What Controls the Mesoscale Variations in Water Isotopic Composition Within Tropical Cyclones and Squall Lines? Cloud Resolving Model Simulations in Radiative‐Convective Equilibrium

Abstract

The isotopic composition of water vapor (HDO or 𝐴𝐴 𝐴𝐴 18 2 𝑂𝑂 ) evolves along the water cycle as phase changes are associated with isotopic fractionation. The isotopic composition of precipitation recorded in paleoclimate archives has significantly contributed to the reconstruction of past hydrological changes across the tropics (Cruz et al., 2009;Wang et al., 2001). Indeed, over tropical oceans, the precipitation is usually more depleted in heavy isotopes as precipitation rate increases, an observation called the amount effect (Dansgaard, 1964). In concert with the precipitation, the water vapor over tropical oceans is also more depleted as precipitation rate increases according to satellite and in situ observations (Kurita, 2013;Worden et al., 2007). Over tropical land, both the precipitation

Is the Isotopic Composition of Precipitation a Robust Indicator for Reconstructions of Past Tropical Cyclones Frequency? A Case Study on Réunion Island From Rain and Water Vapor Isotopic Observations

“…The different cyclonic substructures defined in Section 2.3 are represented for information: not to overload the figure, we gather inner bands (IB), convective inner bands (IBC) and convective system center (C) (the latter never lasts more than 3 hr); and OB2 is a time period where 75% of the hourly data are tagged as corresponding to a outer band (and the other 25% have no assigned substructure). Within the storm system, we clearly observe a progressive isotopic depletion of water vapor toward the system center in agreement with previous studies (Fudeyasu et al, 2008;Gedzelman et al, 2003;Jackisch et al, 2022;Lawrence et al, 2002;Munksgaard et al, 2015;Risi et al, 2023;Sanchez-Murillo et al, 2019;Skrzypek et al, 2019;Sun et al, 2022;Xu et al, 2019). On average, the most depleted domains are the convective outer bands (OBC, δ 18 O v = −21.6 ± 2.5‰, n = 322), the convective inner band (IBC, δ 18 O v = −22.6 ± 2.7‰, n = 29), and the convective system center (C, δ 18 O v = −22.2 ± 3.4‰, n = 18).…”

“…The water vapor also exhibits a systematically stronger depletion during cyclonic periods and, in addition, a steeper amount effect during cyclonic periods with a δ 18 O p /P slope 3-4 times higher. The depleted isotopic ratio in rain during tropical cyclones is thus intimately related to the influence of cyclones on the water vapor isotopic ratio and to interaction between both phases as already suggested by Risi et al (2020Risi et al ( , 2021Risi et al ( , 2023.…”

“…In LMDZ-iso, large-scale precipitation is typically more depleted than convective precipitation (Tuinenburg et al, 2015) because it is formed at a higher altitude and in moister conditions. Previous studies showed that during tropical cyclones the enhanced diffusive exchanges between rain and water vapor and the reduced fraction of rain evaporation, both due to a more humid environment, are the main causes of the isotopic depletion in water vapor and precipitation (Lawrence et al, 2004;Risi et al, 2023). Risi et al (2008) clearly demonstrated that interaction between both water phases controlled the amount effect (Risi et al, 2008).…”

“…Because water vapor is depleted even in the non-precipitating parts of rain bands and convective regions relative to the environment (Figure 10, orange bars) local condensation and precipitation is likely not the dominant control on the pronounced isotopic depletion from the edge to the system center as also shown by Risi et al (2023). However, the convective deepness and the likely presence of a stratiform zone may have an influence as rain bands tagged as convective (IBC and OBC) are more depleted than the non-convective rain bands (IB and OB respectively) (the difference of tagging being on the brightness temperature and thus on the depth of convection).…”

“…Previous observational and simple box modeling studies have highlighted the dominant role of rain evaporation and rain-water vapor diffusive exchanges in depleting water vapor within convective organized systems (Lawrence et al, 2004;Risi et al, 2021Risi et al, , 2023Tremoy et al, 2014;Xu et al, 2019). Rain evaporation acts to deplete the water vapor when the fraction of the evaporated raindrop is low.…”