Atmospheric component of the MPI‐M Earth System Model: ECHAM6

Abstract

[1] ECHAM6, the sixth generation of the atmospheric general circulation model ECHAM, is described. Major changes with respect to its predecessor affect the representation of shortwave radiative transfer, the height of the model top. Minor changes have been made to model tuning and convective triggering. Several model configurations, differing in horizontal and vertical resolution, are compared. As horizontal resolution is increased beyond T63, the simulated climate improves but changes are incremental; major biases appear to be limited by the parameterization of small-scale physical processes, such as clouds and convection. Higher vertical resolution in the middle atmosphere leads to a systematic reduction in temperature biases in the upper troposphere, and a better representation of the middle atmosphere and its modes of variability. ECHAM6 represents the present climate as well as, or better than, its predecessor. The most marked improvements are evident in the circulation of the extratropics. ECHAM6 continues to have a good representation of tropical variability. A number of biases, however, remain. These include a poor representation of low-level clouds, systematic shifts in major precipitation features, biases in the partitioning of precipitation between land and sea (particularly in the tropics), and midlatitude jets that appear to be insufficiently poleward. The response of ECHAM6 to increasing concentrations of greenhouse gases is similar to that of ECHAM5. The equilibrium climate sensitivity of the mixed-resolution (T63L95) configuration is between 2.9 and 3.4 K and is somewhat larger for the 47 level model. Cloud feedbacks and adjustments contribute positively to warming from increasing greenhouse gases.

An abrupt weakening of the subpolar gyre as trigger of Little Ice Age-type episodes

“…The atmosphere general circulation model ECHAM6 (Stevens et al 2013) is run at a horizontal resolution of spectral truncation T63 (1.875°), with 47 vertical levels that resolve the stratosphere up to 0.01 hPa. The ocean/sea-ice model MPIOM (Marsland et al 2003;Jungclaus et al 2013) has a conformal mapping horizontal grid of nominal 1.5° resolution and grid poles over Antarctica and southern Greenland.…”

Fire emission heights in the climate system – Part 2: Impact on transport, black carbon concentrations and radiation

“…Calculations of aerosol optical properties are based on Mie theory for 24 solar spectral bands and provide single scattering albedo, extinction cross section and asymmetry factors. These parameters in turn serve as input for radiation calculations by the ECHAM6 radiation scheme Stevens et al, 2013). Aerosol-cloud interactions are represented by a two-moment cloud micro-physics scheme that is coupled to the aerosol micro-physics (Lohmann et al, 2007).…”

The role of sea-ice albedo in the climate of slowly rotating aquaplanets

“…The studied rotation periods range from one Earth-day to 365 Earth-days in which case the planet would be synchronously rotating. We use the state-of-the-art atmospheric general circulation model (GCM) ECHAM6 (Stevens et al 2013) in an aquaplanet setup in perpetual equinox conditions. The model and the slowly rotating aquaplanet configuration are briefly described in Sect.…”