Aerosol and cloud microphysical properties

Aerosol and cloud microphysical properties characterisation addresses the problem of quantifying the physical and optical attributes of atmospheric aerosol particles and cloud droplets that determine how they interact with radiation. Key aerosol parameters include particle size distribution, particle shape, refractive index and single-scattering albedo; key cloud parameters include droplet effective radius and thermodynamic phase, liquid versus ice. Measuring these properties supports quantification of radiative forcing and aerosol-cloud interaction effects, distinguishing this problem from broader top-of-atmosphere radiative flux accounting and from trace-gas column chemistry retrievals.

Multi-angle polarimetry is the first-choice measurement approach for this problem: instruments observe the same scene from multiple viewing angles and polarisation states to constrain aerosol microphysical parameters and cloud droplet properties ^[1]. Lidar-based atmospheric backscatter measurement is a suitable approach, providing vertically resolved profiles of aerosol and cloud layers via active ranging ^[2]. Multispectral aerosol optical depth retrieval, using multispectral imaging radiometers without polarisation or multi-angle capability, is an adequate route to column aerosol optical depth. Single-view observations characterise aerosols to a lesser degree than multi-angle polarimetry, which alone retrieves aerosol particle size distribution, refractive index, absorption and particle non-sphericity ^[3].

Instruments well suited to this measurement problem include SPEXone and HARP2, both flown on NASA's PACE mission ^[4][1][5], and EUMETSAT's 3MI, the Multi-viewing Multi-channel Multi-polarisation Imager ^[6].

JAXA's SGLI, flown on the GCOM-C, Shikisai, mission, provides adequate capability for this measurement problem ^[7]. The Directional Polarization Camera, DPC, provides adequate capability for aerosol and cloud microphysical retrieval ^[8]. Spaceborne lidar systems provide vertically resolved aerosol and cloud backscatter profiles as a complementary suitable measurement route ^[2].