Commentary on using equivalent latitude in the upper troposphere and lower stratosphere 1National Center for Atmospheric Research, Boulder, Colorado, USA
11 Oct 2012
2Texas A&M University, College Station, Texas, USA
*now at: Forschungszentrum Jülich, Jülich, Germany
**now at: Science System Applications, Greenbelt, Maryland, USA
Received: 10 Oct 2011 – Published in Atmos. Chem. Phys. Discuss.: 15 Dec 2011Abstract. We discuss the use of potential vorticity (PV) based equivalent latitude
(EqLat) and potential temperature (θ) coordinates in the
upper troposphere and lower stratosphere (UTLS) for chemical transport
studies. The main objective is to provide a cautionary note on using
EqLat-θ coordinates for aggregating chemical tracers in the
UTLS. Several examples are used to show 3-D distributions of EqLat together
with chemical constituents for a range of θ. We show that
the use of PV-θ coordinates may not be suitable for several
reasons when tropospheric processes are an important part of a study. Due to
the different static stability structures between the stratosphere and
troposphere, the use of θ as a vertical coordinate does not
provide equal representations of the UT and LS. Since the θ
surfaces in the troposphere often intersect the surface of the Earth, the
θ variable does not work well distinguishing the UT from the
boundary layer when used globally as a vertical coordinate. We further
discuss the duality of PV/EqLat as a tracer versus as a coordinate variable.
Using an example, we show that while PV/EqLat serves well as a transport
tracer in the UTLS region, it may conceal the chemical structure associated
with wave breaking when used as a coordinate to average chemical tracers.
Overall, when choosing these coordinates, considerations need to be made not
only based on the time scale of PV being a conservative tracer, but also the
specific research questions to be addressed.
Revised: 26 Aug 2012 – Accepted: 07 Sep 2012 – Published: 11 Oct 2012
Citation: Pan, L. L., Kunz, A., Homeyer, C. R., Munchak, L. A., Kinnison, D. E., and Tilmes, S.: Commentary on using equivalent latitude in the upper troposphere and lower stratosphere, Atmos. Chem. Phys., 12, 9187-9199, doi:10.5194/acp-12-9187-2012, 2012.