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  • Atmos. Chem. Phys., 13, 79037935, 2013www.atmos-chem-phys.net/13/7903/2013/doi:10.5194/acp-13-7903-2013 Author(s) 2013. CC Attribution 3.0 License.

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    Modelling and interpreting the isotopic composition of water vapourin convective updrafts

    M. Bolot1, B. Legras1, and E. J. Moyer2

    1Laboratoire de Meteorologie Dynamique, IPSL, UPMC/CNRS/ENS/Ecole Polytechnique, UMR8539, Paris, France2Department of the Geophysical Sciences, University of Chicago, Chicago, USA

    Correspondence to: M. Bolot (bolot@lmd.ens.fr)

    Received: 10 May 2012 Published in Atmos. Chem. Phys. Discuss.: 31 August 2012Revised: 4 July 2013 Accepted: 10 July 2013 Published: 16 August 2013

    Abstract. The isotopic compositions of water vapour andits condensates have long been used as tracers of the globalhydrological cycle, but may also be useful for understand-ing processes within individual convective clouds. We re-view here the representation of processes that alter waterisotopic compositions during processing of air in convec-tive updrafts and present a unified model for water vapourisotopic evolution within undiluted deep convective cores,with a special focus on the out-of-equilibrium conditionsof mixed-phase zones where metastable liquid water andice coexist. We use our model to show that a combina-tion of water isotopologue measurements can constrain crit-ical convective parameters, including degree of supersatura-tion, supercooled water content and glaciation temperature.Important isotopic processes in updrafts include kinetic ef-fects that are a consequence of diffusive growth or decay ofcloud particles within a supersaturated or subsaturated en-vironment; isotopic re-equilibration between vapour and su-percooled droplets, which buffers isotopic distillation; anddiffering mechanisms of glaciation (droplet freezing vs. theWegenerBergeronFindeisen process). As all of these pro-cesses are related to updraft strength, particle size distribu-tion and the retention of supercooled water, isotopic mea-surements can serve as a probe of in-cloud conditions of im-portance to convective processes. We study the sensitivity ofthe profile of water vapour isotopic composition to differingmodel assumptions and show how measurements of isotopiccomposition at cloud base and cloud top alone may be suffi-cient to retrieve key cloud parameters.

    1 Introduction

    Because the relative abundances of the stable isotopologuesof water HDO, H218O, and H2O are sensitive to phasechanges that occur within air masses, water isotopic ratioshave long been used as tracers of the atmospheric water cy-cle (since pioneering works in the 1950s; see Dansgaard,1964 for review). Although the primary motivation for iso-topic studies in the past has been the interpretation of pa-leoclimatic measurements (Jouzel and Merlivat, 1984, andreferences herewith), a second motivation is to obtain oth-erwise irretrievable information about convective processes(Jouzel et al., 1975; Risi et al., 2008; Lee et al., 2009; Ku-rita et al., 2011). Constraining convective processes has beena major incentive for recent efforts to include water isotopo-logues in atmospheric general circulation models (AGCMs)(Joussaume et al., 1984; Jouzel et al., 1987; Schmidt et al.,2005; Bony et al., 2008). These modelling studies, alongwith the recent availability of spaceborne observations of theisotopic composition of water vapour (Moyer et al., 1996;Kuang et al., 2003; Nassar et al., 2007; Steinwagner et al.,2007; Worden et al., 2011), have shown the potential of usingwater isotopologues to test our understanding of large-scaleatmospheric processes (Risi et al., 2008; Lee et al., 2009). Atsmaller scales, in situ measurements and inclusion of waterisotopologues in cloud-resolving models may bring insightinto the deep convective transport of water to the tropicaltropopause layer (TTL) (Smith et al., 2006; Hanisco et al.,2007; Blossey et al., 2010; Sayres et al., 2010).

    In the tropics, convective clouds provide the only mech-anism for transporting vapour and condensates across isen-tropes in the free troposphere, and most air parcels, dried

    Published by Copernicus Publications on behalf of the European Geosciences Union.

  • 7904 M. Bolot et al.: Vapour isotopic composition in updrafts

    out within the convective cells, eventually sink back to thesurface owing to radiational heat loss once they reach be-yond the confines of convective anvils (Folkins and Martin,2005). Fast large-scale stirring also redistributes air parcelstowards the subtropics, moving upward in a quasi-isentropicway (Pierrehumbert and Roca, 1998; Couhert et al., 2010)and providing further dehydration (Galewsky et al., 2005).While moisture is fixed by the last dehydration within or out-side the clouds, the water isotopic signature depends on thewhole history of the air parcel since its last contact with theboundary layer. The isotopic signature of free troposphericwater vapour in the tropics and subtropics is therefore largelydetermined by processes within convective clouds and by theevaporation of condensates formed by convection (Wrightet al., 2009; Risi et al., 2012).

    Our ability to model the convective transport of water iso-topologues depends on how well we account for the rele-vant physical processes in convective clouds. In that regard,some current assumptions about water isotopologues madein AGCMs may need to be revisited. The need to reproducethe observed isotopic content of precipitations over very coldregions (Greenland, Antarctica) was the main incentive to in-clude kinetic isotope effects (i.e. the fact that isotope frac-tionation is limited by diffusion of water vapour and heat) inAGCMs (Jouzel and Merlivat, 1984; Ciais and Jouzel, 1994).As clouds and precipitations are parametrised rather than ex-plicitly represented in AGCMs, supersaturation over ice hasbeen commonly represented as a simple function of tem-perature with the dependence chosen to reproduce isotopicsignatures in polar regions (Jouzel et al., 1987; Hoffmannet al., 1998). Temperatures as low as those over the poles arealso found at the top of convective towers, but the conditionswithin vertically-rising convective air parcels are very dif-ferent from those associated with transport of moisture fromevaporation to deposition zones at polar latitudes. Convec-tive towers, unlike stratiform clouds, are characterised by ahigh amount of condensed phase retained in the cloud andsupercooled droplets may persist to temperatures as low as40 C level. Supersaturation in convective clouds dependson updraft velocity, on the size distribution of liquid and iceparticles, and on the retention of supercooled water. The dis-tribution of stable isotopologues is therefore expected to besensitive to the representation of convective processes.

    Much of the material discussed in this paper has alreadybeen treated in the literature, and the basic physics of iso-topologues is not new. Studies of isotopologues in the con-text of convection can be traced back to the pioneering worksof Jouzel et al. (1975) and Federer et al. (1982). Further con-tributions to the field include Gedzelman and Arnold (1994);Moyer et al. (1996); Bony et al. (2008); Lee et al. (2009) andBlossey et al. (2010). Our aim here is to provide a consis-tent picture of how isotopic distributions are linked to cloudmicrophysics and thermodynamics, incorporating elementsthat are spread across several studies. As stated above, wewill revisit some current assumptions on the representation

    of microphysics. In particular, we are interested in the ef-fects related to saturation and glaciation on the distributionof isotopologues within deep convective clouds, which mayopen a way to retrieve quantitative information on these keyprocesses from isotopic observations. This work is motivatedby the recent surge of field and satellite measurements in theupper troposphere (Sayres et al., 2009; Nassar et al., 2007;Steinwagner et al., 2007; Randel et al., 2012) and by risinginterest in using isotopologues to diagnose cloud processesand their i

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