Commentary on using equivalent latitude in the upper troposphere and lower stratosphere

  • Pan L
  • Kunz A
  • Homeyer C
 et al. 
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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.

Author-supplied keywords

  • equivalent latitude
  • lower stratosphere
  • potential temperature
  • potential vorticity
  • pv
  • upper troposphere
  • utls

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  • L L Pan

  • A Kunz

  • C R Homeyer

  • L A Munchak

  • D E Kinnison

  • S Tilmes

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