2.3 Atmospheric River
Detection
2.3.1 Automated global algorithm
We utilize the Guan and Waliser (2019), GW19, global AR detection
algorithm which first specifies a threshold of (vertically) Integrated
Vapor Transport (IVT), and then identifies ARs from the subset of IVT
that conforms to a 3:1 length to width. Here, we use the
85th% IVT for 2018-2022, during which changes in
baseline temperature (and IVT) are minimal (Guan et al, 2023). GW19 is
one of multiple automated global AR detection algorithms, which do not
all agree (ARTMIP: Shields et al., 2018). Algorithm choice can introduce
uncertainty on AR seasonality and changes over time (Payne et al.,
2020).
2.3.2 Atmospheric River Genesis and Transport
Distances
A strength of the GW19 algorithm is that it tracks ARs through their
lifetime including the genesis timestep. We partition each grid box and
time into time categories of all AR (AllAR), non-Atmospheric River
(nonAR), and AR genesis (ARgenesis), defined as the appearance of new
ARs that persist for at least 12 hours. These ARgenesis footprints are
used in the VSD processing.
Each of the VSD tracers are vertically integrated such that tracer 1
(equivalent to q) is identical to IVT. In post processing, we calculate
moisture provenance for each ARgenesis, and we calculate the attendant
AllAR and nonAR moisture provenance (over each ARgenesis footprint)
given a monthly climatological distribution of VSDs over 2018-2022. We
calculate the centroid of the ARgenesis mask and the centroid of the
moisture provenance for each ARgenesis, AllARs, and nonARs to determine
source latitude and distance.