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- ItemThe dynamical core of the Aeolus 1.0 statistical-dynamical atmosphere model: Validation and parameter optimization(Göttingen : Copernicus GmbH, 2018) Totz, S.; Eliseev, A.V.; Petri, S.; Flechsig, M.; Caesar, L.; Petoukhov, V.; Coumou, D.We present and validate a set of equations for representing the atmosphere's large-scale general circulation in an Earth system model of intermediate complexity (EMIC). These dynamical equations have been implemented in Aeolus 1.0, which is a statistical-dynamical atmosphere model (SDAM) and includes radiative transfer and cloud modules (Coumou et al., 2011; Eliseev et al., 2013). The statistical dynamical approach is computationally efficient and thus enables us to perform climate simulations at multimillennia timescales, which is a prime aim of our model development. Further, this computational efficiency enables us to scan large and high-dimensional parameter space to tune the model parameters, e.g., for sensitivity studies.
Here, we present novel equations for the large-scale zonal-mean wind as well as those for planetary waves. Together with synoptic parameterization (as presented by Coumou et al., 2011), these form the mathematical description of the dynamical core of Aeolus 1.0.
We optimize the dynamical core parameter values by tuning all relevant dynamical fields to ERA-Interim reanalysis data (1983-2009) forcing the dynamical core with prescribed surface temperature, surface humidity and cumulus cloud fraction. We test the model's performance in reproducing the seasonal cycle and the influence of the El Niño-Southern Oscillation (ENSO). We use a simulated annealing optimization algorithm, which approximates the global minimum of a high-dimensional function.
With non-tuned parameter values, the model performs reasonably in terms of its representation of zonal-mean circulation, planetary waves and storm tracks. The simulated annealing optimization improves in particular the model's representation of the Northern Hemisphere jet stream and storm tracks as well as the Hadley circulation.
The regions of high azonal wind velocities (planetary waves) are accurately captured for all validation experiments. The zonal-mean zonal wind and the integrated lower troposphere mass flux show good results in particular in the Northern Hemisphere. In the Southern Hemisphere, the model tends to produce too-weak zonal-mean zonal winds and a too-narrow Hadley circulation. We discuss possible reasons for these model biases as well as planned future model improvements and applications. - ItemExploring the sensitivity of Northern Hemisphere atmospheric circulation to different surface temperature forcing using a statistical-dynamical atmospheric model(Göttingen : Copernicus GmbH, 2019) Totz, S.; Petri, S.; Lehmann, J.; Peukert, E.; Coumou, D.Climate and weather conditions in the mid-latitudes are strongly driven by the large-scale atmosphere circulation. Observational data indicate that important components of the large-scale circulation have changed in recent decades, including the strength and the width of the Hadley cell, jets, storm tracks and planetary waves. Here, we use a new statistical-dynamical atmosphere model (SDAM) to test the individual sensitivities of the large-scale atmospheric circulation to changes in the zonal temperature gradient, meridional temperature gradient and global-mean temperature. We analyze the Northern Hemisphere Hadley circulation, jet streams, storm tracks and planetary waves by systematically altering the zonal temperature asymmetry, the meridional temperature gradient and the global-mean temperature. Our results show that the strength of the Hadley cell, storm tracks and jet streams depend, in terms of relative changes, almost linearly on both the global-mean temperature and the meridional temperature gradient, whereas the zonal temperature asymmetry has little or no influence. The magnitude of planetary waves is affected by all three temperature components, as expected from theoretical dynamical considerations. The width of the Hadley cell behaves nonlinearly with respect to all three temperature components in the SDAM. Moreover, some of these observed large-scale atmospheric changes are expected from dynamical equations and are therefore an important part of model validation.
- ItemThree-dimensional parameterizations of the synoptic scale kinetic energy and momentum flux in the Earth's atmosphere(Göttingen : Copernicus GmbH, 2011) Coumou, D.; Petoukhov, V.; Eliseev, A.V.We present a new set of statistical-dynamical equations (SDEs) which can accurately reproduce the three-dimensional atmospheric fields of synoptic scale kinetic energy and momentum flux. The set of equations is closed by finding proper parameterizations for the vertical macro-turbulent diffusion coefficient and ageostrophic terms. The equations have been implemented in a new SD atmosphere model, named Aeolus. We show that the synoptic scale kinetic energy and momentum fluxes generated by the model are in good agreement with empirical data, which were derived from bandpass-filtered ERA-40 data. In addition to present-day climate, the model is tested for substantially colder (last glacial maximum) and warmer (2×CO2) climates, and shown to be in agreement with general circulation model (GCM) results. With the derived equations, one can efficiently study the position and strength of storm tracks under different climate scenarios with calculation time a fraction of those of GCMs. This work prepares ground for the development of a new generation of fast Earth System Models of Intermediate Complexity which are able to perform multi-millennia simulations in a reasonable time frame while appropriately accounting for the climatic effect of storm tracks.