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Discussion papers
https://doi.org/10.5194/acp-2019-302
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-302
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 29 Apr 2019

Research article | 29 Apr 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Impact of convectively lofted ice on the seasonal cycle of tropical lower stratospheric water vapor

Xun Wang1, Andrew E. Dessler1, Mark R. Schoeberl2, Wandi Yu1, and Tao Wang3 Xun Wang et al.
  • 1Department of Atmospheric Sciences, Texas A&M University, College Station, TX, USA
  • 2Science and Technology Corporation, Columbia, MD, USA
  • 3University of Maryland, College Park, MD, USA

Abstract. We use a forward Lagrangian trajectory model to diagnose mechanisms that produce the tropical lower stratospheric (LS) water vapor seasonal cycle observed by the Microwave Limb Sounder (MLS) and reproduced by the Goddard Earth Observing System Chemistry Climate Model (GEOSCCM) in the tropical tropopause layer (TTL). We confirm in both the MLS and GEOSCCM that the seasonal cycle of water vapor is primarily determined by the seasonal cycle of TTL temperatures. However, we find that the seasonal cycle of temperature predicts a smaller seasonal cycle of LS water vapor between 10° N–40° N than observed by MLS. We show that including evaporation of convectively lofted ice in the trajectory model increases the simulated maximum value in the 10° N–40° N water vapor seasonal cycle by 1.9 ppmv (47 %) and increases the seasonal amplitude by 1.26 ppmv (123 %), which improves the prediction of LS water vapor annual cycle. We conclude that the moistening effect from convective ice evaporation in the TTL plays a key role regulating and maintaining the tropical LS water vapor seasonal cycle. Most of the convective moistening in the 10° N–40° N range comes from convective ice evaporation occurring at the same latitudes. A small contribution to the moistening comes from convective ice evaporation occurring between 10° S–10° N. Within 10° N–40° N, the Asian monsoon region is the most important region for convective ice evaporation and convective moistening during boreal summer and autumn.

Xun Wang et al.
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Short summary
We use a trajectory model to diagnose mechanisms that produce the observed and modeled tropical lower stratospheric water vapor seasonal cycle. We confirm that the seasonal cycle of water vapor is primarily determined by the seasonal cycle of tropical tropopause layer (TTL) temperatures. However, between 10° N–40° N, we find that the evaporation of convective ice in the TTL plays a key role contributing to the water vapor seasonal cycle there. The Asian monsoon region is the most important region.
We use a trajectory model to diagnose mechanisms that produce the observed and modeled tropical...
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