Information Content of Ice Cloud Properties from Multi-Spectral, -Angle and -Polarization Observations

Manting Zhang,Xiuqing Hu,Min Min,Chao Liu,Shiwen Teng,Di Di,Husi Letu

doi:10.3390/rs12162548

Abstract

Ice clouds play an important role in the Earth’s radiation budget, while their microphysical and optical properties remain one of the major uncertainties in remote sensing and atmospheric studies. Many satellite-based multi-spectral, -angle and -polarization instruments have been launched in recent years, and it is unclear how these observations can be used to improve the understanding of ice cloud properties. This study discusses the impacts of multi-spectral, -angle and -polarization observations on ice cloud property retrievals by performing a theoretical information content (IC) analysis. Ice cloud properties, including the cloud optical thickness (COT), particle effective radius (Re) and particle habit (defined by the aspect ratio (AR) and the degree of surface roughness level (σ)), are considered. An accurate polarized radiative transfer model is used to simulate the top-of-atmosphere intensity and polarized observations at the cloud-detecting wavelengths of interest. The ice cloud property retrieval accuracy should be improved with the additional information from multi-spectral, -angle and -polarization observations, which is verified by the increased degrees of freedom for signal (DFS). Polarization observations at spectral wavelengths (i.e., 0.87 and 2.13 µm) are helpful in the improvement of ice cloud property retrievals, especially for small-sized particles. An optimal scheme to retrieve ice cloud properties is to comprise radiance intensity information at the 0.87, 1.24, 1.64 and 2.13 µm channels and polarization information (the degree of linear polarization, DOLP) at the 0.87 and 2.13 µm channels. As observations from multiple angles added, DFS clearly increases, while it becomes almost saturated when the number of angles reaches three. Besides, the retrieval of Re exhibits larger uncertainties, and the improvement in total DFS by adding multi-spectral, -angle and -polarization observations is mainly attributed to the improvement of Re retrieval. Our findings will benefit the future instrument design and the improvement in cloud property retrieval algorithms based on multi-spectral, -angle, and -polarization imagers.

Highlights

Ice clouds, which cover 60–70% of the tropics, play an important role in regional and global climate and affect the Earth’s radiation budget by reflecting incoming solar radiation or by blocking outgoing infrared radiation to cool the atmosphere [1,2,3,4]
Simulations are performed for single-layer ice clouds with a cloud optical thickness (COT) of 5.5 at a solar zenith angle (SZA) of 40◦ to ensure the saturation of polarized radiation for thick clouds
The Jacobians of top of atmosphere (TOA) intensity and DOLP with respect to COT at different spectral channels are illustrated in Figure 2l–u; a larger Jacobian value indicates that the TOA intensity or DOLP is more sensitive to COT

Summary

Introduction

Ice clouds, which cover 60–70% of the tropics, play an important role in regional and global climate and affect the Earth’s radiation budget by reflecting incoming solar radiation or by blocking outgoing infrared radiation to cool the atmosphere [1,2,3,4]. The comparison between different satellite Re retrievals suffer from disparities ranging from 2 (20%) to 9 (50%) μm, some of the differences are due to the cloud vertical structure, cloud horizontal homogeneity, viewing geometry and retrieval system [7,8,19]. Such poor accuracy limits our ability to comprehend cloud properties, because the Re accuracy for climate change research requires the uncertainty to be less than 10% [20]

Methods

Results

Conclusion