Elevated Instability

[TalcottWx]

I have often struggled with grasping the concept of it. I remember events with a tremendous amount of lightning affecting BOS and often actually bringing us our best storms of the summer (as most know we often struggle with marine air/cinh/etc that far east)

 

I've been reading this article on events that took place out in the plains in situations of zero cape and no surface instability; (http://www.crh.noaa.gov/crh/ssd/pdf/ta06_02.pdf).

 

I understand that obviously an elevated unstable layer of the atmosphere is what triggers it. But I do not comprehend how cape exists above the surface without daytime heating. Is it always leftover from the daytime hours? If the convection is elevated, is there a higher cloud ceiling? Obviously steep lapse rates,cold pools, and an increase of moisture often keep cells rolling in the nighttime hours or a strong front/lift.

 

Following is an excerpt from the paper:

"The 0600 

UTC sounding showed no surface-based convective available potential energy (CAPE) and a 

surface lifted index (LI) of +9ºC. Using the most unstable parcel, CAPE ranged from 15 J/kg at 

JMS to 356 J/kg at FAR (not shown). The 700-500 hPa lapse rates were 8.2 C/Km at 0600 UTC 

for JMS, indicating any elevated parcel would rise freely in this layer if lifted to this level. In 

addition, the most unstable parcel LI decreased to -2ºC, and the K index, a measure of 850 hPa to 

500 hPa layer moisture, increased from 15 to 25 from 0000 UTC to 0600 UTC."

 

How does the parcel lift to the elevated unstable layer if there is no surface based instability? Parcel stuff confuses me.. I understand it at the surface..

 

Also... Help me understand a situation in which there is elevated instability present.. but no thunderstorms. Like as in a surface based situation, when instability is high and no storms fire.. say to do lack of lift etc.

 

Also.. How can I visually see an elevated instability situation on a skew t or model product?

 

 

 

[weatherwiz]

Getting a bit complex here, first off, when measuring cape (convective available potential energy), there are numerous methods for evaluating the presence of instability in the atmosphere.  The three most common and widely used methods are;

 

surfaced-based (where you use a parcel of air from the surface to evaluate the degree of instability in this atmosphere by lifting this parcel of air to the level of free convection (the LFC),

 

mixed-layer cape (the mean cape available for a parcel of air lifted from the lowest 100mn of the atmosphere to the LFC).  

 

most-unstable cape (this represents the total amount of energy available for the most-unstable parcel of air in the lowest 300-mb of the atmosphere)

 

What can happen is you can have situations where you don't have much or any surfaced-based instability (in which when you look at SBCape plots you see nada), however, above the boundary layer, you can have copious amounts of instability...so why does this occur?  You can have numerous reasons...

 

 At the surface a loss of daytime heating and rapid drop of temperatures creates a stabilizing presence at the sfc and just above the surface.  However, above the boundary-layer you may have steep mid-level lapse rates (indicating a stronger temperature gradient which that by itself leads to an unstable airmass).   

 

You can have a very moist low-level airmass in place, or increasing moisture in the lower levels of the amtosphere (dewpoint advection, theta-e advection), you can have a warming low-level airmass.  

 

You can have cold air advection occurring in the mid-levels of the atmosphere (700-500mb)...this cooling aloft creates a larger temperature gradient between the lower and mid-levels of the atmosphere (also referenced by steepening or steeper mid-level lapse rates) and this leads to a boost in instability...when we have those situations where we see severe at night or the potential this right here is usually the #1 player.  

 

Now how does this help with storms?  

 

Well this leads to what is called "elevated thunderstorms"...all this means is that the thunderstorms initiate from conditions present above the boundary layer and were not formed (or surviving) from lifting parcels from or near the surface, the parcels of air are being lifted from above the boundary layer so if you have instability present here it's enough to feed the storm for a time being.  However, typically you also want some degree of wind shear to help as well.  

 

With regards to your question about the sea-breeze and marine influence typically killing thunderstorms but not these types of storms it's all b/c the instability being used from the storm is not from the surface, therefore, any marine influence has zero effect.  This is why in the hail year of 2008 storms would not only survive all the way to the coast but they thrived...that year we had tons of elevated instability present thanks to the cold pools aloft and steep lapse rates...much of the instability was generated above the boundary layer and the marine influence typically only dominates below the boundary layer.  

 

For your second to last line there regarding where instability is present but no storms fire you hit the nail on the head...no lift.  While instability and strong instability alone generate a degree of lift, you need another forcing mechanism to really enhance the process as these features aid to build the updrafts large enough to where they are able to utilize everything the atmosphere has to offer...they also had in an important ingredient and that can be vorticity advection, divergence, convergence, etc. 

 

For elevated instability a good product to look at is MUcape values...you can also look at K-Index values (anything above 30 is decent...getting above 35-40 and that's a sign of strong instability), TT's index (40-45 is marginal to decent...45-50 is decent, and getting above 50 and that's a sign of stronger instability), and SI (around 0 to slightly below is marginal...-2 to -4 that's pretty decent, and -5 to -6 or lower and that's a pretty strong signal of great instaiblity).  

[CoastalWx]

You can see elevated instability in BUFKIT. That yellow line will show you elevated CAPE...if you have used BUFKIT you'll know what I mean. I'll make it simple. You obviously need air to rise, but how? Well it could be from a warm front or leading edge of a push or warm moist air. Sometimes this is accompanied by a LLJ. That LLJ can be the impetus to get air to rise because the leading edge or nose of it is where you'll have convergence. If the air is unstable..well it has no where to go, but up. Often these low level jets of warm and humid air are shooting in much faster than the atmosphere can adjust. What I mean is upstream mid level temps are warm, but many times the atmosphere becomes unstable to this quick shot of warm and moist air in the lower levels. Mid level temps are relatively cool compared to upstream. So as long as you have a reason for air to rise above the inversion near ground, you'll get precip and/or convection. 

[LocoAko]

Lots of good stuff already in the responses above. Just adding my two cents...

 

I have often struggled with grasping the concept of it. I remember events with a tremendous amount of lightning affecting BOS and often actually bringing us our best storms of the summer (as most know we often struggle with marine air/cinh/etc that far east)

 

I've been reading this article on events that took place out in the plains in situations of zero cape and no surface instability; (http://www.crh.noaa.gov/crh/ssd/pdf/ta06_02.pdf).

 

I understand that obviously an elevated unstable layer of the atmosphere is what triggers it. But I do not comprehend how cape exists above the surface without daytime heating.

Keep in mind CAPE is a potential energy. That is, the amount of CAPE present doesn't actually deal with how it may ultimately be released, etc. and is just a measure of how much energy a parcel COULD have if it were released. The CAPE an elevated unstable parcel has is just a measure of how much energy that parcel has relative to its environment, regardless of any mixing from the surface, sources of lift, etc. See the Skew-T below for an example of the instability relative to a local environment.

 

Is it always leftover from the daytime hours?

It doesn't have to be (i.e. in theory you could get an atmospheric setup where the correct temperature structure advects in and causes elevated instability) but it most often forms from instability that was originally surface based that is then "cut off" from the surface via a temperature inversion at night time due to radiational cooling.

 

If the convection is elevated, is there a higher cloud ceiling? Obviously steep lapse rates,cold pools, and an increase of moisture often keep cells rolling in the nighttime hours or a strong front/lift.

Assuming you mean cloud base, that is very often the case though isn't necessarily true. The cloud base is just a function of when the parcel reaches saturation. You could have an elevated parcel that, when lifted, reaches saturation very quickly, or you could have thunderstorms that are surface based with very high bases. But of course, starting the lifting from a higher point in the atmosphere increases your chances of a higher cloud base.

 

Following is an excerpt from the paper:

"The 0600 

UTC sounding showed no surface-based convective available potential energy (CAPE) and a 

surface lifted index (LI) of +9ºC. Using the most unstable parcel, CAPE ranged from 15 J/kg at 

JMS to 356 J/kg at FAR (not shown). The 700-500 hPa lapse rates were 8.2 C/Km at 0600 UTC 

for JMS, indicating any elevated parcel would rise freely in this layer if lifted to this level. In 

addition, the most unstable parcel LI decreased to -2ºC, and the K index, a measure of 850 hPa to 

500 hPa layer moisture, increased from 15 to 25 from 0000 UTC to 0600 UTC."

 

How does the parcel lift to the elevated unstable layer if there is no surface based instability? Parcel stuff confuses me.. I understand it at the surface..

The parcels do not originate at the surface and break into the unstable layer -- it is the parcels in the unstable layer itself that rise. It is exactly analogous to the surface, except that the lift isn't occurring at the surface, it is occurring aloft. Or to think of it another way -- the entire layer could be rising, but only those with elevated instability will eventually utilize their CAPE.

 

Also... Help me understand a situation in which there is elevated instability present.. but no thunderstorms. Like as in a surface based situation, when instability is high and no storms fire.. say to do lack of lift etc.

As you wrote yourself, it is analogous. CAPE is only a measure of potential energy, but says nothing about actually utilizing it.

 

Also.. How can I visually see an elevated instability situation on a skew t or model product?

Here's an example of a sounding with elevated instability and no surface instability, taken from http://www.estofex.org/guide/1_2_4.html:

As you can see from a parcel lifted at the surface, it saturates quickly and rises along a moist adiabat but has no CAPE as it remains entirely to the left of the environmental temperature trace. However, above the surface cold air and on top of the inversion, a parcel lifted from 940 hPa will ascend, saturate (at around ~875 hPa in this Skew-T) and will rise up the moist adiabat. This time, though, the parcel has positive CAPE values, and could then ascend freely until it reaches its Equilibrium Level.

Everything with elevated instability is pretty much analogous to surface-based instability except that the "surface" is now (usually) above a temperature inversion. Keep in mind that because there is that inversion that exists below the thunderstorm and the source region for the inflow isn't at the surface but is instead aloft, the effects of elevated convection can differ from surface based convection. Generally, the threat for tornadoes and damaging winds is much lower in elevated convection (since the inversion inhibits mixing) but the threat for hail will often be higher because the hail will fall out of the storm and into a relatively cool surface layer, delaying melting.

Hope this helped a bit and wasn't just a repeat of everything above.

[CoastalWx]

What Jake said. Good detailed info. The tricky part is finding the mechanism for air to rise.

[TalcottWx]

This is outstanding. Many of the same triggers exist for both elevated and surface based instability, which is fascinating. Man, I need to make more threads like this!! Awesome guys.

[ROOSTA]

Fantastic discussion and explanations. I often visit the COMET site, no interaction.