Storm Structure - Rain Free Base

[Joe Ireton]

hi all, i'm new to the forum have been interested in storms for a while but i've just started to reasearch them to better understand and find them.....i've been trying to learn about meteorology in general to get a grasp....the more i learn the more i feel like i don't know lol

i live in ontario canada north of toronto and most of the storms i've seen in my area are HP, and they seem to form in clusters a linear fashion or take that appearence as they progress across the region by the time they get to my area, kinda like mini derechos(sorry for that term) coming from the SW, W and NW...based on my little experience and the fact there is so much precip in the storms i have only been able to identify some structure but i have lots to see still.

when i look at diagrams of supercell storms, the rain free base appears to be at the back of the storm if i'm seeing it right, i know not all storms are supercells but i think i experienced the rain free base prior to the rain falling on one storm i was in this year....i assume there are many variables but...

my questions are...

is it possible to have a rain free base/updraft at the front or side of a storm?
in a squall line....can i make assuptions as to the direction of the storm based on the location of the rain free base?

also do non supercell storms take have some of the same structure as supercells except the updraft doesn't tilt and the updraft gets cut off sooner? i know they form in the same way or is it apples and oranges?

thanks.....

 

[Bart_Comstock]

The "mini derechos" you see on radar are called bow echos. The traditional criteria that distinguish a derecho from a severe thunderstorm or a smaller line of storms that make a bow echo are sustained winds of 58 mph (50 kt or 93 km/hr) during the storm, high or rapidly increasing forward speed, and geographic extent (typically 250 nautical miles [280 mi or 460 km] long). They typically take on the appearance of one large or several connected bows.

I think the rain free base you are referring to on the leading edge of these systems is the shelf cloud. Although the underside of the shelf (commonly referred to as the whale's mouth) may be void of rain the typical the term rain free base refers to the base of the updraft on a supercell.

In the image above you see your typical propagation of a bow echo. Cell "C" on the north end will sometimes become a tornadic HP supercell. If you run your hand through a tub of water you will notice on each side that a little vorticity will form. The same thing happens with a bow echo and on the southern side an anticyclonic spin will form and on the north side a cyclonic one will form. These areas are called a book end vorticity. The one on the north side will generally be the strongest and most long lived and sometimes can lead to brief tornadoes that are almost certainly rain wrapped to the point that trying to chase one would be nearly impossible.

A shelf cloud will form on the leading edge of the bow echo and sometimes can take on a smoothed, stacked plate look and to the untrained eye will sometimes resemble a massive wall cloud. Although scarry looking these shelf clouds are totally non tornadic and are a outflow feature as apposed to a inflow feature. They can be a sign of strong straight-line winds.


I assume this is what you were looking at and yes I know I sound like a dork.

Take a look at the next image:

The updraft region of a supercell will be tilted with height. This will deposit the precipitation away from the updraft and thus this also results in less precipitation in the updraft region. The reason it tilts is because the wind speeds are faster as you get higher off the ground and so the updraft will commonly lean in the direction the cell is traveling in and thus deposit its rain ahead of its self. I hope the following image helps. It is a radar grab of a classic supercell. In the image above area G would be the rain free base and F would be the wall cloud that is under the rain free base. Note how the updraft tilts in the image.

 

[Skip Talbot]

is it possible to have a rain free base/updraft at the front or side of a storm?
in a squall line....can i make assuptions as to the direction of the storm based on the location of the rain free base?

Yes, it is definitely possible to have a rain free base on the leading edge of a storm. All storms have an updraft base, and whether that updraft base is a rain free base is determined by whether or not the precipitation has been displaced for the updraft base's location. There are different mechanisms responsible for the features you see on the leading edge of a squall line. The most common feature you see is a shelf cloud, which I would not consider a rain free base. This curtain like cloud is caused by a downdraft moving out ahead of the line and forcing air up and over it.

Sometimes, however, an inflow notch will develop on the leading edge of a line of storms, which can result in more classic supercell structure (a rain free base and wall cloud). You can pick these out on radar as little hook echoes protruding from the front of the line. You can also have hybrid storms, that based on the wind directions at different heights have rain free bases on the leading edge relative to the storms direction of travel. Thus you can't really make sure fire assumptions about storm motion based on the location of the structure.

So yes, squalls can have a rain free base on the front side, however I would say that most squall lines do not have a rain free base or a well defined one. The updraft of a squall is near the leading edge of the storm, but usually the forward flanking downdraft has spread past it on the surface kicking up a shelf cloud instead of revealing a true rain free base.

also do non supercell storms take have some of the same structure as supercells except the updraft doesn't tilt and the updraft gets cut off sooner? i know they form in the same way or is it apples and oranges?

Yes, in fact they can share almost all of the same visual structure. A supercell is a storm with a deep mesocyclone, a storm without one can still posess an anvil, rain free base, and updraft tower. Supercells typically can keep their rain free bases through the mature stage of the storm's lifespan, because of the separation of updraft and downdraft precip. Non-supercells, on the other hand, tend crap on themselves (rain through their updraft), and fall apart quickly once in the mature stage.

 

[Joe Ireton]

thanks guys, I think that i've seen a shelf cloud but it seemed to be more ragged than that......either that or it was a wall cloud......it was so rainy and dark the part that i thought was a shelf was the ragged part was lighter in colour and the only contrast i could see.....

the other time i thought it was a rain free base the clouds were looked a bit like mamma but they werent as smooth but they were dark like i thought it should already be raining......

thanks for the response guys.....

 

[Skip Talbot]

Joe, a good rule of thumb I use when identifying lowerings:
In a wall cloud the pointed end points towards the rain
In a shelf cloud the pointed end points away from the rain.
Ragged clouds are most likely scud.

 

[Bart_Comstock]

Not all shelfs will be as smooth as the one in my video and some can be fairly small height wise. Take a look at these shelfs....

Note that the rain is behind them and like Skip said that they are tilted away from the rain.

Next we have a couple of wall clouds.

In the first image the tail cloud is pointing and growing towards the rain cooled air and the precip which is just off camera to the right and in the second picture you can see the same thing happening.

With scud you will have a very ragged peice of cloud debris that will sometimes rise up into the storm and may briefly look as if it is connected to the base of the storm. These sometimes are easily miskane for other features such as a wall cloud for funnel to the untrained eye.

Here is a good site to look at to see just how tricky identifying tornadoes can be thanks to scud: http://www.stormeyes.org/tornado/faq/notahose.htm

 

[Joe Ireton]

thanks for taking the time Bart.....thats interesting how decieving clouds can be....and i will use that rule of thumb Skip....

the link led me to another question....

one of the pics of a smokenado it said that it was clear that the storm had become outflow dominant....so that means essentially that the rain and cool air from the rain is cutting off the inflow and it is dying pretty much? What would the obvious clues to that be? just a huge amount of rain falling from the whole storm instead of a more concentrated shaft of rain?

 

[Skip Talbot]

Joe, there are a some visual clues that you'll notice when looking at an outflow dominant storm. The updraft base usually bows out into something that resembles your typical squall. There will be lots of rain in the updraft as well as a lot of scud. One feature you see sometimes while underneath is called the whale's mouth:

Here's another example of the transition to an outflow dominant storm:

 

[Lanny Dean]

Posted below are 3 shots of cloud structure. The first being a classic suppercell in which I have outlined with the basic areas of interest ie. updraft, rain free base, RFD, FFd and the like.
The second and third pic are of "shelf" clouds. These "shelf" clouds give stacked look to them, hence the word shelf. NOT TO BE mistaken with Stacked Plates. I see that Skip and Bart have explained the mechanics so no need in going into detail.
Hope these photo's will help as well.
Lan

 

[Greg Blumberg]

One other thing I'd like to add for telling whether or not a storm has become outflow dominant.

I use a little piece of homemade equipment called the Human Skin DX5000 in order to identify the temperature, moisture, and where the wind is coming from. It's really not hard to construct at home or anywhere for that matter, and it cost me absolutely NOTHING! In fact, I am currently constructing more of this equipment.

If the air is feels cold, dry (wet sometimes) and is coming from the storm, then I've got a pretty good idea that the air is outflow. If it feels warm, is moist, and is going into the storm, I know that's inflow. That in relationship to where I am in the storm helps me find out what is going on. I also use the visual clues that the guys posted above me to help verify what my instrument is telling me, but for me the Human Skin DX5000 has never failed me before in telling me what the air feels like.

In conclusion, don't forget that your whole body is an instrument!

 

[Bart_Comstock]

On radar it is easy to identify that a storm has become outflow dominant because you can usually see a large OFB move out from the storm and generally the VIL will start to decrease as the hail core gets dumpped and the updraft weakens.

The gust front along the leading edge of the bow echo in the last image is an OFB (outflow boundary).

An outflow boundary can sometimes act sort of like a cold front and can be the focus of new storm development the day it was formed or the next day if it is able to linger that long.

your whole body is an instrument!

That's what she said.

 

[Greg Blumberg]

That's what she said.

When I wrote that, I just knew someone was gonna fire back with that. Congrats, you win money...

Also I wanted to add that you need to make sure that your OFB is an OFB when checking on radar, so know the position of your synoptic-scale boundaries i.e. drylines, cold fronts! They do show up the same...

 

[Joe Ireton]

thanks everyone....

when storms form ahead of an approaching cold front, does it have to do with the outflow of the storms along the front? do these outflow boudaries only occur from a weakening storm? does the outflow boundary cause additional lift potentially triggering other storms along that boundary in the right environment?

i heard that storms that form ahead of a front can be more potent than the ones that are along the front itself....is this true? and if so why is that?

i'm trying to program my Human Skin DX5000 lol

 

[Chris Allington]

when storms form ahead of an approaching cold front, does it have to do with the outflow of the storms along the front? do these outflow boudaries only occur from a weakening storm? does the outflow boundary cause additional lift potentially triggering other storms along that boundary in the right environment?

i heard that storms that form ahead of a front can be more potent than the ones that are along the front itself....is this true? and if so why is that?

Outflow boundaries are always created by storms, but at times, an existing outflow boundary and a cold front can collide to locally enhance lift and initiate storms. Like you mentioned, it is definately possible for an outflow boundary to increase surface convergence and initiate additional storms. This is why if outflow boundaries persist until the following day, many times they end up being a target for chasers, should a storm initiate and ride that boundary.

Also, typically if storms form on a front, and the front is moving quickly or storm motion is such that the storms and right on the front, they can get undercut by the cooler/ more stable air behind the boundary, often distrupting circulations at the surface and lowering your tornado chances. In an ideal situation you want the front to initiate storms, and then storm motion to carry the storms off of the front into the warm sector ahead of the front.