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Statistical Analysis of the Uncertainties in Cloud Optical Depth Retrievals Caused by Three-Dimensional Radiative Effects

Tamás VárnaiJoint Center for Earth Systems Technology of NASA Goddard Space Flight Center and University of Maryland Baltimore County, Greenbelt, Maryland

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Alexander MarshakJoint Center for Earth Systems Technology of NASA Goddard Space Flight Center and University of Maryland Baltimore County, Greenbelt, Maryland

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Abstract

This paper presents a simple yet general approach to estimate the uncertainties that arise in satellite retrievals of cloud optical depth when the retrievals use one-dimensional radiative transfer theory for heterogeneous clouds that have variations in all three dimensions. For the first time, preliminary error bounds are set to estimate the uncertainty of cloud optical depth retrievals. These estimates can help us better understand the nature of uncertainties that three-dimensional effects can introduce into retrievals of this important product of the Moderate Resolution Imaging Spectroradiometer instrument. The probability distribution of resulting retrieval errors is examined through theoretical simulations of shortwave cloud reflection for a set of cloud fields that represent the variability of stratocumulus clouds. The results are used to illustrate how retrieval uncertainties change with observable and known parameters, such as solar elevation or cloud brightness. Furthermore, the results indicate that a tendency observed in an earlier study—clouds appearing thicker for oblique sun—is indeed caused by three-dimensional radiative effects.

Corresponding author address: Tamás Várnai, Code 913, NASA GSFC, Greenbelt, MD 20771. Email: varnai@climate.gsfc.nasa.gov

Abstract

This paper presents a simple yet general approach to estimate the uncertainties that arise in satellite retrievals of cloud optical depth when the retrievals use one-dimensional radiative transfer theory for heterogeneous clouds that have variations in all three dimensions. For the first time, preliminary error bounds are set to estimate the uncertainty of cloud optical depth retrievals. These estimates can help us better understand the nature of uncertainties that three-dimensional effects can introduce into retrievals of this important product of the Moderate Resolution Imaging Spectroradiometer instrument. The probability distribution of resulting retrieval errors is examined through theoretical simulations of shortwave cloud reflection for a set of cloud fields that represent the variability of stratocumulus clouds. The results are used to illustrate how retrieval uncertainties change with observable and known parameters, such as solar elevation or cloud brightness. Furthermore, the results indicate that a tendency observed in an earlier study—clouds appearing thicker for oblique sun—is indeed caused by three-dimensional radiative effects.

Corresponding author address: Tamás Várnai, Code 913, NASA GSFC, Greenbelt, MD 20771. Email: varnai@climate.gsfc.nasa.gov

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