Cap Question

[Robert Edmonds]

So, I know the cap is a layer of warm air that prevents an air parcel to continue to rise. I have also heard that a cap can lead to potential increase in instability. I also have heard there are a lot of uncertainties with forecasting and dealing with a cap, so I won’t be surprised if there isn’t a clear answer for this question. My question deals with the cap temperature and altitude. Does a “good” cap vary in temperature with respect to altitude? To me it would seem the lower the cap in altitude the stronger you would want the cap to be with respect to temperature. This idea was developed after making some assumptions. First, a larger portion of air will be required to be heated than if there is a higher cap versus a lower cap. A smaller amount of air will be hotter than a large amount of air, if given the same amount of energy. So, from these two assumptions if we were to give a day of heating to air under a high cap versus a low cap, and we were given the same starting temperatures and initial conditions, the air under the low cap should be hotter than the air under the high cap if nothing breaks. Therefore to achieve maximum heating and instability, yet still give the potential for the cap to break we would want a weaker cap at a higher altitude or a strong cap at lower altitudes.

Well so that’s it. Waiting for someone to blow me out of the water, or just say I’m down right wrong. If doesn't make much sense that's cause it's late.

 

[Jason_Politte]

A good article written by Tim Marshall regarding forecasting the cap can be found at:

http://www.stormtrack.org/library/forecast/cap.htm

Jason

 

[Robert Edmonds]

Let me reword one of my fundamental assumptions and perhaps someone can answer my question.

From the article provided through the link:

The strength of the capping inversion changes throughout the day as daytime heating erodes the cap from below.

This makes me ask...

Does the rate at which daytime heating progress change with the cap height?

 

[Fred Gossage]

If you have Yahoo IM man......message me at fredgweather. I'll help ya out....

 

[Bill Tabor]

Originally posted by Robert Edmonds
Does a “goodâ€￾ cap vary in temperature with respect to altitude?

I believe you are basically correct in your analogy. And yes due to adiabatic lapse rate air at higher altitudes typically is a lower temperature than those below. Here's a link that describes it: http://daphne.palomar.edu/jthorngren/adiab...c_processes.htm. Since the temp decreases with altitude the cap temp will typically be a smaller value as well allowing a capping inversion at a lower temperature.

Here's a loaded gun sounding where you can see that to some degree as well. The values of the inversion higher up are lower temp than those further down.
http://www.theweatherprediction.com/thermo.../soundings/gun/

Recall also that heating is not the only variable when it comes to breaking the cap. Upper level mixing, or precipitation can help erode the cap. Short wave troughs and upper jets creating lift can help to break it. Additionally very strong surface convergence near boundaries and drylines can also assist.

The key is to know if it is generally close to being breakable considering what you see on the map and whether those factors will come into play to assist breakage of the cap.

 

[Jeff Snyder]

Originally posted by Robert Edmonds
So, I know the cap is a layer of warm air that prevents an air parcel to continue to rise. I have also heard that a cap can lead to potential increase in instability. I also have heard there are a lot of uncertainties with forecasting and dealing with a cap, so I won’t be surprised if there isn’t a clear answer for this question. My question deals with the cap temperature and altitude. Does a “goodâ€￾ cap vary in temperature with respect to altitude? To me it would seem the lower the cap in altitude the stronger you would want the cap to be with respect to temperature. This idea was developed after making some assumptions. First, a larger portion of air will be required to be heated than if there is a higher cap versus a lower cap. A smaller amount of air will be hotter than a large amount of air, if given the same amount of energy. So, from these two assumptions if we were to give a day of heating to air under a high cap versus a low cap, and we were given the same starting temperatures and initial conditions, the air under the low cap should be hotter than the air under the high cap if nothing breaks. Therefore to achieve maximum heating and instability, yet still give the potential for the cap to break we would want a weaker cap at a higher altitude or a strong cap at lower altitudes.

Well so that’s it. Waiting for someone to blow me out of the water, or just say I’m down right wrong. If doesn't make much sense that's cause it's late.

So you're asking something like this: is a 2F cap at 850mb better or worse than a 2F cap at 700mb? If the LCL occurs below the capping level, then the cap layer will result in lesser (or no) CINH, since the parcel now cools at the moist adiabatic lapse rate instead of the dry (~6.5C/km vs. 9.8C/km). Of course, in terms of convection, there are two forces at play here -- the amount of CINH and the strength of the forcing. For the most part, convective initiation or failure is the result of capping vs. forcing. A weak cap can still suppress convection if forcing is weak, just as a strong cap can still be broken if there is sufficiently strong forcing. Oh yeah, by forcing I'm referring mainly to surface convergence. Remember, even if there is strong DPVA aloft or low-level WAA, vertical motion aloft tends to be quite gentle, on the order of centimeters per second, nothing like the magnitude of deep moist convection, which tends to be on the order of meters or dekameters per second. As Bill noted, on the synoptic scale, vertical motion aloft (such as that caused by DPVA, divergence associated with transverse circulations, etc) can result in a weakened cap owing to the fact that upward motion is a cooling process that can remove a cap sufficiently to allow for convective initiation. I'd explain more, but it's late.

 

[Robert Edmonds]

Sorry for the confusion this question was hard to articulate.

This is what I was thinking...

These are my very imaginary soundings.

Please note that the features are hugely over done to show the features I am asking about, and probably are about to make me look like an idiot. Notice the cap has a higher temp difference in sounding 1 than in sounding 2 (T>t). I was thinking that the lower the cap the higher this difference should be. This is because notice how much more air will have to be heated in sounding 2 than in sounding 1(notice H<h) to erode the cap. Is this true, or am I an idiot like I theorized?

Is this something that I should even think about when looking at a sounding?

 

[Fred Gossage]

The temperatues within a capping inversion closer to the surface probably WILL be higher that those in a capping inversion closer to say 700-mb...... but so with the SURFACE temperatures......by heating anyway. You must also remember, one main thing that causes a cap to be there in the first place, is the advection of warm, dry air above the warm, moist boundary layer....so in the ideal severe weather setup......most of the warm air in the capping inversion isn't from low-level heating......cause low-level heating will help to warm the temperatures below the inversion and help to cause it to erode.

In a good severe weather setup, I would want to see the moisture deeper and richer below the cap....so I would want the cap a little bit off the ground..... This way, the moisture is too deep to be mixed out and the boundary layer become drier once mid-level dry air advection and daytime heating come into play.......