Topic: water spouts

[Mike]

Something that i observed early in the 2005/6 wet season over to the north of Darwin was a water spout.  Obviously I was astounded of such an event here, but could some of you explain the possible conditions of such a storm occurring? ( i just know you're going to talk about wind shear!)

[David C]

Something that i observed early in the 2005/6 wet season over to the north of Darwin was a water spout.  Obviously I was astounded of such an event here, but could some of you explain the possible conditions of such a storm occurring? ( i just know you're going to talk about wind shear!)

Hi Mike,

you would need a surface boundary (source of horizontal vorticity) and a developing updraft to be co-existent with this boundary. The updraft can then 'tilt' this vorticity vertically, stretch it and thereby intensify it into a 'tornado'. A waterspout IS a type of tornado, by defintion, and is associated with relatively small, shallow circulations known as misocyclones (cf. with MESocyclone <-- supercell or MESOcyclonic tornado). When the updraft and misocyclone become uncoupled, the spout craps out.

[Mike]

Hey there Dave.

Thanks very much for that.  I'm getting the gist of when you all talk about vortcity - either horiz or vertical.  As you would well know, if you read some of the forum replies it just goes over one's head completely when you all talk about vorticity, shear and the like, but thankfully you can make a mental picture in your head and 'see' what your'e all talking about. 

So at what level would that 'boundary' be? Within the storm in the middle levels or at low levels because you talked about horizontal vorticity and for the spout to form would the updraft be from the lower ranges of the storm's base to form the funnel (that we can only see because of the moisture being drawn in and around it).

So the storm would be very well formed initially to produce this or it's just a case of surrounding conditions enhancing the same?  For example; when you see lots of scud rotating at the base of a storm - is that the horizontal vorticity that we can actually see because it sometimes travels horizontal to the storm's base or is that just the inflow band of the storm? 

You see where i'm going with this - i really don't want to confuse myself by getting off track.  I guess i'm asking is this:  What visual indicators can one see when looking for vertical or horizontal vorticity without the aid of radar indices?!

(i'm getting a headache)

[David C]

Hi Mike,

While one really needs to get into some pretty decent maths and physics to fully understand and work with these at a professional level, conceptualising things by way of 'mental pictures', as you (and I and most other storm chasers are doing) is more than sufficient to gain a greater understanding and appreciation of what you are seeing. eg, the concept of cyclonic and anticyclonic shear (w/r to jetstream or an RFD surge), simple as it is to someone with a met degree, came to me when I was kayaking and watching the opposing vorticies generated on either side of the paddle as I pushed 'backward' through the water  8)

Anyway, back on topic, I believe it was Fujita who defined misocyclones as shallow circulations with diameters less than 4 km. They are more or less transient eddies along stationary or slow moving wind shift boundaries near the surface. As opposed to supercell tornadoes (we can discuss later), landspouts occur in association with shallow convection (eg congesting cumulus) or during the early stages of deep convection prior to development of a downdraft and surging gustfront that would likely disrupt any misocyclones present. So, you could imagine these shallow invisible misocyclones (not associated with convection per se) lazily spinning away when all of a sudden, by chance, they become vertically aligned with an updraft - it is very much a atmospheric serendipity. In this case, the misocyclone will be stretched vertically to some extent through the updraft, will contract in scale horizontally and hence the rotational velocity will increase. In some cases, this vortex tube will be sufficiently intense to produce a landspout tornado that will connect with the cloud base. When the storm matures or when the updraft - misocyclone pair are 'divorced', then the whole thing breaks down. These are usually, not always very transient events and the tornadoes weak for the most part (not always!). Don't confuse moist convection (that generates the cumulus or cumulonimbus that you see above with the misocylone itself (ie the air current if you will).

Any others please feel free to add more

[David C]

So the storm would be very well formed initially to produce this or it's just a case of surrounding conditions enhancing the same?  For example; when you see lots of scud rotating at the base of a storm - is that the horizontal vorticity that we can actually see because it sometimes travels horizontal to the storm's base or is that just the inflow band of the storm? 



(i'm getting a headache)

Not sure that I answered this above. The answer is no, a storm is not required just an updraft in the right place at the right time.

I guess horizontal vorticity can be seen in the case of a roll cloud (ie rotation about a horizontal axis). Scud rotating at the base of the storm, as you describe, is usually the sign of a strong circulation on the storm scale (ie MESOcyclone) not these weak transient misocyclones. That itself is generated by a complex chain of events beginning with an rotating updraft in the mid-levels of the storm, a special type of downdraft that is thought the by the result of a dynamic process, generation of very strong horizontal vorticity by the storm itself, the subsequent generation of a separate low-level mesocyclone (often with rotating wall cloud and scud visible) and hopefully a tornado!

In eastern Australia there are tons of reports by storm chasers of 'rotation'. I can guarantee that 90% of these reports are people seeing shelf cloud motion / gustfont shear that can promote differential cloud motions. All exciting stuff, but true storm scale rotation is different phenomena indeed. It goes back to what I was saying above - when you develop an understanding of the processes, you can understand storm evolution in the field and you can quickly identify what in fact is really going on - and that knowledge becomes very practical - ie should I ditch this storm and drop south (or north) to the next storm.

Anyway, enough from me 

[Mike]

Ah yes, i understand now .  As you say, not getting into the mathematical and physics side of met calculations (to which I would fail miserably) it's just a matter of knowing the principals and 'basic' foundations of how all types of storms occur.

I have found that when chasing up here since becoming a little more knowledgable that i am running around more after more organised storms since i can to some extent read what is or won't be a decent storm building and what it will do.  Makes for more Kms, but sometimes it's worth it!

I agree with you saying that 90% of storms are not actually 'rotating' per se.  Although it's nice to see the scud and surrounding cloud swirling around, the whole structure is not as we have seen in severe supercell rotation - well i guess there is some rotation actually - but on a smaller scale....

But thank you again.  Your information very useful and am getting a better understanding as i ask questions.