Gliese 581g as a scaled-up version of Earth: atmospheric circulation simulations

We use three-dimensional simulations to study the atmospheric circulation on the first Earth-sized exoplanet discovered in the habitable zone of an M star. We treat Gliese 581g as a scaled-up version of Earth by considering increased values for the exoplanetary radius and surface gravity, while retaining terrestrial values for parameters which are unconstrained by current observations. We examine the long-term, global temperature and wind maps near the surface of the exoplanet --- the climate. The specific locations for habitability on Gliese 581g depend on whether the exoplanet is tidally-locked and how fast radiative cooling occurs on a global scale. Independent of whether the existence of Gliese 581g is confirmed, our study highlights the use of general circulation models to quantify the atmospheric circulation on potentially habitable, Earth-sized exoplanets, which will be the prime targets of exoplanet discovery and characterization campaigns in the next decade.Comment: Accepted by MNRAS. 15 pages, 13 figures. Sample movies of simulations are available at http://www.phys.ethz.ch/~kheng/fms

Calibration and analysis of the uncertainty in downscaling global land use and land cover projections from GCAM using Demeter (v1.0.0)

Demeter is a community spatial downscaling model that disaggregates land use and land cover changes projected by integrated human–Earth system models. Demeter has not been intensively calibrated, and we still lack good knowledge about its sensitivity to key parameters and parameter uncertainties. We used long-term global satellite-based land cover records to calibrate key Demeter parameters. The results identified the optimal parameter values and showed that the parameterization substantially improved the model's performance. The parameters of intensification ratio and selection threshold were the most sensitive and needed to be carefully tuned, especially for regional applications. Further, small parameter uncertainties after calibration can be inflated when propagated into future scenarios, suggesting that users should consider the parameterization equifinality to better account for the uncertainties in Demeter-downscaled products. Our study provides a key reference for Demeter users and ultimately contributes to reducing the uncertainties in Earth system model simulations.</p

Zonal Momentum Balance in the Tropical Atmospheric Circulation during the Global Monsoon Mature Months

In this paper, zonal momentum balances of the tropical atmospheric circulation during the global monsoon mature months (January and July) are analyzed in three dimensions based on the ECMWF Interim Re-Analysis (ERA-Interim). It is found that the dominant terms in the balance of the atmospheric boundary layer (ABL) in both months are the pressure gradient force, the Coriolis force, and friction. The nonlinear advection term plays a significant role only in the Asian summer monsoon regions within the ABL. In the upper troposphere, the pressure gradient force, the Coriolis force, and the nonlinear advection are the dominant terms. The transient eddy force and the residual force (which can be explained as convective momentum transfer over open oceans) are secondary, yet cannot be neglected near the equator. Zonal-mean equatorial upper-troposphere easterlies are maintained by the absolute angular momentum advection associated with the cross-equatorial Hadley circulation. Equatorial upper-troposphere easterlies over the Asian monsoon regions are also controlled by the absolute angular momentum advection but are mainly maintained by the pressure gradient force in January. The equivalent linear Rayleigh friction, which is widely applied in simple tropical models, is calculated and the corresponding spatial distribution of the local coefficient and damping time scale are estimated from the linear regression. It is found that the linear momentum model is in general capable of crudely describing the tropical atmospheric circulation dynamics, yet the caveat should be kept in mind that the friction coefficient is not uniformly distributed and is even negative in some regions

Improving our fundamental understanding of the role of aerosol−cloud interactions in the climate system

The effect of an increase in atmospheric aerosol concentrations on the distribution and radiative properties of Earth’s clouds is the most uncertain component of the overall global radiative forcing from preindustrial time. General circulation models (GCMs) are the tool for predicting future climate, but the treatment of aerosols, clouds, and aerosol−cloud radiative effects carries large uncertainties that directly affect GCM predictions, such as climate sensitivity. Predictions are hampered by the large range of scales of interaction between various components that need to be captured. Observation systems (remote sensing, in situ) are increasingly being used to constrain predictions, but significant challenges exist, to some extent because of the large range of scales and the fact that the various measuring systems tend to address different scales. Fine-scale models represent clouds, aerosols, and aerosol−cloud interactions with high fidelity but do not include interactions with the larger scale and are therefore limited from a climatic point of view. We suggest strategies for improving estimates of aerosol−cloud relationships in climate models, for new remote sensing and in situ measurements, and for quantifying and reducing model uncertainty

The influence of hemispheric asymmetry and realistic basic states on tropical stationary waves in a shallow water model

Thesis (Ph. D.)--University of Washington, 2005.A shallow water model is used to study the stationary waves in the tropical upper troposphere. Realistic zonal-mean winds are generated by imposing a zonally-symmetric topography distribution underneath a thin fluid layer and relaxing the fluid towards its global-mean depth. Basic states with zero mean meridional flow are also constructed by balancing the height field with the equilibrium zonal-mean zonal winds. Both hemispherically-symmetric (equinoctial) and hemispherically-asymmetric (solstitial) basic states are considered. Stationary waves are generated by adding a mass source-sink distribution along or near the equator.Westerly zonal-mean flow in the subtropics amplifies the stationary wave response to tropical eddy forcing and shifts the eddy height and vorticity maxima to the east, bringing the simulated eddies into better agreement with the observed seasonally varying eddy circulations in the tropical upper troposphere. Moving the wave forcing off the equator amplifies the response in the forced hemisphere, but the response in the opposite hemisphere decreases only slightly because the eddy divergent winds act over a wide latitudinal range. The zonal-mean circulation in the solstitial basic state enhances the eddy response in the winter hemisphere and limits the response in the summer hemisphere. Hemispheric asymmetry in either the eddy forcing or the basic state also leads to cross-equatorial eddy momentum fluxes. Linear experiments exhibit stronger subtropical anomalies, weaker variations along the equator, and less hemispheric symmetry than nonlinear integrations.When the eddy forcing is located in the summer hemisphere of the solstitial basic state, the mean meridional winds enhance the propagation of wave activity across the equator, leading to stronger cross-equatorial eddy momentum fluxes and an eddy response with similar amplitudes in both hemispheres. Hence, the anti-correlation between the mean meridional flow and eddy momentum fluxes over the equator and the striking hemispheric symmetry of the tropical stationary waves over the course of the seasonal cycle can both be attributed to the tendency for the maximum eddy and zonal-mean diabatic forcing to occur in the same latitude band. The influence of the mean meridional flow on eddy momentum fluxes at low latitudes is also demonstrated in a simple linear barotropic model

Demeter – A Land Use and Land Cover Change Disaggregation Model

Demeter is an open source Python package that was built to disaggregate projections of future land allocations generated by an integrated assessment model (IAM). Projected land allocation from IAMs is traditionally transferred to Earth System Models (ESMs) in a variety of gridded formats and spatial resolutions as inputs for simulating biophysical and biogeochemical fluxes. Existing tools for performing this translation generally require a number of manual steps which introduces error and is inefficient. Demeter makes this process seamless and repeatable by providing gridded and land cover change (LULCC) products derived directly from an IAM—in this case, the Global Change Assessment Model (GCAM)—in a variety of formats and resolutions commonly used by ESMs. Demeter is publicly available via GitHub and has an extensible output module allowing for future ESM needs to be easily accommodated. Funding statement: This research was supported by the U.S. Department of Energy, Office of Science, as part of research in Multi-Sector Dynamics, Earth and Environmental System Modeling Program. It builds on previous work supported by the National Aeronautics and Space Administration Carbon Monitoring System and ACCESS programs under projects NNH12AU35I and NNH13AW58I, and by the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory (PNNL), a multi-program national laboratory operated by Battelle for the U.S. Department of Energy under Contract DE-AC05- 76RL01830