Application of the First and Second Generation Airborne Multiangle SpectroPolarimetric Imagers (AirMSPI and AirMSPI-2) to Cloud and Aerosol Remote Sensing

NASA is currently formulating the Aerosol-Cloud-Ecosystem (ACE) mission to advance our understanding of the climate impacts of aerosols and clouds. Our development program in support of ACE includes two airborne prototype sensors. The first generation instrument, AirMSPI, is an 8-band ultraviolet-visible-near infrared pushbroom camera, measuring polarization at 470, 660, and 865 nm. It is mounted on a gimbal to acquire multiangular observations over a ±67° along-track range from the NASA ER-2 high-altitude aircraft. In 2013, AirMSPI participated in field campaigns including the ACE Polarimeter Definition Experiment (PODEX) and Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS). Measurement requirements established by the ACE Science Working Group include ±3% radiometric uncertainty and ±0.005 uncertainty in degree of linear polarization (DOLP). AirMSPI radiometric calibration is performed using a combination of laboratory measurements and vicarious field experiments. Polarimetric calibration makes use of a highly accurate Partial Polarization State Generator (PSG). Data acquired with the PSG indicate that systematic errors in AirMSPI DOLP are on the order of ±0.001.

Polarimetry provides valuable constraints on atmospheric particle optical and microphysical properties. AirMSPI imagery over liquid water clouds contain numerous examples in which cloudbow, supernumerary bow, and glory observations in polarized light provide valuable constraints on cloud-top droplet size distributions. Droplet size retrievals based on application of single scattering theory to the spectral and angular structure of polarized radiance of the supernumerary bows (scattering angle from 150° to 170°) can differ from results obtained in the glory region (scattering angle >170°), suggesting the need to account more rigorously for multiple scattering, spatial variability, vertical stratification, or other effects. AirMSPI's ultraviolet channels provide sensitivity to absorbing aerosols, and several datasets acquired in 2013 include smoke above clouds. We will show how multiangle radiometry and polarimetry can work together to simultaneously constrain the aerosol and cloud microphysics in such complex situations.

Construction of the second generation AirMSPI-2 instrument is nearing completion. This sensor adds a cirrus channel at 1888 nm, polarimetric capability at 1620 and 2185 nm, and narrowband radiance channels in the center and wing of the near-infrared O2 A-band. We will discuss how these new features will provide additional information on cloud microphysics and vertical structure. We are aiming for maiden flight of AirMSPI-2 on the ER-2 in late 2014.