Uwe Schulzweida

Sensitivity of Simulated Climate to Horizontal and Vertical Resolution in the ECHAM5 Atmosphere Model

Abstract The most recent version of the Max Planck Institute for Meteorology atmospheric general circulation model, ECHAM5, is used to study the impact of changes in horizontal and vertical resolution on seasonal mean climate. In a series of Atmospheric Model Intercomparison Project (AMIP)-style experiments with resolutions ranging between T21L19 and T159L31, the systematic errors and convergence properties are assessed for two vertical resolutions. At low vertical resolution (L19) there is no evidence for convergence to a more realistic climate state for horizontal resolutions higher than T42. At higher vertical resolution (L31), on the other hand, the root-mean-square errors decrease monotonically with increasing horizontal resolution. Furthermore, except for T42, the L31 versions are superior to their L19 counterparts, and the improvements become more evident at increasingly higher horizontal resolutions. This applies, in particular, to the zonal mean climate state and to the stationary wave patterns i...

Climate predictability experiments with a general circulation model

The atmospheric response to the evolution of the global sea surface temperatures from 1979 to 1992 is studied using the Max-Planck-Institut 19 level atmospheric general circulation model, ECHAM3 at T 42 resolution. Five separate 14-year integrations are performed and results are presented for each individual realization and for the ensemble-averaged response. The results are compared to a 30-year control integration using a climate monthly mean state of the sea surface temperatures and to analysis data. It is found that the ECHAM3 model, by and large, does reproduce the observed response pattern to El Nino and La Niña. During the El Nino events, the subtropical jet streams in both hemispheres are intensified and displaced equatorward, and there is a tendency towards weak upper easterlies over the equator. The Southern Oscillation is a very stable feature of the integrations and is accurately reproduced in all experiments. The inter-annual variability at middle- and high-latitudes, on the other hand, is strongly dominated by chaotic dynamics, and the tropical SST forcing only modulates the atmospheric circulation. The potential predictability of the model is investigated for six different regions. Signal to noise ratio is large in most parts of the tropical belt, of medium strength in the western hemisphere and generally small over the European area. The ENSO signal is most pronounced during the boreal spring. A particularly strong signal in the precipitation field in the extratropics during spring can be found over the southern United States. Western Canada is normally warmer during the warm ENSO phase, while northern Europe is warmer than normal during the ENSO cold phase. The reason is advection of warm air due to a more intense Pacific low than normal during the warm ENSO phase and a more intense Icelandic low than normal during the cold ENSO phase, respectively.

Evaluation of the tropospheric chemistry general circulation model ECHAM5-MOZ and its application to the analysis of the chemical composition of the troposphere with an emphasis on the late RETRO period 1990-2000

The Tropospheric Chemistry General Circulation model ECHAM5-MOZ was developed between 2001 and 2005 and was used to investigate the variability and trends of ozone, CO and NOx in the second half of the 20th century in the framework of the RETRO project. The multi–decadal simulation of the period of 1960 to 2000 was one of the first of that kind. The model captures many features of the seasonal cycle and vertical gradients of trace gas concentrations measured on the ground or from balloons, aircraft or satellite. We diagnose a significant high bias in the simulated ozone concentrations in the 1990s, which can in part be attributed to an overestimated stratosphere troposphere exchange and possibly underestimated dry deposition of ozone. Wintertime CO concentrations in the northern hemisphere are underestimated by up to 30%. The observed interannual variability of the tropospheric NO2 column, surface CO concentrations and ozone is generally captured by the simulation, but the model fails to capture the surface ozone increase observed at several stations around the world during the 1980s and 1990s. The increase in the tropospheric ozone column between the 1960s and 1990s is consistent with model simulations of preindustrial conditions. The global ozone burden and chemical formation and loss are continuously rising during the entire 41-year simulation period. The dry deposition flux increases until the early 1980s and shows a more irregular behavior afterwards. Until around 1980 regionally averaged precursor emissions correlate well with surface ozone changes. Thereafter, the emission trend in Europe and North America is reversed, while ozone levels remain high. Asian emissions and ozone concentrations continue to rise, but the slope of the correlation changes.