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1
Vimont, Daniel J. "The seasonal footprinting mechanism in the CSIRO coupled general circulation models and in observations /." Thesis, Connect to this title online; UW restricted, 2002. http://hdl.handle.net/1773/10074.
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Dubois, Clotilde. "The role of diapycnal mixing in coupled atmosphere-ocean general circulation models." Thesis, University of Southampton, 2006. https://eprints.soton.ac.uk/63133/.
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The value of ocean diapycnal diffusivity (v) sets the rate at which dense bottom water can be mixed up through the stratified water column and thus plays an important role in the meridional overturning circulation (MOC). Previous idealised experiments and simplified theory suggest that the strength of the MOC and the ocean heat transport scale with the v. This study investigates the dependence of the MOC and other parameters on v using atmosphere-ocean general circulation models (AOGCM). Firstly, the dependence of the MOC strength on v is studied using a low resolution AOGCM with realistic geometry, FORTE, with spatially constant v values ranging from 0.1 cm2/s to an unrealistic high value of 5 cm2/s. At the cyclostationary state, global MOC strength is found to scale with v (in agreement with previous studies) according to a power law of 0.5. No power law is found for the MOC in the individual basins. The increase in MOC strength in the Atlantic and Pacific Oceans is associated with an increase in the ocean heat transport. The atmosphere responds to the change in the ocean state by a decrease of its energy transport and surface winds. Only a partial compensation is found between the ocean and atmosphere energy transport. The strength of v is found to have a strong impact on coupled phenomena, such as a cessation of El Niño at high v. Secondly, similar experiments are conducted with a state-of-the-art AOGCM, ECHAM5/ MPIOM. In this model, v is derived from a constant background diapycnal diffusion (b), wind induced mixing, the Richardson number and the convective adjustment. A set of 3 coupled experiments is conducted, with b = 0.1, 0.25 and 1 cm2/s. The scaling law from simple theory and the previous experiments with FORTE is not observed with this coupled model. At the cyclostationary state, the MOC strength weakens by 16% as b increases from 0.1 to 1 cm2/s. This behavior is not found when the experiments are repeated with an ocean-only model. The reduction in MOC in the coupled model is linked to a strong reduction in the convective mixing at high latitudes. The convective mixing is reduced by a continuous strong freshening in the Arctic region due to an increase in surface air temperature and melting of the sea-ice in the coupled experiments, which is not observed in the ocean-only experiments. The responses of the two coupled models show many similarities as b increases. Both models show convection in the Pacific for high values of b. The main difference is the response of the MOC in the Atlantic is linked to the different locations of the deep convection and their relative changes in the models. I conclude that the diapycnal mixing and the ocean-atmosphere interactions both control the strength of the MOC, and their influences cannot be considered separately.
3
Gehlot, Swati, and Johannes Quaas. "Convection–climate feedbacks in the ECHAM5 general circulation model." Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-177611.
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A process-oriented climate model evaluation is presented, applying the International Satellite Cloud Climatology Project (ISCCP) simulator to pinpoint deficiencies related to the cloud processes in the
ECHAM5general circulation model.ALagrangian trajectory analysis is performed to track the transitions of anvil cirrus originating from deep convective detrainment to cirrostratus and thin cirrus, comparing ISCCP observations and the ECHAM5 model. Trajectories of cloudy air parcels originating from deep convection are computed for both, the ISCCP observations and the model, over which the ISCCP joint histograms are used for analyzing the cirrus life cycle over 5 days. The cirrostratus and cirrus clouds originate from detrainment from deep convection decay and gradually thin out after the convective event over 3–4 days. The effect of the convection–cirrus transitions in a warmer climate is analyzed in order to understand the climate feedbacks due to deep convective cloud transitions. An idealized climate change simulation is performed using a+2-K sea surface temperature (SST) perturbation. The Lagrangian trajectory analysis over perturbed climate suggests that more and thicker cirrostratus and cirrus clouds occur in the warmer climate compared to the present-day climate. Stronger convection is noticed in the perturbed climate, which leads to an increased precipitation, especially on day -2 and -3 after the individual convective events. The shortwave and the longwave cloud forcings both increase in the warmer climate, with an increase of net cloud radiative forcing (NCRF), leading to an overall positive feedback of the increased cirrostratus and cirrus clouds from
a Lagrangian transition perspective.
4
Grancini, Carlo. "Initial validation of an agile coupled atmosphere-ocean general circulation model." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2022. http://amslaurea.unibo.it/25439/.
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Mathematical models based on physics, chemistry and biology principles are one of the main tools to understand climate interactions, variability and sensitivity to forcings. Model performance must be validated checking that results are consistent with actual/observed climate. This work describes the initial validation of a new intermediate complexity, coupled climate model based on a set of existing atmosphere, ocean and sea-ice models. The model, developed and made available by the International Centre for Theoretical Physics (ICTP), is based on the widely used SPEEDY atmospheric model. Limited literature is available for its version, coupled to the NEMO ocean model referred to as SPEEDY-NEMO. The focus of this study is on the adaptation and validation of this model.
A long-term spin-up run with constant present-day forcing has been performed to achieve a steady-state climate. The simulated climate has then been compared with observations and reanalyses of the recent past. The initial validation has shown that simulations spanning a thousand years can be easily run. The model does not require many h/w resources and therefore significant size samples can be generated if needed.
Our results prove that long timescale, stable simulations are feasible. The model reproduces the main features of Earth’s mean climate and variability, despite the use of a fairly limited resolution grid, simple parameterizations and a limited range of physical processes.
Ocean model outputs have not been assessed. However, a clear El Niño signal in the simulated Sea Surface Temperatures (SSTs) data and arctic sea ice extent show that the ocean model behaviour is close to observations.
According to the results, the model is a promising tool for climate studies. However, to understand its full potential the validation should be improved and extended with an analysis of ocean variables and targeted simulations with modified conditions to evaluate model behaviour under different conditions
5
Schirber, Sebastian, Daniel Klocke, Robert Pincus, Johannes Quaas, and Jeffrey L. Anderson. "Parameter estimation using data assimilation in an atmospheric general circulation model." Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-177507.
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This study explores the viability of parameter estimation in the comprehensive general circulation model ECHAM6 using ensemble Kalman filter data assimilation techniques. Four closure parameters of the cumulus-convection scheme are estimated using increasingly less idealized scenarios ranging from perfect-model experiments to
the assimilation of conventional observations. Updated parameter values from experiments with real observations are used to assess the error of the model state on short 6 h forecasts and on climatological timescales. All parameters converge to their
default values in single parameter perfect-model experiments. Estimating parameters simultaneously has a neutral effect on the success of the parameter estimation, but applying an imperfect model deteriorates the assimilation performance. With real observations, single parameter estimation generates the default parameter value in one case, converges to different parameter values in two cases, and diverges in the fourth case. The implementation of the two converging parameters influences the model state: Although the estimated parameter values lead to an overall error reduction on short timescales, the error of the model state increases on climatological timescales.
6
Jakob, Christian. "The representation of cloud cover in atmospheric general circulation models." Diss., lmu, 2001. http://nbn-resolving.de/urn:nbn:de:bvb:19-3281.
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Ma, Liang 1962. "On the parameterization of slantwise convection in general circulation models." Thesis, McGill University, 2000. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=37769.
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This study is concerned with the effect of slantwise convection in general circulation models (GCMs). The approach is through the development of a slantwise convective parameterization scheme (SCPS) and its implementation into version 11 of the third generation GCM of the Canadian Climate Centre for modelling and analysis (CCCma gcm11).We first study the characteristics of conditional symmetric instability (CSI) in an environment which is also unstable for conditional upright instability (CUI). The results indicate features common to both upright and slantwise convection. This so called slantwise buoyant instability (SBI) possesses two relevant time scales and its horizontal scale can ranges from tens of km up to over one thousand km.
We then analyze the 15-year ECMWF re-analysis (ERA) data to compute the global distributions of convective available potential energy (CAPE) and slantwise convective available energy (SCAPE). We show that the potential for CSI and CUI indeed co-exists over most areas around the globe. Based on the results of the theoretical study and the data analysis, a parameterization for slantwise convection was developed and implemented into gcm11. It was found that the scheme impacts significantly the simulated general circulation by the development of a direct meridional secondary circulation. The results of the 5-year simulations show that the scheme reduces SCAPE and SCAPE residual rs over the mid-latitudes, leading to a weakening of the thermal wind and the strength of the upper-level jets. The largest improvement in the simulated climate however lies in the reduced meridional transient eddy transports of heat and zonal momentum. With the inclusion of the scheme, the eddy transports agree much more favorably with the observational analysis.
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Chechelnitsky, Michael Y. (Michael Yurievich) 1972. "Adaptive error estimation in linearized ocean general circulation models." Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/58516.
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Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1999.Includes bibliographical references (p. 206-211).
Data assimilation methods, such as the Kalman filter, are routinely used in oceanography. The statistics of the model and measurement errors need to be specified a priori. In this study we address the problem of estimating model and measurement error statistics from observations. We start by testing the Myers and Tapley (1976, MT) method of adaptive error estimation with low-dimensional models. We then apply the MT method in the North Pacific (5°-60° N, 132°-252° E) to TOPEX/POSEIDON sea level anomaly data, acoustic tomography data from the ATOC project, and the MIT General Circulation Model (GCM). A reduced state linear model that describes large scale internal (baroclinic) error dynamics is used. The MT method, closely related to the maximum likelihood methods of Belanger (1974) and Dee (1995), is shown to be sensitive to the initial guess for the error statistics and the type of observations. It does not provide information about the uncertainty of the estimates nor does it provide information about which structures of the error statistics can be estimated and which cannot. A new off-line approach is developed, the covariance matching approach (CMA), where covariance matrices of model-data residuals are "matched" to their theoretical expectations using familiar least squares methods. This method uses observations directly instead of the innovations sequence and is shown to be related to the MT method and the method of Fu et al. (1993). The CMA is both a powerful diagnostic tool for addressing theoretical questions and an efficient estimator for real data assimilation studies. It can be extended to estimate other statistics of the errors, trends, annual cycles, etc. Twin experiments using the same linearized MIT GCM suggest that altimetric data are ill-suited to the estimation of internal GCM errors, but that such estimates can in theory be obtained using acoustic data. After removal of trends and annual cycles, the low frequency /wavenumber (periods> 2 months, wavelengths> 16°) TOPEX/POSEIDON sea level anomaly is of the order 6 cm2. The GCM explains about 40% of that variance. By covariance matching, it is estimated that 60% of the GCM-TOPEX/POSEIDON residual variance is consistent with the reduced state linear model. The CMA is then applied to TOPEX/POSEIDON sea level anomaly data and a linearization of a global GFDL GCM. The linearization, done in Fukumori et al.(1999), uses two vertical mode, the barotropic and the first baroclinic modes. We show that the CMA method can be used with a global model and a global data set, and that the estimates of the error statistics are robust. We show that the fraction of the GCMTOPEX/ POSEIDON residual variance explained by the model error is larger than that derived in Fukumori et al.(1999) with the method of Fu et al.(1993). Most of the model error is explained by the barotropic mode. However, we find that impact of the change in the error statistics on the data assimilation estimates is very small. This is explained by the large representation error, i.e. the dominance of the mesoscale eddies in the TIP signal, which are not part of the 20 by 10 GCM. Therefore, the impact of the observations on the assimilation is very small even after the adjustment of the error statistics. This work demonstrates that simultaneous estimation of the model and measurement error statistics for data assimilation with global ocean data sets and linearized GCMs is possible. However, the error covariance estimation problem is in general highly underdetermined, much more so than the state estimation problem. In other words there exist a very large number of statistical models that can be made consistent with the available data. Therefore, methods for obtaining quantitative error estimates, powerful though they may be, cannot replace physical insight. Used in the right context, as a tool for guiding the choice of a small number of model error parameters, covariance matching can be a useful addition to the repertory of tools available to oceanographers.
by Michael Y. Chechelnitsky.
Ph.D.
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Agarwal, Reema [Verfasser], and Detlef [Akademischer Betreuer] Stammer. "Improving an Atmosphere General Circulation model through Parameter Optimization / Reema Agarwal ; Betreuer: Detlef Stammer." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2017. http://d-nb.info/1124591206/34.
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Shongwe, Mxolisi Excellent. "Performance of recalibration systems of general circulation model forecasts over southern Africa." Pretoria : [s.n.], 2006. http://upetd.up.ac.za/thesis/available/etd-07032007-102650.
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Koster, Randal Dean. "Tracer water transport and subgrid precipitation variation within atmospheric general circulation models." Thesis, Massachusetts Institute of Technology, 1988. http://hdl.handle.net/1721.1/14673.
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Quaas, Johannes. "Evaluating the “critical relative humidity” as a measure of subgrid-scale variability of humidity in general circulation model cloud cover parameterizations using satellite data." Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-177265.
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A simple way to diagnose fractional cloud cover in general circulation models is to relate it to the simulated relative humidity, and allowing for fractional cloud cover above a “critical relative humidity” of less than 100%. In the formulation chosen here, this is equivalent to assuming a uniform “top-hat” distribution of subgrid-scale total water content with a variance related to saturation. Critical relative humidity has frequently been treated as a “tunable” constant, yet it is an observable. Here, this parameter, and its spatial distribution, is examined from Atmospheric Infrared Sounder (AIRS) satellite retrievals, and from a combination of relative humidity from the ECMWF Re-Analyses (ERA-Interim) and cloud fraction obtained from CALIPSO lidar satellite data. These observational data are used to evaluate results from different simulations with the ECHAM general circulation model (GCM). In sensitivity studies, a cloud feedback parameter is analyzed from simulations applying the original parameter choice, and applying parameter choices guided by the satellite data. Model sensitivity studies applying parameters adjusted to match the observations show larger positive cloud-climate feedbacks, increasing by up to 30% compared to the standard simulation.
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Quaas, Johannes, Bjorn Stevens, Philip Stier, and Ulrike Lohmann. "Interpreting the cloud cover: aerosol optical depth relationship found in satellite data using a general circulation model." Copernicus Publications, 2010. https://ul.qucosa.de/id/qucosa%3A13833.
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Statistical analysis of satellite data shows a positive correlation between aerosol optical depth (AOD) and total cloud cover (TCC). Reasons for this relationship have been disputed in recent literature. The aim of this study is to explore how different processes contribute to one model’s
analog of the positive correlation between aerosol optical depth and total cloud cover seen in the satellite retrievals. We compare the slope of the linear regression between the logarithm of TCC and the logarithm of AOD, or the strength of the relationship, as derived from three satellite data sets to the ones simulated by a global aerosol-climate model. We analyse
model results from two different simulations with and without a parameterisation of aerosol indirect effects, and using dry compared to humidified AOD. Perhaps not surprisingly we find that no single one of the hypotheses discussed in the literature is able to uniquely explain the positive relationship. However the dominant contribution to the model’s
AOD-TCC relationship can be attributed to aerosol swelling in regions where humidity is high and clouds are coincidentally found. This finding leads us to hypothesise that much of the AOD-TCC relationship seen in the satellite data is also carried by such a process, rather than the direct effects of the aerosols on the cloud fields themselves.
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Nicholson, W. P. "Studies of the Martian upper atmosphere with the UCL Mars thermosphere and ionosphere general circulation model." Thesis, University College London (University of London), 2011. http://discovery.ucl.ac.uk/1310428/.
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Simulations of the Martian upper atmosphere have been conducted with ‘MarTIM’, University College London’s Martian thermosphere and ionosphere general circulation model (GCM). MarTIM, a finite difference model, solves the coupled non-linear Navier-Stokes equations of continuity and momentum as well as an energy equation with calculations conducted on a fixed co-rotating grid of variable size in the pressure coordinate system. From its lower boundary of 0.883 Pa (~60 km) to its upper boundary of 9.9×10−8 Pa (~200−350 km), it evaluates the main sources of solar forcing (EUV/UV and IR absorption) while self-consistently determining the composition of four of the major gas species, CO2, N2, CO and O. These four major gases are mutually diffused throughout the model in a typical run. Development of MarTIM includes a consideration of the importance of neutral species diffusion and advection on the thermodynamics of the modelled Martian atmosphere. The influence on the modelled atmosphere of including additional neutral species is investigated. Next, a new infrared heating parameterization has been introduced from background research of detailed non-LTE modelling. This has allowed MarTIM to study thermospheric polar warming features as found in Mars Odyssey accelerometer data. MarTIM’s lower boundary is coupled to the Mars Climate Database (MCD v4.3) developed by the University of Oxford, the Open University and Laboratoire de Météorologie Dynamique. This database of GCM results provides MarTIM a physically self-consistent lower boundary derived from multiple runs of the aforementioned circulation models. Consequently the effects of dust storms, non-migrating tides and the influence of Martian topography are studied by prescription of MarTIM’s lower boundary. MarTIM is also compared against density and temperature measurements derived from SPICAM stellar occultation profiles. Lastly, a new ionospheric code has been developed through collaboration with Laboratoire de Planétologie de Grenoble. This has provided a more sophisticated ionosphere model that solves a one-dimensional kinetic Boltzmann transport equation for the suprathermal population of electrons present in the Martian ionosphere. MarTIM can now self-consistently describe an ionosphere produced by both primary (photoionisation) and secondary ionisation (suprathermal electron propagation). This new ionospheric model has been used to study the variation in secondary ionization efficiency (ratio of secondary to primary ion production) through a large range of seasonal and solar conditions.
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Schirber, Sebastian, Daniel Klocke, Robert Pincus, Johannes Quaas, and Jeffrey L. Anderson. "Parameter estimation using data assimilation in an atmospheric general circulation model: Parameter estimation using data assimilation in an atmosphericgeneral circulation model: from a perfect toward the real world." American Geophysical Union (AGU), 2013. https://ul.qucosa.de/id/qucosa%3A13463.
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This study explores the viability of parameter estimation in the comprehensive general circulation model ECHAM6 using ensemble Kalman filter data assimilation techniques. Four closure parameters of the cumulus-convection scheme are estimated using increasingly less idealized scenarios ranging from perfect-model experiments to
the assimilation of conventional observations. Updated parameter values from experiments with real observations are used to assess the error of the model state on short 6 h forecasts and on climatological timescales. All parameters converge to their
default values in single parameter perfect-model experiments. Estimating parameters simultaneously has a neutral effect on the success of the parameter estimation, but applying an imperfect model deteriorates the assimilation performance. With real observations, single parameter estimation generates the default parameter value in one case, converges to different parameter values in two cases, and diverges in the fourth case. The implementation of the two converging parameters influences the model state: Although the estimated parameter values lead to an overall error reduction on short timescales, the error of the model state increases on climatological timescales.
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Anderson, Benjamin Mark. "Feedback mechanisms and constraints on climate sensitivity from a perturbed physics ensemble of general circulation models." Thesis, University of Oxford, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.670120.
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Harris, Matthew James. "A new coupled middle atmosphere and thermosphere general circulation model : studies of dynamic, energetic, and photochemical coupling in the middle and upper atmosphere." Thesis, University College London (University of London), 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.367565.
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Schemann, Vera, Bjorn Stevens, Verena Grützun, and Johannes Quaas. "Scale dependency of total water variance and its implication for cloud parameterizations." Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-177479.
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The scale dependency of variance of total water mixing ratio is explored by analyzing data from a general circulation model (GCM), a numerical weather prediction model (NWP), and large-eddy simulations (LESs). For clarification, direct numerical simulation (DNS) data are additionally included, but the focus is placed on defining a general scaling behavior for scales ranging from global down to cloud resolving. For this, appropriate power-law exponents are determined by calculating and approximating the power density spectrum.
The large-scale models (GCM and NWP) show a consistent scaling with a power-law exponent of approximately 22. For the high-resolution LESs, the slope of the power density spectrum shows evidence of being somewhat steeper, although the estimates are more uncertain. Also the transition between resolved and parameterized scales in a current GCM is investigated. Neither a spectral gap nor a strong scale break is found, but a weak scale break at high wavenumbers cannot be excluded. The evaluation of the parameterized total water variance of a state-of-the-art statistical scheme shows that the scale dependency is underestimated by this parameterization. This study and the discovered general scaling behavior emphasize the need for
a development of scale-dependent parameterizations.
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Gehlot, Swati, and Johannes Quaas. "Convection–climate feedbacks in the ECHAM5 general circulation model: evaluation of cirrus cloud life cycles with ISCCP satellite data from aLagrangian trajectory perspective." American Meteorological Society, 2012. https://ul.qucosa.de/id/qucosa%3A13467.
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A process-oriented climate model evaluation is presented, applying the International Satellite Cloud Climatology Project (ISCCP) simulator to pinpoint deficiencies related to the cloud processes in the
ECHAM5general circulation model.ALagrangian trajectory analysis is performed to track the transitions of anvil cirrus originating from deep convective detrainment to cirrostratus and thin cirrus, comparing ISCCP observations and the ECHAM5 model. Trajectories of cloudy air parcels originating from deep convection are computed for both, the ISCCP observations and the model, over which the ISCCP joint histograms are used for analyzing the cirrus life cycle over 5 days. The cirrostratus and cirrus clouds originate from detrainment from deep convection decay and gradually thin out after the convective event over 3–4 days. The effect of the convection–cirrus transitions in a warmer climate is analyzed in order to understand the climate feedbacks due to deep convective cloud transitions. An idealized climate change simulation is performed using a+2-K sea surface temperature (SST) perturbation. The Lagrangian trajectory analysis over perturbed climate suggests that more and thicker cirrostratus and cirrus clouds occur in the warmer climate compared to the present-day climate. Stronger convection is noticed in the perturbed climate, which leads to an increased precipitation, especially on day -2 and -3 after the individual convective events. The shortwave and the longwave cloud forcings both increase in the warmer climate, with an increase of net cloud radiative forcing (NCRF), leading to an overall positive feedback of the increased cirrostratus and cirrus clouds from
a Lagrangian transition perspective.
20
Entekhabi, Dara 1961. "Landsurface hydrology parameterization for atmospheric general circulation models : inclusion of subgrid scale spatial variability and screening with a simple climate model." Thesis, Massachusetts Institute of Technology, 1990. http://hdl.handle.net/1721.1/17246.
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Schemann, Vera, Bjorn Stevens, Verena Grützun, and Johannes Quaas. "Scale dependency of total water variance and its implication for cloud parameterizations: Scale dependency of total water variance and its implication for cloudparameterizations." American Meteorological Society, 2013. https://ul.qucosa.de/id/qucosa%3A13462.
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The scale dependency of variance of total water mixing ratio is explored by analyzing data from a general circulation model (GCM), a numerical weather prediction model (NWP), and large-eddy simulations (LESs). For clarification, direct numerical simulation (DNS) data are additionally included, but the focus is placed on defining a general scaling behavior for scales ranging from global down to cloud resolving. For this, appropriate power-law exponents are determined by calculating and approximating the power density spectrum.
The large-scale models (GCM and NWP) show a consistent scaling with a power-law exponent of approximately 22. For the high-resolution LESs, the slope of the power density spectrum shows evidence of being somewhat steeper, although the estimates are more uncertain. Also the transition between resolved and parameterized scales in a current GCM is investigated. Neither a spectral gap nor a strong scale break is found, but a weak scale break at high wavenumbers cannot be excluded. The evaluation of the parameterized total water variance of a state-of-the-art statistical scheme shows that the scale dependency is underestimated by this parameterization. This study and the discovered general scaling behavior emphasize the need for
a development of scale-dependent parameterizations.
22
Knochel, Hervé. "Développement et validation d'un modèle numérique de circulation océanique à coordonnées [sigma] pour l'étude climatique de l'Atlantique nord." Université Joseph Fourier (Grenoble), 1998. http://www.theses.fr/1998GRE10256.
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Dans cette these, un modele numerique de circulation generale oceanique est mis en uvre dans le but d'effectuer une etude climatique de l'atlantique nord. Le modele presente la particularite d'utiliser une coordonnee verticale epousant le relief sous-marin (coordonnee ). Il inclut en outre un modele unidimensionnel integral de couche melangee de type krauss-turner pour parametriser la penetration des forcages atmospheriques de surface dans l'ocean. L'etude d'une simulation climatologique type montre la capacite du modele a reproduire les traits dominants de la circulation generale avec en particulier une representation realiste de la trajectoire du gulf stream et de son decollement a cape hatteras. Des etudes de sensibilite du modele sont egalement effectuees, mettant en evidence certains aspects caracteristiques, comme l'importance de la resolution verticale dans la representation de la circulation profonde ou l'amelioration de la penetration des forcages thermohalins du fait de l'utilisation du modele de couche melangee. Ces etudes de simulations climatologiques fournissent un etat climatique moyen de l'ocean, a partir duquel la variabilite interannuelle du modele est etudiee dans le cadre d'une simulation incluant des flux atmospheriques mensuels, issus de la reanalyse ncep/ncar entre 1958 et 1997. L'etude se concentre sur l'eau de la mer du labrador (lsw), montrant une variabilite interannuelle coherente de cette masse d'eau et validant ainsi de maniere tres positive les donnees de forcage du ncep/ncar. La simulation met de plus en evidence une inadequation du parametrage du forcage salin de surface. Malgre cela, le modele reproduit de maniere realiste les periodes observees de forte et faible convection en mer du labrador et fait apparaitre une propagation d'anomalies chaudes et froides dans la circulation cyclonique du gyre subpolaire.
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Thompson, David W. J. "Annular modes in the atmospheric general circulation /." Thesis, Connect to this title online; UW restricted, 2000. http://hdl.handle.net/1773/10057.
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Dugas, Bernard. "Persistent circulation anomalies in observations and in a general circulation model." Thesis, McGill University, 1989. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=74220.
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A comparative diagnostic study of upper-air persistent atmospheric events, as simulated by a general circulation model (GCM) and as observed, is presented. We start with an overview of the several theories that attempt to explain such phenomena. Particular emphasis is put on the model approach of Shutts (1983). We next show that the spatial distributions of persistent events is qualitatively similar in the GCM and observational data. The North-Atlantic events are extracted and a rotated empirical orthogonal function (REOF) analysis is done on the resulting data sets. The two REOF sets that are thus obtained are shown to greatly resemble one another. Both explain roughly 50% of their original data's variance. The relationships between the modes within a set are presented, so as to understand their probable combined evolution. The fourth chapter contains an evaluation of Shutt's theory. There, the third chapter's results are used to isolate a particular class of events, namely the strong +ATL2 dipoles. The time-tendencies associated to short time-scale synoptic waves are evaluated, using an E-vectors approach, taking care to distinguish between the onset, mature and demise phases of the events. It seems that these synoptic waves have a significant impact of the average life-cycle of this +ATL2 type of events, whether they be simulated by a GCM or obtained from a NMC set of analyses.
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Mendonca, Joao M. "Studies of Venus using a comprehensive general circulation model." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:eab33b95-b66a-4d10-8696-548e1d211c9f.
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The profusion of observational data made available by the Venus Express and previous space missions, increases our need to develop numerical tools to interpret the data and improve our understanding of the Venus meteorology. The main objective of this work is to develop an improved Venus general circulation model and to study the most likely mechanisms driving the atmosphere to the current observed circulation. Our new model is an extension of a simplified version and includes a new radiative transfer scheme and convection and an adapted boundary layer scheme and dynamical core that take into account the dependence of the heat capacity with temperature, at constant atmospheric pressure. The new radiative transfer formulation implemented is more suitable for Venus climate studies than previous works due to its easy adaptability to different atmospheric conditions. This flexibility of the model was very important in this work to explore the uncertainties on the lower atmospheric conditions such as the gas absorption and the possible presence of aerosols near the surface. The new general circulation model obtains, after long periods of integration, a super-rotation phenomenon in the cloud region quantitatively similar to the one observed. However, this phenomenon is sensitive to some radiative parameters such as the amount of the solar radiative energy absorbed by the surface and the amount of clouds. The super-rotation in the model is formed due to the combined influence of the zonal mean circulation, thermal tides and transient waves, and the main mechanisms involved are identified and studied. In this process the momentum transported by the semidiurnal tide excited in the upper clouds has a key contribution. These migrating waves transport prograde momentum mainly from the upper atmosphere to the cloud region. In this work we also explored the model parameters to gain a better understanding of the effect of topography, the diurnal cycle and convective momentum mixing. In general the results showed that: the topography seemed capable of sustaining stronger global super-rotation; without diurnal cycle the strong winds in the cloud region are not produced; the convective momentum mixing experiment did not lead to significant changes. A simple experiment done advecting the UV absorber in the atmosphere, qualitatively showed several atmospheric phenomena that are important for the distribution of clouds. Among them is the presence of a region of low permeability isolating the polar vortex. This last experiment also showed that when increasing the amount of UV absorption in the upper cloud region the winds get stronger. Following the interpretation of observational data using numerical models, we also used a simplified version of the general circulation model to assess the accuracy of zonal wind retrievals from measured temperatures using the cyclostrophic thermal wind equation in the Venus mesosphere. From this analysis we suggest a method which better estimates the lower boundary condition, and improves the consistency of the results at high latitudes when compared with cloud tracking measurements.
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Vettoretti, Guido. "Paleoclimate tests of a model of the atmospheric general circulation." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/NQ63759.pdf.
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Privé, Nikki C. 1977. "Zonally symmetric monsoon dynamics in a general circulation model." Thesis, Massachusetts Institute of Technology, 2002. http://hdl.handle.net/1721.1/59100.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2002.Includes bibliographical references (p. 97-98).
The MIT general circulation model is used with simplified setup to study steady zonally averaged monsoon circulations. Two dimensional model runs are made with a zonally symmetric continent north of 15N and a slab ocean of uniform sea surface temperature to study the applicability of axisymmetric theory. Forcing to drive the monsoon is applied by heating the subtropical land surface. The dynamical constraints of axisymmetry prevent low-level cross-equatorial flow and inhibit the northward transport of moisture onto the continent when there is no temperature gradient across the equator. The ocean cannot supply adequate moisture to feed the monsoon, and the ground hydrology strictly controls the behavior of the monsoon. A second set of two dimensional runs with similar continent, but with an SST gradient across the equator, result in a viable steady monsoon with low-level cross-equatorial flow providing moisture to the monsoon. The surface forcing required to induce a monsoon is reasonable given the constraints of the axisymmetric model setup. A series of three dimensional model runs with a zonally symmetric continent are made to study the role of zonally asymmetric flow on the zonal mean monsoon. It is found that greater land surface forcing is required to induce a zonally averaged monsoon circulation in the three dimensional runs than in similar axisymmetric runs. The behavior of the monsoon disturbances in the three dimensional runs is similar to the observed Asian monsoon in that there is low-level cross-equatorial flow which is southwesterly along the coastline, and in that a large-scale angular momentum conserving meridional circulation develops with ascent over the continent and subsidence in the opposite hemisphere. Moisture transport is found to play a very strong role in the monsoon dynamics in all of the model runs.
by Nikki C. Privé.
S.M.
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Su, Lin 1966. "A diagnostic study of the summer southern hemisphere circulation of the CCC general circulation model /." Thesis, McGill University, 1991. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=60493.
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The medium scale planetary wave regime, consisting largely of zonal wavenumbers 5-7, frequently dominate the summer Southern Hemisphere tropospheric circulation. We perform a diagnostic study of this circulation as simulated by the Canadian Climate Centre (CCC) general circulation model (GCM). The analysis of Hovmoller diagrams, space-time and zonal wavenumber spectra shows that the CCC GCM is able to simulate the observed medium scale wave regime.The zonally averaged meridional eddy heat and momentum transports and the associated baroclinic and barotropic energy conversions are also examined. The distributions of the transports on the vertical plane agree well with observations. When compared to the observed summer 1979 distributions, some quantitative differences remain: the vertical structure of the heat transport is too baroclinic, while the momentum transport tends to be too weak. The baroclinic and barotropic conversions all show a medium scale wave signal. The time evolution of the Richardson number of the mean flow suggests that the medium scale wave is due to a finite amplitude baroclinic instability.
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Yagai, Isamu. "NUMERICAL SIMULATION OF ATMOSPHERIC THERMAL TIDES WITH A GENERAL CIRCULATION MODEL." Kyoto University, 1991. http://hdl.handle.net/2433/168744.
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本文データは平成22年度国立国会図書館の学位論文(博士)のデジタル化実施により作成された画像ファイルを基にpdf変換したものであるKyoto University (京都大学)
0048
新制・論文博士
博士(工学)
乙第7629号
論工博第2514号
新制||工||853(附属図書館)
UT51-91-T435
(主査)教授 加藤 進, 教授 深尾 昌一郎, 教授 桜井 健郎
学位規則第4条第2項該当
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Nitsche, Gregor. "Some aspects of planetary-scale atmospheric variability in a low-resolution general circulation model /." Thesis, Connect to this title online; UW restricted, 1996. http://hdl.handle.net/1773/10014.
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Saito, Ryu. "Influence of the surface on the atmospheric circulation of Mars : study with a general circulation model /." Katlenburg-Lindau : Copernicus, 2007. http://www.gbv.de/dms/bs/toc/535034385.pdf.
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Mullen, Steven Lee. "On the maintenance of blocking anticyclones in a general circulation model /." Thesis, Connect to this title online; UW restricted, 1985. http://hdl.handle.net/1773/10094.
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Aider, Rabah. "Skill of monthly and seasonal forecasts using a Canadian general circulation model." Thesis, McGill University, 2009. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=32296.
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An analysis of the co-variability of surface air temperature and precipitation over North America and Pacific SST is conducted using an SVD analysis. The leading SVD mode revealed a strong link between November SST anomalies and winter surface air temperature (SAT) and precipitation anomalies. In summer this relationship is much weaker. In winter GCM3 captured well the Pacific SST forcing and its response, particularly on the SAT pattern, but its response is less accurate in the summer. The monthly and seasonal forecasts using GCM3 are also analyzed. Precipitation forecasts showed little skill, especially in the warm season where the SST forcing is weak. Furthermore, GCM3 has low seasonal predictive skill in forecasting drought events over the Canadian prairies. However, the model has generally good predictive skill for 500 hPa heights and SAT, with higher scores in the winter. The skill is concentrated in the first month of the prediction period and decreases as the lead time is extended to one month.Une analyse de la co-variabilité entre la température de l'air au sol (SAT) ainsi que les précipitations en Amérique du Nord et la température de l'océan Pacifique à la surface (SST), a été faite en utilisant la méthode SVD. Le mode dominant de la SVD a révélé une relation forte entre les anomalies de la SST du mois de novembre et celles de la SAT et des précipitations hivernales. Ce lien est beaucoup plus faible en été. Le modèle GCM3 reproduit assez bien la réponse au forçage de la SST, particulièrement sur les patrons de la SAT, mais sa réponse est beaucoup moins précise en été. Les prévisions mensuelles et saisonnières de GCM3 ont aussi été analysées. Les capacités de GCM3 à prévoir les précipitations sont faibles, surtout en été où le forçage de la SST est aussi faible. De plus, le modèle ne possède pas d'habiletés notables à prédire les sécheresses dans les prairies Canadiennes. Par contre, les capacités prévisionnelles du modèle concernant la SAT et le géopotentiel à 500 hPa sont généralement assez élevées, particulièrement en hivers. Les habiletés de GCM3 sont concentrées dans le premier mois de la période de prévision, puis déclinent lorsque le délai d'émission est prolongé.
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Faleiro, Frederico Augusto Martins Valtuille. "Modelagem de mudanças climáticas: do nicho fundamental à conservação da biodiversidade." Universidade Federal de Goiás, 2016. http://repositorio.bc.ufg.br/tede/handle/tede/5603.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
The climate changes are one of the major threats to the biodiversity and it is expected to increase its impact along the 21st century. The climate change affect all levels of the biodiversity from individuals to biomes, reducing the ecosystem services. Despite of this, the prediction of climate change impacts on biodiversity is still a challenge. Overcoming these issues depends on improvements in different aspects of science that support predictions of climate change impact on biodiversity. The common practice to predict the climate change impact consists in formulate ecological niche models based in the current climate and project the changes based in the future climate predicted by the climate models. However, there are some recognized limitations both in the formulation of the ecological niche model and in the use of predictions from the climate models that need to be analyzed. Here, in the first chapter we review the science behind the climate models in order to reduce the knowledge gap between the scientific community that formulate the climate models and the community that use the predictions of these models. We showed that there is not consensus about evaluate the climate models, obtain regional models with higher spatial resolution and define consensual models. However, we gave some guidelines for use the predictions of the climate models. In the second chapter, we tested if the predictions of correlative ecological niche models fitted with presence-absence match the predictions of models fitted with abundance data on the metrics of climate change impact on orchid bees in the Atlantic Forest. We found that the presence-absence models were a partial proxy of change in abundance when the output of the models was continuous, but the same was not true when the predictions were converted to binary. The orchid bees in general will decrease the abundance in the future, but will retain a good amount of suitable sites in the future and the distance to gained climatic suitable areas can be very close, despite of great variation. The change in the species richness and turnover will be mainly in the western and some regions of southern of the Atlantic Forest. In the third chapter, we discussed the drawbacks in using the estimations of realized niche instead the fundamental niche, such as overpredicting the effect of climate change on species’ extinction risk. We proposed a framework based on phylogenetic comparative and missing data methods to predict the dimensions of the fundamental niche of species with missing data. Moreover, we explore sources of uncertainty in predictions of fundamental niche and highlight future directions to overcome current limitations of phylogenetic comparative and missing data methods to improve predictions. We conclude that it is possible to make better use of the current knowledge about species’ fundamental niche with phylogenetic information and auxiliary traits to predict the fundamental niche of poorly-studied species. In the fourth chapter, we used the framework of the chapter three to test the performance of two recent phylogenetic modeling methods to predict the thermal niche of mammals. We showed that PhyloPars had better performance than Phylogenetic Eigenvector Maps in predict the thermal niche. Moreover, the error and bias had similar phylogenetic pattern for both margins of the thermal niche while they had differences in the geographic pattern. The variance in the performance was explained by taxonomic differences and not by methodological aspects. Finally, our models better predicted the upper margin than the lower margin of the thermal niche. This is a good news for predicting the effect of climate change on species without physiological data. We hope our finds can be used to improve the predictions of climate change effect on the biodiversity in future studies and support the political decisions on minimizing the effects of climate change on biodiversity.
As mudanças climáticas são uma das principais ameaças à biodiversidade e é esperado que aumente seu impacto ao longo do século XXI. As mudanças climáticas afetam todos os níveis de biodiversidade, de indivíduos à biomas, reduzindo os serviços ecossistêmicos. Apesar disso, as predições dos impactos das mudanças climáticas na biodiversidade é ainda um desafio. A superação dessas questões depende de melhorias em diferentes aspectos da ciência que dá suporte para predizer o impacto das mudanças climáticas na biodiversidade. A prática comum para predizer o impacto das mudanças climáticas consiste em formular modelos de nicho ecológico baseado no clima atual e projetar as mudanças baseadas no clima futuro predito pelos modelos climáticos. No entanto, existem algumas limitações reconhecidas na formulação do modelo de nicho ecológico e no uso das predições dos modelos climáticos que precisam ser analisadas. Aqui, no primeiro capítulo nós revisamos a ciência por detrás dos modelos climáticos com o intuito de reduzir a lacuna de conhecimentos entre a comunidade científica que formula os modelos climáticos e a comunidade que usa as predições dos modelos. Nós mostramos que não existe consenso sobre avaliar os modelos climáticos, obter modelos regionais com maior resolução espacial e definir modelos consensuais. No entanto, nós damos algumas orientações para usar as predições dos modelos climáticos. No segundo capítulo, nós testamos se as predições dos modelos correlativos de nicho ecológicos ajustados com presença-ausência são congruentes com aqueles ajustados com dados de abundância nas medidas de impacto das mudanças climáticas em abelhas de orquídeas da Mata Atlântica. Nós encontramos que os modelos com presença-ausência foram substitutos parciais das mudanças na abundância quando o resultado dos modelos foi contínuo (adequabilidade), mas o mesmo não ocorreu quando as predições foram convertidas para binárias. As espécies de abelhas, de modo geral, irão diminuir em abundância no futuro, mas reterão uma boa quantidade de locais adequados no futuro e a distância para áreas climáticas adequadas ganhadas podem estar bem próximo, apesar da grande variação. A mudança na riqueza e na substituição de espécies ocorrerá principalmente no Oeste e algumas regiões no sul da Mata Atlântica. No terceiro capítulo, nós discutimos as desvantagens no uso de estimativas do nicho realizado ao invés do nicho fundamental, como superestimar o efeito das mudanças climáticas no risco de extinção das espécies. Nós propomos um esquema geral baseado em métodos filogenéticos comparativos e métodos de dados faltantes para predizer as dimensões do nicho fundamental das espécies com dados faltantes. Além disso, nós exploramos as fontes de incerteza nas predições do nicho fundamental e destacamos direções futuras para superar as limitações atuais dos métodos comparativos filogenéticas e métodos de dados faltantes para melhorar as predições. Nós concluímos que é possível fazer melhor uso do conhecimento atual sobre o nicho fundamental das espécies com informação filogenética e caracteres auxiliares para predizer o nicho fundamental de espécies pouco estudadas. No quarto capítulo, nós usamos o esquema geral do capítulo três para testar a performance de dois novos métodos de modelagem filogenética para predizer o nicho térmico dos mamíferos. Nós mostramos que o “PhyloPars” teve uma melhor performance que o “Phylogenetic Eigenvector Maps” em predizer o nicho térmico. Além disso, o erro e o viés tiveram um padrão filogenético similar para ambas as margens do nicho térmico, enquanto eles apresentaram diferentes padrões espaciais. A variância na performance foi explicada pelas diferenças taxonômicas e não pelas diferenças em aspectos metodológicos. Finalmente, nossos modelos melhor predizem a margem superior do que a margem inferior do nicho térmico. Essa é uma boa notícia para predizer o efeito das mudanças climáticas em espécies sem dados fisiológicos. Nós esperamos que nossos resultados possam ser usados para melhorar as predições do efeito das mudanças climáticas na biodiversidade em estudos futuros e dar suporte para decisões políticas para minimização dos efeitos das mudanças climáticas na biodiversidade.
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Bessières, Laurent. "Impact des marées sur la circulation générale océanique dans une perspective climatique." Phd thesis, Université Paul Sabatier - Toulouse III, 2007. http://tel.archives-ouvertes.fr/tel-00172154.
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La marée océanique a longtemps été considérée comme un phénomène haute fréquence dont la zone d'influence se limitait aux plateaux continentaux. Ainsi, la marée n'apparaissait pas susceptible d'interagir sur la circulation océanique grande échelle et in fine sur la dynamique basse fréquence du climat. Ce n'est qu'à la fin des années 1990, lorsque la mesure altimétrique et les solutions hydrodynamiques globales de marées ont atteint une précision centimétrique, qu'une connexion entre les marées et le climat est devenue envisageable. Dans cette perspective, l'objectif de cette étude est d'explorer quels sont les effets de la marée sur la circulation océanique grande échelle. Ces effets ont lieu à travers deux processus physiques essentiels liés à la marée : (i) sa dynamique fortement non-linéaire et (ii) la dissipation de son énergie en plein océan, sous la forme de mélange vertical ou de chaleur.Pour examiner chacun de ces aspects, la méthode retenue consiste à paramétriser les effets de la marée dans un modèle tridimensionnel de circulation générale océanique (OGCM) dédié au climat : NEMO. Pour ce faire nous utilisons les sorties 'off line' d'un modèle hydrodynamique bidimensionnel dédié à la marée : MOG2D-G. Dans un premier temps nous déterminons et nous décrivons pour la première fois une carte de la circulation résiduelle de marée (CRM) mondiale générée par la dynamique non-linéaire de la marée. Cette CRM obtenue par l'intermédiaire de MOG2D-G est alors introduite sous la forme d'un forçage extérieur dans l'OGCM NEMO. Dans un second temps, nous examinons la dissipation de l'énergie des marées. Tout d'abord nous quantifions la fraction de l'énergie de marée qui est dissipée en chaleur, ceci afin de déterminer si, à l'instar du flux géothermal, elle est susceptible de jouer un rôle important sur la circulation abyssale. Après avoir écarté cette possibilité, nous considérons la fraction d'énergie de marée qui se dissipe localement en mélange vertical via les ondes internes : le "tidal mixing" (TM). Le TM résulte d'un transfert d'énergie du mode barotope vers les modes baroclines. Ce transfert est diagnostiqué grâce au modèle MOG2D-G et intégré dans NEMO par l'intermédiaire d'une paramétrisation du mélange turbulent vertical.
Nous concluons : (i) que l'effet des marées sur la circulation océanique grande échelle et in fine sur le climat ne peut être significatif qu'à travers le TM, (ii) que l'introduction du TM local dans les OGCM est essentielle pour représenter correctement le transport des masses d'eaux abyssales et (iii) qu'il est désormais crucial de considérer le TM engendré loin du site de génération des ondes internes
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Wang, Yixiong. "Comparative planetary circulation regimes in simple general circulation models." Thesis, University of Oxford, 2014. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.669741.
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This thesis presents the studies of terrestrial planetary atmospheric circula- tion regimes using simplified GCMs with different levels of complexity. Two different versions of the simplified GCM PUMA (Portable University Model for the Atmosphere) are used — PUMA-S with Newtonian cooling scheme and PUMA-G with a semi-grey two-band radiative transfer scheme and dry convective adjustment. A series of controlled experiments are conducted by varying planetary rotation rate and imposed equator-to-pole temperature dif- ference using PUMAS-S, and by varying rotation rate, planetary obliquity, and the ratio of optical depth in long-wave (thermal radiation) band to that in short-wave (stellar radiation) band using PUMA-G. These defining parame- ters are further combined with each other into dimensionless forms to establish parameter spaces, in which the occurences of different circulation regimes are mapped and classified. For the PUMA-S experiments, very coherent trends when varying planetary rotation rate (thermal Rossby number) is found. It is demonstrated that the GRW mechanism is mainly responsible for the equato- rial super-rotation observed in our experiments. Regular baroclinic waves are obtained at intermediate values of thermal Rossby number and depend strongly on the strength of radiative and frictional damping. Global atmospheric ener- getics in terms of Lorenz energy cycle and meridional heat transport efficiency also exhibits strong dependence on planetary rotation rate from our PUMA-S experiments. Theories of geostrophic turbulence (especially the recently in- troduced zonostrophic turbulence) and jet formation are examined using the PUMA-S experiments. For the PUMA-G experiments, Similar trends are ob- served with respect to varying planetary rotation rate, while new regimes like strongly subrotating atmospheres are found when varying obliquity in PUMA- G. Tidally-locked planets are also studied by modifying the incoming stellar irradiation in PUMA-G. It is found that atmospheric optical depth in the long- wave band plays an important role in setting the heat transport efficiency from day-side to night-side. These results provide significant insights into the ter- restrial planetary atmospheric circulation dynamics and the inference of circu- lation regimes of extrasolar planets. Future studies will focus on the effect of seasonal/diurnal cycle, the parametrisation of eddy heat transport efficiency, as well as the modification of the two-band semi-grey radiative transfer scheme to incorporate pressure broadening effects
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Jin, Jiming. "A physically-based snow model coupled to a general circulation model for hydro-climatological studies." Diss., The University of Arizona, 2002. http://hdl.handle.net/10150/289782.
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A Snow-Atmosphere-Soil Transfer (SAST) model has been developed to extend the point snowmelt model to vegetated areas using the parameterization concepts of the Biosphere-Atmosphere Transfer Scheme (Dickinson et al. 1993). The model applications for short-grass and forest fields show that the simulated surface temperature, albedo, and snow depth have close agreement with observations. In addition, because of biases in simulated runoff in the high-latitudes, a Shuffled Complex Evolution (Sorooshian et al. 1993) scheme for automatic calibration has been connected with the SAST model to determine the realistic distribution of runoff components from different soil layers and search the optimized parameter set. The calibrated runoff closely matches observations. Because the Community Climate Model version 3 (CCM3) coupled with the SAST model overestimates snow depth and precipitation and underestimates surface temperature over the Rocky Mountains, remotely sensed snow depth data have been assimilated in the model to alleviate model discrepancies based on energy and mass balances. The improved surface temperature simulations result from the decreased snowmelt and albedo in winter and spring and from the weakened evaporation in summer due to drier soil. Meanwhile, modeled summer precipitation over the Rocky Mountains has a minor improvement. The relationship between the variations of tropical Pacific SST and snowpack anomalies in the western United States (U.S.) has been studied by comparing observations and CCM3 output. The results indicate that in the northwestern U.S., the warm tropical Pacific phase of the El Nino-Southern Oscillation (ENSO) is associated with diminished snowpack while its cool phase is related to enhanced snowpack. This relationship is largely determined by winter precipitation variability for the observations; however, it relies heavily on the variations of temperature due to the biases in atmospheric patterns for the model output. In the southwestern U.S., positive snowpack anomalies for both observations and simulations result from the strong warm phase of the ENSO and negative ones are connected with exaggerated local precipitation in fall.
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Gong, Gavin 1969. "The role of fractional rainfall coverage in atmospheric general circulation model hydrologic processes." Thesis, Massachusetts Institute of Technology, 1994. http://hdl.handle.net/1721.1/35428.
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Liu, Fei. "A sensitivity study of a general circulation model with enhanced shortwave atmospheric absorption." Available to US Hopkins community, 2002. http://wwwlib.umi.com/dissertations/dlnow/3068182.
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Santer, B. D. "Regional validation of General Circulation Models." Thesis, University of East Anglia, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.383549.
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Won, Eric C. "Sensitivity of a general circulation inverse model to sub-grid scale parametrization coefficients." Thesis, Massachusetts Institute of Technology, 1994. http://hdl.handle.net/1721.1/58432.
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Phlips, P. J. "Models of the heat-induced circulation in the tropical atmosphere." Thesis, University of Oxford, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.371572.
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Ring, Michael J. 1979. "Annular mode-like responses to external forcings in a simple atmospheric general circulation model." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/43152.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2008.Includes bibliographical references (p. 217-225).
In this thesis, I investigate the response of a simple atmospheric general circulation model to applied forcings to learn whether the annular mode patterns are a preferred model response to the forcings. The thesis is inspired by the appearance of annular mode patterns in Earth's atmosphere in response to a number of forcings. Climatologies of the model under the influence of applied torques or perturbations to the reference temperature profile are compiled and compared to a control run with neither type of forcing. In most cases the differences in climatologies are annular mode-like, suggesting the patterns are the preferred response of the model to the forcings. The strength of the response typically increases for either an increase in the strength of the forcing, or an increase in the strength of the projection of the forcing on the model's annular mode patterns. Trials with a response which was not annular mode-like usually featured a poor projection of the forcing on the annular modes, or substantial interference with tropical dynamics. A zonally symmetric version of the model is also used to test the direct response of the model to the forcing versus the response caused by changes in eddy feedback processes. The direct forcing alone is found to be insufficient to produce either the correct strength or shape of the annular mode patterns. Instead the changes in eddy fluxes must be included to produce the correct shape and amplitude of the anomalies.
by Michael John Ring.
Ph.D.
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Braine-Bonnaire, Thierry. "Etude de la variabilite intrasaisonniere de l'atmosphere tropicale simulee par un modele de circulation generale." Paris 6, 1989. http://www.theses.fr/1989PA066563.
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Plusieurs experiences annuelles realisees en cycle saisonnier avec le modele de circulation generale du laboratoire de meteorologie dynamique, utilisant divers types de parametrisations physiques et de temperature superficielle de l'ocean sont analysees du point de vue de leur variabilite basse-frequence. L'accent est mis sur la mesure de la capacite du modele a generer l'onde de madden et julian decrite par de nombreuses analyses de donnees meteorologiques de la bande inter-tropicale et sur la sensibilite de cette aptitude d'une experience a l'autre. On note qu'il apparait une oscillation comparable, par bien des aspects, a celle qu'on observe; neanmoins son extraction reclame l'utilisation d'outils numeriques tres selectifs qui sont decrits. Cette oscillation, qui s'apparente a une onde de kelvin epousant, horizontalement le premier mode zonal et, verticalement, le premier mode barocline, apparait clairement dans les variables de vent et de potentiel de vitesse mais est absente des variables physiques de precipitations, d'evaporation et de rayonnement infra-rouge sortant. Ces resultats se retrouvent assez fidelement d'une experience a l'autre. L'onde semble etre alimentee energetiquement par un chauffage convectif moyen plus que modulee par ses inhomogeneites spatiales. L'onde simulee ne presente pas de relation de phase particuliere avec les divers etats de l'ecoulement moyen et en particulier avec les periodes de pause et d'activation de la mousson indienne d'eta. Ce resultat hypotheque serieusement les possibilites d'utiliser cette onde de madden et julian creee par le modele pour une prevision a long terme des fluctuations basse-frequence de tel ou tel systeme tropical que les observations presentent pourtant comme module par le passage de cette oscillation
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Saito, Ryu [Verfasser]. "Influence of the surface on the atmospheric circulation of Mars : study with a general circulation model / von Ryu Saito." Katlenburg-Lindau : Copernicus GmbH, 2007. http://d-nb.info/994484453/34.
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Gough, William A. (William Arthur). "Lateral and isopycnal mixing of passive and active tracers in an ocean general circulation model." Thesis, McGill University, 1991. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=70297.
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The parameterization of isopycnal mixing is examined in an ocean general circulation model. The results are compared to those obtained with lateral mixing.In the equilibrium experiments, the isopycnal case has more gyre kinetic energy, a less intense thermohaline circulation, and less interior downwelling than the lateral case. Convection is replaced by enhanced vertical diffusion in the isopycnal case.
In the time dependent passive tracer experiments, the isopycnal case has smaller depth penetration of a surface released tracer. This is likely due to induced recirculation rather than numerical limitations.
The active tracer experiments examine the long term asymmetric behaviour of warm and cold surface anomalies introduced in an abrupt and gradual fashion for the lateral and isopycnal models. The thermal anomalies produce asymmetric transient responses. The abrupt and gradual changes produce the same equilibrium but different transient responses. The isopycnal case responds more rapidly and energetically than the lateral case.
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Medeiros, Brian Pacheco. "Cloud-climate interactions in general circulation models." Diss., Restricted to subscribing institutions, 2007. http://proquest.umi.com/pqdweb?did=1428862971&sid=1&Fmt=2&clientId=1564&RQT=309&VName=PQD.
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Cosme, Emmanuel. "Cycle du soufre des moyennes et hautes latitudes Sud dans un modèle de circulation générale atmosphérique." Phd thesis, Université Joseph Fourier (Grenoble), 2002. http://tel.archives-ouvertes.fr/tel-00705168.
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L'objectif de ce travail est de contribuer à la compréhension du cycle atmosphérique du soufre en Antarctique par l'utilisation d'un Modèle de Circulation Générale Atmosphérique (MCGA). Les versions "Antarctique" et "soufre" du MCGA LMD-ZT ("Laboratoire de Météorologie Dynamique- Zoom Traceurs") ont été fusionnées pour l'étude à haute résolution Antarctique du cycle du soufre dans les moyennes et hautes latitudes Sud. Une méthode de forçage "latéral" de la circulation atmosphérique antarctique par des analyses du Centre Européen de Prévision Météorologique à Moyen Terme (CEPMMT) a été spécifiquement développée et appliquée. Le modèle a d'abord été évalué. Il représente correctement le cycle saisonnier des espéces soufrées aux sites d'observation en Antarctique. Plusieurs défauts ont cependant été identifiés , discutés, et certains ont été étudiés par des expériences numériques spécifiques. Ceci nous a permis, dans la suite de ce travail, de décrire de manière critique le cycle du soufre en Antarctique à partir des résultats du modèle. Le modèle a été utilisé pour trois applications. La première a été une mise en oeuvre directe du modèle, tel qu'il a été évalué, dans le but d'estimer les distributions spatiales, le cycle saisonnier dans les régions centrales, et le bilan annuel des espèces soufrées en Antarctique. La deuxième se présente sous forme d'études de sensibilité à la formulation des émissions de sulfure de diméthyle (DMS) océanique, que la phase d'évaluation a révélé comme déterminante pour la modélisation du cycle du soufre. Pour la troisième, l'adjoint du modèle de transport (qui, en première approximation, permet de remonter le temps), complété par l'adjoint du module chimique spécifiquement développé pour ce travail, a été mis en oeuvre pour une recherche quantitative de l'origine géographlque et de l'âge des espèces soufrées en Antarctique.
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Kiss, Andrew Elek. "Dynamics of laboratory models of the wind-driven ocean circulation." View thesis entry in Australian Digital Theses Program, 2000. http://thesis.anu.edu.au/public/adt-ANU20011018.115707/index.html.
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Chakraborty, Arindam. "Impact of Orography on the Simulation of Monsoon Climate in a General Circulation Model." Thesis, Indian Institute of Science, 2004. http://hdl.handle.net/2005/76.
Full textAbstract:
Orography plays a major role in the general circulation and climate of the tropics. Although many works have been done on the impact of global orography on summer monsoon, the previous studies have examined the impact on seasonal mean scale or only during the first half of the season. Role of orography on intra-seasonal variability has not been addressed previously. Also, the proximate and remote impacts of orography have not been studied. In this thesis an atmospheric General Circulation Model (GCM) has been used to investigate the impact of global and regional orography on monsoon climate. Two different cumulus schemes have been used to study the sensitivity of the results to the cumulus parameterization scheme. The model was forced with seasonally varying sea surface temperature (SST) for the year 1998. An ensemble simulation of 5 members were performed for each experiment.
The simulations showed that the removal of Himalayas or orography over the entire earth caused a delay of about one month in the onset of the monsoon. The delay in monsoon onset was on account of a more stable atmosphere due to intrusion of mid-latitude cold air into the Indian region in the absence of Himalayas. After the onset, the precipitation rate was comparable in control and no-mountain simulations. The seasonal mean (June-September) precipitation over this region decreased by 25% in the no-mountain case as compared to control.
A comparison of the impact of east and west Himalaya orography showed that orography west of 80E has more impact on the phase and intensity of summer monsoon precipitation over the Indian region than orography east of 80E. The onset of summer monsoon over the Indian region was delayed by about one month with the removal of Himalaya orography west of 80E, but was delayed by just about one week with the removal of Himalaya orography east of 80E. This is because, the cold air intrusion was more when Himalaya orography west of 80E was removed. Seasonal mean precipitation decreased by 22% and 12% with the removal of orography west and east of 80E respectively. Himalaya orography east of 80E showed more influence on precipitation over the north-east Indian region and East Asia.
The removal of orography from the African continent increased the summer monsoon precipitation over the Indian region. This was on account of an increase in the zonal mass flux from the African continent in the absence of East African mountains. This mass flux brings more moisture into the south Asian region and increases precipitation over the Indian region and Bay of Bengal. A higher precipitation over the Bay of Bengal leads to higher wind over the Somalia coast and this acts as a positive feedback to enhance the summer monsoon precipitation by about 28% over the Indian region.
The presence of orography only over the African continent resulted in the largest delay in the monsoon onset (by 50 days) and the lowest amount of seasonal precipitation (decrease by 36%) over the Indian region among all the simulations. This is due to further reduction in zonal mass (and hence, moisture) flux toward the Indian subcontinent with the inclusion of African orography when compared with no-global orography simulation.
The seasonal mean precipitation decreased by 19% over the Indian region with the removal of American orography. The onset of monsoon was delayed by about 3 weeks in this experiment as compared to control. This delay was due to a relative downward motion in the upper troposphere on account of the shift of the Rossby wave with the removal of American mountains.
In this thesis, a new theory has been proposed for monsoon onset based on thermodynamic conditioning (necessary condition) and mechanical trigger (sufficient condition) of the atmosphere. This theory was able to explain the large variation in monsoon onset dates (maximum spread 57 days) in different simulations.
The low level circulation was affected more by Himalaya orography west of 80E, which had a profound influence on precipitation over the Indian region. However, upper level circulation was affected more by Himalaya orography east of 80 E. The northward shift of the upper tropospheric westerly jet during the Northern Hemispheric summer was sudden in presence of the Tibetan Plateau and gradual in its absence. This shift was not related to the onset of monsoon over the Indian region.
Northward propagation of convection was found to be present even in the absence of global orography. But northward extent of this propagation was delayed without orography on account of the absence of a favorite meridional gradient of moist static energy in the lower troposphere in the early summer season due to intrusion of mid-latitude cold air. Space-time spectral analysis showed that the intensity of eastward moving convectively coupled atmospheric waves, known as Madden-Julian oscillation (MJO), decreased in absence of global orography. Moreover, the presence of orography favor the higher zonal wave number for MJO propagation.