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Title: Evaluation of Clear-Sky Solar Fluxes in GCMs Participating in AMIP and IPCC-AR4 from a Surface Perspective

Abstract

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 inmore » 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.« less

Authors:
; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
877571
Report Number(s):
PNNL-SA-45475
Journal ID: ISSN 0747-7309; KP1704010; TRN: US200608%%490
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Geophysical Research - Atmospheres; Journal Volume: 111; Journal Issue: D1
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; ABSORPTION; ACCURACY; CLIMATES; CLOUDS; DIAGNOSIS; DOWNWELLING; EVALUATION; GENERAL CIRCULATION MODELS; INSOLATION; PERFORMANCE; RADIANT HEAT TRANSFER; RADIATIONS; SOLAR RADIATION; radiation budget; GCM; solar radiation; model intercomparison; clear sky; global climate modeling

Citation Formats

Wild, Martin F., Long, Charles N., and Ohmura, Atsumu. Evaluation of Clear-Sky Solar Fluxes in GCMs Participating in AMIP and IPCC-AR4 from a Surface Perspective. United States: N. p., 2006. Web. doi:10.1029/2005JD006118.
Wild, Martin F., Long, Charles N., & Ohmura, Atsumu. Evaluation of Clear-Sky Solar Fluxes in GCMs Participating in AMIP and IPCC-AR4 from a Surface Perspective. United States. doi:10.1029/2005JD006118.
Wild, Martin F., Long, Charles N., and Ohmura, Atsumu. Thu . "Evaluation of Clear-Sky Solar Fluxes in GCMs Participating in AMIP and IPCC-AR4 from a Surface Perspective". United States. doi:10.1029/2005JD006118.
@article{osti_877571,
title = {Evaluation of Clear-Sky Solar Fluxes in GCMs Participating in AMIP and IPCC-AR4 from a Surface Perspective},
author = {Wild, Martin F. and Long, Charles N. and Ohmura, Atsumu},
abstractNote = {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.},
doi = {10.1029/2005JD006118},
journal = {Journal of Geophysical Research - Atmospheres},
number = D1,
volume = 111,
place = {United States},
year = {Thu Jan 12 00:00:00 EST 2006},
month = {Thu Jan 12 00:00:00 EST 2006}
}
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  • The existence and magnitude of a systematic bias in the clear-sky longwave fluxes from the Earth Radiation Budget Experiment (ERBE) is investigated. The bias is apparently introduced because the ERBE method for scene identification does not account for large zonal gradients in longwave absorption by water vapor. The ERBE fluxes are compared to fluxes calculated with a radiative transfer model from ship radiosonde measurements. The comparison is based upon an analysis of 5 yr of coincident satellite and radiosonde observations for equatorial ocean regions. The differences between the ERBE and model fluxes are examined as functions of sea surface temperaturemore » (SST) and relative humidity. The authors use height-mean relative humidity bar-RH as an index of atmospheric moisture. The average offset between and model ERBE fluxes ranges between +2 and +6 W/sq m for SSTs above 295 K, and the gradients with respect to SST are nearly identical. However, the difference between the model and ERBE depends significantly on the tropospheric relative humidity. ERBE fluxes exceed model fluxes for bar-RH above 70%, and the maximum offset of +9 to +12 W/sq m is consistent with previous estimates. There are also indications that the clear-sky fluxes for bar-RH below 25% may be underestimated by about 10-15 W/sq m. Since extreme values of height-mean humidity are relatively infrequent, the net bias introduced in the ERBE monthly mean clear-sky fluxes is generally less than the systematic error in estimates introduced in the ERBE monthly mean clear-sky fluxes is generally less than the systematic error in estimates of the instantaneous fluxes.« less
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