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https://doi.org/10.5194/acp-2020-934
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-934
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  23 Sep 2020

23 Sep 2020

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This preprint is currently under review for the journal ACP.

Sensitivity of stratospheric water vapour to variability in tropical tropopause temperatures and large-scale transport

Jacob W. Smith1, Peter H. Haynes2, Amanda C. Maycock3, Neal Butchart4, and Andrew C. Bushell5 Jacob W. Smith et al.
  • 1Department of Plant Sciences, University of Cambridge, Cambridge, UK
  • 2Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, UK
  • 3School of Earth and Environment, University of Leeds, Leeds, UK
  • 4Met Office Hadley Centre, Exeter, UK
  • 5Met Office, Exeter, UK

Abstract. Concentrations of water vapour entering the tropical lower stratosphere are primarily determined by conditions that air parcels encounter as they are transported through the tropical tropopause layer (TTL). Here we quantify the relative roles of variations in TTL temperatures and transport in determining seasonal and interannual variations of stratospheric water vapour. Following previous studies, we use trajectory calculations with the water vapour concentration set by the Lagrangian Dry Point along trajectories. To isolate the roles of transport and temperatures, the Lagrangian Dry Point calculations are modified by time-shifting temperatures relative to transport, and vice versa, with the shift made by years to investigate interannual variations and by months to investigate seasonal variations. Both ERA-Interim reanalysis data for the 1999–2009 period and data generated by a chemistry-climate model (UM-UKCA) are investigated. Variations in temperatures, rather than transport, dominate interannual variability, typically explaining more than 70 % of variability, including individual events such as the 2000 stratospheric water vapour drop. Similarly seasonal variation of temperatures, rather than transport, is shown to be the dominant driver of the annual cycle in lower stratospheric water vapour concentrations in both model and reanalysis, but it is also shown that seasonal variation of transport plays an important role in reducing the seasonal cycle maximum (reducing the annual range by about 30 %).

The quantitative role in dehydration of sub-seasonal and sub-monthly Eulerian temperature variability is also examined by using time-filtered temperature fields in the trajectory calculations. Sub-monthly temperature variability reduces annual mean water vapour concentrations by 40 % in the reanalysis calculation and 30 % in the model calculation. These results indicate that, whilst capturing seasonal and interannual variation of temperature is a major factor in modelling realistic stratospheric water vapour concentrations, simulation of seasonal variation of transport and of sub-seasonal and sub-monthly temperature variability are also important and cannot be ignored.

Jacob W. Smith et al.

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Short summary
This paper informs realistic simulation of stratospheric water vapour by clearly attributing each of the two key influences on water vapour entry to the stratosphere. Presenting modified trajectory models, the results of this paper show temperatures dominate on annual and inter-annual variations however transport has a significant effect in reducing the annual cycle maximum. Furthermore, sub-seasonal variations in temperature have an important overall influence.
This paper informs realistic simulation of stratospheric water vapour by clearly attributing...
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