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A Better Understanding of Polder's Cloud Droplet Size Retrieval: Impact of Cloud Horizontal Inhomogeneity and Directional Sampling : Volume 8, Issue 7 (01/07/2015)

By Shang, H.

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Book Id: WPLBN0004000516
Format Type: PDF Article :
File Size: Pages 39
Reproduction Date: 2015

Title: A Better Understanding of Polder's Cloud Droplet Size Retrieval: Impact of Cloud Horizontal Inhomogeneity and Directional Sampling : Volume 8, Issue 7 (01/07/2015)  
Author: Shang, H.
Volume: Vol. 8, Issue 7
Language: English
Subject: Science, Atmospheric, Measurement
Collections: Periodicals: Journal and Magazine Collection, Copernicus GmbH
Historic
Publication Date:
2015
Publisher: Copernicus Gmbh, Göttingen, Germany
Member Page: Copernicus Publications

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Wang, Z., Bréon, F., Chen, L., Letu, H., Li, S., Shang, H., & Su, L. (2015). A Better Understanding of Polder's Cloud Droplet Size Retrieval: Impact of Cloud Horizontal Inhomogeneity and Directional Sampling : Volume 8, Issue 7 (01/07/2015). Retrieved from http://www.ebooklibrary.org/


Description
Description: State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing, China. The principles of the Polarization and Directionality of the Earth's Reflectance (POLDER) cloud droplet size retrieval requires that clouds are horizontally homogeneous. Nevertheless, the retrieval is applied by combining all measurements from an area of 150 km × 150 km to compensate for POLDER's insufficient directional sampling. Using the POLDER-like data simulated with the RT3 model, we investigate the impact of cloud horizontal inhomogeneity and directional sampling on the retrieval, and then analyze which spatial resolution is potentially accessible from the measurements. Case studies show that the sub-scale variability in droplet effective radius (CDR) can mislead both the CDR and effective variance (EV) retrievals. Nevertheless, the sub-scale variations in EV and cloud optical thickness (COT) only influence the EV retrievals and not the CDR estimate. In the directional sampling cases studied, the retrieval is accurate using limited observations and is largely independent of random noise.

Several improvements have been made to the original POLDER droplet size retrieval. For example, the measurements in the primary rainbow region (137–145°) are used to ensure accurate large droplet (> 15 Μm) retrievals and reduce the uncertainties caused by cloud heterogeneity. We apply the improved method using the POLDER global L1B data for June 2008, the new CDR results are compared with the operational CDRs. The comparison show that the operational CDRs tend to be underestimated for large droplets. The reason is that the cloudbow oscillations in the scattering angle region of 145–165° are weak for cloud fields with CDR > 15 Μm. Lastly, a sub-scale retrieval case is analyzed, illustrating that a higher resolution, e.g., 42 km × 42 km, can be used when inverting cloud droplet size parameters from POLDER measurements.


Summary
A better understanding of POLDER's cloud droplet size retrieval: impact of cloud horizontal inhomogeneity and directional sampling

Excerpt
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