A24D-04
Cloud Effects on Atmospheric Solar Absorption in Light of Most Recent Surface, Satellite, and GCM Datasets

Tuesday, 15 December 2015: 16:45
3004 (Moscone West)
Maria Zyta Hakuba1, Doris Folini2, Charles N. Long3, Gabriela Schaepman-Strub4, Graeme L Stephens5 and Martin Wild1, (1)ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, (2)ETH Zurich, Zurich, Switzerland, (3)Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (4)University of Zurich, Zurich, Switzerland, (5)NASA Jet Propulsion Laboratory, Pasadena, CA, United States
Abstract:
At 45 stations worldwide, we estimate the atmospheric solar absorption through combining ground-based measurements of surface solar radiation (SSR) with collocated satellite-derived surface albedo and top-of-atmosphere net irradiance under both, all-sky and clear-sky conditions. To derive continuous clear-sky SSR from Baseline Surface Radiation Network (BSRN) in-situ measurements of global and diffuse SSR, we make use of the Long and Ackerman (2000) algorithm that identifies clear-sky measurements and empirically fits diurnal clear-sky irradiance functions using the cosine of the solar zenith angle as the independent variable. The 11-year average (2000-2010) cloud radiative effect (all-sky minus clear-sky) on atmospheric absorption at these sites is overall positive at around +10 Wm⁻² using the ground-based data, and at 3 Wm⁻² in the CERES EBAF product. This discrepancy arises from a potential overestimate in clear-sky absorption by the satellite product, while the all-sky absorption agrees reasonably well. We estimate the multi-model mean cloud effect from 40 CMIP5 historical simulations at 3 Wm⁻², which is in agreement with the CERES EBAF estimates at the surface sites.

Under all-sky conditions, we found the atmospheric solar absorption, derived from CERES EBAF data, to be largely unaffected by variations in latitude (-60deg - 60deg N), remaining nearly constant at its regional mean of 23±2 %, relative to the respective top-of-atmosphere incident radiation. The zonal means follow the imprint of spatial variations in water vapor path, surface albedo, and aerosols. While the clear-sky atmospheric absorption is generally lower over the oceans as compared to the land, the positive effect of clouds is more pronounced. As the cloud radiative effect due to low-level clouds acts stronger in the extra-tropics than in equatorial regions, where predominantly high clouds prevail, the zonal mean distribution of atmospheric solar absorption appears smoother and more constant under all-sky than under clear-sky conditions.