Cloud top observations of LWIR spectro-polarimetry

Ice clouds remain a dominant source of uncertainty in global climate models due to challenges in constraining microphysical properties such as ice water path (IWP). Neglecting polarization effects in retrievals can introduce errors of up to 30%. Long-wave infrared (LWIR) polarimetry is theoretically more sensitive to small ice crystals and low IWP compared to microwave polarimetry, yet no spaceborne instruments currently provide such measurements. To address this gap, we developed the Infrared Channeled Spectro-Polarimeter (IRCSP), which measures linear Stokes parameters across three LWIR wavebands (8.5, 9.5, and 10.5 <i>&mu;</i>m). Following an initial high-altitude balloon demonstration in 2021, we deployed the upgraded instrument from Esrange Space Center (Sweden) in July 2024, obtaining more than 300 minutes of near-space observations at 39 km altitude. Thermal management upgrades stabilized focal-plane array temperatures to within &plusmn;0.1&deg;C, enabling 164 minutes of observations with improved radiometric accuracy. Observed brightness temperatures ranged from 220–270 K and were compared against MODIS cloud-top retrievals, confirming predominantly ice-phase clouds in the instrument field of view. Channeled polarimetry encodes polarization via a spectrally modulated carrier frequency determined by the IRCSP’s high-order retarder. Using the Lomb–Scargle periodogram, polarization carrier frequencies consistent with the instrument design were detected in 15%, 42%, and 15% of measurements in the water vapor, ozone, and clear-sky bands, respectively. Polarized signals exhibited a preferential horizontal orientation, consistent with scattering models for nonspherical ice particles. Degree-of-linear-polarization (DoLP) values reached up to 27% with relatively large retrieval uncertainties of 13–18%. Trends between DoLP and brightness temperature indicate a polarization peak at intermediate cloud-top temperatures, similar to patterns observed in the microwave band. These results provide the first measurements of LWIR polarized signatures from ice clouds with thermally stable detectors, highlighting both the promise of LWIR polarimetry for cloud property retrievals and the enhanced instrumental precision needed for future satellite missions.