Isotopic composition of water in the tropical tropopause layer in cloud‐resolving simulations of an idealized tropical circulation

Abstract

[1] The processes that fix the fractionation of the stable isotopologues of water in the tropical tropopause layer (TTL) are studied using cloud-resolving model simulations of an idealized equatorial Walker circulation with an imposed Brewer-Dobson circulation. This simulation framework allows the explicit representation of the convective and microphysical processes at work in the TTL. In this model, the microphysical transfer of the isotopologues (here, HD 16 O and H 2 18 O) among water vapor and condensed phase hydrometeors is explicitly represented along with those of the standard isotopologue (H 2 16 O) during all microphysical interactions. The simulated isotopic ratios of HD 16 O in water vapor are consistent with observations in both magnitude and the vertical structure in the TTL. When a seasonal cycle is included in the Brewer-Dobson circulation, both the water vapor mixing ratio and the isotopic ratios of water vapor display a seasonal cycle as well. The amplitude and phase of the seasonal cycle in HD 16 O are comparable to those observed. The results suggest that both the sublimation of relatively enriched ice associated with deep convection and fractionation by cirrus cloud formation affect the isotopic composition of water vapor in the TTL and its seasonal cycle.

The isotopic composition of precipitation from a winter storm – a case study with the limited-area model COSMO<sub>iso</sub>

“…Also for the heavy isotopes, an equilibrium approach can be adopted, since the equilibration time with respect to small droplets typically is in the order of seconds (see again Blossey et al, 2010). This leads to a diagnostic equation for the isotope ratio in cloud water, as given by Blossey et al (2010) in their Eq. (B21).…”

“…In the COSMO model, condensation and evaporation of cloud water are parameterised with the help of a saturation adjustment technique, implying thermodynamic equilibrium between vapour and liquid clouds. Also for the heavy isotopes, an equilibrium approach can be adopted, since the equilibration time with respect to small droplets typically is in the order of seconds (see again Blossey et al, 2010). This leads to a diagnostic equation for the isotope ratio in cloud water, as given by Blossey et al (2010) in their Eq.…”

“…The isotopic composition of the particles is assumed to be independent of their size. For all microphysical interactions that do not involve the vapour phase (e.g., autoconversion of cloud particles to form rain or freezing of liquid water), there is no isotopic fractionation, and the transfer rates h S of the heavy isotopes, following Blossey et al (2010), are given by…”

“…Sturm et al (2005) incorporated water isotopes into the regional climate model REMO, which was subsequently used for investigations on long, climatological time scales (e.g., Sturm et al, 2007). Smith et al (2006) and Blossey et al (2010) used cloud resolving models for simulating idealised tropical circulations, focusing on isotope variations in the tropical tropopause layer. Yoshimura et al (2010) simulated the isotopic content of precipitation from an atmospheric river event at the US west coast with the model IsoRSM and compared the results from this case study to observations by Coplen et al (2008).…”

Characterizing moisture exchange between the Hawaiian convective boundary layer and free troposphere using stable isotopes in water

“…The departure of observed and simulated isotopic profiles from pseudoadiabatic models, for instance, suggests a role for lofting of frozen condensate by deep convection in moistening the upper troposphere [Nassar et al, 2007;Bony et al, 2008;Blossey et al, 2010;Sayres et al, 2010]. Gedzelman [1988] used isotope ratios in the lower and mid-free troposphere to distinguish air masses that had risen moist adiabatically but experienced different degrees of rainout and/or turbulent mixing.…”