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Liying Qian - One of the best experts on this subject based on the ideXlab platform.

  • solar extreme ultraviolet irradiance for General Circulation Models
    Journal of Geophysical Research, 2005
    Co-Authors: Stanley C Solomon, Liying Qian
    Abstract:

    [1] Recent measurements of the solar extreme-ultraviolet spectrum provide high-resolution spectral irradiance that can be used for calculating ionization and dissociation rates in the upper atmosphere and for providing improved proxy-based Models of the solar spectrum. These are crucial inputs for global time-dependent General Circulation Models of the thermosphere and ionosphere, but computational economies require that a lower-resolution spectrum be used in the calculations without excessive loss of accuracy. The problem is compounded by the photoelectrons generated by ionization, which cause further ionization and dissociation of atmospheric gases. We describe a method for using solar spectral measurements or Models to calculate ionization and dissociation rates throughout the upper atmosphere, including photoelectron effects, that is more accurate and more efficient than its predecessors. Examples of use with measurements from the Solar EUV Experiment on the TIMED satellite and with the EUVAC model are given, and an example calculation using the National Center for Atmospheric Research thermosphere-ionosphere-electrodynamics General Circulation model is shown.

  • Solar extreme‐ultraviolet irradiance for General Circulation Models
    Journal of Geophysical Research, 2005
    Co-Authors: Stanley C Solomon, Liying Qian
    Abstract:

    [1] Recent measurements of the solar extreme-ultraviolet spectrum provide high-resolution spectral irradiance that can be used for calculating ionization and dissociation rates in the upper atmosphere and for providing improved proxy-based Models of the solar spectrum. These are crucial inputs for global time-dependent General Circulation Models of the thermosphere and ionosphere, but computational economies require that a lower-resolution spectrum be used in the calculations without excessive loss of accuracy. The problem is compounded by the photoelectrons generated by ionization, which cause further ionization and dissociation of atmospheric gases. We describe a method for using solar spectral measurements or Models to calculate ionization and dissociation rates throughout the upper atmosphere, including photoelectron effects, that is more accurate and more efficient than its predecessors. Examples of use with measurements from the Solar EUV Experiment on the TIMED satellite and with the EUVAC model are given, and an example calculation using the National Center for Atmospheric Research thermosphere-ionosphere-electrodynamics General Circulation model is shown.

Judah Cohen - One of the best experts on this subject based on the ideXlab platform.

  • investigating the ability of General Circulation Models to capture the effects of eurasian snow cover on winter climate
    Journal of Geophysical Research, 2008
    Co-Authors: Steven C Hardiman, Paul J Kushner, Judah Cohen
    Abstract:

    [1] The ability of General Circulation Models (GCMs) to reproduce the observed strong correlations of Eurasian snow extent in the fall to wave activity and Northern Annular Mode anomalies in the following winter is studied. The observed correlations have been hypothesized to involve two parts: a Rossby wave pulse generated in the troposphere in response to snow-forced surface cooling and a coupled zonal-mean stratosphere-troposphere response to this Rossby wave pulse involving eddy mean flow interactions. It is found that all coupled ocean atmosphere GCMs used within the Coupled Model Intercomparison Project 3 (CMIP3) fail to capture the observed correlations. Using the CMIP3 GCMs and two versions of a particular GCM forced by prescribed sea surface temperatures, possible reasons for this are considered. The snow forcing, as represented in the spatial extent and interannual variability of snow cover area, is found to be reasonable although somewhat weak in the GCMs, as is the relationship between snow cover and the zonal-mean Circulation. However, the anomaly of eddy geopotential height associated with Eurasian snow cover anomalies is found to be too localized longitudinally in the GCMs. It is proposed that the reduced longitudinal scale of the snow-forced Rossby wave pulse prevents it from propagating into the stratosphere, thus inhibiting the observed wave-driven stratosphere-troposphere response to the pulse.

  • Snow-mass intercomparisons in the boreal forests from General Circulation Models and remotely sensed data sets
    Polar Record, 1996
    Co-Authors: James L. Foster, Helga Behr, Lydia Dumenil, Richard Essery, Glen E. Liston, Randy Koster, David Pollard, Judah Cohen, Starley L. Thompson, Diana Verseghy
    Abstract:

    ABSTRACTIn much of the boreal forests, snow covers the ground for half of the year. Since these boreal forests comprise approximately 15% of the land normally covered by snow during the winter and upwards of 40% of the land surface normally snow-covered during the spring and autumn, reliable measures of snow cover and snow mass are required for improved energy-balance and water-balance estimates. In this study, results from snow-depth climatological data (SDC), passive microwave satellite data, and output from General Circulation Models (GCMs) have been intercompared for the boreal forests of both North America and Eurasia. In Eurasia, during the winter months, snowmass estimates from these data sets correspond rather well; however, in North America, the passive microwave estimates are smaller than the estimates from the climatological data and the modeled data. The underestimation results primarily from the effects of vegetation on the microwave signal. The reason why the underestimation is a bigger problem in North America than in Eurasia is likely due to the use of global microwave algorithms that have not accounted for regional differences in the size of snow grains. The GCMs Generally produce too much snow in the spring season. This is a result of the Models having moisture amounts that are greater and temperatures that are slightly lower than observed, in the late winter and early spring periods. The Models compare more favorably with the SDC in the Eurasian boreal forest than in the forests of North America during the winter season. However, in the spring, the model results for the North America boreal forest are in better agreement with the SDC than are the forests of Eurasia.

  • Snow cover and snow mass intercomparisons of General Circulation Models and remotely sensed datasets
    Journal of Climate, 1996
    Co-Authors: James L. Foster, Helga Behr, Lydia Dumenil, Starly Thompson, Diana Verseghy, Richard Essery, Glen E. Liston, Randy Koster, David Pollard, Judah Cohen
    Abstract:

    Abstract Confirmation of the ability of General Circulation Models (GCMs) to accurately represent snow cover and snow mass distributions is vital for climate studies. There must be a high degree of confidence that what is being predicted by the Models is reliable, since realistic results cannot be assured unless they are tested against results from observed data or other available datasets. In this study, snow output from seven GCMs and passive-microwave snow data derived from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) are intercompared. National Oceanic and Atmospheric Administration satellite data are used as the standard of reference for snow extent observations and the U.S. Air Force snow depth climatology is used as the standard for snow mass. The reliability of the SMMR snow data needs to be verified, as well, because currently this is the only available dataset that allows for yearly and monthly variations in snow depth. [The GCMs employed in this investigation are the United Ki...

Minghua Zhang - One of the best experts on this subject based on the ideXlab platform.

  • the cgils experimental design to investigate low cloud feedbacks in General Circulation Models by using single column and large eddy simulation Models
    Journal of Advances in Modeling Earth Systems, 2012
    Co-Authors: Minghua Zhang, Christopher S Bretherton, Sandrine Bony, Peter N Blossey, Florent Brient, Jean-christophe Golaz
    Abstract:

    [1] A surrogate climate change is designed to investigate low cloud feedbacks in the northeastern Pacific by using Single Column Models (SCMs), Cloud Resolving Models (CRMs), and Large Eddy Simulation Models (LES), as part of the CGILS study (CFMIP-GASS Intercomparison of LES and SCM Models). The constructed large-scale forcing fields, including subsidence and advective tendencies, and their perturbations in the warmer climate are shown to compare well with conditions in General Circulation Models (GCMs), but they are free from the impact of any GCM parameterizations. The forcing fields in the control climate are also shown to resemble the mean conditions in the ECMWF-Interim Reanalysis. Applications of the forcing fields in SCMs are presented. It is shown that the idealized design can offer considerable insight into the mechanisms of cloud feedbacks in the Models. Caveats and advantages of the design are also discussed.

  • toward understanding the double intertropical convergence zone pathology in coupled ocean atmosphere General Circulation Models
    Journal of Geophysical Research, 2007
    Co-Authors: Xuehong Zhang, Minghua Zhang
    Abstract:

    [1] This paper first analyzes structures of the double Intertropical Convergence Zone (ITCZ) in the central equatorial Pacific simulated by three coupled ocean-atmosphere General Circulation Models in terms of sea surface temperatures, surface precipitation, and surface winds. It then describes the projection of the double ITCZ in the equatorial upper ocean. It is shown that the surface wind convergences, associated with the zonally oriented double rainbands on both sides of the equator, also correspond to surface wind curls that are favorable to Ekman pumping immediately poleward of the rainbands. The pumping results in a thermocline ridge south of the equator in the central equatorial Pacific, causing a significant overestimation of the eastward South Equatorial Counter Current that advects warm water eastward. A positive feedback mechanism is then described for the amplification of the double ITCZ in the coupled Models from initial biases in stand-alone atmospheric Models through the following chain of interactions: precipitation (atmospheric latent heating), surface wind convergences, surface wind curls, Ekman pumping, South Equatorial Counter Current, and eastward advection of ocean temperature. This pathology provides a possible means to address the longstanding double ITCZ problem in coupled Models.

  • comparing clouds and their seasonal variations in 10 atmospheric General Circulation Models with satellite measurements
    Journal of Geophysical Research, 2005
    Co-Authors: Minghua Zhang, A D Del Genio, Richard T Cederwall, James J Hack, S A Klein, Julio T Bacmeister, Sandrine Bony, Norman G Loeb, Ulrike Lohmann
    Abstract:

    [1] To assess the current status of climate Models in simulating clouds, basic cloud climatologies from ten atmospheric General Circulation Models are compared with satellite measurements from the International Satellite Cloud Climatology Project (ISCCP) and the Clouds and Earth's Radiant Energy System (CERES) program. An ISCCP simulator is employed in all Models to facilitate the comparison. Models simulated a four-fold difference in high-top clouds. There are also, however, large uncertainties in satellite high thin clouds to effectively constrain the Models. The majority of Models only simulated 30–40% of middle-top clouds in the ISCCP and CERES data sets. Half of the Models underestimated low clouds, while none overestimated them at a statistically significant level. When stratified in the optical thickness ranges, the majority of the Models simulated optically thick clouds more than twice the satellite observations. Most Models, however, underestimated optically intermediate and thin clouds. Compensations of these clouds biases are used to explain the simulated longwave and shortwave cloud radiative forcing at the top of the atmosphere. Seasonal sensitivities of clouds are also analyzed to compare with observations. Models are shown to simulate seasonal variations better for high clouds than for low clouds. Latitudinal distribution of the seasonal variations correlate with satellite measurements at >0.9, 0.6–0.9, and −0.2–0.7 levels for high, middle, and low clouds, respectively. The seasonal sensitivities of cloud types are found to strongly depend on the basic cloud climatology in the Models. Models that systematically underestimate middle clouds also underestimate seasonal variations, while those that overestimate optically thick clouds also overestimate their seasonal sensitivities. Possible causes of the systematic cloud biases in the Models are discussed.

  • cloud feedback in atmospheric General Circulation Models an update
    Journal of Geophysical Research, 1996
    Co-Authors: Robert D Cess, Minghua Zhang, W J Ingram, G L Potter, V Alekseev, Howard W Barker, E Cohensolal, R Colman, D A Dazlich, A D Del Genio
    Abstract:

    Six years ago, we compared the climate sensitivity of 19 atmospheric General Circulation Models and found a roughly threefold variation among the Models; most of this variation was attributed to differences in the Models' depictions of cloud feedback. In an update of this comparison, current Models showed considerably smaller differences in net cloud feedback, with most producing modest values. There are, however, substantial differences in the feedback components, indicating that the Models still have physical disagreements.

  • diagnostic study of climate feedback processes in atmospheric General Circulation Models
    Journal of Geophysical Research, 1994
    Co-Authors: Minghua Zhang, J T Kiehl, James J Hack, Robert D Cess
    Abstract:

    A method is proposed to diagnose climate feedbacks of water vapor, temperature lapse-rate, and cloud variations in atmospheric General Circulation Models. It is then applied to study differences in sensitivity of the National Center for Atmospheric Research community climate model (CCM2) and two hybrid versions of CCM2 with different cumulus-convection schemes. Water vapor feedback and temperature lapse-rate feedback differ among the Models due to different efficiencies of heat and moisture transport by cumulus convections. A large compensation occurs between water vapor feedback and temperature lapse-rate feedback. This leads to similar clear-sky sensitivities in the Models. Cloud-radiative feedback is negative in CCM2 with a ΔSST climate change due to the vigorous cumulus-convective scheme. Stronger convection warms the upper troposphere and reduces its cloudiness more, resulting in negative longwave cloud-radiative feedback. In Models where a moist-adiabatic-adjustment scheme and then a decoupling of the atmospheric boundary layer are subsequently used, intensity of cumulus convection is successively reduced and cloud-radiative feedback changes to either neutral or positive.

Stanley C Solomon - One of the best experts on this subject based on the ideXlab platform.

  • solar extreme ultraviolet irradiance for General Circulation Models
    Journal of Geophysical Research, 2005
    Co-Authors: Stanley C Solomon, Liying Qian
    Abstract:

    [1] Recent measurements of the solar extreme-ultraviolet spectrum provide high-resolution spectral irradiance that can be used for calculating ionization and dissociation rates in the upper atmosphere and for providing improved proxy-based Models of the solar spectrum. These are crucial inputs for global time-dependent General Circulation Models of the thermosphere and ionosphere, but computational economies require that a lower-resolution spectrum be used in the calculations without excessive loss of accuracy. The problem is compounded by the photoelectrons generated by ionization, which cause further ionization and dissociation of atmospheric gases. We describe a method for using solar spectral measurements or Models to calculate ionization and dissociation rates throughout the upper atmosphere, including photoelectron effects, that is more accurate and more efficient than its predecessors. Examples of use with measurements from the Solar EUV Experiment on the TIMED satellite and with the EUVAC model are given, and an example calculation using the National Center for Atmospheric Research thermosphere-ionosphere-electrodynamics General Circulation model is shown.

  • Solar extreme‐ultraviolet irradiance for General Circulation Models
    Journal of Geophysical Research, 2005
    Co-Authors: Stanley C Solomon, Liying Qian
    Abstract:

    [1] Recent measurements of the solar extreme-ultraviolet spectrum provide high-resolution spectral irradiance that can be used for calculating ionization and dissociation rates in the upper atmosphere and for providing improved proxy-based Models of the solar spectrum. These are crucial inputs for global time-dependent General Circulation Models of the thermosphere and ionosphere, but computational economies require that a lower-resolution spectrum be used in the calculations without excessive loss of accuracy. The problem is compounded by the photoelectrons generated by ionization, which cause further ionization and dissociation of atmospheric gases. We describe a method for using solar spectral measurements or Models to calculate ionization and dissociation rates throughout the upper atmosphere, including photoelectron effects, that is more accurate and more efficient than its predecessors. Examples of use with measurements from the Solar EUV Experiment on the TIMED satellite and with the EUVAC model are given, and an example calculation using the National Center for Atmospheric Research thermosphere-ionosphere-electrodynamics General Circulation model is shown.

Robert D Cess - One of the best experts on this subject based on the ideXlab platform.

  • cloud feedback in atmospheric General Circulation Models an update
    Journal of Geophysical Research, 1996
    Co-Authors: Robert D Cess, Minghua Zhang, W J Ingram, G L Potter, V Alekseev, Howard W Barker, E Cohensolal, R Colman, D A Dazlich, A D Del Genio
    Abstract:

    Six years ago, we compared the climate sensitivity of 19 atmospheric General Circulation Models and found a roughly threefold variation among the Models; most of this variation was attributed to differences in the Models' depictions of cloud feedback. In an update of this comparison, current Models showed considerably smaller differences in net cloud feedback, with most producing modest values. There are, however, substantial differences in the feedback components, indicating that the Models still have physical disagreements.

  • diagnostic study of climate feedback processes in atmospheric General Circulation Models
    Journal of Geophysical Research, 1994
    Co-Authors: Minghua Zhang, J T Kiehl, James J Hack, Robert D Cess
    Abstract:

    A method is proposed to diagnose climate feedbacks of water vapor, temperature lapse-rate, and cloud variations in atmospheric General Circulation Models. It is then applied to study differences in sensitivity of the National Center for Atmospheric Research community climate model (CCM2) and two hybrid versions of CCM2 with different cumulus-convection schemes. Water vapor feedback and temperature lapse-rate feedback differ among the Models due to different efficiencies of heat and moisture transport by cumulus convections. A large compensation occurs between water vapor feedback and temperature lapse-rate feedback. This leads to similar clear-sky sensitivities in the Models. Cloud-radiative feedback is negative in CCM2 with a ΔSST climate change due to the vigorous cumulus-convective scheme. Stronger convection warms the upper troposphere and reduces its cloudiness more, resulting in negative longwave cloud-radiative feedback. In Models where a moist-adiabatic-adjustment scheme and then a decoupling of the atmospheric boundary layer are subsequently used, intensity of cumulus convection is successively reduced and cloud-radiative feedback changes to either neutral or positive.

  • uncertainties in carbon dioxide radiative forcing in atmospheric General Circulation Models
    Science, 1993
    Co-Authors: Robert D Cess, Minghua Zhang, G L Potter, Howard W Barker, R Colman, D A Dazlich, A D Del Genio, Monika Esch, J R Fraser, V Galin
    Abstract:

    Global warming, caused by an increase in the concentrations of greenhouse gases, is the direct result of greenhouse gas-induced radiative forcing. When a doubling of atmospheric carbon dioxide is considered, this forcing differed substantially among 15 atmospheric General Circulation Models. Although there are several potential causes, the largest contributor was the carbon dioxide radiation parameterizations of the Models.

  • Interpretation of Snow-Climate Feedback as Produced by 17 General Circulation Models
    Science, 1991
    Co-Authors: Robert D Cess, Minghua Zhang, G L Potter, R Colman, D A Dazlich, J P Blanchet, S. Chalita, A. D. Del Genio, V. P. Dymnikov, V Galin
    Abstract:

    Snow feedback is expected to amplify global warming caused by increasing concentrations of atmospheric greenhouse gases. The conventional explanation is that a warmer Earth will have less snow cover, resulting in a darker planet that absorbs more solar radiation. An intercomparison of 17 General Circulation Models, for which perturbations of sea surface temperature were used as a surrogate climate change, suggests that this explanation is overly simplistic. The results instead indicate that additional amplification or moderation may be caused both by cloud interactions and longwave radiation. One measure of this net effect of snow feedback was found to differ markedly among the 17 climate Models, ranging from weak negative feedback in some Models to strong positive feedback in others.

  • intercomparison and interpretation of climate feedback processes in 19 atmospheric General Circulation Models
    Journal of Geophysical Research, 1990
    Co-Authors: Robert D Cess, G.j. Boer, G L Potter, A D Del Genio, V Galin, J P Blanchet, V. P. Dymnikov, Michel Deque, W L Gates, Steven J. Ghan
    Abstract:

    The need to understand differences among General Circulation model projections of CO2-induced climatic change has motivated the present study, which provides an intercomparison and interpretation of climate feedback processes in 19 atmospheric General Circulation Models. This intercomparison uses sea surface temperature change as a surrogate for climate change. The interpretation of cloud-climate interactions is given special attention. A roughly threefold variation in one measure of global climate sensitivity is found among the 19 Models. The important conclusion is that most of this variation is attributable to differences in the Models' depiction of cloud feedback, a result that emphasizes the need for improvements in the treatment of clouds in these Models if they are ultimately to be used as reliable climate predictors. It is further emphasized that cloud feedback is the consequence of all interacting physical and dynamical processes in a General Circulation model. The result of these processes is to produce changes in temperature, moisture distribution, and clouds which are integrated into the radiative response termed cloud feedback.