DOI: 10.5061/dryad.8w9ghx3mr
title: Enhancing understanding of the hydrological cycle via pairing of process‐oriented and isotope ratio tracers

description: This dataset contains monthly average output files from the iCAM6
 simulations used in the manuscript "Enhancing understanding of the
 hydrological cycle via pairing of process-oriented and isotope ratio
 tracers," in review at the Journal of Advances in Modeling Earth
 Systems. A file corresponding to each of the tagged and isotopic variables
 used in this manuscript is included. Files are at 0.9° latitude x 1.25°
 longitude, and are in NetCDF format. Data from two simulations are
 included: 1) a simulation where the atmospheric model was
 "nudged" to ERA5 wind and surface pressure fields, by adding an
 additional tendency (see section 3.1 of associated manuscript), and 2) a
 simulation where the atmospheric state was allowed to freely evolve, using
 only boundary conditions imposed at the surface and top of atmosphere.
 Specific information about each of the variables provided is located in
 the "usage notes" section below. Associated article abstract:
 The hydrologic cycle couples the Earth's energy and carbon budgets
 through evaporation, moisture transport, and precipitation. Despite a
 wealth of observations and models, fundamental limitations remain in our
 capacity to deduce even the most basic properties of the hydrological
 cycle, including the spatial pattern of the residence time (RT) of water
 in the atmosphere and the mean distance traveled from evaporation sources
 to precipitation sinks. Meanwhile, geochemical tracers such as stable
 water isotope ratios provide a tool to probe hydrological processes, yet
 their interpretation remains equivocal despite several decades of use. As
 a result, there is a need for new mechanistic tools that link variations
 in water isotope ratios to underlying hydrological processes. Here we
 present a new suite of “process-oriented tags,” which we use to explicitly
 trace hydrological processes within the isotopically enabled Community
 Atmosphere Model, version 6 (iCAM6). Using these tags, we test the
 hypotheses that precipitation isotope ratios respond to parcel rainout,
 variations in atmospheric RT, and preserve information regarding
 meteorological conditions during evaporation. We present results for a
 historical simulation (1980-2004 for publication, but since extended to
 2021), forced with winds from the ERA5 reanalysis. We find strong evidence
 that precipitation isotope ratios record information about atmospheric
 rainout and meteorological conditions during evaporation, but little
 evidence that precipitation isotope ratios vary with water vapor RT. These
 new tracer methods will enable more robust linkages between observations
 of isotope ratios in the modern hydrologic cycle or proxies of past
 terrestrial environments and the environmental processes underlying these
 observations.
description: Details about the simulation setup can be found in section 3 of
 the associated open-source manuscript, "Enhancing understanding of
 the hydrological cycle via pairing of process‐oriented and isotope ratio
 tracers." In brief, we conducted two
 simulations of the atmosphere using the isotope-enabled version of the
 Community Atmosphere Model 6 (iCAM6) at 0.9x1.25° horizontal resolution,
 and with 30 vertical hybrid layers spanning from the surface to ~3 hPa. In
 the first simulation, wind and surface pressure fields were
 "nudged" toward the ERA5 reanalysis dataset by adding a nudging
 tendency, preventing the model from diverging from observed/reanalysis
 wind fields. In the second simulation, no additional nudging tendency was
 included, and the model was allowed to evolve 'freely' with only
 boundary conditions provided at the top (e.g., incoming solar radiation)
 and bottom (e.g., observed sea surface temperatures) of the model. Both
 simulations were run between 1980-2004 for the publication, but in the
 2025 release the "nudged" simulation has been expanded to run
 from 1980-2021. In addition to the
 isotopic variables, our simulation included a suite of
 'process-oriented tracers,' which we describe in section 2 of
 the manuscript. These variables are meant to track a property of water
 associated with evaporation, condensation, or atmospheric
 transport.
description: Metadata are provided about each of the files below; moreover,
 since the attached files are NetCDF data - this information is also
 provided with the data files. NetCDF metadata can be accessed using
 standard tools (e.g., ncdump). Each file has 4
 variables: the tagged quantity, and the associated coordinate variables
 (time, latitude, longitude). The latter three are identical across all
 files, only the tagged quantity changes. Twelve files
 are provided for the nudged simulation, and an additional three are
 provided for the free simulations: Nudged
 simulation files iCAM6_nudged_1980-2021_mon_RHevap:
 Mass-weighted mean evaporation source property: RH (%) with respect to
 surface temperature.
 iCAM6_nudged_1980-2021_mon_Tevap: Mass-weighted mean evaporation
 source property: surface temperature in Kelvin
 iCAM6_nudged_1980-2021_mon_Tcond: Mass-weighted mean
 condensation property: temperature (K)
 iCAM6_nudged_1980-2021_mon_columnQ: Total (vertically
 integrated) precipitable water (kg/m2).  Not a tagged quantity, but
 necessary to calculate depletion times in section 4.3 (e.g., Fig. 11 and
 12). iCAM6_nudged_1980-2021_mon_d18O: Precipitation
 d18O (‰ VSMOW) iCAM6_nudged_1980-2021_mon_d18Oevap_0:
 Mass-weighted mean evaporation source property - d18O of the evaporative
 flux (e.g., the 'initial' isotope ratio prior to condensation),
 (‰ VSMOW) iCAM6_nudged_1980-2021_mon_dxs:
 Precipitation deuterium excess (‰ VSMOW) - note that precipitation d2H can
 be calculated from this file and the precipitation d18O as d2H = d-excess
 - 8*d18O. iCAM6_nudged_1980-2021_mon_dexevap_0:
 Mass-weighted mean evaporation source property - deuterium excess of the
 evaporative flux iCAM6_nudged_1980-2021_mon_lnf:
 Integrated property - ln(f) calculated from the constant-fractionation
 d18O tracer (see section 3.2).
 iCAM6_nudged_1980-2021_mon_precip: Total precipitation rate in
 m/s. Note there is an error in the metadata in this file - it is total
 precipitation, not just convective precipitation.
 iCAM6_nudged_1980-2021_mon_residencetime: Mean atmospheric water
 residence time (in days).
 iCAM6_nudged_1980-2021_mon_transportdistance: Mean atmospheric
 water transport distance (in km).
 iCAM6_nudged_1980-2021_mon_precc.nc: Local mean convective
 precipitation rate (m/s).
 iCAM6_nudged_1980-2021_mon_precl.nc: Local mean large-scale
 (e.g., stratiform) precipitation rate (m/s).
 iCAM6_nudged_1980-2021_mon_t2m.nc: Local 2-m air temperature
 (K). iCAM6_nudged_1980-2021_mon_Tsrf.nc: Local
 surface temperature (K). Free
 simulation files iCAM6_free_1980-2004_mon_d18O:
 Precipitation d18O (‰ VSMOW)
 iCAM6_free_1980-2004_mon_dxs: Precipitation deuterium excess (‰
 VSMOW) - note that precipitation d2H can be calculated from this file and
 the precipitation d18O as d2H = d-excess - 8*d18O.
 iCAM6_free_1980-2004_mon_precip: Total precipitation rate in
 m/s. Note there is an error in the metadata in this file - it is total
 precipitation, not just convective precipitation.
description: Readme for dataset associated with "Enhancing understanding of the
 hydrological cycle via pairing of process‐oriented and isotope ratio
 tracers" by Fiorella, Siler, Nusbaumer, and Noone, published in 2021
 in the Journal of Advances in Modeling Earth Systems (doi:
 10.1029/2021ms002648). Contact information: Richard Fiorella,
 [rfiorella@newmexicoconsortium.org](mailto:rfiorella@newmexicoconsortium.org); [rfiorella@lanl.gov](mailto:rfiorella@lanl.gov) Abstract This dataset contains monthly average output files from the iCAM6 simulations used in the manuscript "Enhancing understanding of the hydrological cycle via pairing of process-oriented and isotope ratio tracers," in review at the Journal of Advances in Modeling Earth Systems. A file corresponding to each of the tagged and isotopic variables used in this manuscript is included. Files are at 0.9° latitude x 1.25° longitude, and are in NetCDF format. Data from two simulations are included: 1) a simulation where the atmospheric model was "nudged" to ERA5 wind and surface pressure fields, by adding an additional tendency (see section 3.1 of associated manuscript), and 2) a simulation where the atmospheric state was allowed to freely evolve, using only boundary conditions imposed at the surface and top of atmosphere. Specific information about each of the variables provided is located in the "usage notes" section below. Associated article abstract: The hydrologic cycle couples the Earth's energy and carbon budgets through evaporation, moisture transport, and precipitation. Despite a wealth of observations and models, fundamental limitations remain in our capacity to deduce even the most basic properties of the hydrological cycle, including the spatial pattern of the residence time (RT) of water in the atmosphere and the mean distance traveled from evaporation sources to precipitation sinks. Meanwhile, geochemical tracers such as stable water isotope ratios provide a tool to probe hydrological processes, yet their interpretation remains equivocal despite several decades of use. As a result, there is a need for new mechanistic tools that link variations in water isotope ratios to underlying hydrological processes. Here we present a new suite of “process-oriented tags,” which we use to explicitly trace hydrological processes within the isotopically enabled Community Atmosphere Model, version 6 (iCAM6). Using these tags, we test the hypotheses that precipitation isotope ratios respond to parcel rainout, variations in atmospheric RT, and preserve information regarding meteorological conditions during evaporation. We present results for a historical simulation (1980-2004 for publication, but since extended to 2021), forced with winds from the ERA5 reanalysis. We find strong evidence that precipitation isotope ratios record information about atmospheric rainout and meteorological conditions during evaporation, but little evidence that precipitation isotope ratios vary with water vapor RT. These new tracer methods will enable more robust linkages between observations of isotope ratios in the modern hydrologic cycle or proxies of past terrestrial environments and the environmental processes underlying these observations. Methods Details about the simulation setup can be found in section 3 of the associated open-source manuscript, "Enhancing understanding of the hydrological cycle via pairing of process‐oriented and isotope ratio tracers." In brief, we conducted two simulations of the atmosphere using the isotope-enabled version of the Community Atmosphere Model 6 (iCAM6) at 0.9x1.25° horizontal resolution, and with 30 vertical hybrid layers spanning from the surface to ~3 hPa. In the first simulation, wind and surface pressure fields were "nudged" toward the ERA5 reanalysis dataset by adding a nudging tendency, preventing the model from diverging from observed/reanalysis wind fields. In the second simulation, no additional nudging tendency was included, and the model was allowed to evolve 'freely' with only boundary conditions provided at the top (e.g., incoming solar radiation) and bottom (e.g., observed sea surface temperatures) of the model. Both simulations were run between 1980-2004 for the publication, but in the 2025 release the "nudged" simulation has been expanded to run from 1980-2021. In addition to the isotopic variables, our simulation included a suite of 'process-oriented tracers,' which we describe in section 2 of the manuscript. These variables are meant to track a property of water associated with evaporation, condensation, or atmospheric transport. Usage notes Metadata are provided about each of the files below; moreover, since the attached files are NetCDF data - this information is also provided with the data files. NetCDF metadata can be accessed using standard tools (e.g., ncdump). Each file has 4 variables: the tagged quantity, and the associated coordinate variables (time, latitude, longitude). The latter three are identical across all files, only the tagged quantity changes. Twelve files are provided for the nudged simulation, and an additional three are provided for the free simulations: ``` 1. Nudged simulation files a. iCAM6_nudged_1980-2021_mon_RHevap.nc: Mass-weighted mean evaporation source property: RH (%) with respect to surface temperature. b. iCAM6_nudged_1980-2021_mon_Tevap.nc: Mass-weighted mean evaporation source property: surface temperature in Kelvin c. iCAM6_nudged_1980-2021_mon_Tcond.nc: Mass-weighted mean condensation property: temperature (K) d. iCAM6_nudged_1980-2021_mon_columnQ.nc: Total (vertically integrated) precipitable water (kg/m2). Not a tagged quantity, but necessary to calculate depletion times in section 4.3 (e.g., Fig. 11 and 12). e. iCAM6_nudged_1980-2021_mon_d18O.nc: Precipitation d18O (‰ VSMOW) f. iCAM6_nudged_1980-2021_mon_d18Oevap_0.nc: Mass-weighted mean evaporation source property - d18O of the evaporative flux (e.g., the 'initial' isotope ratio prior to condensation), (‰ VSMOW) g. iCAM6_nudged_1980-2021_mon_dxs.nc: Precipitation deuterium excess (‰ VSMOW) - note that precipitation d2H can be calculated from this file and the precipitation d18O as d2H = d-excess - 8*d18O. h. iCAM6_nudged_1980-2021_mon_dexevap_0.nc: Mass-weighted mean evaporation source property - deuterium excess of the evaporative flux i. iCAM6_nudged_1980-2021_mon_lnf.nc: Integrated property - ln(f) calculated from the constant-fractionation d18O tracer (see section 3.2). j. iCAM6_nudged_1980-2021_mon_precip.nc: Total precipitation rate in m/s. Note there is an error in the metadata in this file - it is total precipitation, not just convective precipitation. k. iCAM6_nudged_1980-2021_mon_residencetime.nc: Mean atmospheric water residence time (in days). l. iCAM6_nudged_1980-2021_mon_transportdistance.nc: Mean atmospheric water transport distance (in km). m. iCAM6_nudged_1980-2021_mon_precc.nc: Local mean convective precipitation rate (m/s). n. iCAM6_nudged_1980-2021_mon_precl.nc: Local mean large-scale (e.g., stratiform) precipitation rate (m/s). o. iCAM6_nudged_1980-2021_mon_t2m.nc: Local 2-m air temperature (K). p. iCAM6_nudged_1980-2021_mon_Tsrf.nc: Local surface temperature (K). 2. Free simulation files a. iCAM6_free_1980-2004_mon_d18O: Precipitation d18O (‰ VSMOW) b. iCAM6_free_1980-2004_mon_dxs: Precipitation deuterium excess (‰ VSMOW) - note that precipitation d2H can be calculated from this file and the precipitation d18O as d2H = d-excess - 8*d18O. c. iCAM6_free_1980-2004_mon_precip: Total precipitation rate in m/s. Note there is an error in the metadata in this file - it is total precipitation, not just convective precipitation. ```