Commentary on using equivalent latitude in the upper troposphere and lower stratosphere
We discuss the use of potential vorticity (PV) based equivalent latitude\n(EqLat) and potential temperature (θ) coordinates in the upper troposphere\nand lower stratosphere (UTLS) for chemical transport studies. The\nmain objective is to provide a cautionary note on using EqLat-θ coordinates\nfor aggregating chemical tracers in the UTLS. Several examples are\nused to show 3-D distributions of EqLat together with chemical constituents\nfor a range of θ. We show that the use of PV-θ coordinates may not\nbe suitable for several reasons when tropospheric processes are an\nimportant part of a study. Due to the different static stability\nstructures between the stratosphere and troposphere, the use of θ\nas a vertical coordinate does not provide equal representations of\nthe UT and LS. Since the θ surfaces in the troposphere often intersect\nthe surface of the Earth, the θ variable does not work well distinguishing\nthe UT from the boundary layer when used globally as a vertical coordinate.\nWe further discuss the duality of PV/EqLat as a tracer versus as\na coordinate variable. Using an example, we show that while PV/EqLat\nserves well as a transport tracer in the UTLS region, it may conceal\nthe chemical structure associated with wave breaking when used as\na coordinate to average chemical tracers. Overall, when choosing\nthese coordinates, considerations need to be made not only based\non the time scale of PV being a conservative tracer, but also the\nspecific research questions to be addressed.