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Comparison of model noise in AGCM independent ensemble runs and continuous simulation
Decremer, D.,Chung, C. E. SPRINGER SCIENCE AND BUSINESS MEDIA 2014 Asia-Pacific Journal of Atmospheric Sciences Vol.50 No.3
Atmospheric General Circulation models (AGCMs) forced by prescribed sea surface temperature (SST) climatological seasonal cycle simulate interannual variabilities that have cyclic characteristics. Such cyclic characteristics generate relationships between one year and the next in the model output. We document these relationships by computing lag-1-year autocorrelation in hundreds of years of CAM3 and ECHAM5 simulations. The autocorrelation is found to be generally less than 0.2, but contain robust structures. In case of zonal averaged zonal wind and air temperature the winter hemisphere is characterized by negative autocorrelation and the summer hemisphere characterized by positive autocorrelation. The presence of autocorrelation means that an average over a 10 similar to 25 year AGCM simulation in an effort to reduce the influence of interannual variability on externally-driven climate change might not be very effective. In view of this, we investigate if ensemble runs instead of a continuous simulation is more effective in reducing such influences. The reduction gain by using N 1-year long ensemble runs over N years of continuous run is generally less than 30% and mainly limited to the areas where the autocorrelation is positive. We thus conclude that each year in a continuous simulation can generally be treated as largely independent of the next year in an AGCM run with fixed SST forcing.
On the possibilities to use atmospheric reanalyses to evaluate the warming structure in the Arctic
Chung, C. E.,Cha, H.,Vihma, T.,Rä,isä,nen, P.,Decremer, D. Copernicus GmbH 2013 Atmospheric chemistry and physics Vol.13 No.22
<P>Abstract. There has been growing interest in the vertical structure of the recent Arctic warming. We investigated temperatures at the surface, 925, 700, 500 and 300 hPa levels in the Arctic (north of 70° N) using observations and four reanalyses: ERA-Interim, CFSR, MERRA and NCEP II. For the period 1979-2011, the layers at 500 hPa and below show a warming trend in all seasons in all the chosen reanalyses and observations. Restricting the analysis to the 1998-2011 period, however, all the reanalyses show a cooling trend in the Arctic-mean 500 hPa temperature in autumn, and this also applies to both observations and the reanalyses when restricting the analysis to the locations with available IGRA radiosoundings. During this period, the surface observations mainly representing land areas surrounding the Arctic Ocean reveal no summertime trend, in contrast with the reanalyses whether restricted to the locations of the available surface observations or not. In evaluating the reanalyses with observations, we find that the reanalyses agree better with each other at the available IGRA sounding locations than for the Arctic average, perhaps because the sounding observations were assimilated into reanalyses. Conversely, using the reanalysis data only from locations matching available surface (air) temperature observations does not improve the agreement between the reanalyses. At 925 hPa, CFSR deviates from the other three reanalyses, especially in summer after 2000, and it also deviates more from the IGRA radiosoundings than the other reanalyses do. The CFSR error in summer T925 is due mainly to underestimations in the Canadian-Atlantic sector between 120° W and 0°. The other reanalyses also have negative biases in this longitude band. </P>