Plotting CAPE

[Sam Kennedy]

In Forecast Lab the temperature of an ascending parcel isn't plotted on the soundings. Here is an example(from a past chase case )

The surface temperature was 50°F so that is 10°C, how would you plot the temperature of a parcel as it ascends so that you can get some estimate of the amount of CAPE?

 

[Jeff Duda]

Sam,

This is something you should be learning in meteorology classes. Otherwise, I refer you to www.theweatherprediction.com/thermo for a fairly in-depth education on the topic.

To answer your question, the parcel path (i.e., the temperature of a parcel as it ascends from a given level) is drawn by lifting the parcel dry adiabatically from its starting point to its LCL (determined by following a constant mixing ratio line from the starting point up to where that line intersects the dry adiabat along which the parcel is ascending), and then lifting the parcel along a moist adiabat after that point. Both dry and moist adiabats are drawn on any standard skew-T, so you should learn which lines are which and practice drawing some parcel paths. You don't even need actual data plotted on a skew-T to practice.

 

[Paul Austin]

Here's another great resource. It's an interactive module. I love the COMET modules. There's one on just about any weather-related topic you can think of. And they are rated 1 to 3 on level of difficulty/technicality. You have to register, but it is free, and they send out notices via email for new module postings.

https://www.meted.ucar.edu/loginForm.php?urlPath=mesoprim/skewt#

I tried to draw the lifted parcel lines on your given graphic and post here, but I screwed up the drawing. Maybe someone with better graphics tools can assist. Basically, CAPE would be the area bounded by the lifted parcel line along its moist adiabat and the temperature line. Jeff's description is correct with the "starting point" of the mixing ratio line corresponding to the surface dewpoint.

Hope this is helpful. Both links should be.

EDIT for clarification: The graphic calculation of CAPE described above (the area between two curved lines) only applies if the lifted parcel line actually crosses over the temp line. It is the area under the curved moist adiabat portion of the lifted parcel's path, but bounded below by the temp line, not the other way around.