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; 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.