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

Abstract

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. These processes are not yet fully understood and lead to large uncertainties in climate projections. 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 convection-permitting simulations with the regional Consortium for Small-Scale Modelling isotope-enabled (COSMOiso) model for the period from June to July 2016. First, we evaluate our model simulations using space-borne observations of mid-tropospheric water vapour isotopes, monthly station data of precipitation isotopes, and satellite-based precipitation estimates. Next, we explore the isotope signatures of tropical deep convection in atmospheric water vapour and ice based on a case study of a mesoscale convective system (MCS) and a statistical analysis of a 1-month period. The following five key processes related to tropical ice clouds can be distinguished based on isotope information: (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 downdraughts that enriches the environmental vapour, and (5) the freezing of liquid water just above the 0 ∘C isotherm in convective updraughts. Importantly, we note large variations in the isotopic composition of water vapour in the upper troposphere and lower tropical tropopause layer, ranging from below −800 ‰ to over −400 ‰, which are strongly related to vertical motion and the moist processes that take place in convective updraughts and downdraughts. In convective updraughts, the vapour is depleted by the preferential condensation and deposition of heavy isotopes, whereas the non-fractionating sublimation of ice in convective downdraughts enriches the environmental vapour. An opposite vapour isotope signature emerges in thin-cirrus cloud regions where the direct transport of enriched (depleted) vapour prevails in large-scale ascent (descent). Overall, this study demonstrates that isotopes can serve as useful tracers to disentangle the role of different processes in the West African monsoon water cycle, including convective transport, the formation of ice clouds, and their impact on the tropical tropopause layer.

Water isotopic characterisation of the cloud-circulation coupling in the North Atlantic trades. Part 1: A process-oriented evaluation of COSMOiso simulations with EUREC4A observations

“…For the major part of the simulated period, the model output matches the diurnal cycle of the satellite-based precipitation product and earlier studies (Vial et al, 2019). The flight-to-flight, as well as the in-flight variability of the humidity and isotope signals at cloud base and in the subcloud layer, are well captured by the model (similar to earlier studies looking at the meso-to synoptic-scale variability in humidity and isotope variables; Dahinden et al, 2021;de Vries et al, 2022).…”

“…Furthermore, isotope-enabled models have been shown to be useful in looking into tropical deep convection (Bony et al, 2008;Risi et al, 2008;Blossey et al, 2010;Risi et al, 2020Risi et al, , 2021de Vries et al, 2022), entrainment of free-tropospheric air into the tropical marine boundary layer (Risi et al, 2019), or precipitation-driven cold pool dynamics (Torri and Kuang, 2016a, b;Torri, 2021).…”

“…However, the day-to-day variability of COSMO iso 's humidity and δ 2 H agreed well with ground-based remote sensing observations from Teneriffe (FTIR). de Vries et al (2022) performed three COSMO iso simulations in the West African monsoon region and tested the effect of parameterised versus explicit convection and different spatial resolutions. Independent of the modelling setup, COSMO iso produced a distinct bias towards too high δ 2 H at 4.2 km compared to satellite-based remote sensing observations from IASI.…”

“…The literature summarised above suggests that simulations should be done by using an explicit representation of convection and for variables such as the cloud fraction a fine spatial resolution (Ban et al, 2014;Vergara-Temprado et al, 2020;Heim et al, 2021) albeit without expecting a significant improvement for water vapour isotope variables (de Vries et al, 2022).…”

“…Furthermore, we anticipate a depletion bias in the near-surface vapour (Thurnherr et al, 2021) and an enriched bias in the free-tropospheric vapour but overall a good performance in terms of the mesoscale to synoptic-scale variability in humidity and isotope variables (Dahinden et al, 2021;de Vries et al, 2022). Motivated by Thurnherr et al (2021)'s consideration, we used the smooth regime of Merlivat and Jouzel (1979) in our simulations to formulate non-equilibrium fractionation during evaporation from the ocean surface.…”