Preprints
https://doi.org/10.5194/acp-2021-902
https://doi.org/10.5194/acp-2021-902

  03 Dec 2021

03 Dec 2021

Review status: this preprint is currently under review for the journal ACP.

Stable water isotope signals in tropical ice clouds in the West African monsoon simulated with a regional convection-permitting model

Andries Jan de Vries1, Franziska Aemisegger1, Stephan Pfahl2, and Heini Wernli1 Andries Jan de Vries et al.
  • 1ETH Zürich, Institute for Atmospheric and Climate Science, Zürich, Switzerland
  • 2Institute of Meteorology, Freie Universität 5 Berlin, Berlin, Germany

Abstract. Tropical ice clouds have an important influence on the Earth’s radiative balance. They often form as a result of tropical deep convection, which strongly affects the water budget of the tropical tropopause layer. Ice cloud formation involves complex interactions on various scales, which are not fully understood yet and lead to large uncertainties in climate predictions. In this study, we investigate the formation of tropical ice clouds related to deep convection in the West African monsoon, using stable water isotopes as tracers of moist atmospheric processes. We perform simulations using the regional isotope-enabled model COSMOiso with different resolutions and treatments of convection for the period of June–July 2016. First, we evaluate the ability of our simulations to represent the isotopic composition of monthly precipitation through comparison with GNIP observations, and the precipitation characteristics related to the monsoon evolution and convective storms based on insights from the DACCIWA field campaign in 2016. Next, a case study of a mesoscale convective system (MCS) explores the isotope signatures of tropical deep convection in atmospheric water vapour and ice. Convective updrafts within the MCS inject enriched ice into the upper troposphere leading to depletion of vapour within these updrafts due to the preferential condensation and deposition of heavy isotopes. Water vapour in downdrafts within the same MCS are enriched by non-fractionating sublimation of ice. In contrast to ice within the MCS core regions, ice in widespread cirrus shields is isotopically in approximate equilibrium with the ambient vapour, which is consistent with in situ formation of ice. These findings from the case study are supported by a statistical evaluation of isotope signals in the West African monsoon ice clouds. The following five key processes related to tropical ice clouds can be distinguished based on their characteristic isotope signatures: (1) convective lofting of enriched ice into the upper troposphere, (2) cirrus clouds that form in situ from ambient vapour under equilibrium fractionation, (3) sedimentation and sublimation of ice in the mixed-phase cloud layer in the vicinity of convective systems and underneath cirrus shields, (4) sublimation of ice in convective downdrafts that enriches the environmental vapour, and (5) the freezing of liquid water in the mixed-phase cloud layer at the base of convective updrafts. Importantly, the results show that convective systems strongly modulate the humidity budget and the isotopic composition of the lower tropical tropopause layer. They contribute to about 40 % of the total water and 60 % of HDO in the 175–125 hPa layer in the African monsoon region according to estimates based on our model simulations. Overall, this study demonstrates that isotopes can serve as useful tracers to disentangle the role of different processes in the Earth’s water cycle, including convective transport, the formation of ice clouds, and their impact on the tropical tropopause layer.

Andries Jan de Vries et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-902', Anonymous Referee #1, 01 Jan 2022
  • RC2: 'Comment on acp-2021-902', Anonymous Referee #3, 07 Jan 2022
  • RC3: 'Comment on acp-2021-902', Anonymous Referee #2, 07 Jan 2022

Andries Jan de Vries et al.

Andries Jan de Vries et al.

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Short summary
The Earth’s water cycle contains the common H2O water molecules, but also less abundant heavier HDO molecules. We use their different physical characteristics combined with model simulations to study how tropical cirrus clouds form related to deep convection in the African monsoon. We find that these processes strongly influence the water budget of the tropical tropopause layer with implications for a better understanding of the Earth’s radiative budget and water transport into the stratosphere.
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