The Cap: How Do You Know What It's Going to Do?

Jul 2, 2004
Hastings, Michigan
From today's (7/13) target area discussion:

"Expect some supercells at the start of the event, but with the cap weakening, linear convection should organize. Derecho event very possible..."--rdewey

I understand how too weak a cap can lead to widespread, linear convection versus supercells. What I don't get is, how do you determine what the cap is doing or is going to do? Soundings are only done twice in a 24-hour period, right? Is that enough for you to extrapolate cap tendencies? How do you figure it out?
You have to think about it in terms of what the atmosphere is going to do between 12 and 00Z. The most obvious change would be surface heating. As the surface heats, the LFC is lower and thus the parcels begin to life moist adiabatically much earlier, so they stay warmer and can break the cap. Increasing the surface dewpoint does the same thing.

Also of importance is synoptic scale lift. A passing shortwave at 700 mb will lift the layer and cause the cap to weaken a little bit. It's kind of hard to write about, but if you go to the link below, it does a good job of explaining it.

Finally, you have to think about mesoscale influences such as cold pool/shear interactions where opposing vorticities at the edge of the cold pool enhance lifting which can overcome a capping inversion. Outflow boundaries, confluence lines, drylines, jet maxima, etc. can all contribute to generating enough dynamic lift to break the cap.

Then again, I'm just a kid so what do I know.
Regarding the correlation between cap strength and convective mode... I think it's a slight misconception to think that a weak cap will always yield linear convection, while a strong cap will yield supercells. The mode of convection depends upon many things -- strength and 'shape' of forcing, etc. Imagine it as a continuum box:

WF - WC --------------------- WF - SC
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SF - WC --------------------- SF - SC

where WF = weak forcing, SF = strong forcing, WC = weak capping, SC = strong capping...

Now, in cases of strong forcing (say linear too) and weak cap, there will be a great tendency for MCS / linear development, and in cases of weak forcing and strong cap, there may very well be no convective development (cap bust)... The rest of the possible combinations of forcing and capping yield the many different tendencies, beit mostly discrete cells or mostly linear convection.

Why do I not think cap strength alone has too much to do with convective mode? Even when you have a weak cap, you still need convergence (forcing). You can have no cap and still not have convection. Likewise, even if you have a really strong cap, you can have widespread convection if the forcing / convergence is high enough to overcome the convective inhibition.

Of course, it's important to remember that many other factors play into this... For example, the orientation of the shear vector relative to a boundary providing convergence ... Shear vectors perpendicular to a boundary (presumably, the boundary is providing the convergence) will tend to yield discrete cells, while shear vectors parallel to the boundary tend to yield convective seeding and MCS development...

Now, back to the topic.... The strength of the cap is a function of the temperature (technically, virtual temperature, but lets start basic) difference between the parcel and it's environment, assuming the parcel is "cooler" than the environment. Now, lets just say, for simplicity, that the atmosphere does not change through the day except for the surface layer. Now, lets say we warm and/or moisten the surface parcel. This will effective reduce the strength of the cap since the parcel will have a "warmer" temperature than it had before -- so, relatively, it is less cool than before, and thus the strength of the cap is weaker. Now, in the case of moistening the parcel (say, via increasing the dewpoints), the parcel attains saturation at a lower level (it's lifted condensation level is lower), and thus it cools at a slower rate (moist adiabatic -- ~5.8c/km -- vs. dry adiabatic -- 9.8 C/km). So, above the LCL level, and below the level of free convection, the moistened parcel will be 'warmer' than the previous unmoistened parcel. Now, in the case of increasing the temperature of the surface parcel, semi-obviously, if you raise the original temperature of the parcel, the parcel at a particular level below the LFC will have a higher temperature than the non-heated parcel would have...

So.... On a normal summer day east of the Rockies, given insolation (incoming solar radiation -- aka the sun is out), the surface tends to both increase in temperature and moisture (via evapotranspiration -- moisture from plants), and thus the relative cap strength tends to decrease. Now, remember that the example in the previous paragraph assumed that we kept the atmosphere of alone save the surface. Obviously, it is seldom that the temperature and moisture profile of the atmosphere above a given location doesn't change at all in a 6-12 hr period. However, also remember that solar radiation impacts, largely, only the lowest portion of the atmosphere (say, the lowest couple of km). There are other important processes, however, that should be accounted for to get an accurate forecast of the temperature/moisture profiles -- such as subsidence (which tends to warm the 500-850mb layer), synoptic-scale lift (which tends to cool the 500-850mb layer), upwind mid-level precipitation (which tends to cool the 500/700 - 850mb layer). Yes, there obviously are exceptions, but this is a simple explanation / example...

So... How to do this with only the typical 2-launch day? Well, first off, on a "big days", NWSFO's tend to release special 18z soundings, which usually help a great deal in determining cap strength a few hours before the typical convective initiation time of late afternoon (again, when the temperatures are usually highest, and thus cap strength, with all that we are assuming, is weakest). A lot of times, model data is used to help shed light on forecast cap strength. Othertimes, since the mid-upper levels TEND not to change drastically in 6-12 hours, 12z soundings can be adjusted for the forecast afternoon surface temperature - dewpoint.

Much of this is best seen on skew-T log-P charts... Since I don't know your level of meteorological knowledge, I apologize if you find anything above obvious and are offended. Likewise, if the above is still confusing, I'm sure Mike G (the "links guy"), can post some good websites about skew-t log-p charts.
Hey, guys, thanks!

While some of what you've shared is over my head--i.e. the interaction of shear vectors with boundaries--the bulk of it is something I think I can chew on and digest. Very helpful.

Jeff, regarding my knowledge of skew-T, log-P, I've been reading up on that subject lately, to the point where I'm developing a basic grasp. One of the guys at the NOAA office down the road kindly lent me a publication by Doswell et al on basic convection, and its coverage of the skew-T, log-P is the most thorough I've found so far. Of course, it begets plenty of questions that I won't get into here--except for one :) : When I pull up soundings on the Internet, I find a map with what seems like a relative handful of sources. For southern Michigan, for instance, there's DTX; for Illinois, ILX; for Iowa, DVN, and so forth. Nothing at all in Indiana. Obviously, there are a lot more local weather stations than that! And some of them seem to become per-need secondary sources. But do meteoroligists have access to soundings that I'm not aware of, or are you able to put together a national picture based on what I can access through, say, Robert Lattery's RAOB link?
There are no sounding observations in Indiana... For forecast soundings you use model output or modify an observed one for conditions you expect in your area.

- Rob
There's a program here on campus that allows you to manipulate the Skew-Ts. Using current datas, you can make adjustments as needed to see at what thresholds various things could happen. GARP I believe is the name (or filename) of this program.
There's a program here on campus that allows you to manipulate the Skew-Ts. Using current datas, you can make adjustments as needed to see at what thresholds various things could happen. GARP I believe is the name (or filename) of this program.

NSHARP allows you to modify soundings and view model generated soundings...

As for the cap in this event... Given the NW flow aloft, and the cooler air at mid levels sliding SE into the threatened region, along with very strong SFC heating and moisture pooling... The CAP would weaken significantly between 12Z and 00Z, so that any weak linear convergence would have created more of a line, rather than individual storms. Also, HP supercell and bow echo environments overlap, and with the flow becoming unidirectional as storms moved SE... I expected more of an MCS...

Just too bad my prediction for 7/13 didn't pan out, at least the last half. I expected supercell type storms at the beginning (not as early as they started, I was thinking a bit later actually)... Then I expected a pretty solid line of convection from northern MI SWestward into northern IL/IN/OH. The northern part did develop, but the early morning supercells created subsidence, and supressed any significant development across IL (the intiation zone)...