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A question about the cap....

I'm still trying to fill some holes in my 'self study' of meteorology.

How does one determine the cap strength. I know that the cap is usually seen around the 700-800mb level on a Skew-T, but I've also heard mention of using a 700mb chart to determine cap strength.

Any help would be appreciated. I'm trying to make the best of my time before chase season get's cooking.

Thanks,

brianb
N5ACN
 
I'd use cap strength (Tsomelevel - Tparcel) AND CINH... The "cap strength" is usually the max temperature difference between the temperature of the environment at a particular level and the temperature of the parcel at that level below the LFC. CINH is kind of like the opposite of CAPE -- it's the area between the temperature trace and the parcel trace where the parcel trace is cooler than the temperature trace. In other words, it's the amount of energy require to lift a parcel through a capping layer.

So, why use both? Let's say you have a tall, skinny CINH profile. In other words, suppose that the temperature is just barely warmer than the parcel trace. In this case, the "cap strength" may be quite low, since the Tlevel-Tparcel is low. However, if this is the case over a deep layer (say, 900-600mb), you may not get initiation unless you have deep convergence. In another case, suppose that you have a sharp cap, where the temp at 800mb is 7C warmer than the parcel at that level, but the cap is shallow (steep inversion). Such a profile may have the same amount of CINH as the previous example, but the "cap strength" would be much higher. Despite this, very strong forcing may be able to lift surface parcels through that capping layer enough to initiate and sustain deep convection.

Here's how I use it -- if there is a sharp inversion, then chances are good that the "cap strength" and CINH are pretty significant. In addition, a little warming /moistening of the parcel layer may not remove the CINH / capping very quickly. On the other hand, if there is no sharp inversion, the "cap strength" may be low, but we don't really know a lot about CINH since we don't know the depth of the capping layer. If it's deep (like the first example in the previous paragraph), you may need deep-layer convergence to initiation convection (e.g. not a shallow outflow boundary). However, a deep, skinny CINH profile is usually more easily removed by warming / moistening the low-levels or cooling the capping layer.

I'm not a fan of using 700mb temp to determine 'cap strength'... The 'cap strength' is a function of both the environment and the parcel (in that the cap strength is the relative difference between the two). 15C 700mb temps may be 'too warm' for convection one day, but 'not warm enough' the next (if, for example, the low-levels warm / moisten). There are some rule-of-thumbs out there, but I've never really followed them.
 
Jeff had a outstanding discussion on it so I won't write too much here. But I will add some 'rules of thumb' or 'magic numbers' that I'm a fan of. I've been using these pretty religiously for my 12 years of chasing and they've worked out really well for me. The numbers come primarily from research meteorologists that established said rules, but some of it is also from my own experience.

1) Check if any of the below are in the "capped" range. If any are, expect a capped day. It only takes one of the following to classify it as a "capped" day.

2) The following are "capped" values. You want values that are NOT in this range.

* 850mb temperature > 24c
* 700mb temperature > 12c (see this web page for more specifics, courtesy of research done by Jon Davies: http://members.cox.net/jondavies1/700mbTca...mbTcapguide.gif)
* SBCINH (Surface Based) > |-50| 'note that CIN is sometimes represented as a positive number, and sometimes as a negative number. By definition it's a negative value, but if we make it a positive number you can understand my logic of saying anything greater than 50 is capped.
* MLCINH (mixed layer) > |-50|
* Lid Strength (aka Cap Strength) > 2c

Other rules that are useful although I don't use them religiously:
* DNFC (Degrees Needed for Convection) > 5 f
* LFC-LCL Depth > 500m
 
Originally posted by Tyler Allison
so is it pronounced cap as in ball cap or cape as in cape of good hope?

CAP, as in baseball CAP...

CAPE refers to Convective Available Potential Energy, which isn't the same as the "CAP".
 
Originally posted by rdewey+--><div class='quotetop'>QUOTE(rdewey)</div>
<!--QuoteBegin-Tyler Allison
so is it pronounced cap as in ball cap or cape as in cape of good hope?

CAP, as in baseball CAP...

CAPE refers to Convective Available Potential Energy, which isn't the same as the "CAP".[/b]

okay...is CAPE pronounced cape as in "good hope"?
 
Originally posted by Tyler Allison+--><div class='quotetop'>QUOTE(Tyler Allison)</div>
Originally posted by rdewey@
<!--QuoteBegin-Tyler Allison

so is it pronounced cap as in ball cap or cape as in cape of good hope?


CAP, as in baseball CAP...

CAPE refers to Convective Available Potential Energy, which isn't the same as the "CAP".

okay...is CAPE pronounced cape as in "good hope"?[/b]

Not sure about good hope, but how about Supermans cape? LOL
 
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