Comments on “A Third-Law Isentropic Analysis of a Simulated Hurricane”

Abstract

The atmospheric overturning can be estimated by computing an isentropic streamfunction, defined as the net upward mass transport of all air parcels with the potential temperature less than a given threshold. Here, the streamfunctions for the equivalent potential temperature and the entropy potential temperature are compared in a numerical simulation of a hurricane. It is shown that, when condensate is not taken into account, the two streamfunctions are equivalent and can be related to one another by a coordinate transformation. When condensate content is included, the streamfunctions differ substantially in the upper troposphere because of the large amount of ice water found in some updrafts. It is also shown that using an equivalent potential temperature over ice avoids this problem and offers a more robust way to compute the atmospheric overturning when precipitation is included. While it has been recently recommended to limit the isentropic analysis to the entropy potential temperature, it is argued here that more insights can be gained by comparing a circulation averaged in multiple coordinates over limiting oneself to one specific choice.