The global mean energy balance under cloud-free conditions: an assessment based on direct observations and CMIP5 models

Abstract:

In previous studies we derived new estimates for the magnitude of the components of the global, land and ocean mean energy balance (Wild et al. 2013 / 2015, Clim. Dyn. 40/44). Here we establish complementary estimates for the global mean energy fluxes under cloud free conditions. The energy fluxes in and out of the climate system under cloud free conditions at the Top of Atmosphere (TOA) can be determinend with high accuracy from satellite-based measurements (CERES EBAF). For the estimation of the global mean clear-sky radiative fluxes at the Earth’s surface we follow the approach presented in Wild et al. (2013 / 2015), based on a combination of more than 40 state of the art global climate models from CMIP5 and their biases compared to a comprehensive set of high quality surface observations from the Baseline Surface Radiation Network (BSRN). Thereby the clear sky radiative flux biases in the various models are lineraly related to their respective global means. The associated clear sky reference climatologies at the BSRN sites have been composed based on Long and Ackermann (2000) and Hakuba et al. (2015). From the linear regression we inferred a best estimate (with zero bias against the surface observations) of 249 Wm^-2 for the global mean insolation at the surface under cloud free conditions, and a corresponding surface absorption of 215 Wm^-2, assuming a global mean surface albedo of 13.5%. Combined with a best estimate for the net influx of solar radiation at the TOA under cloud free skies from CERES-EBAF of 287 Wm^-2, this leaves an amount of 72 Wm^-2 absorbed solar radiation in the cloud free atmosphere. The 72 Wm^-2coincide with our earlier estimate for this quantity in Wild et al. (2006, JGR) based on older models and much fewer direct observations, suggesting that this estimate is fairly robust. We intend to pursue a similar approach for the estimation of the thermal fluxes of the cloud-free global mean energy balance and aim at determining the global mean cloud effects by relating the all sky and clear sky global mean energy balances.

References:

Wild et al. 2013: The global energy balance from a surface perspective, Clim. Dyn., 40, 3107-3134,

Wild et al. 2015: The energy balance over land and oceans: An assessment based on direct observations and CMIP5 climate models. Clim. Dyn., Dyn., 44, 3393–3429.