1) The formation of mid-level mesocyclones are thought to be very-well understood. It forms as a result of updraft-tilting of horizontal vorticity, or, more specifically, sufficient amounts of storm-relative streamwise vorticity.

2) The formation of a low-level mesocyclone is not as well understood. It is thought that it forms in a similar manner to that of the mid-level mesocyclone except that the relative roles of horizontal vorticity from vertical wind shear and that from baroclinic generation of horizontal vorticity is still being debated.

3) The formation of rotation near the ground (surface) is not well-understood at all. The tilting and stretching of horizontal vorticity is not adequate for explaining rotation near the ground. The updraft that participates in the stretching and tilting also would advect vertical vorticity upward, preventing rotation near the surface. So it is thought that a downdraft is necessary to keep a circulation near the ground.

4) The formation of the tornado after rotation has been established is better understood. It results from large increases in cyclonic vertical vorticity resulting from stretching in the updraft. With enough convergence and angular momentum, a vortex forms at mid-levels first and propagates downward by drawing in air towards its lower end. The convergence associated with the drawn in air helps to amplify vertical vorticity and cylclostrophic balance is established at the lower level. This process repeats until a vortex reaches the ground. Friction at the ground also appears to play a role by generating strong radial inflow near the ground in the vortex.

Many theories have been offered as explanations for #3, though there really is no consensus at this point.

Speaking in terms of roation near the surface...would that be harder to study b/c there are alot of isolated local factors that play a role in that

Yes. It is thought that small-scale environmental heterogeneities, for example boundaries, may have a large influence in helping to produce rotation near the ground. Of course, obtaining observations of anything on such a small scale is difficult, let alone in an environment where tornadoes form.

yes its very difficult. I'm really interested in this concept and this is one major thing I want to research...especially in the NE where small scale forces play a major role

Well, that'd be an interesting area of study. There are very few observations of supercells and/or tornadoes in New England, for a couple of reasons:

1) Terrain-This would become an issue especially if Doppler radar was used as an observing tool. In the Plains, the terrain is mostly flat, so one could be ~1-2 km away from an area and still obtain observations of that area very close to the ground. The same would not be true in New England, where the terrain is far from flat. This means either using other observing tools, or getting extremely close to an updraft or tornado, which is usually not ideal.

2) Predictability-Finding an isolated supercell (where you know there is a fighting chance for a tornado) in the Plains can be difficult, so doing the same in NE would be even harder. Guessing where a tornado may be embedded at the southern end of a squall line may be the best bet, but my guess is those opportunities don't occur very often.

our best bet right now in the NE is probably based on computer data (radar's and other data sources). Hopefully more people in this area become interested and maybe the more eyes the better. As technology advances hopefully that will help with these causes in the NE.

I think the best way to study convection in New England on small scales is to ditch extremely high resolution observations and concentrate on larger scales, such as squall lines. Chasing tornadoes in that environment just doesn't seem reasonable. Better to focus on the storm scale and hope for the best.

In the near future we will have enough computer resources to both simulate a tornado at "high-enough" resolutions as well as have a domain large enough to adequately capture the larger scale flow. I believe the beginnings of this have already started at the U. of OK

Have their been studies done of the major NE tornadoes? (Windsor Locks, Hamden, Worcester, Mechanicville).

If not maybe we can study the synoptic setups, temp, dp, and wind obs for the area and see if there are any comparasions.
Basically just get a ton of data from all of the major events in the NE. If nobody has done this yet I would love to work on this

There has been a peer-reviewed study done on the Windsor Locks tornado: Riley, G.T. and L.F. Bosart, 1987: The Windsor Locks, Connecticut Tornado of 3 October 1979: An Analysis of an Intermittent Severe Weather Event. Monthly Weather Review, 115, 1655–1677.

Here is a copy of it if you'd like to read it: http://ams.allenpress.com/archive/1520-049...-115-8-1655.pdf

There has also been a peer-reviewed study of the Mechanicville event: LaPenta, K.D., L.F. Bosart, T.J. Galareneau, and M.J. Dickinson, 2005: A Multiscale Examination of the 31 May 1998 Mechanicville, New York, Tornado. Weather and Forecasting, 20, 494–516.

Because its from 2005, you can't read that one unless you have AMS journal access. I assume Lyndon State has AMS access on their department computers, so you can probably access it there: http://ams.allenpress.com/archive/1520-043...34-20-4-494.pdf

Not surprisingly, both studies include Lance Bosart. I am sure they are both of high quality, but I have not yet read either one of them.

I did not find anything peer-reviewed for the Hamden or Worcester tornadoes.

Because it is relevant to this thread, I am going to repeat here some questions I posed in this post in the main forum (and I might add that I am somewhat disappointed that the only response I got was from someone else wondering the same thing), namely:

Given the seemingly less than optimal synoptic environment (persistent strato-deck from decaying Midwestern MCS and possible marine influence) present at mid-day in Eastern NY and Western CT, how can we explain the numerous supercells and suspected and confirmed tornados or funnel clouds that erupted in mid-afternoon in that region on July 19?

Weatherwiz, I'd be really interested in your take on this given your interest in New England tornadogenesis. AtticaFanatica, I'd of course be interested in your take given your obvious knowledge of things tornadic.

I know a major reason for this board is to stimulate discussion of ongoing and future meteorological phenomena (and to give other members a hard time ), but I think one of its most valuable purposes is for education. And there is no better way to learn than to "reverse engineer" a particular event and determine why it happened, even when it seemed like it should not have occurred, like yesterday's event.

Paul:

Between your reply here and BillG's response in my original thread, I have a much better understanding. Your last paragraph gives the lie to all those statements we always see in posts regarding the need for insolation. I think, if I am reading both of your posts correctly, that insolation is simply one, but only one, of several different ways to achieve the lift necessary for violent thunderstorms.

Have their been studies done of the major NE tornadoes? (Windsor Locks, Hamden, Worcester, Mechanicville).

If not maybe we can study the synoptic setups, temp, dp, and wind obs for the area and see if there are any comparasions.
Basically just get a ton of data from all of the major events in the NE. If nobody has done this yet I would love to work on this

At BOX, I'm working on a climatology of SNE tornado events dating back to 1995 with one of our summer students (essentially when the KBOX WSR-88D became operational) and we will be presenting our findings at this year's SNE Weather Conference in October.

We've classified all the events into different types based upon the prevailing synoptic pattern (500 mb) and are currently working on radar analysis for each type, to see if there are any clues which will help us improve accuracy for tornado warnings. The weaker F-0/EF-0 ones are notoriously tough to warn on, but we have seen some subtle radar signatures, which are "warnable" if the forecast team is aware of the potential through larger scale pattern recognition. The F-1/EF-1 and F-2/EF-2 events are obviously a bit more evident on radar, but since coverage is less than perfect near the Connecticut Valley where most of the events have occurred (sampling too high in the storm), none of the area radars (BOX, OKX, ENX) show much detail. The best cases we have are the June 17, 2001 Worcester County tornado (Allison remnants) - as well as the Cheshire County NH storm back in 1997 (bookend vortex).

When complete, we will also have a copy of this study on the BOX website.

Joe

Have their been studies done of the major NE tornadoes? (Windsor Locks, Hamden, Worcester, Mechanicville).

If not maybe we can study the synoptic setups, temp, dp, and wind obs for the area and see if there are any comparasions.
Basically just get a ton of data from all of the major events in the NE. If nobody has done this yet I would love to work on this

There was a recent article in the December 2006 issue of Weather and Forecasting that featured a study of the Great Barrington, MA tornado of 29 May 1995. It was hypothesized that topographic configurations and the associated channeling of ambient low-level flows may have played a crucial role in mesocyclone intensification and resultant tornadogenesis during that event....which can probably be said for the vast majority of tornadogenesis events in the Northeast. The study was performed by Lance Bosart, Anton Seimon, Kenneth LaPenta, and Michael Dickinson.

I am going to make sure I get to that conference just to really see this presentation. Have you talked to local television meteorologists about if they have any radar images saved? For example, WFSB3 in Hartford, NBC30 in West Hartford, WTNH in New Haven all have state radars and maybe those radars detect more of the lower levels of the storm.

There was a recent article in the December 2006 issue of Weather and Forecasting that featured a study of the Great Barrington, MA tornado of 29 May 1995. It was hypothesized that topographic configurations and the associated channeling of ambient low-level flows may have played a crucial role in mesocyclone intensification and resultant tornadogenesis during that event....which can probably be said for the vast majority of tornadogenesis events in the Northeast. The study was performed by Lance Bosart, Anton Seimon, Kenneth LaPenta, and Michael Dickinson.

We limited our data to WSR-88Ds only, since we also need to look at velocity images. But, it would be interesting to look at any archived data, if it is available!

If your answer included words such as dull or tiresome, i.e., boring, think again. Or better yet, click on the image below to see an undular bore in action:





Above: Undular bore waves over Iowa, Oct. 3, 2007. Movies: 5 MB mov, 5 MB gif, 13 MB gif.

Those giant waves—"undular bore waves"—were photographed Oct. 3rd flowing across the skies of Des Moines, Iowa. (Credit: KCCI-TV Des Moines and Iowa Environmental Mesonet SchoolNet8 Webcam.)

"Wow, that was a good one!" says atmospheric scientist Tim Coleman of the National Space Science and Technology Center (NSSTC) in Huntsville, Alabama. Coleman is an expert in atmospheric wave phenomena and he believes bores are more common and more important than previously thought.

But first, Iowa: "These waves were created by a cluster of thunderstorms approaching Des Moines from the west," he explains. "At the time, a layer of cold, stable air was sitting on top of Des Moines. The approaching storms disturbed this air, creating a ripple akin to what we see when we toss a stone into a pond."

Undular bores are a type of "gravity wave"—so called because gravity acts as the restoring force essential to wave motion. Analogy: "We're all familiar with gravity waves caused by boats in water," points out Coleman. "When a boat goes tearing across a lake, water in front of the boat is pushed upward. Gravity pulls the water back down again and this sets up a wave."

Playing the role of boat, the thunderstorms tearing across Iowa on Oct. 3rd spawned a train of four waves. "They're beautifully shown in this NEXRAD radar image."



"Green denotes winds coming towards the radar while red means the winds are moving away," explains Coleman. People in Des Moines actually felt this back-and-forth breeze as the waves passed overhead. "Flags few one way during the crest of the wave and swung around 180o to fly in the opposite direction during the trough."

What's so important about all this?

"Undular bores may play a surprising role in severe weather," says Coleman.

"For one thing, we believe undular bores can amplify tornadoes." He cites as an example an F5 tornado that struck the outskirts of Birmingham, Alabama, in April 1998. "At first the tornado was doing relatively little damage. But our research shows that, just before the tornado reached Birmingham, it was hit by an undular bore." The wave spun up the twister, increasing its intensity and size; the tornado went on to wreck more than 1000 homes and business totaling $200 million in damage. Tornado-wave interactions are the subject of Coleman's PhD dissertation, which he is completing now under the direction of University of Alabama-Huntsville professor Kevin Knupp.



"Furthermore," he says, "undular bores may be a source of thunderstorms." That's right, thunderstorms make undular bores and undular bores return the favor. "These waves churn up the atmosphere, causing instabilities that can initiate and sustain severe storms."

Although few people have witnessed undular bores with their own eyes, Coleman believes they're common. "An undular bore passes over any given point in the United States about once a month," he estimates. Often they occur at night when temperature inversions create a layer of cool stable air near the ground—much like the layer over Des Moines—ripe for rippling.

"Last year I saw a nighttime undular bore lit up by the nearly full Moon right outside my front door—that was cool," says Coleman.

Typical waves measure 5 miles from peak to peak and race across the sky at 10 to 50 mph. "Yes, you could chase them in your car—although I wouldn't recommend it." The waves don't always travel along established roadways.

But just in case they do want to chase one, Coleman's colleagues led by Kevin Knupp have a mobile weather station waiting in the parking lot of the NSSTC. The MIPS—short for Mobile Integrated Profiling System—is equipped with a radar, a laser, a microwave radiometer and other instruments which can measure wind, temperature, pressure, aerosols and water vapor content in vertical columns up to 10 km high.

"I can't wait until the next wave comes by," says Coleman, not bored at all.

Abstract

We present a novel explanation of the physical processes behind one type of cloud and ground-level tornadogenesis within a supercell. We point out that the charge separation naturally found in these large thunderstorms can potentially serve to contract the preexisting angular momentum through the additional process of the electric force. On the basis of this, we present a plausible geometry that explains why many tornado vortices begin at storm midlevel and build downward into ground-level tornadoes. A simple model based on this geometry is used to demonstrate the strength of the electric force compared to the required centripetal acceleration to maintain cloud midlevel tornado vortices measurable as tornado vortex signatures (TVSs). Furthermore, a model based on this geometry is used to get a time estimate for tornado vortex formation. From this we are able to identify a plausible value for the threshold charge density that would lead to tornadogenesis and tornado maintenance on the timescale of a few minutes. We show that the proposed geometry can explain the observations that ground-level tornadoes thrive in regions with high shear and large convective available potential energy (CAPE) and are able to make some predictions of specific measurable quantities.

ABSTRACT

This paper investigates whether the descending rain curtain associated with the hook echo of a supercell can instigate a tornado through a purely barotropic mechanism. A simple numerical model of a mesocyclone is constructed in order to rule out other tornadogenesis mechanisms in the simulations. The flow is axisymmetric and Boussinesq with constant eddy viscosity in a neutrally stratified environment. The domain is closed to avoid artificial decoupling of a vortex from the storm-scale circulation. In the principal simulation, the initial condition is a balanced, slowly decaying, Beltrami flow describing an updraft that is rotating cyclonically at midlevels around a low pressure center surrounded by a concentric downdraft that revolves cyclonically but has anticyclonic vorticity. The boundary conditions are no slip on the tangential wind and free slip on the radial or vertical wind to accommodate this initial condition and to allow strong interaction of a vortex with the ground.

Precipitation is released through the top above the updraft and falls to the ground near the updraft–downdraft interface in an annular curtain. The downdraft enhancement induced by the precipitation drag upsets the balance of the Beltrami flow. The downdraft and its outflow toward the axis increase low-level convergence beneath the updraft and transport air with moderately high angular momentum downward and inward where it is entrained and stretched by the updraft. The resulting tornado has a corner region with an intense axial jet and low pressure capped by a vortex breakdown and a transition to a broader vortex aloft (a tornado cyclone). A clear slot of subsiding air with anticyclonic vorticity surrounds the vortex. The vertical kinetic energy of the entire circulation declines dramatically prior to tornado formation.

I think the best way to study convection in New England on small scales is to ditch extremely high resolution observations and concentrate on larger scales, such as squall lines. Chasing tornadoes in that environment just doesn't seem reasonable. Better to focus on the storm scale and hope for the best.

It might only happen once in a while, but a couple years ago I was watching the radar on a day with severe storms, and one thunderstorm aquired an almost textbook hook shape as it moved westward of the New Hampshire coast (and I know I wasn't making it up, because a tornado warning was issued).

On what "scale" would a severe storm cluster start rotating when it hits a sea breeze? (if that's what happened)