Friday, April 4, 2014

What is CAPE.

One term you will hear when listening to NOAA Weather Radio, Myself, and other weather outlets during severe storm season is CAPE. Here is a little write up about what CAPE is and how it pertains to severe weather.

This is a very intricate subject, and involves a lot of math and science. I will try to stay away for most of the math and science, as much as I can.  This will be quick lesson in plain speak. When you're done with it, you should have a good understanding of what CAPE is.

CAPE is an acronym that stands for Convective Available Potential Energy.  The amount of CAPE is a measure of just how much energy is available convection.

In meteorology, convection is the vertical movement of heat and moisture in an unstable atmosphere.   Clouds are a visible signal that convection is taking place. There is a process called dry convection, but that is beyond the scope of this post. For our purpose in this write up, convection has to do the updrafts needed for thunderstorm development. Another term for this type of convection is called  moist convection.

CAPE represents the amount of buoyant energy available to speed up a parcel vertically in an updraft. The potential energy available for convection is expressed mathematically using a standard measurement of energy represented as Joules Per Kilogram (J/Kg).  

one more thing I think I need to make clear.

One Joules is the amount of energy expended in moving an object through one meter when it is opposed by a force of one  Newton ( a Joules is equal to one Newton-meter).  If you remember your high school science , you will recall that energy is stored work, Work being the application of a force through a distance. Power is the rate of flow of energy, or the rate at which work is done. Stored energy becomes working energy when it is used.

CAPE is a measure of instability in the atmosphere. Therefore, the higher the value the greater the potential for severe weather.

CAPE Values:
0001-1000 J/kg: Marginally (Weakly) Unstable
1000-2500 J/kg: Moderately Unstable
2500-3500 J/kg: Very Unstable
3500 J/kg and higher: Extremely Unstable
CAPE values of 5000 -6000 J/Kg  show staggering instability.

There are no threshold values above which severe weather becomes imminent. So, you can still have values of CAPE and have no thunderstorms.

Without diving into the laws of thermodynamics, this will be basic, but you will still have a good understanding.

An air parcel is a small volume of air that has the same general temperature, air pressure, and amount of moisture. There is no exact definition of the size of the parcel, but for our purpose   a cubic foot of air will work.
A few more acronyms:

Lifting Condensation Level (LCL) is altitude at which clouds begin in a rising parcel of air. LCL can be said to be the level to which an unsaturated air parcel can be lifted adiabatically before it becomes saturated.
Level of Free Convection (LFC): the pressure level at which the lifted parcel becomes warmer than its environment, and therefore becomes positively buoyant.

Level of Free Convection (LFC): the pressure level at which the lifted parcel becomes warmer than its environment, and therefore becomes positively buoyant.

Equilibrium Level (EL): the pressure level at which the lifted parcel becomes colder that its environment, and therefore becomes negatively buoyant.

Equilibrium Level (EL): the pressure level at which the lifted parcel becomes colder than its environment, and therefore negatively buoyant.

Convective inhibition (CIN) is a numerical measure That shows an area in the atmosphere that hinders the updraft necessary to produce convective weather.  Most of you have heard it more often called the Cap on the evening weather segment. The stronger the Cap the more difficult it is to get convective thunderstorms.

CAPE can be visualized on a thermodynamic diagram by lifting a parcel dry adiabatically until it becomes saturated this is the LCL. From there, lifted until the parcel temperature crosses the environmental temperature the LFC. The area enclosed by the two curves between the LCL and the LFC is the CIN, or negative area. From the LFC up to where the parcel temperature again crosses the environmental temperature and becomes cooler than the environment This is the EL. This area often is called positive area, but normally just called CAPE.

A Skew-T

Here is a link to a post I did, that went into a little detail on the Skew-T.
How to read a skew-t-log-p

Types of CAPE:

MLCAPE: Mean Layer CAPE  this is sometimes referred to as Mixed Layer CAPE. This type of
CAPE is calculated using a parcel consisting of mean layer values of temperature and moisture from the lowest 100 mb above the ground level.

MUCAPE: Most Unstable CAPE:
This type of CAPE is calculated using a parcel from a pressure level in the lowest 300 mb that will give you the most unstable CAPE there is.

SBCAPE: Surface-Based CAPE
This type of CAPE is calculated using a surface based parcel.

Other Types of CAPE

DCAPE: Downdraft CAPE used to estimate the potential strength of rain-cooled downdrafts

NCAPE: Normalized CAPE is CAPE that is divided by the depth of the buoyancy layer.

NCAPE is very important.   NCAPE is found by taking CAPE and dividing it by the distance between the LFC and EL in meters.

There are two aspects of CAPE that are important in trying to forecast severe weather. The size and distribution of the area of CAPE, describes the potential strength of the updrafts.

The width of the area of CAPE shown on the sounding, is important as it describes the potential strength of the speed of the updraft needed for thunderstorm development. 


Skinny CAPE

A larger number indicates what we call Fat CAPE.. Fat CAPE indicates the possibility of stronger updrafts as compared to Skinny CAPE.

Well  I most likely went in a bit too deep, but I wanted to give you a thorough understanding of what CAPE is and how we use it.    As always I will answer any questions you may have.

I will add other segments to this series that will talk more about instability, and other severe weather indices, such as:  Helicity, LI, BRN, EHI.

Other post you might like.

The thunderstorm lifecycle

Types of thunderstorms

Types of thunderstorms part 2

Severe thunderstorm structure
The Tornado

Hope you enjoyed this........and maybe came away with a better understanding of the subject.

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