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.