Atmospheric Stability - Thunderstorms

[JD Jaya]

When the adiabatic lapse rate becomes warmer than the environmental lapse rate at, say, 5000 feet, is that the point where an uplifting force is no longer necessary? My understanding is that below 5000 feet, the air remains stable, so an uplifting force is needed to reach that altitude. Once at 5000 feet, however, the adiabatic air would be warmer and less dense than the surrounding air, causing it to rise on its own.

In addition, In an absolutely unstable atmosphere, where the adiabatic lapse rate consistently exceeds the environmental lapse rate, at what point do clouds stop forming? Do they stop once they reach stable air in the stratosphere/tropopause?

 

[Dan Thomas]

The lapse rate has to do with the temperature decease with altitude, from the ground up. Normally, it's about 2°C per thousand feet. At this rate, or less, the air is stable and won't support thermal lift. But if that lapse rate gets a bit steeper, more than 2°C per thousand feet, air that rises will cool at 2°C per 1000 as its pressure drops, so it ends up warmer than the surrounding air at any altitude and is less dense, so less heavy, and it continues to rise. If that air has enough water vapor in it, that water condenses as the pressure and temperature fall, releasing MORE heat into the air and making it rise even more aggressively. That heat was the energy it took to turn the water into water vapor in the first place. This is where thunderstorms develop.

When that rising air hits the tropopause, the meeting place of the troposphere and stratosphere, the rise usually slows or stops. That's where you see the classic "anvil" head of the storm. At the tropopause, the lapse rate reverses and the air begins to get warmer with more altitude, so it won't support a bunch more lift. But if the thunderstorm was enthusiastic enough, the momentum of that rapidly rising air will punch through the tropopause and go thousands of feet into the stratosphere, making some really damaging thunderstorms.

 

[dmspilot]

Not a meteorologist but I'll try.

When the adiabatic lapse rate becomes warmer than the environmental lapse rate at, say, 5000 feet, is that the point where an uplifting force is no longer necessary? My understanding is that below 5000 feet, the air remains stable, so an uplifting force is needed to reach that altitude. Once at 5000 feet, however, the adiabatic air would be warmer and less dense than the surrounding air, causing it to rise on its own.

Where did this 5,000 feet come from? Is that just an example? If so, yes, the environmental lapse rate can change at different altitudes. In your example, a stable layer below 5,000 and unstable above 5,000 is possible. In such a scenario, fronts (e.g. a cold front pushing up air in front of it) would be the main lifting source.

In addition, In an absolutely unstable atmosphere, where the adiabatic lapse rate consistently exceeds the environmental lapse rate, at what point do clouds stop forming? Do they stop once they reach stable air in the stratosphere/tropopause?

Typically yes, because the ambient lapse rate changes at the tropopause. It's a stable layer that acts like a lid for the vast majority of weather activity.

 

[bbchien]

Jd, it's about 16,000 feet (in summer).

 

[Dan Thomas]

Jd, it's about 16,000 feet (in summer).

What is? The tropopause? Never. It's found between 30,000 feet at the poles and 56,000 feet at the equator, with some seasonal variations from that, but never as low as 16,000 feet.

 

[Dan Thomas]

Dan,

The tropopause comes in three flavors, thermal tropopause, dynamical tropopause and if you want to get really picky, there's also the chemical tropopause (stability-based, dynamics-based and composition-based). For now, let's focus on the former since that's the emphasis of the OP's question. Its height varies from 9 to 12 miles in the tropics and 5 to 6 miles (~25,000 feet) in polar regions. But if the atmosphere is extremely cold (during a polar vortex scenario), it's not out of the question that the thermal tropopause can be as low as 20,000 feet in the most northern parts of the contiguous U.S. (frankly, when it is this cold it can be difficult to determine the exact height of the tropopause). I doubt you'll ever see a tropopause height as low as 16,000 feet over the U.S., it may occur in the polar regions on a very rare occasion.

Thanks for the details. Most pilots will never get that much (I didn't) and it will likely be of interest only to meteorologists like yourself. For me as a CPL and instructor (Canadian), the basics as I described them were plenty to let me and my students know when to expect thunderstorms and determine their potential ferocity. I taught them to look at the upper winds, which give speeds, directions and temperatures, and to see what the temp differences were between altitudes. More than 6 degrees per 3000-foot reporting level was a concern. Then check the temp and dewpoint to see what moisture was available. That tells me what cloud ceilings to expect. That was usually enough to know whether thunderstorms were likely or not. Sometimes I could expect those storms even though the forecasts didn't mention them.

Even my Air Command Weather Manual, the official text used in the training of Canadian military pilots, doesn't give the detail about the various tropopauses as you do. It does discuss the seasonal variations and the expected turbulence there, particularly that associated with the jet streams.

 

[scottd]

Most pilots will never get that much (I didn't) and it will likely be of interest only to meteorologists like yourself. For me as a CPL and instructor (Canadian), the basics as I described them were plenty to let me and my students know when to expect thunderstorms and determine their potential ferocity. I taught them to look at the upper winds, which give speeds, directions and temperatures, and to see what the temp differences were between altitudes. More than 6 degrees per 3000-foot reporting level was a concern. Then check the temp and dewpoint to see what moisture was available. That tells me what cloud ceilings to expect. That was usually enough to know whether thunderstorms were likely or not. Sometimes I could expect those storms even though the forecasts didn't mention them.

Dan,

If the environmental lapse rate approaches the dry adiabatic lapse rate (3°/1000 ft), the air is indeed unstable. However, to get deep, moist convection, three things are usually needed.
(1) Instability
(2) Moisture
(3) Some kind of outside energy contribution (or anything that creates ascending air).

You can have plenty of #1 and #2 with zero risk of deep, moist convection because #3 is unavailable in sufficient quantity. For example, below is an example from my Skew-T book where the surface-based CAPE is over 7600 J/kg (surface-based lifted index is -8), so there is a huge of instability and plenty of moisture...but in this case, there's zero chance for any convection...even shallow convection. In fact, this is a forecast for clear skies.



This is due to the fact that this all occurred under a ridge and subsidence (sinking air) dominated the area. This created a formidable cap with convective inhibition (CINH) of -167 J/kg. So the big weather picture is quite important here which is what I typically use to determine whether or not convection is likely. By the way, the tropopause in this case was likely above 50,000 feet.