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Combining the independent pixel and point-spread function approaches to simulate the actinic radiation field in moderately inhomogeneous 3D cloudy media

Kniffka, Anke and Trautmann, Thomas (2008) Combining the independent pixel and point-spread function approaches to simulate the actinic radiation field in moderately inhomogeneous 3D cloudy media. In: International Radiation Symposium (IRS2008), Abstracts Booklet. International Radiation Symposium 2008 (IRS2008), 2008-08-03 - 2008-08-08, Foz do Iguazu, Brazil.

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Abstract

Field observations and three-dimensional (3D) radiative transfer (RT) simulations of the actinic flux density show strongly pronounced local variations in the presence of inhomogeneous cloud fields which depend on the spatial distribution of the clouds and their optical properties. While exact three-dimensional RT models like, for example, Monte-Carlo techniques or the Spherical Harmonics Discrete Ordinate Method (SHDOM), despite their large computational burden are fully capable of resolving the observed 3D structures of the actinic radiation field, the often employed independent pixel approximation (IPA) for RT can not treat this situation realistically. For inhomogeneous 3D optical property fields of clouds this lack of horizontal photon transport may lead to significant local errors with up to 30% in the present study. Therefore, a very fast method was developed which realistically reproduces the actinic radiation field for 3D cloud fields having moderate spatial variability. This method employs a combination of the IPA, including an exact 3D treatment of the direct sunlight, and a Gaussian point spread function (PSF) to mimic the smoothing properties caused by 3D diffuse radiative transport. As a post-processing step the effect of horizontal photon transport is approximated by means of a Gaussian smoothing filter which accounts for characteristic optical properties of the medium under consideration. Compared to exact 3D RT simulations, the new method delivers more realistic results than the pure IPA approach in all cases analysed. Local deviations of the actinic flux density from 3D RT simulations are significantly reduced in comparison to IPA RT simulations. Depending on the inhomogeneity of the treated cloud fields, the computing time is reduced by a factor of about 20 relative to a full 3D RT simulation. Due to the employment of the IPA in which the RT is handled independently in each column, the size of the simulated model domain is practically unlimited. We also illustrate how this method can improve IPA RT simulations over orographically structured terrain.

Document Type:Conference or Workshop Item (Poster)
Title:Combining the independent pixel and point-spread function approaches to simulate the actinic radiation field in moderately inhomogeneous 3D cloudy media
Authors:
AuthorsInstitution or Email of Authors
Kniffka, AnkeLeipzig Institute for Meteorology, Leipzig, Germany, University of Leipzig
Trautmann, ThomasUNSPECIFIED
Date:June 2008
Journal or Publication Title:International Radiation Symposium (IRS2008), Abstracts Booklet
Refereed publication:No
In ISI Web of Science:No
Status:Published
Keywords:Three-dimensional radiative transfer, actinic radiation, clouds, independent pixel approximation
Event Title:International Radiation Symposium 2008 (IRS2008)
Event Location:Foz do Iguazu, Brazil
Event Type:international Conference
Event Dates:2008-08-03 - 2008-08-08
Organizer:International Radiation Commission (IRC), International Association of Meteorology and Atmospheric Sciences (IAMAS)
HGF - Research field:Aeronautics, Space and Transport (old)
HGF - Program:Space (old)
HGF - Program Themes:W EO - Erdbeobachtung
DLR - Research area:Space
DLR - Program:W EO - Erdbeobachtung
DLR - Research theme (Project):W - Vorhaben Entwicklung von Atmosphärenprozessoren (old)
Location: Oberpfaffenhofen
Institutes and Institutions:Remote Sensing Technology Institute > Atmospheric Processors
Deposited By: Prof.Dr. Thomas Trautmann
Deposited On:16 Feb 2009
Last Modified:12 Dec 2013 20:36

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