We discuss the use of potential vorticity (PV) based equivalent latitude<br />(EqLat) and potential temperature (θ) coordinates in the upper troposphere<br />and lower stratosphere (UTLS) for chemical transport studies. The<br />main objective is to provide a cautionary note on using EqLat-θ coordinates<br />for aggregating chemical tracers in the UTLS. Several examples are<br />used to show 3-D distributions of EqLat together with chemical constituents<br />for a range of θ. We show that the use of PV-θ coordinates may not<br />be suitable for several reasons when tropospheric processes are an<br />important part of a study. Due to the different static stability<br />structures between the stratosphere and troposphere, the use of θ<br />as a vertical coordinate does not provide equal representations of<br />the UT and LS. Since the θ surfaces in the troposphere often intersect<br />the surface of the Earth, the θ variable does not work well distinguishing<br />the UT from the boundary layer when used globally as a vertical coordinate.<br />We further discuss the duality of PV/EqLat as a tracer versus as<br />a coordinate variable. Using an example, we show that while PV/EqLat<br />serves well as a transport tracer in the UTLS region, it may conceal<br />the chemical structure associated with wave breaking when used as<br />a coordinate to average chemical tracers. Overall, when choosing<br />these coordinates, considerations need to be made not only based<br />on the time scale of PV being a conservative tracer, but also the<br />specific research questions to be addressed.
Pan, L. L., Kunz, A., Homeyer, C. R., Munchak, L. A., Kinnison, D. E., & Tilmes, S. (2012). Commentary on using equivalent latitude in the upper troposphere and lower stratosphere. Atmospheric Chemistry and Physics, 12(19), 9187–9199. https://doi.org/10.5194/acp-12-9187-2012