DS01 - Gestion sobre des ressources et adaptation au changement climatique

Effects of convection and cirrus clouds on the Tropical Tropopause Layer over the Indian Ocean – CONCIRTO

The CONCIRTO project highlights the importance of balloon measurements in understanding the impact of stratospheric water vapor on climate change

CONCIRTO is a 62-month young researcher project funded by the National Research Agency. It aims to study the impact of deep convection and cirrus on the water vapor balance in the tropical UTLS of the southern hemisphere.

Stratospheric water vapor composition: Reunion island, a sentinel of climate in the SH?

Stratospheric water vapor plays an important role on climate. Predictions of changes in stratospheric humidity are uncertain because of gaps in our understanding of physical processes occurring in the Tropical Tropopause Layer (TTL), the atmospheric layer that controls the entry of trace species into the stratosphere. The CONCIRTO project funded two field campaigns organized at the high-altitude Maïdo Observatory on Réunion Island to 1) Acquire a groundbreaking balloon borne in situ and remotely sensed dataset that will help improve the representation of cirrus clouds, deep convection and stratospheric water vapor in models, 2) Characterize ?cirrus properties and formation mechanism and their effect on TTL humidity, 3) ?Assess whether deep convection hydrate the TTL. Furthermore, unique circumstances allowed the measurement of the massive eruption of the submarine volcano Hunga Tonga which produced major perturbations in atmospheric composition, increasing the stratospheric water vapor burden by 10%. This project highlighted the importance of balloon borne in situ measurements besides LIDARs and satellites for climate survey of the atmospheric composition in the stratosphere.

This project was twofold, encompassing both data analysis with balloon sondes (?a cost-effective mean to study TTL and the stratosphere), RADAR and satellite-sensed constituent data, and high resolution model runs (Meso-NH/FLEXPART) designed to elucidate physical mechanisms affecting the TTL and the stratosphere over the IO.
Leveraging on the existing CNRS-INSU Maïdo observation infrastructure, the first campaign (12/2018 to 03/2019), focused on deep convection, cirrus clouds and how they affect TTL humidity over the Indian Ocean. The convective outflows were targeted using a dedicated forecasting tool. The second campaign, delayed after the COVID time period, was a rapid response experiment for the period 20-25 January 2022 to provide observations of the evolution of the volcanic plume in the stratosphere less than a week after the eruption of the submarine volcano Hunga Tonga (HT). The HT eruption produced major perturbations in atmospheric composition, increasing the stratospheric water burden by 10%.

A nascent in situ cirrus with a high ice supersaturation, beyond remote sensing capabilities, was observed on January 11 (Reinares et al., 2020). Then, the humidification of the TTL by deep convection from the Enawo tropical cyclone was analyzed using a high-resolution Meso-NH model (Héron et al., 2020). It showed the maximum of humidification occurred during the intensification phase of the tropical cyclone. Finally, we established that the Hunga-Tonga volcanic eruption led to unprecedented high water vapor anomaly in the tropical stratosphere and a rapid ozone loss at a speed never observed before (Evan et al., 2022). The project helped initiate a long term survey of the atmospheric composition within the B2SAP project at Reunion island.

The measurements and results of campaigns dedicated to the TTL and the Hunga Tonga volcano have shown the importance of balloon measurements for observing the TTL and the stratosphere and their complementarity with RADAR, LIDAR, and satellite measurements. The results of the project have enabled the integration of the B2SAP project (NOAA funding) and the sustainability of measurements for 2 more years (2023-2024). Furthremore, the project has allowed the coordinator of the project, S. Evan, to strengthen her expertise in measuring water vapor using hygrometers and ozone using ECC probes. She is now an integral part of the GRUAN community and the SHADOZ network. She will participate in the development of new hygrometers. She organized the GRUAN-ICM14 conference in Réunion in November-December 2022. The results of this project have convinced her to continue her study of stratospheric water vapor, with a new research focus on the heterogeneous chemistry of stratospheric ozone in the tropics.

Impact of convection on the TTL: Evan et al., 2020; Héron et al., 2020. A third paper will be corrected and submitted in 2023.
Cirrus formation and impact on the TTL: Reinares Martinez et al. (2021). A second paper using mesoscale modeling has been written and will be submitted in 2023.
Impact of Hunga Tonga eruption on stratospheric water vapor: Evan et al., 2022 (submitted).

Stratospheric water vapor is important for the radiative budget of the climate. Climate models predict a ~1ppmv increase in stratospheric water vapor with a radiative feedback responsible for at least 10% of the global warming [Dessler et al., 2013]. Predictions of changes in stratospheric humidity are uncertain because of gaps in our understanding of physical processes occurring in the Tropical Tropopause Layer (TTL, ~14-19km altitude), the primary gateway of trace species into the stratosphere. The composition and dynamics of the TTL is controlled by a complex interplay between large-scale stratospheric circulations, atmospheric waves, deep convection, cirrus clouds and radiation. Current models show biases in the tropical cold point tropopause (CPT) and its variability as well as deep convection and cirrus cloud microphysics that lead to inadequate representation of TTL water vapor.
TTL physical processes have been the focus of field campaigns in the Pacific Ocean and South America, but less attention has been given to the tropical Indian Ocean (IO), an important but under sampled part of the world. The IO has seen an unprecedented rise in heat content and is now home to 70% of the global ocean heat gain during the past decade. It is possible that the warmer water in the IO could increase deep convection and tropical cyclones activity over the basin which could make southern IO deep convection a significant source of moisture to the TTL in the years to come. This project aims to further our understanding of deep convection and cirrus clouds and how they affect the TTL over the IO. Balloon sondes, a cost-effective mean to study TTL processes compared to high-altitude aircraft that can reach the TTL, reveal fine-scale features that are below the vertical resolution of satellite sounding systems. The proposal will fund novel coincidental high-resolution balloon in situ measurements of water vapor, aerosol and cirrus cloud with a Doppler polarimetric cloud radar observations at Reunion Island (21oS, 55oE). These new observations will complement existing instruments at the Maïdo observatory to create holistic picture of the processes impacting the temperature and humidity of the TTL. We also propose to apply an innovative modeling approach to understand how cirrus clouds and deep convection affect TTL humidity over the Southern Hemisphere (SH) tropics.
The scientific objectives are:
1) Acquire a groundbreaking in situ and remotely sensed dataset that will help improve the representation of cirrus clouds, deep convection and water vapor in models
2) Characterize cirrus properties and formation mechanism and their effect on TTL humidity
3) Assess whether deep convection hydrate the TTL over the IO
This study will be twofold, encompassing both data analysis with balloon sondes, RADAR and satellite-sensed TTL constituent data, and model runs (Meso-NH/FLEXPART) designed to elucidate physical mechanisms affecting the TTL over the IO. This proposal will support my research as a young researcher at LACy and develop collaborations with national and international scientific partners. This project will provide, for the first time, observations of a complete set of water vapor and cirrus properties needed to assess the dominant mechanism for cirrus formation over the IO. These measurements will benefit the French national research on cirrus clouds and water vapor that are key topics of the national ACTRIS FR infrastructure. They will help elevate Maïdo Observatory to an essential observatory for the French and international communities within NDACC, GRUAN (GCOS Reference Upper Air Network). An objective of GRUAN is to provide long-term high-quality upper-air climate records. The project will help obtain the international network GRUAN certification on water vapor measurements and will help achieve the perennity of in situ measurements of important climate variables in the tropical SH.

Project coordinator

Madame Stephanie Evan (Laboratoire de l'Atmosphère et des Cyclones)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LACy Laboratoire de l'Atmosphère et des Cyclones

Help of the ANR 277,560 euros
Beginning and duration of the scientific project: October 2017 - 42 Months

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