CAPE: How Fast Can It Build?

[Bob Hartig]

Let's say you're sitting in an area with great moisture, great shear, great everything except for two things: CAPE and insolation. You're socked in with a crapload of convective waste. But a clear slot is gradually working its way toward your area from the southwest. It's still hours away, but when it arrives...

That's actually my thinking starting off on Chase Case #7. What I'm wondering is, just how fast can CAPE build from nothing to a good, fat profile once the clouds burn away and the hot sun shines? I realize that plenty of variables are involved, but what's a reasonable expectation? Minutes? An hour? Several hours?

 

[Rich Thompson]

Bob,

The CAPE increase will occur at the same rate as the surface temperature increase, assuming nothing else changes aloft and surface dewpoints are maintained. Surface temperatures can easily warm 5-10 F in an hour due to surface heating after clouds move away during the afternoon, which can translate to 500-1000 J/kg of sbCAPE in about an hour.

You might make a point of watching the OK mesonet meteograms this spring. They provide measured insolation (incoming solar radiation), along with temperature, wind, and dewpoint traces. Plus, you can see plots of 3 hour temperature change on the public site:

http://www.mesonet.org/

This should give you a better feel for the rate of change of surface temperatures and the resultant changes in instability.

Rich T.

 

[Chris Allington]

Another thing that could cause some rapid destabilization is strong CAA in the upper/ mid levels of the atmosphere. Since CAPE is the measure of an unstable parcel of air rising to its equilibrium, either warmer surface air... or colder air aloft can create stronger instability becasue the warm moist, parcel of air will be more bouyant.

 

[Bob Hartig]

Rich and Chris--Thanks, guys. You've been most helpful. It's good to have some sense of a rough time frame. And your suggestions are really useful, Rich, as is the link to the OK mesonet.

Bottom-line, it sounds like CAPE can build pretty quickly under the right circumstances, be that surface heating or cold air advection at mid-levels.

 

[Bobby Prentice]

Most significant changes to atmospheric stability (i.e., CAPE) are caused by an increase of low level moisture (i.e., surface dewpoint). During a typical severe storm event, a one degree Farenheight (1 F) increase in surface dewpoint is equivalent to a two or three degree Farenheight (2-3 F) increase in surface temperature.

Cold air advection (CAA) in the mid- and upper-levels is usually not the driving factor which destabilizes the atmosphere. In fact, studies have shown the mid- and upper-level temperatures typically increase during synoptically-driven tornado outbreaks.

A slight tangent to Bob's original inquiry, but important to emphasize nonetheless.

 

[Mike Johnston]

From a practical standpoint, one thing you could do on active days is check the SPC mesoanalysis. Put some of the CAPE analyses in loop mode, and it will give you some feel for how quickly, and to what extent, instability can build (and CIN can erode.)

 

[Matt Gingery]

From a practical standpoint, one thing you could do on active days is check the SPC mesoanalysis. Put some of the CAPE analyses in loop mode, and it will give you some feel for how quickly, and to what extent, instability can build (and CIN can erode.)

Thats how I do it. Earl Barker is a great site for that. . .

 

[Rich Thompson]

Don't overlook the new 3 hour change fields that are part of the SPC mesoanalysis sectors. Three CAPE change fields are shown, and these changes will directly estimate the rate of destabilization.

Rich T.