Abstract
In recent studies we quantified the global mean Earth energy balance based on direct observations from surface and space. Here we infer complementary reference estimates for its components specifically under cloud-free conditions. While the clear-sky fluxes at the top of atmosphere (TOA) are accurately known from satellite measurements, the corresponding fluxes at the Earth’s surface are not equally well established, as they cannot be directly measured from space. This is also evident in 38 global climate models from CMIP5, which are shown to greatly vary in their clear-sky surface radiation budgets. To better constrain the latter, we established new clear-sky reference climatologies of surface downward shortwave and longwave radiative fluxes from worldwide distributed Baseline Surface Radiation Network sites. 33 out of the 38 CMIP5 models overestimate the clear-sky downward shortwave reference climatologies, whereas both substantial overestimations and underestimations are found in the longwave counterparts in some of the models. From the bias structure of the CMIP5 models we infer best estimates for the global mean surface downward clear-sky shortwave and longwave radiation, at 247 and 314 Wm−2, respectively. With a global mean surface albedo of 13.5% and net shortwave clear-sky flux of 287 Wm−2 at the TOA this results in a global mean clear-sky surface and atmospheric shortwave absorption of 214 and 73 Wm−2, respectively. From the newly-established diagrams of the global energy balance under clear-sky and all-sky conditions, we quantify the cloud radiative effects not only at the TOA, but also within the atmosphere and at the surface.
Highlights
The global energy balance fundamentally constrains the energy fluxes in the Earth’s climate system
The comparison is complicated by the fact that the monthly shortwave clear-sky BSRN reference climatologies are derived from measurements under truly cloud-free conditions, whereas the global climate models (GCMs) clear-sky fluxes are calculated continuously at every time-step solely by removing the clouds, yet otherwise retaining the prevailing atmospheric composition during the cloudy conditions
In previous studies we derived estimates for the magnitudes of the components of the “all-sky” global mean energy balance, taking into account the information provided by direct observations from surface and space as well as climate model estimates (Wild et al 2013, 2015)
Summary
The global energy balance fundamentally constrains the energy fluxes in the Earth’s climate system. Thanks to the sophisticated satellite observing systems in operation since the turn of the millennium, the radiative fluxes in and out of the climate system at the TOA are well determined (Loeb et al 2012, 2018) This allows an accurate estimation of the impact of clouds on the TOA radiation budget through the comparison of satellite measurements representing all-sky and clear-sky conditions, and an adequate assessment of their representation in climate models (e.g., Potter and Cess 2004; Wang and Su 2013). By combining these estimates with the corresponding all-sky estimates determined in our earlier studies we will quantify the global cloud radiative effects at the TOA, and within the atmosphere and at the surface
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