garage or out buildings as well.

The EF3 tornado is much stronger, and does much more damage to wooden/ brick structures, on a foundation. An EF2 tornado has winds up to 115 knots or round 219 km/hr. An EF3 has winds from 219- 265 km/hr. That is nearly 160 knots. A category 5 hurricane is another example.

Typical EF3 Damage (From NOAA-NWS)

Heavy cars will be thrown a good distance by an EF3 and walls of well built structures will collapse. Damage in the Sylvania area, of Dekalb County, indicates that the tornado Friday was at EF3 strength. Late word from the NWS survey team tonight, of one home that was picked up, and slammed 4 feet into the dirt in Sylvania. Read the preliminary storm survey here.

The methods of rating a tornado is explained in this slide show. If everyone who lived in a mobile home, left for safer shelter during a tornado watch, the death toll from tornadoes would drop by half in this country. Don’t think that tie downs make any difference. They almost certainly do not.

Thanks to all of our viewers here in Alabama and Tennessee whose pictures and reports helped us track the storms on Friday. If you see your picture in this post-please email me and I will add a credit. We have so many- the names have been separated!

UPDATE SUNDAY EVENING: Donnie Charles of Albertville emailed us this picture. It’s a GIANT wall cloud. Taken in 2006, but a good example.

Huge Wall Cloud Near Albertville, Al 2006.

Notice the striations. Wall clouds like that almost always accompany killer tornadoes.

Now the bad news. More storms Monday.

Keep safe,

Dan

His name was Richardson.

Lewis Fry Richardson. He was a British mathematician, and as a Quaker, also a pacifist. When World War one began, he enlisted as an ambulance driver to serve his country while adhering to his ideals.

Richardson had the bright idea that the mathematical equations that govern the air, could be used to predict the weather. He decided to make a forecast, and divided the forecast area into squares.  Then he assigned such variables as current temperature, pressure, and humidity to each of these grids. He believed that if he solved all the equations for each grid, he would come up with a forecast of the weather 8 hours in the future.
It took him months! No computers around in World War One, and no one really wanted to help him!

Lewis Fry Richardson- Ctsy UK Met. Office

Did it work??

Total failure dude. (I can relate my friend, I can definitely relate!)

He did not give up though, and kept working on it for years. He had the idea of having thousands of people in a stadium, who would each do one calculation, and pass it on to the next person. This would make it possible to solve all of the complex calculus equations, in a short period of time, and produce an accurate forecast!

With the advent of computers in the 1950′s, this idea took on a whole new meaning.  While Richardson’s first forecast failed spectacularly, the attempts showed much promise by 1960. If only Richardson could see the models I look at every day now!

The rapid increase in computing power has revolutionized weather forecasting. In 1979, I was told that an accurate 7 day forecast would not be possible in my lifetime. Computers were just not fast enough. In 1980, an extended forecast was 3 days. I produce a 7 day forecast everyday now, and some stations put on a 10 day forecast. The 5 day forecast is as accurate as a 3 day forecast was just 20 years ago!

Let me explain how they work. No math, I promise!!

Grab a map of North America or Europe, or wherever you are in the world. A map that covers several thousand kilometers. Now get a box of sugar cubes. The kind you put in your tea or coffee. Put them down on the map. Now imagine that the cubes cover the entire map. Now, we tell the computer model what the weather conditions are in every single one of those cubes. We then use the mathematical equations that describe the atmosphere, to move the weather from one cube to another.

If there is a south wind over Texas, then the cubes over Oklahoma will get warmer. How fast they get warmer, will depend on the wind speed, the elevation, and whether or not the sun is shining. Actually there are a ton of variables, and they can all be described using equations of math. Some better than others. We have to decide how often we are going to move the weather, from one cube to another. Modern weather models use a time period of just a few minutes. We call this the time step of the model. We also have to decide how big are cubes are going to be.

Imagine a map covered with sugar cubes

This is very important, because the smaller the cubes, the more accurate the forecast. Imagine we have a cube that is 50 kilometers wide on each side. We will give one value of the current weather to that cube, when we tart the model. Will the weather be the same across that entire cube? No, of course not. We will have to average the weather over the cube and this will introduce error into the forecast. The smaller the cube, the less error.

Here’s the rub.

The smaller the cube, and the shorter the time step, the more time it takes to run the model!

I also neglected to tell you something. We have a map covered with a a layer of sugar cubes right? What about the rest of the atmosphere? What about the weather up high. What about the jet stream? We have to have a bunch of sugar cubes in layers on our map. Each one has to be given a value for the weather happening now, before we can run the model!

The best resolution model run by NOAA over North America is called the NAM. North American Mesoscale model. The cubes are 12 km square in the NAM and there are 70 layers of them. The top layer is over 20 miles high! The model is run 4 times a day on a multi-million dollar supercomputer. It makes a forecast out to 84 hours in the future. Other models have bigger cubes and forecast the atmosphere out to 10 or twelve days. They take longer to run of course.

NWP model forecast of temperatures and winds at 5000 ft. From NOAA and weather.unisys.com

So why isn’t the three day forecast always perfect??

The cubes are still too big for one thing. We also do not know the weather over all of North America exactly is another. The NAM covers large areas of ocean, where there may be no weather observations at all! We have to guess. We do a pretty good job of it too.

There is more error. The equations are not perfect, so even if we did know the weather everywhere, and could tell the model exactly what the weather is doing right now, it would still make a less than perfect forecast! There is more though. what about clouds?? How do you tell the model what a cloud looks like, and how do you tell the model what to do with rain and snow that falls.

The rain will affect the temperature and humidity. If the rainfall is wrong, more error!

Lastly, there is chaos.

If we had perfect equations, and perfect knowledge of the initial weather, the forecast would still go wrong!

The atmosphere is a chaotic thing. This was described brilliantly by Ed Lorenz. He famously asked the question “Does a butterfly flapping it’s wings in Mexico cause a blizzard in Kansas”? Tiny changes at molecular level will eventually add up to big errors.

The current thinking is that the limit to very accurate numerical weather model forecasts is somewhere between 2 and 4 weeks. We have a long, long way to go, to get there though!

You might wonder about Climate models used by the IPCC to forecast climate change. They are similar, but since they must run for hundreds of years, the cubes are huge. Climate scientists are not interested in the weather the model comes up with. They ARE interested in the climate it comes up with, and the changes in climate that it comes up with.
How good are these Climate models??
Look at the image below.

Climate Model Hindcast from IPCC

When we start the models back in the early 1900′s, they do a very good job of predicting the Earths temperature through the 20th century. Notice the difference when we do not include greenhouse gases rising. The model blows it. Only when we include natural and human caused changes in the atmosphere, does the model get it right.

Those same models say we will see a catastrophic rise in temperature in the next 100 years, if we do not do smething to reduce CO2, and other greenhouse gases.

Yea, it’s scary isn’t it. Now you know why so many climate scientists have a bad feeling in the pit of their stomach, and they cannot seem to convince the public why.

See, I told you this was interesting!

Later,

Dan

Credits:
The Forgiving Air by Richard Somerville (He goes into great detail, and also with no Math! Buy the book.)
COMET NWP Model Matrix
NCEP (National Centers for Environmental Prediction-NOAA)
Unisys

A lot of news about the Arctic Sea ice in the past few days. NASA and the National Snow and Ice Data Center issued a report Monday morning that showed the average ice coverage in March was 15.16 million Sq. KM. This is well below the 1979-2000 average for March. The ice is well above the record low in 2006, but the news is generally bad.

Arctic Sea Ice from NSIDC. Click image for full size.

Most of the ice is much thinner than it should be. This thin (First year ice), will melt quickly as Summer comes. The Arctic ice reaches it’s maximum extent in late February, or early March. This year the maximum was reached on Feb. 28th, but the ice held steady for most of March.

ARCTIC ICE IS MELTING MUCH FASTER THAN PREDICTED

The thickness of the ice is key to how quickly and how much it will melt in the Summer. The Arctic ice reaches it’s minimum in early September. 2007 saw a record low in the ice. As one scientist put it, “The Arctic Ice has fallen off a cliff”. More importantly, the ice is not following the IPCC forecasts. It’s disappearing much faster!

Arctic Ice Thickness Time Series- NASA/NOAA Click image for large version.

Greenhouse gases are almost certainly responsible for the rapid decline, but other things control the ice as well. Ocean currents and wind patterns in the atmosphere. This past Winter has been characterized by La Nina conditions. This cold water over a large part of the Pacific actually cools the planet slightly, and it might have been the reason the 2008 melt did not quite reach the drastic levels of 2007.

I read a fascinating paper over the weekend, that is being published in the well respected Science Journal Geophysical Research letters. (GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L07502, doi:10.1029/2009GL037820, 2009)  Muyin Wang of the U. of Washington, and James Overland of NOAA Pacific Marine Environmental Laboratory, went back, and looked at the climate models used to forecast the Arctic ice. The IPCC had forecasted most of the ice to disappear by 2100. It’s apparently melting much faster than that.

The way climate modeling works is this. You run the model many times with slightly different initial conditions. You will get an ensemble of different results. A few crazy solutions yes, but the models will cluster around certain values. The average of these clusters is usually the most reliable. I look at ensembles like this daily when making my forecasts for the next 7 days here in Huntsville.

The bright idea here, was to go back and see which of the IPCC models, did the best job over the last 20 years with the ice, and then see what those models predicted for the next 90 years. The answer? Most of the models bring the date of nearly total melt in Summer forward to around 2040. Just 30 years from now, and the North Pole will be water in September, not ice.

Arctic Ice Projections from "A Sea Ice Free Summer Arctic Within 30 Years"- Wang and Overland Geo. Phy Res. Letters.

You might wonder about how that will affect our weather patterns. How will it change the temperature in the Arctic- North of 60. What kind of changes will we see further South??

A lot of Meteorologists, and Climate scientists are wondering the same thing…

Later,
Dan