How is the tropopause defined?
From Office Note 438: The post processor can generate the following tropopause level fields: pressure, temperature (ambient and potential), horizontal winds, and vertical wind shear. The greatest difficulty was coding an algorithm to locate the tropopause above each mass point. The procedure used in the Eta post processor is based on that in the NGM. Above each mass point a surface-up search is made for the first occurrence of three adjacent layers over which the lapse rate is less than a critical lapse rate. In both the NGM and Eta model the critical lapse rate is 2 K/km. The midpoint (in log pressure) of these two layers is identified as the tropopause. A lower bound of 500 mb is enforced on the tropopause pressure. If no two layer lapse rate satisfies the above criteria the model top is designated the tropopause. Very strong horizontal pressure gradients result from this algorithm. Horizontal averaging over neighboring grid points prior to or during the tropopause search might minim
