Open AccessApparent absorption of solar spectral irradiance in heterogeneous ice clouds
Author(s) -
Schmidt K. Sebastian,
Pilewskie Peter,
Mayer Bernhard,
Wendisch Manfred,
Kindel Bruce,
Platnick Steven,
King Michael D.,
Wind Gala,
Arnold G. Tom,
Tian Lin,
Heymsfield Gerald,
Kalesse Heike
Publication year - 2010
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2009jd013124
Subject(s) - irradiance , absorption (acoustics) , atmospheric sciences , shortwave , cirrus , environmental science , radiative transfer , radiometer , atmospheric radiative transfer codes , flux (metallurgy) , solar irradiance , remote sensing , spectroradiometer , atmosphere (unit) , optical depth , physics , effective radius , optics , meteorology , astrophysics , materials science , geology , reflectivity , aerosol , galaxy , metallurgy
Coordinated flight legs of two aircraft above and below extended ice clouds played an important role in the Tropical Composition, Cloud and Climate Coupling Experiment (Costa Rica, 2007). The Solar Spectral Flux Radiometer measured up‐ and downward irradiance on the high‐altitude (ER‐2) and the low‐altitude (DC‐8) aircraft, which allowed deriving apparent absorption on a point‐by‐point basis along the flight track. Apparent absorption is the vertical divergence of irradiance, calculated from the difference of net flux at the top and bottom of a cloud. While this is the only practical method of deriving absorption from aircraft radiation measurements, it differs from true absorption when horizontal flux divergence is nonzero. Differences between true and apparent absorption are inevitable in any inhomogeneous atmosphere, especially clouds. We show, for the first time, the spectral shape of measured apparent absorption and compare with results from a three‐dimensional radiative transfer model. The model cloud field is created from optical thickness and effective radius retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator and from reflectivity profiles from the Cloud Radar System, both on board the ER‐2. Although the spectral shape is reproduced by the model calculations, the measured apparent absorption in the visible spectral range is higher than the model results along extended parts of the flight leg. This is possibly due to a net loss of photons into neighboring cirrus‐free areas that are not contained within the model domain.