What Controls the 3D Distribution of Water Vapor Isotopic Composition in East Asia?

Directeurs.rices de thèses : Risi C. & Tian L.

Date 2023-11-20
Diplôme Sorbonne Université & China


Composition du jury

Président : M. Aymeric Spiga, Professeur, Sorbonne Université (France)
Rapporteurs : M. David Noone, Professeur, University of Auckland (New Zealand) et M. Hongxi Pang Professeur, Nanjing University (China)
Examinateur-ice-s : Mme Franziska Aemisegger, Professeur, ETH Zurich (Switzerland), Mme Adriana Bailey, Professeur associé, University of Michigan (USA), et M. Peter Blossey, Professeur associé, Washington University (USA)
Directeur-ice-s : Mme Camille Risi, Chargée de recherche CNRS, HDR (France) et M. Lide Tian, Scientifique senior, Yunnan University (China)


Unlike polar ice core records, the isotope variations in Tibetan ice cores challenge the interpretation of temperature signals. One of the main reasons is that in monsoon regions at low and middle latitudes, water isotopes are influenced by convective and cloud processes. A deeper understanding of water isotope behavior and the dynamical controls involved in moisture transpiration and convection is needed. Large-scale in-situ observations and vertical profiles of vapor isotopes during convection would be helpful. However, such data are rare. The aim of this thesis was to document horizontal, vertical, and temporal variations in the isotopic composition of water in East Asian monsoon region.
First, to document the horizontal and seasonal variations of water isotopes near the surface across China, we made in-situ observations of near-surface vapor isotopes over a large region (over 10000 km) across China in both pre-monsoon and monsoon seasons, using a newly-designed vehicle-based vapor isotope monitoring system. We found that the observed spatial variations of vapor δ18O are mainly controlled by Rayleigh distillation along air mass trajectories during the pre-monsoon period, but are significantly influenced by different moisture sources, continental recycling processes, and convection along moisture transport during the monsoon period. These results provide an overview of the spatial distribution and seasonal variability of water isotopic composition in East Asia and their controlling factors and emphasize the need to interpret proxy records in the context of the regional system and moisture sources.
Second, to better understand the physical processes that control the vertical distribution of vapor isotopes and its intra-seasonal and seasonal variability, we observed the vertical profiles of atmosphere vapor isotopes up to the upper troposphere (from the ground surface at 3856m up to 11000m a.s.l.) from June to October in the southeastern Tibetan Plateau using a specially-designed unmanned-aerial-vehicle (UAV) system. For the sampling, we chose to carry air bags on UAVs as a portable sampling device, but encountered the permeability problem commonly associated with these bags. To corrected for this problem, we developed a diffusion model with diffusion parameters calibrated through laboratory experiments. This allows us to document forthe first time the vertical distribution of atmospheric water vapor isotopes across the entire monsoon period up to the upper troposphere, boasting an unprecedented vertical resolution and altitude range. We find that the vertical profiles of water vapor isotopic composition reflect a combination of large-scale processes, in particular deep convection and continental recycling along trajectories, and local convective processes, in particular convective detrainment, and sublimation of ice crystals. The observed seasonal and intra-seasonal variations are generally vertically coherent, due to the strong vertical convective mixing and local convective detrainment of vapor originating from the low levels, and are mainly due to deep convection along trajectories.