Use In General Circulation Models Research Articles

Modeling Noncondensing Compositional Convection for Applications to Super-Earth and Sub-Neptune Atmospheres

Compositional convection is atmospheric mixing driven by density variations caused by compositional gradients. Previous studies have suggested that compositional gradients of atmospheric trace species within planetary atmospheres can impact convection and the final atmospheric temperature profile. In this work, we employ 3D convection-resolving simulations using Cloud Model 1 (CM1) to gain a fundamental understanding of how compositional variation influences convection and the final atmospheric state of exoplanet atmospheres. We perform 3D initial value problem simulations of noncondensing compositional convection for Earth-air, H2, and CO2 atmospheres. Conventionally, atmospheric convection is assumed to mix the atmosphere to a final, marginally stable state defined by a unique temperature profile. However, when there is compositional variation within an atmosphere, a continuous family of stable end states is possible, differing in the final state composition profile. Our CM1 simulations are used to determine which of the family of possible compositional end states is selected. Leveraging the results from our CM1 simulations, we develop a dry convective adjustment scheme for use in general circulation models (GCMs). This scheme relies on an energy analysis to determine the final adjusted atmospheric state. Our convection scheme produces results that agree with our CM1 simulations and can easily be implemented in GCMs to improve modeling of compositional convection in exoplanet atmospheres.

Transport parameterization of the Polar SWIFT model (version 2)

Abstract. The Polar SWIFT model is a fast scheme for calculating the chemistry of stratospheric ozone depletion in the polar vortex in winter. It is intended for use in general circulation models (GCMs) and earth system models (ESMs) to enable the simulation of interactions between the ozone layer and climate when a full stratospheric chemistry scheme is computationally too expensive. In addition to the simulation of chemistry, ozone has to be transported in the GCM. As an alternative to the general schemes for the transport and mixing of tracers in the GCMs, a parameterization of the transport of ozone can be used in order to obtain the total change of ozone as the sum of the change by transport and by chemistry. One of the benefits of this approach is the easy and self-contained coupling to a GCM. Another potential advantage is that a transport parameterization based on reanalysis data and measurements can avoid deficiencies in the representation of transport in the GCMs, such as deficits in the representation of the Brewer–Dobson circulation caused by the gravity wave parameterization. Hence, we present a transport parameterization for the Polar SWIFT model that simulates the change in vortex-averaged ozone by transport in a fast and simple way without the need for a complex transport scheme in the GCM.

Impact of Clouds and Hazes on the Simulated JWST Transmission Spectra of Habitable Zone Planets in the TRAPPIST-1 System

Abstract The TRAPPIST-1 system, consisting of an ultracool host star having seven known Earth-sized planets, will be a prime target for atmospheric characterization with the James Webb Space Telescope (JWST). However, the detectability of atmospheric molecular species may be severely impacted by the presence of clouds and/or hazes. In this work, we perform 3D general circulation model (GCM) simulations with the LMD-G model supplemented by 1D photochemistry simulations at the terminator with the Atmos model to simulate several possible atmospheres for TRAPPIST-1e, 1f, and 1g: (1) modern Earth, (2) Archean Earth, and (3) CO2-rich atmospheres. The JWST synthetic transit spectra were computed using the GSFC Planetary Spectrum Generator. We find that the TRAPPIST-1e, 1f, and 1g atmospheres, with clouds and/or hazes, could be detected using JWST’s NIRSpec Prism from the CO2 absorption line at 4.3 μm in less than 15 transits at 3σ or less than 35 transits at 5σ. However, our analysis suggests that other gases would require hundreds (or thousands) of transits to be detectable. We also find that H2O, mostly confined in the lower atmosphere, is very challenging to detect for these planets or similar systems if the planets’ atmospheres are not in a moist greenhouse state. This result demonstrates that the use of GCMs, self-consistently taking into account the effect of clouds and subsaturation, is crucial to evaluate the detectability of atmospheric molecules of interest, as well as for interpreting future detections in a more global (and thus robust and relevant) approach.

Artificial Neural Network Technique for Statistical Downscaling of Global Climate Model

The nineteenth century, an era of industrialization, witnessed huge utilization of natural resources extensively for the construction activities besides emission of nitrous oxide and methane (green house gases). Rises in the GHG play an important role in the impact on climate change. Rapid and dynamic changes of earth climate have affected human life physically, psychologically, and emotionally. General circulation models (GCMs) are the numerical models developed using a set of linear and nonlinear partial differential equations. Climate and weather forecasting reliability can be possible with the use of GCMs. Statistical downscaling is popular among the research community on an account of building a strong and accurate relationship between GCM and local level information. Among the wide range of regression models, artificial neural network (ANN), a multilinear regression method, is the most popular approach, which works on the basis of transfer function statistically relating predictors and predictands. ANN is developed particularly to address present requirements in global environmental change research and necessitate for more detailed temporal and spatial information from GCM. For the present work, CanCM4 at grid size 2.8° × 2.8° developed by Canadian Centre for Climate Modelling and Analysis has been chosen based on the skill score. It gives historical data from 1971 to 2005 as well as the data consisted of future simulations by emission scenarios RCP4.5 from 2006 to 2035. The data were extracted to cover the entire Bhima basin with nine grid points. Observed meteorological data provided by Indian Meteorological Department (IMD) are used for calibration and validation of ANN model. Karl Pearson’s coefficient is taken as guideline to test the sensitiveness of predictors. To evaluate the performance of downscaling model, Nash–Sutcliffe coefficient and root-mean-square error performance indices have been adopted. With the good predictor and predictand correlation, the work moves on to investigate the application of the transfer function in the form of statistical downscaling. The downscaled results can be applied to watershed management such as flood and drought management studies of the Bhima basin.

Role of vertical velocity in improving finer scale statistical downscaling for projection of extreme precipitation

Increase in human-induced climate warming is unequivocal and is subsequently causing an increase in the magnitude of precipitation extremes around the globe, inducing substantial damages to the socioeconomic sectors. Thus, a reliable projection of extreme precipitation scenarios is crucial for designing suitable adaptation strategies. At present, this is being attempted with the use of general circulation models (GCMs) projections. However, the GCMs simulate the extreme precipitation events rather poorly, especially over the tropical regions, and also suffer from frequent lack of reliability at local/regional scales. Therefore, it is of paramount significance to identify the relevant physical parameters for the extreme precipitation scenarios, and further implement these parameters in downscaling approaches to significantly improve the impact assessment at a regional scale. Previous studies have reported that the dynamic component (mainly vertical wind velocity) has a significant influence on the precipitation extremes over South Asian regions, along with the thermodynamic component. This indicates that the consideration of vertical wind velocity for projecting the precipitation extremes may significantly increase the efficacy of the downscaling approach, a contemplation that provoked its inclusion in statistical downscaling in this study. The methodology was demonstrated over the Mahanadi river basin (India), which often experiences heavy rainfall during the monsoon and post-monsoon disturbances originating from the Bay of Bengal (BoB). We observed that the dynamic component plays a crucial role in changing the pattern of precipitation extremes over the basin. Further, we noticed that inclusion of this dynamic component in statistical downscaling significantly improved the extreme precipitation projections. Based on these observations, we projected (2026–2055) the mean and extreme precipitation events, considering six most efficient CMIP5 models for the Indian subcontinent under RCP 4.5 and RCP 8.5 scenarios. The outcomes of this study can be utilized in deriving reliable water resource management practices.

A rank-based approach for correcting systematic biases in spatial disaggregation of coarse-scale climate simulations

Use of General Circulation Model (GCM) precipitation and evapotranspiration sequences for hydrologic modelling can result in unrealistic simulations due to the coarse scales at which GCMs operate and the systematic biases they contain. The Bias Correction Spatial Disaggregation (BCSD) method is a popular statistical downscaling and bias correction method developed to address this issue. The advantage of BCSD is its ability to reduce biases in the distribution of precipitation totals at the GCM scale and then introduce more realistic variability at finer scales than simpler spatial interpolation schemes. Although BCSD corrects biases at the GCM scale before disaggregation; at finer spatial scales biases are re-introduced by the assumptions made in the spatial disaggregation process. Our study focuses on this limitation of BCSD and proposes a rank-based approach that aims to reduce the spatial disaggregation bias especially for both low and high precipitation extremes.BCSD requires the specification of a multiplicative bias correction anomaly field that represents the ratio of the fine scale precipitation to the disaggregated precipitation. It is shown that there is significant temporal variation in the anomalies, which is masked when a mean anomaly field is used. This can be improved by modelling the anomalies in rank-space. Results from the application of the rank-BCSD procedure improve the match between the distributions of observed and downscaled precipitation at the fine scale compared to the original BCSD approach. Further improvements in the distribution are identified when a scaling correction to preserve mass in the disaggregation process is implemented. An assessment of the approach using a single GCM over Australia shows clear advantages especially in the simulation of particularly low and high downscaled precipitation amounts.

Assessing the performance of thermospheric modeling with data assimilation throughout solar cycles 23 and 24

Data assimilation procedures have been developed for thermospheric models using satellite density measurements as part of the EU Framework Package 7 ATMOP Project. Two models were studied; one a general circulation model, TIEGCM, and the other a semi-empirical drag temperature model, DTM. Results of runs using data assimilation with these models were compared with independent density observations from CHAMP and GRACE satellites throughout solar cycles 23 and 24. Time periods of 60 days were examined at solar minimum and maximum, including the 2003 Hallowe'en storms. The differences between the physical and the semi-empirical models have been characterised. Results indicate that both models tend to show similar behaviour; underestimating densities at solar maximum, and overestimating them at solar minimum. DTM performed better at solar minimum, with both models less accurate at solar maximum. A mean improvement of ~4% was found using data assimilation with TIEGCM. With further improvements, the use of general circulation models in operational space weather forecasting (in addition to empirical methods currently used) is plausible. Future work will allow near-real-time assimilation of thermospheric data for improved forecasting.

Correcting for systematic biases in multiple raw GCM variables across a range of timescales

Summary Many hydro-climatological applications require use of General Circulation Models (GCMs) outputs. However, the raw information as available from GCMs often contains significant systematic biases when compared with observations. This necessitates some kind of statistical adjustment to be carried out on the GCM fields before their use in any application. It is common to correct the GCM simulations by removing the systematic biases at the time scale of interest, usually individually for each GCM variable that is needed. The outcome is a set of bias corrected variables that are not assessed for bias in their joint dependence structure, nor biases in all attributes at other time scales (such as daily and monthly and annual) except the one under consideration. In this paper, we present a bias correction approach that simultaneously adjusts for the biases in multiple variables across multiple time scales. The proposed Multivariate Recursive Nesting Bias Correction (MRNBC) approach simultaneously corrects many GCM variables and repeats the procedure across different levels of temporal aggregation to impart observed distributional and persistence properties at multiple time scales. The bias corrected series exhibits improvements across all variables and over all the time scales considered. The use of the approach in hydrology and water resources related downscaling applications is expected to deliver better results.

The roles of diurnal forcing and large‐scale moisture transport for initiating rain over northwest Australia in a GCM

The representation of rainfall, and in particular its diurnal cycle, is generally poor in general circulation models (GCMs). Nonetheless, studies make use of GCMs in future climate projections in regions where precipitation has a strong diurnal cycle. In this study we evaluate whether one GCM (ACCESS1.3) can represent the rainfall in such a region (Australia) where the diurnal cycle of rainfall is produced as a result of both the destabilisation of the boundary layer (convection) and a larger‐scale reorganisation of the low‐level flow, the latter of which may be resolved by a GCM. In northern and eastern Australia, where the diurnal cycle of rainfall is controlled by convective processes, the GCM produces rain 3 to 6 h too early in the day. Nevertheless, the model represents the continental‐scale reorganisation of the low‐level circulation that results from the diurnal cycle of surface heating and cooling. A nocturnal low‐level jet forms over the western half of the continent with strong convergence at the jet exit, which initiates rain overnight in the continental northwest, in agreement with previous work. The model also captures the change in the air‐flow direction, from a southeasterly to a northeasterly, responsible for bringing the necessary moisture for precipitation to occur. Thus, while the model may have a tendency to initiate convection too early, it is able to represent the larger‐scale nocturnal reorganisation of the flow and the associated rainfall.

Development of a parameterization of black carbon aging for use in general circulation models

Abstract. A parameterization of black carbon (BC) aging in the atmosphere is developed for use in general circulation models (GCMs) that separately treat hydrophobic BC and hydrophilic BC modes but do not explicitly calculate the aging processes of BC. The rate of BC aging is expressed as the conversion rate from hydrophobic BC to hydrophilic BC modes (i.e., inverse of the e-folding time of the conversion, 1/τBC). In this study, the conversion rates are estimated using results of detailed calculations by a size and mixing state resolved aerosol box model with numerous initial conditions. We introduce a new concept, the hydrophobic-BC-mass-normalized coating rate (VBC), defined as the rate of increase of the mass concentration of condensed materials on hydrophobic BC normalized by the hydrophobic BC mass concentration. Although the conversion rate largely varies depending on the atmospheric conditions and the concentrations of chemical species, we find that the variations of the conversion rate are generally expressed well by a unique function of VBC for given lognormal size distributions of hydrophobic BC. The parameterized conversion rate is expressed as a function of VBC, which enables the representation of diurnal and seasonal variations of the BC aging rate and its spatial differences in polluted and clean air, while other widely used constant conversion rates cannot. Application of our newly developed parameterization to GCMs will provide more reliable estimates of the spatial distribution of BC and its radiative effects at regional and global scales.

On the stability and accuracy of the harmonic and biharmonic isoneutral mixing operators in ocean models

Ocean models usually rely on a tracer mixing operator which diffuses along isoneutral directions. This requirement is imposed by the highly adiabatic nature of the oceanic interior, and a numerical simulation needs to respect these small levels of dianeutral mixing to maintain physically realistic results. For non-isopycnic models this is however non-trivial due to the non-alignment of the vertical coordinate isosurfaces with local isoneutral directions, rotated mixing operators must therefore be used. This paper considers the numerical solution of initial boundary value problems for the harmonic (Laplacian) and biharmonic rotated diffusion operators. We provide stability criteria associated with the conventional space–time discretizations of the isoneutral Laplacian operator currently in use in general circulation models. Furthermore, we propose and study possible alternatives to those schemes. A new way to handle the temporal discretization of the rotated biharmonic operator is also introduced. This scheme requires only the resolution of a simple one-dimensional tridiagonal system in the vertical direction to provide the same stability limit of the non-rotated operator. The performance of the various schemes in terms of stability and accuracy is illustrated by idealized numerical experiments of the diffusion of a passive tracer along isoneutral directions.

Cloud optical thickness and liquid water path – does the <i>k</i> coefficient vary with droplet concentration?

Abstract. Cloud radiative transfer calculations in general circulation models involve a link between cloud microphysical and optical properties. Indeed, the liquid water content expresses as a function of the mean volume droplet radius, while the light extinction is a function of their mean surface radius. There is a small difference between these two parameters because of the droplet spectrum width. This issue has been addressed by introducing an empirical multiplying correction factor to the droplet concentration. Analysis of in situ sampled data, however, revealed that the correction factor decreases when the concentration increases, hence partially mitigating the aerosol indirect effect. Five field experiments are reanalyzed here, in which standard and upgraded versions of the droplet spectrometer were used to document shallow cumulus and stratocumulus topped boundary layers. They suggest that the standard probe noticeably underestimates the correction factor compared to the upgraded versions. The analysis is further refined to demonstrate that the value of the correction factor derived by averaging values calculated locally along the flight path overestimates the value derived from liquid water path and optical thickness of a cloudy column, and that there is no detectable relationship between the correction factor and the droplet concentration. It is also shown that the droplet concentration dilution by entrainment-mixing after CCN activation is significantly stronger in shallow cumuli than in stratocumulus layers. These various effects are finally combined to produce the today best estimate of the correction factor to use in general circulation models.

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.

The 158-Year Climate Experiment

An analysis of the impact of carbon dioxide on global warming was performed using global average temperature data covering the last 158 years, along with corresponding atmospheric CO2 concentration data. A statistically significant correlation between the two was developed and used directly to calculate values of the key CO2 climate sensitivity factors (CSFs). When CO2 was treated as a proxy for all possible radiative and feedback forcing agents, an upper bound for the temperature rise due to a doubling of CO2 concentration was found to be 1.92 K. When the impact of CO2 alone was separated out from that of other radiative forcing agents, its CSF was found to be 0.250 KW−1m2, and the 2xCO2 temperature rise is 0.96 K. These values are less, by more than a factor of 3, than those typically reported from use of general circulation models.

European Forests and Carbon Sequestration Services: An Economic Assessment of Climate Change Impacts

This paper reports an original economic valuation of the impact of climate change on the provision of forest regulating services in Europe. To the authors’ knowledge the current paper represents the first systematic attempt to estimate human well-being losses with respect to changes in biodiversity and forest regulating services that are directly driven by climate change. First, selected 34 European countries are grouped by their latitude intervals to capture the differentiated regional effects of forests in response to climate change. Moreover, the future trends of forest areas and stocked carbon in 2050 are projected through the construction and simulation of global circulation models such as HADMC3 following four different future developing paths described by the four IPCC scenarios. Finally, the valuation exercise is anchored in an ecosystem service based approach, involving the use of general circulation models and integrated assessment models. Our findings address two dimensions in the evaluation of climate impacts on European forests: Firstly, future projections yield different states of the world depending upon the IPCC scenario adopted. Secondly, spatial issues matter in an assessment of the distributional impacts of climate change, as these impacts are not distributed in a uniform way across the European countries under consideration.

The radiative energy budget of the middle atmosphere and its parameterization in general circulation models

The thermal regime of the middle atmosphere is determined to a great extent by the balance between the incoming solar and outgoing infrared radiation. To account for these processes in numerical models of the middle atmosphere, parameterizations that are capable of quickly and accurately calculating infrared cooling and solar heating rates are required. These parameterizations should include the breakdown of local thermodynamic equilibrium (LTE) conditions and allow for feedbacks by ensuring that dependencies on all input parameters are accounted for. This paper discusses the major mechanisms responsible for maintaining the radiative energy budget of the middle atmosphere and presents a brief review of approaches and numerical schemes currently available for use in general circulation models. The main focus of the paper is on the approaches and schemes designed for non-LTE treatment.

Performance comparison of some dynamical and empirical downscaling methods for South Africa from a seasonal climate modelling perspective

AbstractThe ability of advanced state‐of‐the‐art methods of downscaling large‐scale climate predictions to regional and local scale as seasonal rainfall forecasting tools for South Africa is assessed. Various downscaling techniques and raw general circulation model (GCM) output are compared to one another over 10 December‐January‐February (DJF) seasons from 1991/1992 to 2000/2001 and also to a baseline prediction technique that uses only global sea‐surface temperature (SST) anomalies as predictors. The various downscaling techniques described in this study include both an empirical technique called model output statistics (MOS) and a dynamical technique where a finer resolution regional climate model (RCM) is nested into the large‐scale fields of a coarser GCM. The study addresses the performance of a number of simulation systems (no forecast lead‐time) of varying complexity. These systems' performance is tested for both homogeneous regions and for 963 stations over South Africa, and compared with each other over the 10‐year test period. For the most part, the simulations method outscores the baseline method that uses SST anomalies to simulate rainfall, therefore providing evidence that current approaches in seasonal forecasting are outscoring earlier ones. Current operational forecasting approaches involve the use of GCMs, which are considered to be the main tool whereby seasonal forecasting efforts will improve in the future. Advantages in statistically post‐processing output from GCMs as well as output from RCMs are demonstrated. Evidence is provided that skill should further improve with an increased number of ensemble members. The demonstrated importance of statistical models in operation capacities is a major contribution to the science of seasonal forecasting. Although RCMs are preferable due to physical consistency, statistical models are still providing similar or even better skill and should still be applied. Copyright © 2008 Royal Meteorological Society

Middle Miocene tectonic boundary conditions for use in climate models

Utilizing general circulation models (GCMs) for paleoclimate study requires the construction of appropriate model boundary conditions. We present a middle Miocene paleotopographic and paleobathymetric reconstruction geographically constrained at 15 Ma for use in GCMs. Paleotopography and paleogeography are reconstructed using a published global plate rotation model and published geological data. Paleobathymetry is reconstructed through application of an age‐depth relationship to a middle Miocene global digital isochron map, followed by the overlay of reconstructed sediment thickness and large igneous provinces. Adjustments are subsequently made to ensure our reconstruction may be utilized in GCMs.

Comparison of stochastic parametrization approaches in a single-column model

We discuss and test the potential usefulness of single-column models (SCMs) for the testing of stochastic physics schemes that have been proposed for use in general circulation models (GCMs). We argue that although single-column tests cannot be definitive in exposing the full behaviour of a stochastic method in the full GCM, and although there are differences between SCM testing of deterministic and stochastic methods, SCM testing remains a useful tool. It is necessary to consider an ensemble of SCM runs produced by the stochastic method. These can be usefully compared with deterministic ensembles describing initial condition uncertainty and also with combinations of these (with structural model changes) into poor man's ensembles. The proposed methodology is demonstrated using an SCM experiment recently developed by the GCSS (GEWEX Cloud System Study) community, simulating transitions between active and suppressed periods of tropical convection.

Climate model boundary conditions for four Cretaceous time slices

Abstract. General circulation models (GCMs) are useful tools for investigating the characteristics and dynamics of past climates. Understanding of past climates contributes significantly to our overall understanding of Earth's climate system. One of the most time consuming, and often daunting, tasks facing the paleoclimate modeler, particularly those without a geological background, is the production of surface boundary conditions for past time periods. These boundary conditions consist of, at a minimum, continental configurations derived from plate tectonic modeling, topography, bathymetry, and a vegetation distribution. Typically, each researcher develops a unique set of boundary conditions for use in their simulations. Thus, unlike simulations of modern climate, basic assumptions in paleo surface boundary conditions can vary from researcher to researcher. This makes comparisons between results from multiple researchers difficult and, thus, hinders the integration of studies across the broader community. Unless special changes to surface conditions are warranted, researcher dependent boundary conditions are not the most efficient way to proceed in paleoclimate investigations. Here we present surface boundary conditions (land-sea distribution, paleotopography, paleobathymetry, and paleovegetation distribution) for four Cretaceous time slices (120 Ma, 110 Ma, 90 Ma, and 70 Ma). These boundary conditions are modified from base datasets to be appropriate for incorporation into numerical studies of Earth's climate and are available in NetCDF format upon request from the lead author. The land-sea distribution, bathymetry, and topography are based on the 1°×1° (latitude × longitude) paleo Digital Elevation Models (paleoDEMs) of Christopher Scotese. Those paleoDEMs were adjusted using the paleogeographical reconstructions of Ronald Blakey (Northern Arizona University) and published literature and were then modified for use in GCMs. The paleovegetation distribution is based on published data and reconstructions and consultation with members of the paleobotanical community and is represented as generalized biomes that should be easily translatable to many vegetation-modeling schemes.