Landspouts and Tornadoes

[Andrew Khan]

I am striving to figure out if there is a way to differ a landspout and a tornado, visually. I am also wanting to know, does the NWS put out LSR's for landspouts. I see the NWS doesn't have warnings for landspouts as well...landspouts however, can be damaging, and deadly. There is a conspiracy about how they're different, some people think they're the same, but I'm trying to be able to figure out how they differ based on visual evidence. Another thing, do Landspouts have a specific Reflectivity on Radar, that is unique?

Thanks

 

[Marko Korosec]

Andrew,
if this helps; taken from NWS tornado FAQ:

• 
  Marko

 

[Andrew Khan]

Oh, okay. That definition helps me a lot. Thanks!

 

[rdale]

"Another thing, do Landspouts have a specific Reflectivity on Radar, that is unique?"

Neither landspouts nor tornadoes show up on radar...

- Rob

 

[Andrew Khan]

I really was not insinuating a tornado did. But, there are some specific reflectivity signs, that show tornado initiation, not whether or not is in contact with the ground or not. For a tornado the signature would be a Hook Echo, and the tornadogenesis Couplet, and I am trying to figure out if the landspout has a specific radar unique to itself.

 

[rdale]

Again a slight correction... A hook echo has nothing to do with tornado initiation, it's just a sign of rotation in the storm. You'd want to look at velocity imagery for a better view, but still no matter how impressive the rotation looks in either image you still cannot conclude that a tornado is forming.

Landspouts do not have the associated storm-scale rotation that a tornado has...

- Rob

 

[Andrew Khan]

Right, a hook echo deals with extremely tight strong rotation, that could in fact produce a tornado, nothing is for sure though, I know this. But it is a trademark of a lot of tornadoes, whether it be an indicator or not, it does in many cases, mark tornadogenesis. I realize a good way is to look for a couplet in a velocity scan...

 

[Andrew Khan]

"A "landspout" is storm observer slang for a non-supercell tornado, most often seen in relatively dry enviromnents with weak wind shear and high-based thunderstorms. On this day, vertical shear favored supercells (and one did develop farther south); however, the process behind this tornado was not that of a mesocyclone. Such tornadoes typically take shape as a translucent dust column, like this, sometimes beneath funnel clouds, unlike this."

Looks like a lot of landspouts do not have full condensation funnels reaching to the ground.

 

[Andy Wehrle]

To clarify what Rob is saying, landspouts do not have the radar signatures normally associated with tornado production. The storms often have weak reflectivity values and no hook echo, and if a velocity couplet shows up it will be very small and only show up right at or after the time of landspout formation.

 

[Andrew Khan]

So, do people report landspouts to the NWS office at ALL, and if they do, do they say 'tornado' or 'landspout'? Why are these so common in Colorado?

 

[Andy Wehrle]

Yes, landspouts are (and should be) reported to NWS offices, as "tornado" reports. Landspouts are tornadoes even though they do not originate from storm-scale rotation and tend to be weaker than those that do.

A landspout is not to be confused with a gustnado which rotates only at ground level, not at cloud level as well. Therefore, a gustnado is not a true tornado.

I don't know exactly why landspouts are common in Colorado. Someone who chases there can answer that question.

 

[Mikey Gribble]

A landspout is not associated with a mesocyclone and it is not going to show a velocity couplet like a supercell tornado. The way I understand it, landspouts usually form because of small circulations(often times terrain induced) along a boundary. When these small circulations are under a convective updraft landspouts can form throgh vortex stretching. The landspout would visually look different for a couple reasons. With tornadoes, there is usually a broad area of rotation in the updraft base before and during the tornado. With a landspout there would not be much if any rotation in the updraft base above the landspout. There is no mesocyclone. Also, a landspout is typically weak when compared to a tornado.

 

[Mikey Gribble]

"I don't know exactly why landspouts are common in Colorado."

They are common because of the terrain. The Palmer Divide causes a convergence zone and these small circulations along the boundary(that I mentioned in my previous post) lead to landspouts when they get pulled into a convective updraft.

 

[Jeff Snyder]

Michael covered it pretty well. Landspouts derive their rotation from strong pre-existing horizontal shear along a boundary. Oftentimes, there are relatively small-scale circulation centers (misocyclones, with an "i" instead of an "e") that develop... If a convective updraft moves or develops over such an area, the updraft can stretch the existing shear and produce a landspout. Unlike a supercell tornado, a landspout can develop from showers or even towering cumulus (cumulus congestus).

Bruce D, and Robert B. Wilhelmson, 1996:The Numerical Simulation of Non-Supercell Tornadogenesis. Part I: Initiation and Evolution of Pretornadic Misocyclone Circulations along a Dry Outflow Boundary


A Google search for "non-supercell tornado" and "misocyclone" turns up quite a few hits, though mainly AMS publications, which require a username/password to view anything other than the absract.

 

[Marko Korosec]

Michael, Jeff,
so if I understood you right...landspouts can also develop from showers with a usual front pass. I posted this topic: http://www.stormtrack.org/forum/viewtopic....ighlight=#90548 few weeks ago with some pics of rotating wall cloud, we came to conclusion that it was a tornado, probably a multi-vortex tornado.

But later I realised that there were no supercells (or a mesocyclone) that day, only a normal cold front pass. But I've noticed a few condensation funnels that reached the ground, there were heavy showers the whole day too.

So now I am a bit confused, seems those were landspouts...the thing that confuses me is that there were few of them, so can they occur as multi-vortex (landspouts) even there is no supercell storms?

Thanks,
Marko