CalculateCAPE - Reading 4: Procedure Summary-Calcluating...

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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 Skew-T 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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Note 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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CalculateCAPE - Reading 4: Procedure Summary-Calcluating...

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