OK You Knowledgeable Weather People

[Alex Lamers]

Here's an excerpt from an MD. I was wondering if you could explain what the bolded parts mean. It was issued today...

ASSOCIATED LEADING EDGE OF STRONGEST LARGE SCALE ASCENT
ALOFT WILL OVERSPREAD DIABATICALLY DESTABILIZING AIR MASS ACROSS
CENTRAL MS...AMIDST FAVORABLE DEEP-LAYER SHEAR...HELPING TO MAINTAIN
ORGANIZED SEVERE POTENTIAL.

I've heard the term used a lot. Diabatic heating. What does it mean? Are there any variations on this theme? I probably know of the concept...just have never associated it with the term.

By the way, I love Roger Edwards' SWOMCD's. I either learn a vocab word or a new concept seems like everytime I read them

 

[rdale]

After sunset, the upper-levels cool faster than low levels in many instances...

 

[Kiel Ortega]

Originally posted by rdale
After sunset, the upper-levels cool faster than low levels in many instances...

Actually, levels near the ground cool a lot faster than upper levels (unless advection is stronger at the upper levels), that's why radiational inversions set up overnight. Air doesn't really like changing temperature (it's a decent insulator), you can see this from soundings at 12Z to soundings at 00Z (and 00Z to 12Z). As long as there isn't much advection, one will see that the surface temperature and low level temperatures change way more than levels farther from the ground.

 

[nickgrillo]

Originally posted by Kiel Ortega+--><div class='quotetop'>QUOTE(Kiel Ortega)</div>

<!--QuoteBegin-rdale

After sunset, the upper-levels cool faster than low levels in many instances...

Actually, levels near the ground cool a lot faster than upper levels (unless advection is stronger at the upper levels), that's why radiational inversions set up overnight. Air doesn't really like changing temperature (it's a decent insulator), you can see this from soundings at 12Z to soundings at 00Z (and 00Z to 12Z). As long as there isn't much advection, one will see that the surface temperature and low level temperatures change way more than levels farther from the ground.[/b]

Correct...

At night, the air closest to the surface will cool the most (leading to boundary layer decoupling) in the majority of situations and a layer of warm air will form up on top of the cooled layer (radiational inversion) -- which is what often leads to the demise of surface-based storms into the overnight hours (absolutely not the case within some situations, and a good example would the 11-6-2005 Evansville, IN tornado) as the increasing CINH stops the ascent of surface-based parcels (i.e. storms then have to feed off elevated parcels rooted above the inversion layer... "elevated" convection... which often decreases the chances for tornadoes).

Originally posted by nickgrillo
...are becoming elevated and CINH strengthens through diabatic sfc cooling...

In the quote above (from the 1/2/06 NOW thread) I was referring to CINH strengthening through nocturnal cooling of the surface layer (which eventually leaded to boundary layer decoupling and destroyed the chances for any more surface-based storms with associated tornado/damaging wind risk) -- i.e. a diabatic process -- as this was occuring from longwave radiation (e.g. term "diabatically-driven boundary layer decoupling").

An excerpt from the 5-13-05 northwest TX MCD:

Originally posted by Roger Edwards
THESE FACTORS COMBINE WITH DIABATICALLY DESTABILIZING
BOUNDARY LAYER AND STEEP MIDLEVEL LAPSE RATES -- EVIDENT IN 18Z AMA
RAOB AND RUC SOUNDINGS

The sun is the biggest contributer to diabatic heating -- as the sun will warm parcels of air at the surface -- without them ascending or descending (i.e. NOT an adiabatic process). Therefore, the process has nothing to do with a sinking or rising motion (an adiabatic process) to a parcel -- therefore the "diabatically destabilizing" or "diabatic heating" terms will be used.

I hope this helps answer your question

 

[rdale]

Sorry - I read diabatic and answered diurnal!

"Actually, levels near the ground cool a lot faster than upper levels (unless advection is stronger at the upper levels), that's why radiational inversions set up overnight."

I'm completely off this thread now ;> but there are instances in the summer when the low levels are quite humid and upper-level cooling results in growing instability and storm development - many times unforecast. That was where I was going with my answer.

 

[Bill Tabor]

Originally posted by Alex Lamers
Here's an excerpt from an MD. I was wondering if you could explain what the bolded parts mean. It was issued today...

ASSOCIATED LEADING EDGE OF STRONGEST LARGE SCALE ASCENT
ALOFT WILL OVERSPREAD DIABATICALLY DESTABILIZING AIR MASS ACROSS
CENTRAL MS...AMIDST FAVORABLE DEEP-LAYER SHEAR...HELPING TO MAINTAIN
ORGANIZED SEVERE POTENTIAL.

Alex, I don't think anyone really answered your original question - that of diabatically destabilizing air. Apparently SPC uses this term from time to time. Since diabatic air is air that is heated or cooled without rising or falling in height, I would assume the term means that the air mass mentioned is becoming unstable due to diabatic temperature process, and in this case an apparent short wave or similar feature of 'large scale ascent aloft' is overspreading the lower airmass. What I can't tell from the term in this case is if they are inferring instability of the diabatic airmass in conjunction with the overspreading upward vertical velocities, or separate from them. My guess however is that some process is contributing to diabatic heating of the lower airmass which is then particularly becoming unstable as the large scale ascent aloft overspreads that airmass. Does this sound right to you?

Edit: Or perhaps as Dale mentions above it is evaporative cooling above the airmass that is making it unstable & that partially due the the new airmass (perhaps drier) entering the area overhead? Thoughts?

 

[Mike Johnston]

Bill, it sounds to me like you are on the right track. I read over the question again, looked at the forecast in question. Obviously the inference was a destabilizing airmass at lower levels. Nine out of ten times, such a forecast would probably read just thus without the qualifying term. Introduction of the term in question was probably superflous although technically accurate. I suppose one could use the term, in a similar situation, anytime when either diurnal heating, WAA or whatever destabilizes the lower atmosphere.

Re: Nick's comments on nocturnal de-coupling of the boundary layer and the Evansville event, I am convinced this tornadic outbreak was as much because of, and not in spite of, the boundary layer de-coupling. As is often the case, this process enhances the speed of the low level jet. We have seen it over and over again at night this cool season; the LLJ feeding and invigorating the front nose of an MCS. Clearly, mesocyclone circulations evolve in these environments. Although I am completely unable to explain tornadogenesis in these situations (can anyone, really?), observation alone shows a vigorous nocturnal LLJ is to be reckoned with as a synoptic factor.

 

[nickgrillo]

Actually, Bill... I would have to think my explanation above would have answered his question about diabatic heating (and destabilizing) pretty well. What Edwards was likely implying in this MD was that sun's energy was warming the surface layer diabatically. In addition, the large-scale ascent was likely provided by the increasingly divergent flow aloft / H5 s/w moving into the LA/MS region today (this was MD #111) -- which provided for synoptic-scale ascent (there was numerous supercells that produced large hail across the Gulf Coastal states today). Edwards (and I always like to myself) likes to use and elaborate on a situation with techniqual terms in his products (always very well-written).

Originally posted by Bill Tabor


Alex, I don't think anyone really answered your original question - that of diabatically destabilizing air.

Edit: Or perhaps as Dale mentions above it is evaporative cooling above the airmass that is making it unstable & that partially due the the new airmass (perhaps drier) entering the area overhead? Thoughts?

 

[Bill Tabor]

Originally posted by nickgrillo+--><div class='quotetop'>QUOTE(nickgrillo)</div>

Actually, Bill... I would have to think my explanation above would have answered his question about diabatic heating (and destabilizing) pretty well. What Edwards was likely implying in this MD was that sun's energy was warming the surface layer diabatically. In addition, the large-scale ascent was likely provided by the increasingly divergent flow aloft / H5 s/w moving into the LA/MS region today (this was MD #111) -- which provided for synoptic-scale ascent (there was numerous supercells that produced large hail across the Gulf Coastal states today). Edwards (and I always like to myself) likes to use and elaborate on a situation with techniqual terms in his products (always very well-written).
[/b]

You know what I might have missed your last paragraph somehow:
<!--QuoteBegin-nickgrillo

The sun is the biggest contributer to diabatic heating -- as the sun will warm parcels of air at the surface -- without them ascending or descending (i.e. NOT an adiabatic process). Therefore, the process has nothing to do with a sinking or rising motion (an adiabatic process) to a parcel -- therefore the "diabatically destabilizing" or "diabatic heating" terms will be used.

Looks like you did define 'diabatic / heating destabilization' - I was also kind of looking for the meaning in the context, or trying to explain it's use in the context Edwards was using it in. But it's probably just straightforward like you say - heating at the surface and overhead divergence or something similar adding to the situation or combining to promote an increased risk of severe.

 

[Rich Thompson]

I'm jumping in a little late, but I know Roger's intent in the MD:

"diabatically destabilizing air mass" simply refers to the impact of daytime heating on surface temperatures, low-level lapse rates, and lifted parcel CAPE. Daytime heating was warming the ground and adjacent air mass across central MS, resulting in destabilization. The approaching belt of ascent is largely an adiabatic process, while the development of the "cold pools" with the storms is considered diabatic.

Adiabatic refers to a "reversible" process - in the absence of condensation, if you lift a parcel its temperature will cool, and if the same parcel sinks back to the same level, the temperature returns to the original value. Diabatic processes result in a net gain (like surface heating with sunshine) or loss (evaporative cooling by rain falling into unsaturated air) of heat for the air parcel. "Moist adiabats" on a skew-T diagram represent a pseudo adiabatic process where you assume that the condensate (cloud/rain drops) doesn't fall out, and the parcel doesn't feel any impact of the outside environment. In other words, the parcel rises and saturates, it carries the condensed water with it. When sinking back down to the original level, evaporation of the condensate keeps the temperature from warming as fast as unsaturated (dry abiabatic) descent. Does that make sense?

Rich T.

 

[Alex Lamers]

Originally posted by Rich Thompson
I'm jumping in a little late, but I know Roger's intent in the MD:

"diabatically destabilizing air mass" simply refers to the impact of daytime heating on surface temperatures, low-level lapse rates, and lifted parcel CAPE. Daytime heating was warming the ground and adjacent air mass across central MS, resulting in destabilization. The approaching belt of ascent is largely an adiabatic process, while the development of the "cold pools" with the storms is considered diabatic.

...

Rich T.

Yup makes sense Rich, and everyone else. Thanks! I knew the process but as usual, sometimes the lingo gets me caught up...only been a major weather follower for 4 years (approx.) so I'm still trying to learn it all but I'm starting to nail down a lot of the concepts.

BTW, keep up the good work SPC!

Thanks again all!

 

[Rich Thompson]

Alex,

I made a mistake in my earlier post about pseudoadiabatic processes. I actuallydescribed the saturated adiabatic process in which an parcel keeps the condensate and the process is reversible. The "pseudo" part means that the condensate falls out and takes heat with it. As you'll see in thermodynamics, the heat content of the condensate is small compared to the heat content of the air parcel, thus a saturated adiabat is essentially the same as a pseudoadiabat.

Rich T.