A New Retrieval of Aerosol Optical Depth under Partly Cloudy Conditions with Multi-Spectral Measurements of Reflectance
The three-dimensional (3D) radiative effects may cause large uncertainties of satellite aerosol retrievals under partly cloudy conditions [1,2]. For example, analysis of multi-year aerosol statistics derived from the MODerate-Resolution Imaging Spectroradiometer (MODIS) data in clear patches of cloud fields suggests that aerosol product may be in a large error (up to 140%) as a result of 3D cloud-induced enhancement of clear sky reflectance [3]. Retrievals of AOD τa from satellite observations consist of two basic steps: (1) sampling, which includes detection of clear pixels and (2) and application of an algorithm, which estimates AOD in these pixels. The quality of the final product depends on both steps [4]. The largest errors occur for pixels located within areas of sunlight and shadows where the 3D radiative effects have the greatest impacts on the AOD retrievals [2]. To reduce the 3D radiative effects, clear pixels have to be selected far away (~1-2 km) from clouds and their shadows [3]. For selected clear pixels, the independent pixel approximation approach (IPA) [5] is used to estimate the AOD. Since the IPA ignores the 3D cloud-induced enhancement, the IPA-based retrievals can substantially overestimate AOD even for these clear pixels. To take into account such enhancement, a simple parameterization has been suggested [6]. Here we introduce an approach [7], that provides an effective way to avoid the 3D cloud effects, and illustrate with a model-output inverse problem its capability to detect clear pixels (outside of shadows) and estimate their AOD.
- Research Organization:
- Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 977332
- Report Number(s):
- PNNL-SA-61627; KP1701000
- Country of Publication:
- United States
- Language:
- English
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