Infrared Instrument Radiance Modeling from Large Eddy Simulations to Access Sensitivity to Marine Planetary Boundary Layer Processes

Infrared radiance simulations from a LES (large eddy simulation) experiment were generated and examined to understand sensitivity to fine structure arising from turbulent processes in the PBL. The LES run supported the Rain in Cumulus over the Ocean (RICO) field campaign, which was undertaken to understand how rain from shallow cumulus develop and evolve. The LES run corresponds to the composite case of observations collected during RICO. The model has 40 m horizontal and vertical resolution and includes binned cloud microphysics. Radiances were generated using the Stand-alone AIRS (Atmospheric Infrared Sounder) Radiative Transfer Algorithm (SARTA) radiative transfer model with a d 4-stream multiple scattering cloud model. The cloud optical depth and effective cloud particle radius were calculated from the binned microphysics fitting to a G size distribution. Two sets of radiances were calculated: one with scattering on and a second clear sky case with the scattering turned off. This was so we could assess the relative contributions of clouds, water vapor and temperature. The grid was 20.4 x 20.4 km and extended to 4 km; a single atmospheric state was tacked onto the top of the grid. The satellite was positioned above the center of the grid at an altitude of 825 km, and a whisk-broom scan was assumed; each grid point has a slightly different viewing angle and path through the atmosphere. Different telescope (field-of-view) geometries were studied from 40 m to 4 km footprint size to determine information content versus footprint size. The most sensitive water vapor channels were window channels having the strongest water vapor continuum signals. Most of the variability is smoothed from the image, once the footprint size exceeds 500 m. The additions of clouds add more variability, but distinguishing between clouds, temperature and water vapor is more difficult, especially once footprint size exceeds 120 m. In this talk we discuss the tradeoffs between spatial resolution, spectral resolution and the capabilities of IR sounders to characterize PBL turbulent processes.