2.2 NASA Goddard Institute for Space Studies ModelE version
2.1,
GISS-E2.1
2.2.1 Base Model
GISS-E2.1 (Kelley et al., 2020), submitted to CMIP6 with a resolution of
2°x2.5°x40-layers, top at 0.1mb prescribes sea surface temperature (SST)
and ice from Hadley1.1 (Rayner et al., 2003). We ‘nudged’ the horizontal
wind field to conform with MERRA-2 reanalysis (Bosilovich et al., 2015)
to reproduce historical climate, denoted GISS-E2.1+MERRA2u/v. Nudging to
MERRA-2 ends at 1000hPa, while surface pressure is
~1030hPa; below, the wind field can vary consistent with
the planetary boundary layer (PBL) scheme. Simulations are performed
over the 1980-2022 period; we focus only on the CYGNSS 2018-2022 period.
From 1980-2016, CMIP6 atmospheric constituent conditions are applied.
Unfortunately, at the time of this writing, CMIP6 has not been extended,
thus we use the RCP6.5 scenario as a reasonable approximation for
atmospheric constituents 2016-2022. We save variables at 6-hour time
intervals and all analyses are on the GISS-E2.1 grid.
2.2.2 Moisture Source
Tracers
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).
Please find an expanded explanation of VSD tracers in the Supplemental
Text.