Evaluation of Clear-Sky Solar Fluxes in GCMs Participating in AMIP and IPCC-AR4 from a Surface Perspective
Solar fluxes at the Earth’s surface calculated in General Circulation Models (GCMs) contain large uncertainties, not only in the presence of clouds, but, as shown here, even under cloud-free (i.e. clear-sky) conditions. Adequate observations to constrain these clear-sky fluxes at the surface have long been missing. The present study provides newly derived observational clear-sky climatologies at worldwide distributed anchor sites with high accuracy measurements from the Baseline Surface Radiation Network (BSRN) and the Atmospheric Radiation Measurement Program (ARM). These data are used to systematically assess the performance of a total of 36 GCMs with respect to solar surface downwelling clear-sky fluxes over more than a decade of model development, from the atmospheric model intercomparison projects AMIP I and AMIP II to the state of the art models participating in the 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR4). Results show that earlier model versions tend to largely overestimate the surface insolation under cloud-free conditions. This identifies an overly transparent cloud-free atmosphere as a key error source for the excessive surface insolation in GCMs noted in previous studies. Similar biases remain in a number of current models with comparatively low atmospheric clearsky solar absorption (around 60 Wm-2 in the global mean). However, there are now several models participating in IPCC-AR4 with higher atmospheric clear-sky absorption (70 Wm-2 and up, globally averaged) which are in excellent agreement with the newlyderived observational clear-sky climatologies. This underlines the progress made in the solar radiative transfer through the cloud-free atmosphere as well as in the observation and diagnosis of solar radiation under cloudless atmospheres.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 877571
- Report Number(s):
- PNNL-SA-45475; KP1704010; TRN: US200608%%490
- Journal Information:
- Journal of Geophysical Research - Atmospheres, Vol. 111, Issue D1; ISSN 0747-7309
- Country of Publication:
- United States
- Language:
- English
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