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A New "Cloud Mode" for the AERONETA key to predicting climate change is to observe an understand the global distribution of clouds and their physical properties such as optical thickness and droplet size. Since clouds change rapidly over short time and space intervals, they are difficult to simulate in computer models. But it is essential that global climate models predict realistic spatial and temporal distribution of cloud optical depth. The best way to verify these distributions is to infer optical depth from global coverage satellite data. However, satellite methods have many sources of uncertainty; thus, independent and reliable ground-based estimates are essential for validation. For aerosol, there is the AERONET - a ground based monitoring network that consists of identical multi-channel radiometers for assessing aerosol optical properties and validating their satellite retrievals. In addition to AEROSOL, we want the network monitoring CLOUD optical properties. It will use AERONET "time" (inappropriate for aerosol studies) to make basic new measurements related to cloud physics. In the presentation we will report on a new technique that retrieves cloud optical thickness for even broken clouds above green vegetation from surface measurements of zenith radiance in the visible (VIS) and near-IR (NIR) spectral regions. The idea of the method is simple: since green vegetation reflects 40-50% of incoming radiation in the NIR and only 5-10% in the VIS region, ground measurements under thin clouds have little spectral contrast between VIS and NIR, while thick clouds reflect much more of the surface-reflected radiation in the NIR than in VIS. Based on this idea, we use a combination of measurements (spectral indices) in VIS and NIR to estimate cloud optical thickness. By analogy with NDVI, the simplest index that can be defined is the Normalized Difference Cloud Index (NDCI) which is a ratio between the difference and the sum of two radiances measured for two narrow spectral bands in VIS and NIR.
Document ID
20020004194
Acquisition Source
Goddard Space Flight Center
Document Type
Conference Paper
Authors
Marshak, Alexander
(Maryland Univ. Baltimore County Catonsville, MD United States)
Barker, H.
(Meteorological Service of Canada Canada)
Knyazikhin, Y.
(Boston Univ. Boston, MA United States)
Wiscombe, W.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Einaudi, Franco
Date Acquired
August 20, 2013
Publication Date
January 1, 2001
Subject Category
Meteorology And Climatology
Meeting Information
Meeting: IAMAS Meeting
Location: Innsbruck
Country: Austria
Start Date: July 10, 2001
End Date: July 18, 2001
Sponsors: International Association of Meteorology and Atmospheric Sciences
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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