Preparing for GeoXO: Capabilities of Shortwave Infrared Water Vapor Bands with Low-Level Moisture Boundary Detection and Total Precipitable Water Retrievals

NOAA’s next generation of geostationary satellites will feature a constellation of three satellites under the Geostationary Extended Observations (GeoXO) program. The GeoXO Imager (GXI), which will be carried on the eastern and western members of this constellation, is the flagship sensor of GeoXO for use in operational weather forecasting. GXI will bring several improvements over the Advanced Baseline Imager (ABI) instrument currently on the GOES-R series of satellites in terms of spatial, spectral, and temporal resolution. One such improvement is the addition of a new shortwave infrared (IR) water vapor narrow spectral band centered near 0.91 μm. Absorption by water vapor, primarily residing in the lowest levels of the atmosphere, results in a dampening of the 0.91 μm surface reflectance compared to a dry atmosphere. We are therefore able to relate the observed reflectances at 0.91 μm to the total precipitable water (TPW) in the column, as well as highlight fluctuations in lower tropospheric moisture. It is anticipated that this new band will offer valuable information to forecasters regarding clear-sky, daytime water vapor structure and distributions in the low levels; useful for identifying moisture boundaries, areas of low-level moisture convergence, and other features relevant to convective forecasting. Radiative Transfer simulations using the MODerate resolution atmospheric TRANsmission (MODTRAN) model provide insight into the physics of the band and highlight the channel’s sensitivity to column water vapor in the context of variable surface reflectance. The Solar Moisture Transmittance (SMT), a ratio of the reflectance of a shortwave water vapor-absorbing channel to that of an adjacent atmospheric-window band positioned at 0.865 μm, has been used previously to emphasize moisture features by removing surface influences. However, the SMT remains sensitive to surface albedo spatial variability. Case study analysis using NASA’s MODerate-resolution Imaging Spectroradiometer (MODIS) and simulated imagery is improving our understanding of the relationship between SMT and TPW, potentially leading to higher-order algorithms that can further isolate the water vapor features in the imagery. Highlighting the capabilities and possible applications of this new GXI band will help prepare forecasters to make optimal and informed use of GXI data at its first availability.