Reading 4:
Procedure Summary
Calcluating CAPE, Lifted Index and Strength of Maximum
Convective Updraft
CAPE Calculation
Lifted Index Calculation
Maximum Updraft Strength Calculation
I. Convective Available Potential Energy (CAPE)
A. Background
(i) The acceleration an air parcel experiences due to density differences at a given level
can be related to the difference in temperature of the air parcel , T
ap
, with respect to
the temperature of the surrounding air, T
e
, AT A GIVEN LEVEL.:
(ii) A true measure of the
potential buoyancy
is a measure of the "positive" area on a
SkewT Ln P diagram. This represents the portion of the parcel ascent curve in which
the parcel is warmer and, thus, less dense than the air surrounding it.
The positive area represents a
potential
source of energy for parcels at the ground that
are lifted to the elevation (LFC) above which they become warmer than their
surroundings. To obtain this, one needs to algebraically add this parameter at every level
of the parcel's ascent until it reaches the point at which it becomes the same temperature
as its surroundings again (Equilibrium Level).
The parameter is known as Convective Available Potential Energy (CAPE) or Positive
Buoyancy (B+).
Object 1
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View Full DocumentNote that this equation really states that CAPE is directly proportion to the total acceleration a parcel would experience
due to buoyancy from the LFC to the EL.
Equation 1
The equation tells you that CAPE is a function of the difference in temperature between
the rising (or sinking) air parcel (T
ap
) relative to the air around it at the same elevation
(T
e
). Note that the temperatures should be measured in Kelvin. CAPE is a measure of
the area on a Skew T/Ln P diagram bounded by the curve of the ascending air parcel on
the right and the environmental lapse rate (ELR) (sounding) on the left from the Level of
Free Convection (LFC) (or, in the case of a forecast sounding for the afternoon based
upon heating, the Convective Condensation Level c CCL) to the Equilibrium Level
(EL). (Note: a similar expression is defined for Convective Inhibition Energy (CIN or
CINH) in which the parcel curve lies to the left of the ELR.).
The summation sign in the equation means that the value of the expression in brackets
needs to be evaluated at every level (and, of course, there are an infinite number of
them) between the LFC (or CCL) and the EL. When you take calculus you will learn
how to approximate the summation (called, finite differences) without making an
extravagant number of calculations. But for the purposes of Homework 1, you will
simply divide the atmosphere into 50 mb layers, from the LFC to the EL and calculate a
"mean" CAPE for each of the layers and then sum those up to get the total or net CAPE.
B. Procedure
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 Spring '11
 Ahmed
 Energy, Potential Energy, Vapor pressure, Atmospheric thermodynamics, MB, Cape

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