CAPE question

[BBauer]

I know CAPE is needed in order to at least initiate storms. Once storms fire with the proper moisture, instability, and triggering mechanism can they survive in areas that have little to no instability? Or will storms dissipate once they enter such an environment. It would seem that warm parcels of air would no longer rise if the temperature gradient no longer existed but maybe I am missing additional factors that would help the storm survive. Looking at the various models it seems CAPE is sometimes very localized hence my question.

 

[rdale]

Model CAPE does not always equal real-world CAPE, so you're starting from the wrong end by using that for your analysis. But your summary is correct, if you don't have all three ingredients (moisture / lift / instability) then the storm will weaken.

 

[Rob H]

At a high level your understanding is spot on - you need some degree of instability for updrafts to form. Jon Davies has a nice write up on narrow instability axes, and upslope events can get by with as low as 300 J/Kg MLCAPE, but for the most part, your storm needs to be in an environment with ample instability. What defines 'ample' is of course, up to the storm I usually look for at least 1000 J/Kg and that the storm will be in an area at least 100 miles wide with some instability. This gives the storm 2 hours or more to produce.

A complicating factor is that even the HRRR only has a resolution of 3km, and radiosondes might only go up twice on a potential chase day, so models can't ever resolve the exact instability that an environment has. On normal synoptic setups where you have a surface low, a warm front, and a cold front/dryline you can pretty much assume that upper air destabilization will be taking place to some degree, and that low-level warm air advection will be helping from the ground up.

Helicity can also play a role in minimizing the amount of CAPE that is necessary for a strong, persistent updraft. Near low convergence can also pull in a small tongue of CAPE just long enough for a quick tornado to form. I'll leave the storm scale dynamics to those with meteorology backgrounds however.

 

[Dan Robinson]

A strongly forced squall line on a plowing cold front can maintain itself with very little instability, however these tend to be very narrow and have very little lightning. On models you need to check for instability at different levels - you can have no surface-based CAPE, but lots of elevated instability overhead.

 

[Jason Foster]

Side question/confirmation:

SB vs. ML CAPE doesn't change much forecasting wise for me on the east coast (at a average elevation of 300' above sea level) versus the central plains (with elevations averaging 3000' plus above sea level). Trying to get my head out of the Mid-Atlantic dynamics and back into Plain State dynamics.

 

[BBauer]

Thanks for the replies. So is having elevated instability more important than surface based CAPE as long as there is a mechanism to feed air into the upper level environment? I understand there are many variables, but as a general rule where do you want CAPE to be the highest?

 

[BBauer]

Model CAPE does not always equal real-world CAPE, so you're starting from the wrong end by using that for your analysis. But your summary is correct, if you don't have all three ingredients (moisture / lift / instability) then the storm will weaken.

Is there a way to measure real-world CAPE, how do I start from the "right end"? Thanks for the help, I'm a big time newbie (not that anyone could not tell). My goal is learn a little bit of severe weather forecasting before the peak of the season hits here in Iowa when I'll start chasing locally. It hurts since I have not been able to get out on these early season storms. Hopefully the opportunities keep coming come late May/June!

 

[rdale]

The best way is to look at an upper-air sounding. Other ways include looking at 00hr plots from the RUC and/or HRRR. But a forecast CAPE is bound to be bad.

 

[BBauer]

The best way is to look at an upper-air sounding. Other ways include looking at 00hr plots from the RUC and/or HRRR. But a forecast CAPE is bound to be bad.

Awesome, thanks! Pieces of the puzzle coming together very slowly, but surely

 

[Greg Blumberg]

I would also say that the concept of "real-world CAPE" is difficult to measure, regardless of whether or not it comes from a model or observations.

The level at which thunderstorms draw their parcels of air can vary the amount of CAPE a thunderstorm is realizing. CAPE is only a measure of positive buoyancy over a distance. Positive buoyancy is when the parcel is warmer than it's environment. The units of CAPE are Joules per kilogram (J/kg.) (Ignore the kg part of that for now.) A Joule is a measurement of energy. To calculate energy, you multiply a force by a distance. In the case of CAPE, the force is positive buoyancy and the distance is the vertical extent of the atmosphere in which you are positively buoyant.

If you take parcels from the surface (surface based CAPE or SBCAPE) you will often get a larger amount of CAPE a.) because the surface often contains the warmest (and most moist) parcels in the sounding and b.) because you sometimes are looking at a longer distance the air travels while it is positively buoyant. If you look at MLCAPE or mean layer CAPE, you are taking the using the average temperature and moisture content in the lowest 100 mb of the sounding and then using that to compute the CAPE. These average value will likely not be the one from the surface and will reduce your CAPE value. Whatever level you are using to compute the CAPE will give you different CAPE values. In conclusion, you can't determine the exact value of CAPE a thunderstorm is realizing because you have no idea of the exact location it's lifting it's parcels from. Because of this, CAPE calculations in forecasting often serve more of an qualitative estimate of instability (weak, moderate, strong, extreme).

CAPE is a theoretical computation of the amount of kinetic energy an updraft could have. Because we're not considering any other forces that will act against (or with) a parcel's rising motion (we're only considering buoyancy), we get a theoretical value for maximum updraft speed by finding out the square root of 2*CAPE. A more correct value of the updraft speed is MUCH more difficult to pull off because we have to consider lots of other contributions to vertical motion that are hard to account for at each level of the atmosphere.

You are absolutely correct in saying that CAPE is very localized! This is one of the reasons it's hard to predict thunderstorms. Our upper air network is so spread out, that we cannot correctly measure how CAPE is changing at individual locations. Additionally, certain small scale meteorological features that are hard to resolve can significantly alter the amount of CAPE over a few kilometers.

 

[J Allen]

Theres also the issue of storm modification of the ingested CAPE environment, depending on whose theory you believe, which is so far below the model or radiosonde scale that its really impossible to take any real meaningful CAPE value. So even in a marginal environment the actual CAPE experienced may be well above what the environment might otherwise indicate. In essence CAPE is just an indicator of instability, probably one of the better ones (say compared to a fixed layer index like LI). There is a huge issue with what I have seen thus far in this thread and its the concept of balance. For the type of storms the majority of us seek to chase the instability is just one piece of the puzzle. Seeking out the highest possible CAPE is also not the answer, especially in a weak shear environ (storm goes up, storm goes down). I've seen real rapid rotating supercells in what would technically be a warm season environment with only 700 CAPE, granted they were balanced by 40 knots of shear, but realistically the CAPE becomes academic...If instability exists, if a trigger exists and some sort of shear is juxtaposed upon it there is a potential for convection, and then I worry about balance and mode to ascertain if its worth my while. The same can be said for any day with CAPE over 3000, these environments just become academic all you really need to ascertain for this is there is a crapload of energy there. Sure, there are cases where you get an exceptional CAPE value doing some incredible things with not that bigger shear, but the environment still balances (Vis. Bowdle, Jarrel, Plainview)...generally these cases are the exception rather than the norm.

As Greg above says, CAPE is Convective Available Potential Energy, rarely entirely realised (and definitely not without the presence of VWS) and a theoretical conception of the energy available within the atmosphere. Like all indices its a reasonable way to get a picture but the numbers probably aren't going to be listened to when a storm surprises you by changing the rules .

I suppose on a more grand conceptual front you have the basic idea there, storms are a means to an ends of mixing instability resulting from the differential between the surface and upper troposphere. However, the model you seem to suggest is that upon consuming just its local environment the storm would collapse as it mixes itself out? By generating that lift the storm causes convergence, air rushes in from nearby to fill the void. Provided the storm can maintain its updraft, it continues to feed and cycle this air until such time as it becomes unavailable. The inflow level winds can also accentuate this (think turning on gas, running a richer mixture into your engine).

Personally I would take CAPE and just use it to get a picture of the instability side of things, make your decisions based on the combination of ingredients and a trigger, and not just one variable.

Hope this rather rambling post helps.

 

[BBauer]

Huge help guys, thanks for all the great info!