The first paragraph is repeated from Main Text Methods 2.2.2 so
that this supplemental text can be read independently.
The GISS model uses Vapor Source Distribution tracers (VSDs) for
explicit evaluation of the initial source of water for a given air
parcel (e.g., Lewis et al., 2010; Nusbaumer et al., 2019). VSDs are
assigned an initial distribution upon evaporation using spherical
harmonics basis functions, yielding computationally efficient ‘painted
water’ tracers. We resolve wave number 11, ~8°×10°
horizontal resolution, using 144 tracers (Lewis et al., 2010), versus
the 828 required for a grid-box approach. The concentration of each of
these 144 tracers in post processing is used to solve for the
evaporative source at each level in the model, and here, these tracers
are integrated vertically analogous to the calculation of integrated
water transport used to flag ARs. Because there is no a prioridefinition of evaporative source, the GISS VSD tracers can provide a
quantitative assessment of distance traveled and average latitude and
longitude of distance traveled for any location from a single grid box
to a dynamically moving AR (e.g., Fig. 4), to a fixed region during
post-processing.
S3.2 Background Moisture Source Tracers
Previous ‘painted water’ tracking was limited to a predefined set of
regions (<20: Joussaume et al., 1986; Koster et al., 1986;
Sodemann & Stohl, 2013; Nusbaumer & Noone, 2018); our VSDs are not
anchored to any particular geographic boundary and require no prior
regional definition. Back trajectory analysis has also been used to
infer moisture sources (Brown et al., 2013; Sodemann et al., 2008) –
these analyses have difficulty tracking moisture source across the
boundary layer. Both Sodemann & Stohl (2013) and Nusbaumer & Noone
(2018) used the ‘painted water’ approach specific to atmospheric rivers;
however, each tagged large swaths of the ocean, principally by latitude.
Sodemann & Stohl (2013) were able to track moisture sources persisting
through several ETCs – along with attendant climate background, wind,
and other changes – in the North Atlantic only which highlights the
power of this method to discriminate moisture fueling ARs. Nusbaumer &
Noone (2018) were able to surmise that ARs carry ~15%
of far-field moisture in the North Pacific with more intense storms
having more distant sources, but their tracer concentration cannot
provide a quantitative estimate for distance or latitude in the same
way. Others have used self-organizing maps (Mattingly et al., 2016) to
assess moisture provenance; these are a promising technique but do not
include explicit treatment for source. GISS VSDs remain robust across
the boundary layer and through time allowing for far-field moisture
sources to be explicitly examined.