Cap erosion

[chrisbray]

Hey all,

I was reading the SPC day 2 today and it said "BOTH THE NAM AND GFS SUGGEST INTENSE BOUNDARY LAYER
HEATING WILL OCCUR AHEAD OF THE FRONT ACROSS KS/NWRN OK/TX
PANHANDLE. THIS WILL WEAKEN THE CAP AND COMBINED WITH FRONTAL
CONVERGENCE SHOULD PROVE ADEQUATE FOR TSTMS TO DEVELOP ALONG/JUST
BEHIND THE ADVANCING WIND SHIFT TOWARD EARLY EVENING."

Which got me thinking, while I understand the concept of a cap, I'm not sure I get the process of how surface heating would weaken the cap. Is it more that the continuous heating of the boundary layer causes the air to rise and displace the cap above it, or is it that when surface air rises in a thermal it is hot enough to be bouyant despite the warm air inversion above it? Or is it something else? Thanks for the help,

Chris

 

[chrisbray]

From what I've been studying it seems to be more that the air is hot enough to be bouyant even through the inversion, but I wanted to make sure there aren't other factors at play here.

 

[Zach Young]

During the day, the atmosphere warms from the ground up. This is because the atmosphere is mostly transparent to the wavelengths of light that the sun radiates most strongly. Early in the day the atmosphere usually has an inversion (warm air over cool air) near the surface which suppresses convection. This is the most common location for the cap, but there can also be inversions higher up that cap things, but surface heating doesn't contribute as much to overcoming those.

Since the atmosphere is transparent, sunlight will shine straight through the warmer layer aloft and so its temperature remains unchanged (unless different temps blow in from elsewhere). The sunlight hits the ground which is not transparent. The ground absorbs the radiation and releases its thermal energy into the air. As the day progresses, the layer beneath the inversion heats up. It will tend to evenly because there is convection happening within the confines of the inversion, even if there are no clouds present. At the beginning of the day the inversion might be warm over cold, by mid-morning it might be warm over cool, and sometime in the afternoon/evening the inversion is no longer there as the temps become warm over warm, or warm over hot. This process where the inversion gets weaker throughout the day is what people mean when they refer to the cap "eroding". It's not that the parcels of air become bouyant enough to break the cap, it's that the cap itself gradually becomes non-existant.

Hope that helps! I don't have any example skew-T's available or I could probably explain this more clearly...

 

[GPhillips]

Albany has some pretty nice animations from COMET of soundings illustrating ways to weaken the convective inhibition/cap. Look at 9 through 11 at this link:

http://www.atmos.albany.edu/daes/atmclasses/atm301/CAPE.htm

 

[chrisbray]

Albany has some pretty nice animations from COMET of soundings illustrating ways to weaken the convective inhibition/cap. Look at 9 through 11 at this link:

http://www.atmos.albany.edu/daes/atmclasses/atm301/CAPE.htm

Thanks to both of you!

 

[samuel stone]

simply put, a cap is is a layer of air above a cooler layer of air. for air to rise. you need the air above it to be cooler. so the below air does not move upward in that situation., as the lower level of air is heated up through the day, it becomes warmer than the air above it. so then it rises. and the cap is broken. that is just the very fundamental process. which it seams you were not understanding