Novice Guide to Caps...

[Andrew Burnett]

I need some help learning more about identifying caps and how to tell if and where they may weaken enough to allow storm formation. Any tidbit would be helpfull, but I am primarily talking about using the models. Don't assume I know anything.

Thanks for any help.

 

[ahaberlie]

Here is my understanding...

A cap is an inversion in the ambient temperature lapse rate..

So lets say temps at the surface are 75 degrees.. but temps at 950 mb are 78 degrees.. surface parcels will be unable to rise past this "Cap" until they are sufficiently heated or lifted above the cap with a lifting mechanism (cold front/warm front)

So a cap can "weaken" enough to the point where an available lifting mechanism can lift air from the surface above this inversion layer... or the surface heats up enough and the environment has absolute instability.. and into layers with favorable lapse rates that allow for explosive vertical development.

Caps are generally "good" in the sense that they can inhibit crapvection and allow for discrete cells or organized squall lines based on the atmospheric conditions above the cap and moisture content of the surface parcel..

I would take a look at radiosonde (weather balloon) Stuve data and read up on LCL, dry moist and environmental adiabatic rates, and CAPE as it applies to these charts.

One instance where you can "see" a cap is when there is a smog issue..

Once again, please correct me if I'm wrong.

 

[Jason Berner]

CAP really refers to the amount of CIN (convective inhibition) there is in the atmosphere at any one point. CIN can be thought of as the opposite of CAPE (to a point) as on a sounding it is the area found where the parcel is cooler than its environment i.e. negatively buoyant. CAPE is the area where the parcel is warmer than its environment and therefore buoyant. The thing to know is that while they seem opposite you can't just simply add the two together to determine the strength of the CAP as, simply put, the atmosphere doesn't work like that. Basically, a warm inversion layer above the surface acts as a lid to rising parcels which keeps rising parcels from rising enough to turn into storms. CIN is one of the indexes we use to determine how strong that inversion layer (i.e. how strong of a lid) is relative to a rising air parcel.

You can find model runs that display the amount of CIN all over. Normally, you will see an inversion around 850-700 mb which the parcel will encounter before finding its LFC. As a day warms up, normally daytime heating and mixing in the atmosphere works to weaken the CAP which is what we look for in terms of initiation. Once the CAP breaks, storms can fire by either having an updraft strong enough to punch thruogh weak CIN or by just not having any CIN at all. Areas along front and boundaries can sometimes have enough forcing to help push particles through a weak CAP but in general you want to see none to very small values of CIN to get storms to fire.

You normally like to see a strong CAP during the day to prevent storms from firing too early in the day when the atmosphere is more stable. By capping through the day, you can think of it like corking a coffee pot and letting the pressure build up. Letting the steam out slowly is much less explosive than all at once and a similar idea holds in the atmosphere, where storms that can hold off until the afternoon to fire will have more unstable atmosphere to work with.

Hope that helps.

 

[Andrew Burnett]

Great stuff, guys that really helps. So you have primarily used CIN forcasts as a determination of when a cap may break?

So if I have it right.. High CAPE and High CIN would be favorable for formation as long as the CIN is forcast to degrade later in the day. (assuming the other factors are there)

 

[Jason Berner]

In general, when you see the CIN decrease throughout the day and then get low to zero, you can start predicting the breaking of the cap. Again, initiation of storms is one of those research topics being currently conducted because we don't understand it that great cause there are days when things look great and even the CIN gets low and you don't get storms. It's one of those things that can be extremely difficult to figure out and busts many chases every year. Personally, CIN is mostly what I look at to determine if the cap is going to get broken but I am sure that more experienced forecasters and chasers may have some other factors they look at.

I would be careful of saying high CIN being good for storms because, in general, the atmosphere isn't that capable of eliminating a real high CIN. High CAPE is of course good. Low to moderate CIN which holds initiation off during the day is also good but that moderate CIN has to be low enough to be broken to actually get initiation.

 

[Karla Dorman]

As an aside to Andrew's question, have one of my own: our LWO (FWD) said in their latest AFD:

LATEST 00Z/7 PM/ FWD SOUNDING SHOWS CAPPING INVERSION STILL PRESENT AND RELATIVELY LOW/DRY ... WITH ONLY A THIN SLIVER OF MOISTURE AROUND 800 MB/6 KFT. LSI/LID STRENGTH INDEX ... A CAP STRENGTH PARAMETER JUST INTRODUCED VERIFIES THESE THOUGHTS ... NOT TO MENTION 700 MB TEMPS AROUND 15 DEG C ...

Where is a map, or what model would the meteorologists/storm chasers use to find the LSI (Lid Strength Index)?

Thank you in advance for any assistance or answers.

 

[Jeff Duda]

As an aside to Andrew's question, have one of my own: our LWO (FWD) said in their latest AFD:

LATEST 00Z/7 PM/ FWD SOUNDING SHOWS CAPPING INVERSION STILL PRESENT AND RELATIVELY LOW/DRY ... WITH ONLY A THIN SLIVER OF MOISTURE AROUND 800 MB/6 KFT. LSI/LID STRENGTH INDEX ... A CAP STRENGTH PARAMETER JUST INTRODUCED VERIFIES THESE THOUGHTS ... NOT TO MENTION 700 MB TEMPS AROUND 15 DEG C ...

Where is a map, or what model would the meteorologists/storm chasers use to find the LSI (Lid Strength Index)?

Thank you in advance for any assistance or answers.

http://www.aprweather.com/pages/calc.htm

Look for it partway down the page.

 

[Karla Dorman]

As originally posted by Jeff Duda

http://www.aprweather.com/pages/calc.htm Look for it partway down the page.

Okay, in laymen's terms. I do not understand math AT all. In part why my dreams of becoming a Meteorologist didn't come true.

 

[Jeff Duda]

Honestly it looks somewhat poorly explained to me, too, but I think what it says is that the Lid Strength Index is just the difference between two theta-e values. One of those theta-e values is the maximum theta-e value in the lowest 100 mb of the sounding (kind of like the process used to find the most-unstable parcel). The other is the maximum theta-e in the lowest 500 mb of the sounding. I dont' have time right now to show a skew-T and how to find theta-e, but perhaps another member can enlighten us. Also, please fix my errors if I'm making any, someone.

 

[Andrew Burnett]

I've seen it listed like this: http://www.srh.noaa.gov/fwd/?n=convectiveparameterscin as J/KG... I've also see it kg-1... What is the prefered method and why?

Thanks for the help.. I've learned alot.

 

[Andrew Burnett]

I found this interesting link on CAPE and CIN.

https://www.e-education.psu.edu/meteo361/cape_primer.html

 

[Jeff Duda]

I've seen it listed like this: http://www.srh.noaa.gov/fwd/?n=convectiveparameterscin as J/KG... I've also see it kg-1... What is the prefered method and why?

Thanks for the help.. I've learned alot.

That's convective inhibition (CIN), the negative area to the right of a parcel's path in a sounding. See Jason's post about it or ask if you have more questions.

 

[Kris Hair]

http://www.jondavies.net/700mbTcap/700mbTcap.htm

This is a great read on cap...

 

[Andrew Ryan]

I've seen it listed like this: http://www.srh.noaa.gov/fwd/?n=convectiveparameterscin as J/KG... I've also see it kg-1... What is the prefered method and why?

Thanks for the help.. I've learned alot.

I'm not quite sure what you are asking...J/Kg and JKg-1 are the same thing, just written differently. CIN and LSI are measuring the same thing. CIN/CAPE and LSI/LI (Lifted Index) are "inverses" (not really for the reasons mentioned in other posts but I can't think of another word) of each other with one measuring the cap and the other measuring instability, if that makes any sense.

 

[W. Grant Dade]

The LSI lines on the FWD page is being measured in degrees C. This has been an on going discussion I have been having with the Fort Worth office on trying to find a current analysis of LSI. They agreed and put it on their CIN analysis. There have been a few studies that have shown looking at the LSI during peak afternoon heating during the warm season is a better indicator of when the cap could break. The 2C line seems to be a good threashold. Any value less than 2 and there is a chance, especially with added forcing, surface based convection can form. Values greater than 2C and the Lid seems to be too strong. Yesterday in SW OK SPC analysis was showing very little CIN for surface based convection along with SBCAPE over 6000 and MLCAPE over 5000. The FWD convection parameters page was showing the LSI was over 4C. The two cells that tried to form south of the warm front, did not mature.