Some questions about storm initiation and CIN

[Verhaegen Yoni]

Hi all,

I have been doing some self-research on internet concerning thunderstorms and their initiation. I do, however, have some questions that are still not clear to me. Can anyone help me out and/or confirm that I got things right? Thanks!

First:
So, given an unstable atmosphere, thunderstorm initiation can occur when (a) the convection temperature is reached or (b) CIN is absent. However, in some situations deep moist convection still will not occur given that one of these 2 requirements is fulfilled (e.g. with high vertical wind shear beneath the cloud base or entrainment of dry air). Sometimes, initiation does occur when there is still some CIN left (e.g. with elevation convection). Can anyone confirm that I got things right?

Second:
When a rising parcel does not come across CIN, it will spontaneously rise from the surface to the LCL and further until hitting the equilibrium level. However, I was thinking that the LCL is only used for lifted air parcels (contrary to the CCL). So isn't this some kind of contradiction: with no CIN, the parcels rise "spontaneously" to the LCL, which actually is a level that requires "forced lifting" to be reached?

Third:
When CIN is present, it can be reduced by (a) heating below (until convection temperature reached), (b) moistening below, and (c) vertical motion, which then cools air adiabatically (cool the cap layer). However, for some mechanical lifting mechanisms, CIN does not need to be reduced for thunderstorm initiation, as the air can be lifted mechanically to a level above (or "through") the CIN (orographic lifting, lifting with fronts/outflow boundaries).

Thanks for your time and effort! I would really appreciate it!

 

[Jeff Duda]

First:
So, given an unstable atmosphere, thunderstorm initiation can occur when (a) the convection temperature is reached or (b) CIN is absent. However, in some situations deep moist convection still will not occur given that one of these 2 requirements is fulfilled (e.g. with high vertical wind shear beneath the cloud base or entrainment of dry air). Sometimes, initiation does occur when there is still some CIN left (e.g. with elevation convection). Can anyone confirm that I got things right?

This is very close, but you're missing an important detail: a thunderstorm can form when an air parcel reaches its level of free convection (LFC). But let's back up a bit.

First, your conditions (a) and (b) are actually directly related - if the convective temperature is reached, then (assuming the PBL mixed completely above it) by definition the value of CIN will be 0 J/kg. This relationship falls out of how the convective temperature is defined/calculated.

Second, the convective temperature should be regarded as more of a diagnostic than a prognostic feature. Relying too much on the convective temperature to discern when storms should fire is more likely to confuse you than to help you understand what the atmosphere is doing. I believe your reliance on the convective temperature idea is the cause of your confusion on the below-quoted statement; it is very confusing for me to read. Try not to think about thunderstorm initiation this way.

Second:
When a rising parcel does not come across CIN, it will spontaneously rise from the surface to the LCL and further until hitting the equilibrium level. However, I was thinking that the LCL is only used for lifted air parcels (contrary to the CCL). So isn't this some kind of contradiction: with no CIN, the parcels rise "spontaneously" to the LCL, which actually is a level that requires "forced lifting" to be reached?

The most important thing to remember here is that a parcel needs to reach its LFC. That can be done many ways, but as long as a parcel reaches its LFC, you're in business.

Third:
When CIN is present, it can be reduced by (a) heating (of the air parcel) below (until convection temperature reached), (b) moistening (of the air parcel) below, and (c) (large-scale) vertical motion, which then cools air adiabatically (cool the cap layer). However, for some mechanical lifting mechanisms, CIN does not need to be reduced (fully eliminated) for thunderstorm initiation, as the air can be lifted mechanically to a level above (or "through") the CIN (orographic lifting, lifting with fronts/outflow boundaries) its LFC.

I have modified the above quote to contain more proper language that describes the means of reducing or eliminating CIN and promoting thunderstorm initiation.

Please respond with questions and clarifications!

 

[Zack Cooper]

First of all- remember that "parcels" of air are theoretical. With that said parcel theory is the best thing we have so moving forward I'll refer to them. A parcel of air is free to rise if there is no inhibition present, but a key here is that parcels will not RESIST vertical motion in the absence of inhibition, not that they are guaranteed t automatically ascend in the absence of it. Some source of lift is still necessary to get a parcel to its level of free convection. This is the level at which a parcel will continue to rise if some source of ascent has already gotten it there. Sources of lift can be obvious (sharp cold front) or very subtle and not easily seen on a map (thermals). The key here- is getting a parcel to the LFC.

Inhibition can be reduced in a lot of ways- but on most typical plains severe weather days the stage is set by background vertical ascent which happens slowly and then usually ascent along a convergent boundary is enough to overcome it. Again- there are other ways. If you heat to convective temperatures as you mentioned, you will be eliminating CIN. The problem is- as Jeff stated- if you are relying on something like the convT on a 12z observed sounding to determine whether or not you are going to get initiation, you're using a diagnostic tool to make a prognosis. It is best to try and determine if you think you are going to get initiation using other factors.

 

[Verhaegen Yoni]

This is very close, but you're missing an important detail: a thunderstorm can form when an air parcel reaches its level of free convection (LFC). But let's back up a bit.

First, your conditions (a) and (b) are actually directly related - if the convective temperature is reached, then (assuming the PBL mixed completely above it) by definition the value of CIN will be 0 J/kg. This relationship falls out of how the convective temperature is defined/calculated.

Second, the convective temperature should be regarded as more of a diagnostic than a prognostic feature. Relying too much on the convective temperature to discern when storms should fire is more likely to confuse you than to help you understand what the atmosphere is doing. I believe your reliance on the convective temperature idea is the cause of your confusion on the below-quoted statement; it is very confusing for me to read. Try not to think about thunderstorm initiation this way.



The most important thing to remember here is that a parcel needs to reach its LFC. That can be done many ways, but as long as a parcel reaches its LFC, you're in business.



I have modified the above quote to contain more proper language that describes the means of reducing or eliminating CIN and promoting thunderstorm initiation.

Please respond with questions and clarifications!

Hi, thanks for this explanation. So the basic things to remember:

- Convective temperature and CCL are just diagnostic tools, only use them to forecast the possibility of occurrence of convective clouds
(is this because rising due to convection alone is usually too slow/weak for thunderstorms to grow vertically?)
- For thunderstorm initiation, look if the parcels reach their LFC, which can be done by eliminating CIN (heating and moistening of the air parcel, large-scale vertical motion) or in some cases, air parcels get lifted mechanically to their LFC without fully eliminating CIN (fronts, outflow boundaries, orographic lifting, I guess in these cases it is best to rely on models?). To check this, just use the LCL in stead of the CCL on soundings.

 

[Jeff Duda]

Hi, thanks for this explanation. So the basic things to remember:

- Convective temperature and CCL are just diagnostic tools, only use them to forecast the possibility of occurrence of convective clouds
(is this because rising due to convection alone is usually too slow/weak for thunderstorms to grow vertically?)

The utility of the convective temperature lies in the fact that it implies a dry-adiabatic lapse rate all the way to the LFC, which is why it is indicative of the ability of storms to form in the absence of forcing. In a situation in which the convective temperature is reached and the PBL above it is perfectly well mixed, any parcel that rises for any reason will not stop rising, continuing through the PBL to its LFC, at which point it will be able to buoyantly accelerate upward until it his its equilibrium level.

- For thunderstorm initiation, look if the parcels reach their LFC, which can be done by eliminating CIN (heating and moistening of the air parcel, large-scale vertical motion) or in some cases, air parcels get lifted mechanically to their LFC without fully eliminating CIN (fronts, outflow boundaries, orographic lifting, I guess in these cases it is best to rely on models?). To check this, just use the LCL instead of the CCL on soundings.

Not quite. The level of free convection (LFC) is an independent vertical level from LCL and CCL. It is merely defined as the lowest level at which a lifted parcel becomes warmer than the environment such that it has a consistently buoyant path to near the tropopause (we have to add all those little notes in there for cases with thin layers with rapidly varying temperature typically lower to the ground...in cases of "cleaner" profiles, the definition simplifies to "the level at which a lifted parcel first becomes warmer than the environment"). The LCL and CCL have their own, separate definitions. It is possible that the LFC may be at the same height as the LCL and/or CCL (and in fact, in almost every case the LFC = CCL), but again, in instances of forced lifting, which are common, storms develop well before the convective temperature is reached (which is the only occasion in which the CCL has any importance).

 

[Verhaegen Yoni]

The utility of the convective temperature lies in the fact that it implies a dry-adiabatic lapse rate all the way to the LFC, which is why it is indicative of the ability of storms to form in the absence of forcing. In a situation in which the convective temperature is reached and the PBL above it is perfectly well mixed, any parcel that rises for any reason will not stop rising, continuing through the PBL to its LFC, at which point it will be able to buoyantly accelerate upward until it his its equilibrium level.



Not quite. The level of free convection (LFC) is an independent vertical level from LCL and CCL. It is merely defined as the lowest level at which a lifted parcel becomes warmer than the environment such that it has a consistently buoyant path to near the tropopause (we have to add all those little notes in there for cases with thin layers with rapidly varying temperature typically lower to the ground...in cases of "cleaner" profiles, the definition simplifies to "the level at which a lifted parcel first becomes warmer than the environment"). The LCL and CCL have their own, separate definitions. It is possible that the LFC may be at the same height as the LCL and/or CCL (and in fact, in almost every case the LFC = CCL), but again, in instances of forced lifting, which are common, storms develop well before the convective temperature is reached (which is the only occasion in which the CCL has any importance).

Hi Jeff,

Thanks for your time and answers and correcting me where needed. Sorry for my late reply, it has been quite busy lately.

Thus, in a proper thunderstorm forecast assessment, you should check for each level in the lowest 300 hPa the amount of CAPE (MUCAPE) and CIN of a parcel that initiates at the corresponding level, to check if convective initialization is possible. Concerning CIN reduction: mechanical lifting is the forced lifting of air parcels towards their LFC which does not necessarily eliminates CIN (air mass boundaries + orography), while dynamical lifting is large scale vertical motion which cools the cap layer and reduces CIN (convergence zones, PVA, etc.), correct?

So basically, when you look at numerical model output and they both forecast a significant amount of SB CIN as well as thundery showers, you know you're dealing with either elevated convection OR air that has been lifted mechanically to their LFC? If it is elevated convection, you need to find the level of the parcel with the lowest CIN, and this is likely to be the level from which thunderstorm initiation should occur (if this CIN approaches zero and there is still some CAPE left, of course)? If it is mechanical lifting, parcels can still develop from the surface because air is forced to its LCL and LFC?

I hope I got most things right this time...
Thanks!

 

[Jeff Duda]

Thus, in a proper thunderstorm forecast assessment, you should check for each level in the lowest 300 hPa the amount of CAPE (MUCAPE) and CIN of a parcel that initiates at the corresponding level, to check if convective initialization is possible. Concerning CIN reduction: mechanical lifting is the forced lifting of air parcels towards their LFC which does not necessarily eliminates CIN (air mass boundaries + orography), while dynamical lifting is large scale vertical motion which cools the cap layer and reduces CIN (convergence zones, PVA, etc.), correct?

Checking each individual level in a human way would be a little cumbersome. That's why the "most unstable" parcel was derived...it takes care of most of that work for you. So, yes, I would evaluate the MUCAPE and, if available (on many websites that produce NWP output graphics, this product is not), MUCIN.

As far as categorizing the type of forced lifting, you are generally correct. However, I tend to group mechanical and dynamical sources of lift under just the umbrella term "forced lift". But you are right about what each does to promote storm development.

So basically, when you look at numerical model output and they both forecast a significant amount of SB CIN as well as thundery showers, you know you're dealing with either elevated convection OR air that has been lifted mechanically to their LFC? If it is elevated convection, you need to find the level of the parcel with the lowest CIN, and this is likely to be the level from which thunderstorm initiation should occur (if this CIN approaches zero and there is still some CAPE left, of course)? If it is mechanical lifting, parcels can still develop from the surface because air is forced to its LCL and LFC?

Again, this is generally correct. However, if you see substantial SBCIN and thunder, I would definitely check SBCAPE as well just for good measure.

For completeness, your second statement is also generally correct, but is missing some detail (that may not be that important). The vertical level that has the lowest CIN is the level from which the initial parcels that reach their LFCs and start the thunderstorm are most likely to emerge. Parcels may enter a burgeoning updraft from other vertical levels, though. Then, once a thunderstorm updraft is firmly established, parcels are likely to enter the updraft from just about any layer that has substantial CAPE, as the kinematic activity within the updraft itself will usually be enough to get parcels with large CIN to overcome that and subsequently reach their own LFCs.

Also keep in mind that mechanical lifting is not limited just to the surface. Low-level convergence, say along the nose of a low-level jet at the 925- or 850-hPa level, can represent mechanical lift as well.

 

[Verhaegen Yoni]

Also keep in mind that mechanical lifting is not limited just to the surface. Low-level convergence, say along the nose of a low-level jet at the 925- or 850-hPa level, can represent mechanical lift as well.

Hi!
Thanks! Very glad to hear! Concerning your last statement: does this mean that low-level convergence can be categorized into both "mechanical" and "dynamical" lift, depending on whether you're dealing with an air mass boundary or not?

Thanks!

 

[Jeff Duda]

Thanks! Very glad to hear! Concerning your last statement: does this mean that low-level convergence can be categorized into both "mechanical" and "dynamical" lift, depending on whether you're dealing with an air mass boundary or not?

It is your choice to separate forced lift into "mechanical" vs. "dynamical", so I see that question as your prerogative to answer.