48 research outputs found
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Consistent Differences in Climate Feedbacks between Atmosphere–Ocean GCMs and Atmospheric GCMs with Slab-Ocean Models
Climate sensitivity is generally studied using two types of models. Atmosphere-ocean general circulation models (AOGCMs) include interactive ocean dynamics and detailed heat uptake. Atmospheric GCMs (AGCMs) with slab ocean models (SOMs) cannot fully simulate the ocean's response to and influence on climate. However, AGCMs are computationally cheaper and thus are often used to quantify and understand climate feedbacks and sensitivity. Here, physical climate feedbacks are compared between AOGCMs and SOM-AGCMs from the Coupled Model Intercomparison Project phase 3 (CMIP3) using the radiative kernel technique. Both the global-average (positive) water vapor and (negative) lapse-rate feedbacks are consistently stronger in AOGCMs. Water vapor feedback differences result from an essentially constant relative humidity and peak in the tropics, where temperature changes are larger for AOGCMs. Differences in lapse-rate feedbacks extend to midlatitudes and correspond to a larger ratio of tropical- to global-average temperature changes. Global-average surface albedo feedbacks are similar between models types because of a near cancellation of Arctic and Antarctic differences. In AOGCMs, the northern high latitudes warm faster than the southern latitudes, resulting in interhemispheric differences in albedo, water vapor, and lapse-rate feedbacks lacking in the SOM-AGCMs. Meridional heat transport changes also depend on the model type, although there is a large intermodel spread. However, there are no consistent global or zonal differences in cloud feedbacks. Effects of the forcing scenario [Special Report on Emissions Scenarios A1B (SRESa1b) or the 1% CO2 increase per year to doubling (1%to2x) experiments] on feedbacks are model dependent and generally of lesser importance than the model type. Care should be taken when using SOM-AGCMs to understand AOGCM feedback behavior.This is the publisher’s final pdf. The published article is copyrighted by the American Meteorological Society and can be found at: http://www.ametsoc.org/pubs/journals/jcli/.Keywords: Equilibrium, General circulation models, Carbon dioxide, State, Radiative kernel technique, Transient responses, Adjustment, CO2, Sensitivity, Mean respons
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Model-Specific Radiative Kernels for Calculating Cloud and Noncloud Climate Feedbacks
Radiative kernels have become a common tool for evaluating and comparing radiative feedbacks to climate change in different general circulation models. However, kernel feedback calculations are inaccurate for simulations where the atmosphere is significantly perturbed from its base state, such as for very large forcing or perturbed physics simulations. In addition, past analyses have not produced kernels relating to prognostic cloud variables because of strong nonlinearities in their relationship to radiative forcing. A new methodology is presented that allows for fast statistical optimizing of existing kernels such that accuracy is increased for significantly altered climatologies. International Satellite Cloud Climatology Project (ISCCP) simulator output is used to relate changes in cloud-type histograms to radiative fluxes. With minimal additional computation, an individual set of kernels is created for each climate experiment such that climate feedbacks can be reliably estimated even in significantly perturbed climates.
This methodology is applied to successive generations of the Community Atmosphere Model (CAM). Increased climate sensitivity in CAM5 is shown to be due to reduced negative stratus and stratocumulus feedbacks in the tropics and midlatitudes, strong positive stratus feedbacks in the southern oceans, and a strengthened positive longwave cirrus feedback. Results also suggest that CAM5 exhibits a stronger surface albedo feedback than its predecessors, a feature not apparent when using a single kernel. Optimized kernels for CAM5 suggest weaker global-mean shortwave cloud feedback than one would infer from using the original kernels and an adjusted cloud radiative forcing methodology.Keywords: Clouds, Optimization, Cloud retrieval, Statistical techniques, Climate models, Climate sensitivit
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Comparison of Short-Term and Long-Term Radiative Feedbacks and Variability in Twentieth-Century Global Climate Model Simulations
The climate sensitivity uncertainty of global climate models (GCMs) is partly due to the spread of individual feedbacks. One approach to constrain long-term climate sensitivity is to use the relatively short observational record, assuming there exists some relationship in feedbacks between short and long records. The present work tests this assumption by regressing short-term feedback metrics, characterized by the 20-yr feedback as well as interannual and intra-annual metrics, against long-term longwave water vapor, longwave atmospheric temperature, and shortwave surface albedo feedbacks calculated from 13 twentieth-century GCM simulations. Estimates of long-term feedbacks derived from reanalysis observations and statistically significant regressions are consistent with but no more constrained than earlier estimates. For the interannual metric, natural variability contributes to the feedback uncertainty, reducing the ability to estimate the interannual behavior from one 20-yr time slice. For both the interannual and intra-annual metrics, uncertainty in the intermodel relationships between 20-yr metrics and 100-yr feedbacks also contributes to the feedback uncertainty. Because of differences in time scales of feedback processes, relationships between the 20-yr interannual metric and 100-yr water vapor and atmospheric temperature feedbacks are significant for only one feedback calculation method. The intra-annual and surface albedo relationships show more complex behavior, though positive correspondence between Northern Hemisphere surface albedo intra-annual metrics and 100-yr feedbacks is consistent with previous studies. Many relationships between 20-yr metrics and 100-yr feedbacks are sensitive to the specific GCMs included, highlighting that care should be taken when inferring long-term feedbacks from short-term observations.Keywords: Anomalies, Feedback, Seasonal cycle, Climate models, Energy budget/balanc
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Abrupt Transition to Strong Superrotation in an Axisymmetric Model of the Upper Troposphere
Abrupt transitions to strongly superrotating states have been found in some idealized models of the troposphere. These transitions are thought to be caused by feedbacks between the eddy momentum flux convergence in low latitudes and the strength of the equatorial flow. The behavior of an axisymmetric shallow-water model with an applied tropical torque is studied here to determine if an abrupt transition can be realized without eddy feedbacks. The upper-tropospheric layer is relaxed to a radiative equilibrium thickness, exchanging mass and thus momentum with the nonmoving lower layer. For low values of the applied torque, the circulation is earthlike; however, for larger values, an abrupt transition to a strongly superrotating state can occur. In some cases, the system remains superrotating as the torque is subsequently decreased. A simple analytical model is used to better understand the system. The bifurcation is caused by a feedback between the applied torque and the strength of the Hadley cell. As the torque increases, the strength of the cell decreases, reducing the damping caused by momentum transfer from the lower layer
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Using Relative Humidity as a State Variable in Climate Feedback Analysis
An approach to climate change feedback analysis is described in which tropospheric relative humidity replaces specific humidity as the state variable that, along with the temperature structure, surface albedos, and clouds, controls the magnitude of the response of global mean surface temperature to a radiative forcing. Despite being simply a regrouping of terms in the feedback analysis, this alternative perspective has the benefit of removing most of the pervasive cancellation between water and lapse-rate feedbacks seen in models. As a consequence, the individual feedbacks have less scatter than in the traditional formulation. The role of cloud feedbacks in controlling climate sensitivity is also reflected more clearly in the new formulation.Keywords: Climate sensitivity, Climate change, Water vapor, Humidity, Feedbac
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A culture of silence: modes of objectification and the silencing of disabled bodies
Throughout history different practices have attempted to silence the experiences of disabled people. In this paper we explore some of these practices including the medical, familial, and self-subjugating practices English-speaking Canadian polio survivors experienced throughout their lives. We analyze participant’s experiences of silence and silencing through a Foucauldian lens, drawing on the three modes of objectification to explain the institutional and cultural discourses around polio subjects that acted upon and through the polio body to silence it. Participants’ oral history accounts demonstrate how sociocultural and medical practices effectively silenced survivors from speaking about their polio experiences. However, the trope of silence is also uprooted within oral history traditions. We will demonstrate how participants broke their silence and shifted their perspectives on polio and disability, and how this process contributed to their resistance of hegemonic conceptualizations of disability as defective
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Direct radiative effect of mineral dust and volcanic aerosols in a simple aerosol climate model
Airborne mineral dust can influence the climate by altering the radiative properties of the atmosphere, but the magnitude of the effect is uncertain. An idealized global model is developed to study the dust-climate system. The model determines the dust longwave and shortwave direct radiative forcing, as well as the resulting temperature changes, based on the specified dust distribution, height, and optical properties. Comparisons with observations and general circulation results indicate that the model produces realistic results for the present-day dust distribution as well as for volcanic aerosols. Although the model includes many simplifications, it can still provide insight into dust-climate system behavior. Recent observations suggest that dust may absorb less solar radiation than previously thought. Experiments with the model suggest that previous studies which used more absorbing dust may be underestimating the effect of dust. Increasing the solar single scattering albedo value from 0.85 to 0.97, corresponding to recent measurements, more than doubles the modeled global average top-of-the-atmosphere (TOA) shortwave direct forcing for the present-day dust distribution, while the surface shortwave forcing is halved. The corresponding temperature decreases are larger for the larger single scattering albedo, and the latent and sensible heat fluxes decreases are smaller. The dust forcing and climate response are approximately linear with respect to optical depth. However, the relationship depends on the relative magnitudes of shortwave versus longwave TOA forcing. Thus the net TOA forcing alone does not determine the steady state climate response
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Sensitivity of climate forcing and response to dust optical properties in an idealized model
An idealized global climate model is used to explore the response of the climate to a
wide range of dust radiative properties and dust layer heights. The top-of-the-atmosphere
(TOA) shortwave forcing becomes more negative as the broadband shortwave single
scattering albedo increases and the broadband shortwave asymmetry parameter decreases,
but the sensitivity is highly dependent on the location of the dust layer with respect to
clouds. The longwave TOA forcing is most affected by the height of the dust layer. The
net TOA forcing is most sensitive to the shortwave single scattering albedo and shortwave
asymmetry parameter. The surface and atmospheric temperature responses are
approximately linear with respect to the TOA forcing, as opposed to the surface or
atmospheric forcings. Thus the TOA forcing can be used to estimate both the surface and
atmospheric temperature responses to dust. The corresponding changes in latent and
sensible heat fluxes are essential for the close relationship of the surface temperature
response to the TOA forcing. Estimating the hydrological cycle response requires
knowledge of the vertical distribution of dust with respect to clouds or other reflective
particles. The sensitivity of the latent heat flux to variations in the shortwave single
scattering albedo changes sign with dust height. The latent heat flux change becomes less
negative as the shortwave single scattering albedo increases if the dust layer is below
clouds. However, when the dust is above clouds, the latent heat response becomes more
negative as the single scattering albedo increases.Keywords: climate model, mineral dust, hydrological cycleKeywords: climate model, mineral dust, hydrological cycl
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Climate Feedbacks in CCSM3 under Changing CO2 Forcing. Part II: Variation of Climate Feedbacks and Sensitivity with Forcing
Are equilibrium climate sensitivity and the associated radiative feedbacks a constant property of the climate system, or do they change with forcing magnitude and base climate? Using the radiative kernel technique, feedbacks and climate sensitivity are evaluated in a fully coupled general circulation model (GCM) for three successive doublings of carbon dioxide starting from present-day concentrations. Climate sensitivity increases by 23% between the first and third COâ‚‚ doublings. Increases in the positive water vapor and cloud feedbacks are partially balanced by a decrease in the positive surface albedo feedback and an increase in the negative lapse rate feedback. Feedbacks can be decomposed into a radiative flux change and a climate variable response to temperature change. The changes in water vapor and Planck feedbacks are due largely to changes in the radiative response with climate state. Higher concentrations of greenhouse gases and higher temperatures lead to more absorption and emission of longwave radiation. Changes in cloud feedbacks are dominated by the climate response to temperature change, while the lapse rate and albedo feedbacks combine elements of both. Simulations with a slab ocean model (SOM) version of the GCM are used to verify whether an SOM-GCM accurately reproduces the behavior of the fully coupled model. Although feedbacks differ in magnitude between model configurations (with differences as large as those between COâ‚‚ doublings for some feedbacks), changes in feedbacks between COâ‚‚ doublings are consistent in sign and magnitude in the SOM-GCM and the fully coupled model.Keywords: Cloud Feedback, Ocean Model, General circulation model, Radiative kernel techniqu
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Climate Feedbacks in CCSM3 under Changing COâ‚‚ Forcing. Part I: Adapting the Linear Radiative Kernel Technique to Feedback Calculations for a Broad Range of Forcings
Climate feedbacks vary strongly among climate models and continue to represent a major source of uncertainty in estimates of the response of climate to anthropogenic forcings. One method to evaluate feedbacks in global climate models is the radiative kernel technique, which is well suited for model intercomparison studies because of its computational efficiency. However, the usefulness of this technique is predicated on the assumption of linearity between top-of-atmosphere (TOA) radiative fluxes and feedback variables, limiting its application to simulations of small climate perturbations, where nonlinearities can be neglected. This paper presents an extension of the utility of this linear technique to large forcings, using global climate model simulations forced with CO₂ concentrations ranging from 2 to 8 times present-day values. Radiative kernels depend on the model’s radiative transfer algorithm and climate base state. For large warming, kernels based on the present-day climate significantly underestimate longwave TOA flux changes and somewhat overestimate shortwave TOA flux changes. These biases translate to inaccurate feedback estimates. It is shown that a combination of present-day kernels and kernels computed using a large forcing climate base state leads to significant improvement in the approximation of TOA flux changes and increased reliability of feedback estimates. While using present-day kernels results in a climate sensitivity that remains constant, using the new kernels shows that sensitivity increases significantly with each successive doubling of CO₂ concentrations.This is the publisher’s final pdf. The published article is copyrighted by the American Meteorological Society and can be found at: http://journals.ametsoc.org/loi/clim