Global annual averages of the incoming solar radiation at TOA .

1. Differences in solar insolation fields at TOA among numerical models for the projects AMIP-2 and IPCC-FAR, Ehrhard Raschke (University of Hamburg), Yoko Tsuchima (JAMSTEC, Japan) Stephan Bakan, Marco Giorgetta, Stefan Kinne (Max Planck Institute for Meteorology, Hamburg), Martin Wild(ETH Zürich) Solar radiation is the only external energy source for all (!) processes within the climate system. Therefore its spatial distribution over the entire globe should at any time be modeled accurately in all models and radiation climatologies. However, when comparing regional and zonal monthly averages of daily sums of the incoming solar radiative fluxes reaching the top of the atmosphere (TOA), we found that there are significant differences in the solar insolation fields of GCMs contributing to international projects AMIP-2 (Atmospheric Model Intercomparison Project) and IPCC-FAR (Fourth Assessment Report). We deduce considerable seasonal deviations from (1) the astro-mechanical model applied, (2) the handling of the leap-year, (3) the cut-off angle at sun-rise and sunset and (4) an inability to compensate for the coarse temporal and spatial resolution in global modeling and climate data-sets Global annual averages of the incoming solar radiation at TOA. Left panel: AMIP 2 project model data, where solar constant (1365 Wm-2)and orbital data were prescribed. Note the different effects of the leap year. Right panel: IPCC-FAR project model data: only some models account for the sun-spot cycle which is not always in phase (observations in red), one model even adjusts for tropopause losses after major volcanic eruptions (green curve) IPCC-FAR AMIP 2 - Models ccc Victoria ccs Tokyo cnr Toulouse col Calverton dnm Moscow ecm Reading gis New York gla Greenbelt jma Tokyo mgo St.Petersburg mpi Hamburg mri Tsukuba nca Boulder nce La Jolla pnn Richland sun Albany uga Reading uiu Champaign ukm Bracknell yon Yonsei IPCC-far - Models CCc Toronto CCC Toronto CCs Tokyo CCS Tokyo CNR Toulouse CSI Melbourne GFD Princeton DNM Moscow GI1 New York GIS New York IAP Beijing IPS Paris MPI Hamburg MRI Tsukuba NCA Boulder PCM Boulder UKO Exeter Climate Data-sets isc ISCCP cte CERES-terra ca3 CERES-aqua ca4 CERES-aqua Differences between monthly averages (1985 to 1988) of zonal means for the downward solar radiation at TOA between models and CERES data (cte, ca3, ca4) with respect to data used in the ISCCP climatology. Data are shown from models participating in ISCCP-FAR. Considerable deviations (to ISCCP) of up to 10 Wm-2 occur during the transitional seasons, when the angular speed of the Earth’s orbit is highest. These profiles suggest, that some models experience a meridional forcing changing seasonally from others. Deviations are largest at higher latitudes during extended periods of low sun-elevation. Why are we concerned? Seasonal differences in meridional gradients for the solar insolation may affect the computed atmospheric circulation ! Sensitivity studies have shown, that reductions or increases of the insolation cause significant changes in cloud fields near both polar regions. Left table: In all IPCC models slightly different values for the Earth‘s orbital parameters were used, mostly constant over the period of simulations (20 years). A simulation with the model MIROC (Tsuchima, 2006, priv. comm.) showed, that only differences in the obliquity caused small changes in insolation. The large zonal anomalies, reproduced above, could not be verified yet. A search for other possible error sources is underway. Deviations ot the local incoming solar radiation at TOA from its zonal averages for March (based on data a 1olat*1olon spatial resolution). Left: Models participating in the AMIP-2 project (the ISCCP data-set behavior is given in the lower right [isc]). Right: Models participating in IPCC-FAR (the ISCCP data-sets [isc] and data used in conjunction with CERES [cte, ca3, ca4] are also given). Note: the (common) quadrupole-like pattern of areal anomalies, which can amount up to ±1 Wm-2 during the months March and September is produced only by models (and data-sets) which consider an elliptical Earth orbit. These anomalies are apparently due to too long time steps (G. Schmidt, GISS, priv. comm.). Phase shifts by 6 and 12h are due to different starting times of the integration.. The vertical pattern with wave numbers 8 and 24 are due to finite time steps with solar insolation kept constant over 3 or 1 hour. TAKE HOME MESSAGE: DIFFERENCES FOR TOA SOLAR INSOLATION ARE NOT ACCEPTABLE ! We suggest to tie all numerical models for studies of the climate at various time periods to incoming solar radiation fields based on the same method, which accurately considers known realistic astro-mechanical facts. Earlier details in: E. Raschke, M. Giorgetta, S. Kinne, M. Wild (2005): How accurate did GCMs compute the insolation at TOA for AMIP-2? Geophys. Res. Letters, VOL. 32, L23707, doi:10.1029/2005GL024411; E. Raschke, S. Bakan, S. Kinne (2006): An assessment of radiation budget data provided by the ISCCP and GEWEX-SRB. Geophys. Res. Letters, in print Corresponding e-mail addresses:ehrhard.raschke@zmaw.de, stefan.kinne@zmaw.de, , martin.wild@env.ethz.ch