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A Simple Model for the Evaporation of Hydrometers and Their Isotopes
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  • Simon P. de Szoeke,
  • Mampi Sarkar,
  • Estefanía Quiñones Meléndez,
  • Peter N. Blossey,
  • David C Noone
Simon P. de Szoeke
Oregon State University

Corresponding Author:[email protected]

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Mampi Sarkar
University of Houston
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Estefanía Quiñones Meléndez
Oregon State University
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Peter N. Blossey
University of Washington
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David C Noone
University of Auckland
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Evaporation decreases the mass and increases the isotope composition of falling drops. Combining and integrating the dependence of the evaporation on the drop diameter and on the drop-environment humidity difference, the square of drop diameter is found to decrease with the square of vertical distance below cloud base. Drops smaller than 0.5 mm evaporate completely before falling 700 m in typical subtropical marine boundary layer conditions. The effect on the isotope ratio of equilibration with the environment, evaporation, and kinetic molecular diffusion is modeled by molecular and eddy diffusive fluxes after Craig and Gordon (1965), with a size-dependent parameterization of diffusion that enriches small drops more strongly, and approaches the rough aerodynamic limit for large drops. Rain shortly approaches a steady state with the subcloud vapor by exchange with a length scale of 40 m. Kinetic molecular diffusion enriches drops up to as they evaporate by up to +5~\permil~for deuterated water (HDO) and +3.5~\permil~for H$_2$$^{18}$O. Rain evaporation enriches undiluted subcloud vapor by +12~\permil~per mm rain, explaining enrichment of vapor in evaporatively cooled downdrafts that contribute to cold pools. Microphysics enriches the vapor lost by the early and complete evaporation of smaller drops in the distribution. Vapor from hydrometeors is more enriched than it would be by Rayleigh distillation or by mixtures of liquid rain and vapor in equilibrium with rain.
07 Mar 2024Submitted to ESS Open Archive
08 Mar 2024Published in ESS Open Archive