Cirrus cloud optical and microphysical property retrievals from eMAS during SEAC4RS using bi-spectral reflectance measurements within the 1.88 μm water vapor absorption band.

K Meyer,S Platnick,G Thomas Arnold,C Wang,J Yorks,Robert E Holz,P Veglio,G T Arnold,R E Holz

doi:10.5194/amt-9-1743-2016

Abstract

Previous bi-spectral imager retrievals of cloud optical thickness (COT) and effective particle radius (CER) based on the Nakajima and King (1990) approach, such as those of the operational MODIS cloud optical property retrieval product (MOD06), have typically paired a non-absorbing visible or near-infrared wavelength, sensitive to COT, with an absorbing shortwave or midwave infrared wavelength sensitive to CER. However, in practice it is only necessary to select two spectral channels that exhibit a strong contrast in cloud particle absorption. Here it is shown, using eMAS observations obtained during NASA's SEAC4RS field campaign, that selecting two absorbing wavelength channels within the broader 1.88 μm water vapor absorption band, namely the 1.83 and 1.93 μm channels that have sufficient differences in ice crystal single scattering albedo, can yield COT and CER retrievals for thin to moderately thick single-layer cirrus that are reasonably consistent with other solar and IR imager-based and lidar-based retrievals. A distinct advantage of this channel selection for cirrus cloud retrievals is that the below-cloud water vapor absorption minimizes the surface contribution to measured cloudy TOA reflectance, in particular compared to the solar window channels used in heritage retrievals such as MOD06. This reduces retrieval uncertainty resulting from errors in the surface reflectance assumption, as well as reduces the frequency of retrieval failures for thin cirrus clouds.

Highlights

Reflectance measurements at spectral channels centered within the water vapor absorption bands at 1.38 and 1.88 μm have been shown to be well suited for detecting cirrus clouds (Gao et al, 1993)
Because the non-absorbing visible (VIS), near-infrared (NIR), or shortwave infrared (SWIR) wavelength channels typically used for cloud optical thickness (COT) retrievals, as well as the absorbing SWIR and midwave infrared (MWIR) wavelength channels used for cloud effective particle radius (CER) retrievals, are sensitive to reflection by the underlying surface, such approaches are subject to larger retrieval uncertainty and increased frequency of retrieval failures for thin cirrus cases
Previous bi-spectral imager retrievals of COT and CER based on the Nakajima and King (1990) approach, such as those of the operational MODIS cloud optical property product (MOD06), have typically paired a non-absorbing VIS or NIR wavelength channel, sensitive to COT, with an absorbing SWIR or MWIR wavelength channel sensitive to CER

Summary

Introduction

Reflectance measurements at spectral channels centered within the water vapor absorption bands at 1.38 and 1.88 μm have been shown to be well suited for detecting cirrus clouds (Gao et al, 1993). TOA reflectance of cirrus at 1.38 μm is sensitive to cloud optical thickness (COT), with only a small sensitivity to cloud effective particle radius (CER) due to weak ice crystal absorption (Kou et al, 1993; Yang et al, 2000). This sensitivity has been exploited using 1.38 μm reflectance measurements from the Moderate-resolution Imag-. Because the non-absorbing visible (VIS), near-infrared (NIR), or shortwave infrared (SWIR) wavelength channels typically used for COT retrievals, as well as the absorbing SWIR and midwave infrared (MWIR) wavelength channels used for CER retrievals, are sensitive to reflection by the underlying surface, such approaches are subject to larger retrieval uncertainty and increased frequency of retrieval failures for thin cirrus cases

Methods

Results

Conclusion