Cap/CINH Question

[Dan Rupnow]

Maybe a dumb question, but I'd like to know why the capping layer, which is warmer than air above it, doesn't rise?

Also, this is my first post on here, so a big hello to everyone!

 

[Dann Cianca]

I think that a better way of thinking of the capping layer would be that it is warmer than the air below it. Generally speaking (without inversions), temperature decreases with height (lapse rate) until you start getting into the stratosphere and higher.

On days where there is no capping inversion, you still find that temps decrease with height. Now, exactly how steep a lapse rate you have speaks to how stable the atmosphere is. A steeper lapse rate, the more unstable (the more likely the air is to rise).

Hope that helps, didn't have a lot of time to reply.

Anyone want to add anything?

 

[Bobby Prentice]

Maybe a dumb question, but I'd like to know why the capping layer, which is warmer than air above it, doesn't rise?

The quick lay-person answer is...this can happen when the air within the "capping layer" is more dense ("heavier") "than air above it."

where R is the universal gas constant, P is the pressure, M the molar mass, and T the absolute temperature.

From the density equation above, we can see that density is inversely related to temperature, but directly related to pressure. Yes, temperature decreases with height above an inversion layer (by definition), but pressure decreases too.

Actually, showers and thunderstorms sometimes form from rising air parcels based at the top of an inversion layer if they are convectively unstable. This is a common occurrence with fronts.

WES Case for 8 June 2002 with Elevated Severe Thunderstorms by Andrew Kleinsasser
WFO Glasgow, Montana

Glasgow, Montana Elevated Thunderstorm Sounding - June 8, 2002

 

[Dan Rupnow]

Thanks for the replies. Very insightful, it makes sense when you account for pressure.

 

[Jeff Snyder]

Bobby took care of noting that density is a function of temperature AND pressure, so I'll take a bit of a different view. A vertical profile of virtual potential temperature would help explain it a bit too. Let's assume an unsaturated air parcel... If the environmental lapse rate is less than 10C/km in magnitude (i.e. 5C/km), a parcel that's forced to rise will be colder (more dense, technically) than the surrounding environment. This negative buoyancy will cause the parcel to accelerate downward. For what it's worth, the now descending parcel will tend to "overshoot" its original level, being pushed downward, where it will be warmer (less dense) than the surrounding environment. Now, it will accelerate upward again, and so on, in an oscillating manner. Such a process demonstrates thermal stability -- no matter whether the parcel is originally forced upward or downward, it will be forced back to its original level. If virtual potential temperature increases with height, there is implied stability for an unsaturated parcel; virtual potential temperature that is constant with height is nearly equivalent to dry adiabatic lapse rate (~9.8C/km), so virt. pot. temp. that increases with height signifies that the temperature decreases with height less than the dry adiabatic lapse rate. Vertical changes in mixing ratio affect the density of the parcel too, which is important to keep in mind too, and it is the reason why we look at the virtual temperature when trying to assess 'dry' stability.

But what happens if the environmental lapse rate is greater than 10C/km in magnitude? Suppose, for example, that the temperature at 1km AGL is 20C, and the temperature at 2km AGL is 0C, representing an environmental lapse rate of 20C/km. If the parcel at 1km is pushed upward to 2km, its temperature will be 10C. Uh oh -- the parcel temperature is 10C higher than the ambient temperature, and this positive bouyancy will result in an upward acceleration. These "superadiabatic" lapse rates are relatively common immediately above the surface during the day (assuming there's sunshine). The lapse rate between 1mm above an asphalt parking lot and 10 m above that parking lot can be much, much greater than the dry adiabatic lapse rate. In such cases, auto-convection occurs -- the "air" immediately above the asphalt accelerates upward, while the air farther above the asphalt sinks (in a way, to fill the "void" caused by the rising near-surface air)... Imagine a pot of water on the stove -- water at the bottom of the pan, immediately adjacent to the heating element, is heated more quickly and to a higher temperature than the water at the top of the pot. If you watch closely, you'll see convection in action.

FWIW, similar things happen in the boundary layer when the sun rises and diabatic heating commences. Sensible heating at the earth's surface leads to the creation of what we call the "convective boundary layer" (CBL). As solar heating continues, the boundary layer tends to deepen (air mixes from above as heated air at the surface rises, leading to the familiar characteristic of boundary layers -- constant potential temperature and mixing ratio with height, at least above the surface layer). I'm going off a tangent here, so I'll stop.