Spectrally-invariant behavior of zenith radiance around cloud edges simulated by radiative transfer

J C Chiu,Y Knyazikhin,W J Wiscombe,A Marshak

doi:10.5194/acp-10-11295-2010

J C Chiu, Y Knyazikhin + Show 2 more

Open Access

https://doi.org/10.5194/acp-10-11295-2010

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Abstract

Abstract. In a previous paper, we discovered a surprising spectrally-invariant relationship in shortwave spectrometer observations taken by the Atmospheric Radiation Measurement (ARM) program. The relationship suggests that the shortwave spectrum near cloud edges can be determined by a linear combination of zenith radiance spectra of the cloudy and clear regions. Here, using radiative transfer simulations, we study the sensitivity of this relationship to the properties of aerosols and clouds, to the underlying surface type, and to the finite field-of-view (FOV) of the spectrometer. Overall, the relationship is mostly sensitive to cloud properties and has little sensitivity to other factors. At visible wavelengths, the relationship primarily depends on cloud optical depth regardless of cloud phase function, thermodynamic phase and drop size. At water-absorbing wavelengths, the slope of the relationship depends primarily on cloud optical depth; the intercept, by contrast, depends primarily on cloud absorbing and scattering properties, suggesting a new retrieval method for cloud drop effective radius. These results suggest that the spectrally-invariant relationship can be used to infer cloud properties near cloud edges even with insufficient or no knowledge about spectral surface albedo and aerosol properties.

Highlights

The transition zone between cloudy and clear air, sometimes called the “cloud halo”, is a region of strong aerosolcloud interactions
We further extend the above analysis to various cloud optrary, a much wider spectral region is taken into account in tical depths in the transition zone (Fig. 5)
Comparison of C to B shows when the cloud drop size increases in moving from cloud to clear, the inter- 3.2.2 Cloud phase function cept decreases substantially. This suggest s that the intercept at 2110–2220 nm wavelengths contains information on the We study sensitivity to cloud phase function by replacing the growth/evaporation tendency of cloud drops, which can be Henyey-Greenstein phase function with a Mie phase funcapplied to actual observation and provide evidence of homo- tion

Summary

Introduction

The transition zone between cloudy and clear air, sometimes called the “cloud halo”, is a region of strong aerosolcloud interactions. One-second-resolution zenith radiance measurements from Atmospheric Radiation Measurement program ShortWave Spectrometer (SWS) provide a unique opportunity to analyze the cloud-clear transition zone. Chiu et al (2009) rigorously defined the transition zone and found a remarkable linear relationship between the sum and difference of radiances at 870 and 1640 nm wavelengths; this relationship allows us to separate radiative signatures of aerosols and clouds. Chiu et al.: Spectrally-invariant behavior of zenith radiance around cloud edges. By extending this work to all wavelengths in the SWS spectra, Marshak et al (2009) discovered a surprising spectrally-invariant relationship between ratios of zenith radiance spectra in the transition zone. We study factors that could, in principle, affect the spectrally-invariant behavior These sensitivity results will provide insights for the improvement of aerosol and cloud property retrievals near cloud edges

Radiative transfer simulations

The spectrally-invariant relationship

Control run simulations

Underlying surface type