ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-10-22093-2010 Impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing: NICAM simulation results Sohn B. J. 1 Nakajima T. 2 Satoh M. 2 Jang H.-S. 1 School of Earth and Environmental Sciences, Seoul National University, Seoul, 151-747, Korea Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8568, Japan 24 09 2010 10 9 22093 22107 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/10/22093/2010/acpd-10-22093-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/10/22093/2010/acpd-10-22093-2010.pdf

Using one month of the cloud-resolving Nonhydrostatic Icosahedral Atmospheric Model (NICAM) simulations, we examined the impact of different definitions of clear-sky flux on the determination of longwave cloud radiative forcing (CRF). Because the satellite-like cloud-free composite preferentially samples drier conditions relative to the all-sky mean state, the conventional clear-sky flux calculation using the all-sky mean state in the model may represent a more humid atmospheric state in comparison to the cloud-free state. The drier bias is evident for the cloud-free composite in the NICAM simulations, causing an overestimation of the longwave CRF by about 10% compared to the NICAM simulated longwave CRF. Overall, water vapor contributions of up to 10% of the total longwave CRF should be added to make the NICAM-generated cloud forcing comparable to the satellite measurements.

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