Available potential energy in the northern hemisphere during the FGGE year

Northern Hemispheric twice daily data from the First GARP Global Experiment (FGGE) year are used to compare available potential energy estimates obtained using the so-called exact (isentropic) form of the equations with two commonly used approximate (isobaric) forms. The two approximate forms, one using a fixed lapse rate and the other a lapse rate which varies in the vertical, overestimate the total available potential energy except in mid-summer.

The greatest difference between the exact and approximate forms is in the eddy term, which is considerably smaller in the approximate forms, especially in spring and summer. Partitioning the eddy term into its standing and transient components shows that in the exact form the standing component exceeds the transient term. This is in contrast to the approximate form in which the transient term is the larger in every month except July. The latitudinal and monthly variations of the standing and transient terms show different patterns, especially in the standing component. In the approximate form the latitude of maximum standing eddy values is near 60° N in winter but shifts equatorward to about 30°N and decreases in summer. While the exact form shows a weak winter maximum near 60° N, the dominant maximum develops near 20° N in spring and shifts poleward to about 30° N and increases in summer.

The hemispheric distribution of the vertically integrated grid point values from which the eddy available potential energy was calculated and the standard deviation of these twice daily values produce results which show that the exact form values better conform to the pattern of cyclone activity than the approximate form. In general, the results indicate that the exact form of the available potential energy equations should be more extensively used in diagnostic work.