Abstract

We present an improved remote sensing technique to infer an optimal habit/shape model for ice particles in cirrus clouds using multi-angle polarimetric measurements at 865 nm made by the Airborne Multi-angle SpectroPolarimeter Imager (AirMSPI) instrument. The common method of ice model inference is based on intensity (total reflectivity) measurements, which is generally not applicable to optically thin ice clouds (i.e., cirrus clouds) where single scattering dominates. The new approach is able to infer an ice model in clouds with optical thicknesses smaller than 5. The improvement is made by first assuming the optical thickness retrieved using total reflectivity. Subsequently, the polarized reflectivity is calculated based on look-up tables generated from simulated polarized reflectances computed for cirrus clouds in conjunction with eight ice particle models. The ice particle model that leads to the closest fit to the measurements is regarded as the optimal ice particle model. Additionally, an alternative method is applied that does not consider polarized reflectivity. These two methods are applied to a data sample as a proof-of-concept study where AirMSPI observed a single cirrus layer. In this case study, the hexagonal column aggregate model works for most pixels both with and without considering polarized reflectivities. The retrieval cost function is high when the camera pairs with large zenith angles are included in the retrievals. This result suggests that further studies will be necessary to have a better understanding of all eight selected ice particle models at scattering angles smaller than 100°.

Highlights

  • The selection of an appropriate ice particle habit model and subsequent generation of the corresponding bulk single-scattering properties are critical to the implementation of remote sensing cirrus cloud property retrievals and, subsequently, to the assessment of the cirrus radiative forcing effect [1,2,3,4]

  • A bulk ice particle habit model should lead to consistent optical thickness and effective particle size retrievals from measurements across different scattering viewing geometries

  • We developed a remote sensing technique to infer an optimal ice particle model for cirrus using multidirectional polarimetric observations at 865 nm

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Summary

Introduction

The selection of an appropriate ice particle habit model and subsequent generation of the corresponding bulk single-scattering properties are critical to the implementation of remote sensing cirrus cloud property retrievals and, subsequently, to the assessment of the cirrus radiative forcing effect [1,2,3,4]. A bulk ice particle habit model should lead to consistent optical thickness and effective particle size retrievals from measurements across different scattering viewing geometries (i.e., viewing zenith angle and viewing azimuth angles) Towards this goal of developing a model that leads to greater consistency, satellite or airborne multi-angle imaging sensors, including the POLarization and Directionality of the Earth’s Reflectances (POLDER) [5] and Multi-angle Imaging SpectroRadiometer (MISR) instruments [6], as well as similar airborne instruments, provide unique observations. These sensors use fixed cameras to view a given pixel at multiple angles along the satellite or airplane path. The off-nadir cameras from multi-angle imaging sensors could capture the escaped radiation

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