Australian Severe Weather Forum

 Greetings gang,

  just as an intro .....
Jimmy added this section to hopefully fill a niche for those of us that are not so versed in the deeper understandings of various meteorological terms and conditions.
  With that in mind I would like to get some feedback on some things spoken about in "The CAPE Debate"
in another section of the forum.

to quote Dave C ....
 " In addition to thermodynamic instability, vertical windshear is also important -- you can't exclude either since there is a sort of inverse relationship between the two. ie higher-end CAPE and weaker vertical windshear can produce supercells as can lower-end CAPE and stronger vertical windshear. "

OK questions ....
 1) why is there this inverse relationship between the two ?

 2)  could "vertical windshear"  be clarified   and could I identify its presence visually during the cell growth ?

 3)  several times on various forums I have seen the term   "CIN"  used   what is that  ?   haven't found reference
      to it in any of the glossaries I have looked through.

thanks 
Dave N   

Hi Dave,

my several cents worth:

(1) My use of 'inverse relationship' is probably a bit misleading - 'compensatory effects' might have been a better way to put it.
 
Scatter plots (CAPE v Windshear') of past events have revealed the relationship between these parameters that I mentioned (an example attached). The reasoning behind this, in fairly simple terms is:

Higher values of CAPE (or, let's say a lower lifted index (more negative)) would promote a stonger updraft resulting in more vertical stretching (the ballerina analogy is useful to think of here), hence leading to more rotation on the storm scale for a given amount of ambient vertical wind shear.

At the other end of the spectrum, when the surface through to 6km windshear exceeds approximately 50 knots, an updraft will tend to rotate as a result of horizontal vorticity being tilted into the vertical. A product of this mid-level rotation is the development of a pressure pertubation gradient, a dynamic effect that increases with the amount of rotation (I discussed this over on TWZ in the thread on storms in the top-end and extratropics). This is a kind of feedback mechanism whereby the strengthening updraft lowers the pressure in the mid-levels which in turn further strengthens the updraft - this dynamic effects is equally as important , if not more important than buoyancy alone if determining updraft strength and rotation.

(2) Windshear refers to a change in winds (speed and / or direction) with height. It is revealed when clouds at different altitudes are seen to be moving in different directions. With respect to cell growth, vertical windshear might be revealed as towers which 'lean' downshear.

(3) CIN = Convective Inhibition. On a skew-T plot, CAPE reflects positive buoyancy, CIN reflects negative buoyancy (anyone feel free to post a skew-t here to illustrate this!)

Well its good your willing to ask in order to learn, and id be more than happy to take a stab at explaining these to you.

Ill go backwards:
3) CIN: Convective INhibition. The energy required for a parcel of air(as identified on a sounding diagram) to be lifted to its level of free convection(the point at which the parcel becomes positively buoyant). Also refered to as the Cap. When the forcing mechanism: something like say solar heating inducing lifting, or a cold front lifting the layer overcomes this energy requirement convective formation begins.

2) Vertical windshear: the change of wind direction and strength with height. There are two types of windshear as such: Unidirectional shear: a change of windspeed with height with the wind blowing from a constant direction. Eg 10knots from the SE at the surface, and 30kts at 5km gives a windshear of 20 knots.
Rotational Shear: where the windspeed remains constant, but the wind changes direction with height. Eg SE at the surface, NE at 5km.(To calculate take the vector components of the wind and resolve).

In reality windshear is often a combination of these two factors: it is mechanisms like these that produce the rotating updrafts forming organised convection. Shear itself can be a lifting mechanism for convective formation. Shear can also destroy convection: At moderate windspeeds a rampant change in wind direction with height can literally shear the top off a cloud, as no doubt many of you have seen.

An example of shearing the tops off is a sky with many orphan anvils , or cumulus that is very bent over.
An example of strong shearing enviroment is a rapidly rotating storm.(Although this is not the only formation mechanism)
An example of a weak shearing enviroment is normal isolated thunderstorms, that pile up over one location.

1)  " In addition to thermodynamic instability, vertical windshear is also important -- you can't exclude either since there is a sort of inverse relationship between the two. ie higher-end CAPE and weaker vertical windshear can produce supercells as can lower-end CAPE and stronger vertical windshear. " Dave C.

This is a subject about which we still dont fully understand. To explain somewhat: Supercells are very energetic storms, and therefore should require large amounts of CAPE(Convective available potential energy), however in certain situations, lower end values of CAPE with extreme windshear can produce supercells as well. Shear is very important in the formation of organised convective systems or cells, however how exactly supercell formation occurs within such vast and varied enviroments is not fully understood. The statement that the relationship is inverse is not strictly correct: supercells can form with moderate shear and moderate CAPE. We know approximately what SHOULD induce supercell formation, however mother nature continues to amaze us by producing supercellular events when we least expect it: just look at the Brisbane hailstorms this year. If you ask Harald Richter from the Bureau's storm forecasting section, supercells form in enviroments of upwards 40knots shear, in excess of 2000J/kg cape and in an area of LIs somewhere near -6. But this is not the be all and end all....some members of this forum have seen SCs in <40knots or <2000J/kg.

If you would like to know more id have a look at some of the links on the Australian severe weather site, or a good book reference is Atmospheric Science an Introductory Survey, Wallace and Hobbs

Hope this explains it somewhat to you

NB: Dave C Youd better carefully mention what downshear is...dont want to get it sheared the wrong way round.

John I think you missed the point of Daves comment about the relationship between cape & shear. I don't think Dave was suggesting that supercells can only form in either high cape/low shear or high shear/low cape enviroments.

He was i believe suggesting that vertical stretching in high cape/low shear enviroments creates rotation in the updraft without the need of windshear.

You can go no further for an example of this than May 27th 1997 in Texas.

Hi Jeff,  ta, indeed that's what I meant (and thought I clarified in the above post  ).


I can see I'll have to become more discerning in my choice of words 

John, what I'm saying is that since westerly momentum usually increases with height here in the mid-latitudes, new convection will tend to lean in a downshear (forward) direction. We haven't yet really discussed vorticity which you more or less eluded to in your notes on windshear.....if that's where you're going you are most welcome to start the discussion!

 Thanks guys,
  happy new year to all 

   am learning lots from your combined answers. Appreciate your thoughts.

 on one day in NW Texas and Oklahoma (May 26, 2006)  I saw the creation of a good few orphaned
anvils soon after their formation.
  At the time I attributed this maybe to the lack of significant updraught/convection to sustain
the anvil creation. As the afternoon progressed convection improved and we had numerous individual
cells to chase.

look forward to more Q's 

cheers
Dave N

I think you might be bang on the money from the sound of it, id Suspect the CAPE may have been to blame. Often if you have a high value, but elongated CAPE area, you will get a burst of convection and then it peters out, resulting in orphaned anvils. It could have also been just weak air mass storms with distinct lack of organisation too, Thats the great thing about the nature of these beauties: we just dont know how they are gonna behave.

NB: your so lucky having been to the states *jealous vibes*

Heh....my reference for downshear Dave is mirthful....my lecturer took great delight in explaining it to the class....6 times because of all the puzzled faces. Damn shear being a frame of reference.

Good stuff John. If you have time, would you be able to explain that in more detail - as soon as you mention 'frame of reference' I think there are some puzzled faces browsing the forum   I think we should keep this discussion going forward and as I said delve into vorticity and other aspects that will help storm chasers, who dont have a extensive physics background (including me), to understand some of the key physical processes (without going into the Navier-Stokes equations ). Good stuff!!

Dear God, Navier-Stokes are just about the most evil eqns know to mankind squared. To explain: basically the eqns allow the description of the movement of fluid between points, factoring in viscousity(fluid resistance), gravity, drag, and any other possible thing that can effect the motion of a fluid. As you can guess what comes out is an impossible to solve eqn with too many variables to solve...so hence we remove terms: but then it isnt truthful: Hence the Navier-Stokes phenomena and general evilness.

Now....frame of reference. Basically say you are moving at the same rate as a thunderstorm. (Eg 20km/hr east), now to you the storm would relatively stay still and hence be RELATIVELY stationary. Now shear also occurs within these confines. So hence downshear is in the direction of propagation of the storm:even though the relative shear might be upshear. This is particular evident in squall line type systems. The relative shear is negative(hence relatively upshear) allowing the cold pool emanating from the gust front to bulge at the leading edge of the storm(as the moving cold pool runs into the opposing shear), which generates new convection, and hence propagation.

Now for an example. Say our storm is moving to the right at 70km/hr, Now lets say that the shear is 50km/hr also to the right. This means that the RELATIVE SHEAR(effective) is to the left at 20km/hr.

Thats frame of reference. Vorticity generation is a far more detailed concept and im going to have to do some rereading to make sure i get it right before discussing it. I will also look into discussing shear convective intiation: a very important concept in lower cape enviros for organised convective development.

John   

Just a thought: In a CAPE environment of 5000 and LIs in the extreme -8 and beyond is the atmosphere that 'thick' so to speak that one can actually feel it?  Is the environment similar to the feel of tropical humidity or what?

Was just thinking about it and decided to ask. 

Mike

Edited starting comment

The lifted index has no bearing: its just a stability measure. The CAPE yes values of 5000+ would be considered oppressive or 'thick': to produce it you need very warm, very moist air. I suspect it would be close to tropical feeling, although the atmosphere is more primed in a non-tropical location(excluding wet season), and will mix out with the slightest forcing.