The Model for Interdisciplinary Research on Climate (MIROC), which is the coupled generalcirculation model used in the K-1 project, consists of five componet models: atmosphere,land, river, sea ice, and ocean. The atmospheric component interacts with the land and seaice components. The air-sea exchange is realized exclusively between the atmosphere andsea ice components, not directly between the atmosphere and ocean components, and theocean component interacts only with the sea ice component. That is, air-sea flux at ice-freegrids is consequently passed to the ocean component without modification, but it is firstpassed to the sea ice component. The river component receives ground runoff water fromthe land component and drains riverine runoff water into the sea ice component. Lakes aredealt with by the sea ice and ocean components.Two MIROC setups of different resolution are used in the K-1 project and are describedherein. The higher resolution setup is referred to as ¡ÈHI¡É and the lower resolution one as¡ÈMID¡É hereafter.
A new cloud parameterization based on prognostic equations for the subgrid-scale fluctuations in temperature and total water content is introduced for global climate models. The proposed scheme, called hybrid prognostic cloud (HPC) parameterization, employs simple probability density functions (PDFs) to the horizontal subgrid-scale inhomogeneity, allowing them to vary in shape in response to small-scale processes such as cumulus detrainment and turbulent mixing. Simple tests indicate that the HPC scheme is highly favorable as compared to a diagnostic scheme in terms of the cloud fraction and cloud water content under either uniform or non-uniform forcing. The relevance of the HPC scheme is investigated by implementing it in an atmospheric component model of the climate model MIROC with a coarse resolution of T42. A comparison of the short-term integrations between the T42 model and a global cloud resolving model (GCRM) reveals that the HPC scheme can reproduce, to a certain degree, the subgrid-scale variance and skewness of temperature and total water content simulated in the GCRM. It is also found that the HPC scheme significantly alters the climatological distributions in cloud cover, precipitation, and moisture, which are all improved from the model using a conventional diagnostic cloud scheme.
A new version of the atmosphere-ocean general circulation model cooperatively produced by the Japanese research community, known as the Model for Interdisciplinary Research on Climate (MIROC), has recently been developed. A century-long control experiment was performed using the new version (MIROC5) with the standard resolution of the T85 atmosphere and 1¡ë ocean models. The climatological mean state and variability are then compared with observations and those in a previous version (MIROC3.2) with two different resolutions (medres, hires), coarser and finer than the resolution of MIROC5.A few aspects of the mean fields in MIROC5 are similar to or slightly worse than MIROC3.2, but otherwise the climatological features are considerably better. In particular, improvements are found in precipitation, zonal mean atmospheric fields, equatorial ocean subsurface fields, and the simulation of El Niño-Southern Oscillation. The difference between MIROC5 and the previous model is larger than that between the two MIROC3.2 versions, indicating a greater effect of updating parameterization schemes on the model climate than increasing the model resolution. The mean cloud property obtained from the sophisticated prognostic schemes in MIROC5 shows good agreement with satellite measurements. MIROC5 reveals an equilibrium climate sensitivity of 2.6 K, which is lower than that in MIROC3.2 by 1 K. This is probably due to the negative feedback of low clouds to the increasing concentration of CO2, which is opposite to that in MIROC3.2.
The results of a T213L250 gravity wave (GW) resolvinggeneral circulation model (GWR-GCM) are usedto constrain the GW source spectra of a non-orographicGW drag parameterization (GWDP) proposed by Hines.In this study, the following two constraints were placedon Hines¡Çs GWDP: 1) the launch level at which GWsource spectra are specified, and 2) the GW-sourcespectra, that is, the seasonally varying geographical andazimuthal distribution of GW momentum flux and horizontalwind amplitude. Considering the importance ofthe lateral propagation of GWs, which is ignored inHines¡Çs GWDP, the GW source spectra are prescribedusing information from the GWR-GCM at 70 hPa, wherethe GWs have already propagated laterally somedistance from their source regions. The GW-sourcespectra have significant geographical variations andanisotropy, reflecting source distribution and the effectsof critical level filtering due to the background flows.Although the effects of the lateral propagation andintermittency of GWs are ignored, a T42L80 chemistrycoupled climate model using the GWDP with the constraintsdeveloped in this study realistically reproducedthe meridional structures of the zonal wind jets in thestratosphere and mesosphere.
An earth system model (MIROC-ESM 2010) isfully described in terms of each model component and theirinteractions. Results for the CMIP5 (Coupled Model IntercomparisonProject phase 5) historical simulation are presentedto demonstrate the model¡Çs performance from severalperspectives: atmosphere, ocean, sea-ice, land-surface,ocean and terrestrial biogeochemistry, and atmosphericchemistry and aerosols. An atmospheric chemistry coupledversion of MIROC-ESM (MIROC-ESM-CHEM 2010) reasonablyreproduces transient variations in surface air temperaturesfor the period 1850-2005, as well as the presentdayclimatology for the zonal-mean zonal winds and temperaturesfrom the surface to the mesosphere. The historicalevolution and global distribution of column ozone and theamount of tropospheric aerosols are reasonably simulated inthe model based on the Representative Concentration Pathways¡Ç(RCP) historical emissions of these precursors. Thesimulated distributions of the terrestrial and marine biogeochemistryparameters agree with recent observations, whichis encouraging to use the model for future global change projections.
This paper introduces an atmospheric general circulation model (AGCM) that better simulates the dynamical and physical processes and is used in the framework of our integrated earth system modeling. In particular, the dynamical and physical processes in the stratosphere are greatly improved compared to the pre-vious version of the climate model. In this study, the top of the AGCM is extended to the mesopause height, and a hybrid ¦Ò-pressure coordinate system, which is suited for the simulation of transport phenomena near the tropopause, is introduced. An improved radiative transfer scheme dramatically decreases the cold bias near the tropical tropopause and the extratropical lower stratosphere. Incorporation of the non-orographic gravity wave drag parameterization with a source function based on the results of a high-resolution AGCM simula-tion allows the model to reproduce a realistic general circulation in the stratosphere and mesosphere.
Analysis of modern and historical observations demonstrates that the temperature of the intermediate-depth (150-900 m) Atlantic water (AW) of the Arctic Ocean has increased in recent decades. The AW warming has been uneven in time; a local 1¡ëC maximum was observed in the mid-1990s, followed by an intervening minimum and an additional warming that culminated in 2007 with temperatures higher than in the 1990s by 0.24¡ëC. Relative to climatology from all data prior to 1999, the most extreme 2007 temperature anomalies of up to 1¡ëC and higher were observed in the Eurasian and Makarov Basins. The AW warming was associated with a substantial (up to 75-90 m) shoaling of the upper AW boundary in the central Arctic Ocean and weakening of the Eurasian Basin upper-ocean stratification. Taken together, these observations suggest that the changes in the Eurasian Basin facilitated greater upward transfer of AW heat to the ocean surface layer. Available limited observations and results from a 1D ocean column model support this surmised upward spread of AW heat through the Eurasian Basin halocline. Experiments with a 3D coupled ice-ocean model in turn suggest a loss of 28-35 cm of ice thickness after 50 yr in response to the 0.5 W m-2 increase in AW ocean heat flux suggested by the 1D model. This amount of thinning is comparable to the 29 cm of ice thickness loss due to local atmospheric thermodynamic forcing estimated from observations of fast-ice thickness decline. The implication is that AW warming helped precondition the polar ice cap for the extreme ice loss observed in recent years.
The impact of a new cumulus parameterization developed in Part I of this paper on climatology in an atmospheric general circulation model (AGCM) is compared with that of the Arakawa-Schubert scheme. The parameterization is characterized by a vertically variable entrainment rate depending on the surrounding environment. Two kinds of formulations on entrainment rate are tested and produce similar results in the AGCM. The results show reduction of precipitation over land and increase over the sea, weakening of the southern side of the double intertropical convergence zone (ITCZ) over the southeastern Pacific, and better representation of the South Pacific convergence zone (SPCZ), all of which are consistent with observations. The population of cumulus congestus is significantly increased, thereby inducing additional heating in the lower troposphere. The diurnal variation over land shows that deep convection tends to be suppressed earlier because of the reduction of convective available potential energy and tropospheric humidity caused by the convective activity itself. An analysis of the daily outputs suggests that a better representation of the cumulus congestus and sensitivity of the scheme to tropospheric humidity are important for the realistic representation of the precipitation over the double ITCZ and SPCZ.
Precipitation reproducibility over the tropical oceans in climate models is examined. Models participating in phase 3 of the Coupled Model Intercomparison Project (CMIP3) and the current (fifth) version Model for Interdisciplinary Research on Climate (MIROC5) developed by the Atmosphere and Ocean Research Institute, National Institute for Environmental Studies, and Research Institute for Global Change (AORI/NIES/RIGC) are analyzed. Scores of a pattern similarity between precipitation in the models and that in observations are evaluated. The low score models (LSMs) overestimate (underestimate) precipitation over large-scale subsidence (ascending) regions compared to the high score models (HSMs). The sensitivity of deep convection to sea surface temperature (SST) and large-scale subsidence is examined; analysis suggests that dynamical suppression of deep convection by the entrainment of environmental dry air over the subsidence region is very weak, and deep convection follows SST closely in LSMs. For example, deep convective activity is identified over the southeastern Pacific in LSMs, which corresponds to the double intertropical convergence zone (ITCZ) problem. It is suggested that the double ITCZ is associated not only with the local SST but also with the precipitation schemes that control deep convection over the entire tropical oceans. The current version, MIROC5, reproduces precipitation distributions significantly better than the older versions. Precipitation in MIROC5 has a weaker correlation with SST and a stronger correlation with environmental humidity than that in LSMs. The realistic representation of entrainment in regions with dynamical suppression is suggested to be a key factor for better reproducibility of precipitation distributions.
The overall performance of general circulation models is often investigated on the basis of the synthesis of a number of scalar performance metrics of individual models that measure the reproducibility of diverse aspects of the climate. Because of physical and dynamic constraints governing the climate, a model¡Çs performance in simulating a certain aspect of the climate is sometimes related closely to that in simulating another aspect, which results in significant intermodel correlation between performance metrics. Numerous metrics and intermodel correlations may cause a problem in understanding the evaluation and synthesizing the metrics. One possible way to alleviate this problem is to group the correlated metrics beforehand. This study attempts to use simple cluster analysis to group 43 performance metrics. Two clustering methods, the K-means and the Ward methods, yield considerably similar clustering results, and several aspects of the results are found to be physically and dynamically reasonable. Furthermore, the intermodel correlation between the cluster averages is considerably lower than that between the metrics. These results suggest that the cluster analysis is helpful in obtaining the appropriate grouping. Applications of the clustering results are also discussed.
The gas absorption process scheme in the broadband radiative transfer code ¡Èmstrn8¡É, which is used to calculate atmospheric radiative transfer efficiently in a general circulation model, is improved. Three major improvements are made. The first is an update of the database of line absorption parameters and the continuum absorption model. The second is a change to the definition of the selection rule for gas absorption used to choose which absorption bands to include. The last is an upgrade of the optimization method used to decrease the number of quadrature points used for numerical integration in the correlated k-distribution approach, thereby realizing higher computational efficiency without losing accuracy. The new radiation package termed ¡ÈmstrnX¡É computes radiation fluxes and heating rates with errors less than 0.6 W/m2 and 0.3 K/day, respectively, through the troposphere and the lower stratosphere for any standard AFGL atmospheres. A serious cold bias problem of an atmospheric general circulation model using the ancestor code ¡Èmstrn8¡É is almost solved by the upgrade to ¡ÈmstrnX¡É.
The performance of several state-of-the-art climate model ensembles, including two multi-model ensembles (MMEs) and four structurally different (perturbed parameter) single model ensembles (SMEs), are investigated for the first time using the rank histogram approach. In this method,¡¡the reliability of a model¡¡ensemble¡¡is¡¡evaluated¡¡from¡¡the point of view of whether the observations can be regarded as being sampled from the ensemble. Our analysis reveals that, in the MMEs, the climate variables we investigated are broadly reliable on the global scale, with a tendency towards overdispersion. On the other hand, in the SMEs, the reliability differs depending on the ensemble and variable field considered. In general, the mean state and historical trend of surface air temperature, and mean state of precipitation are reliable in the SMEs. However, variables such as sea level pressure or top-of-atmosphere clear-sky shortwave radiation do not cover a sufficiently wide range in some. It is not possible to assess whether this is a fundamental feature of SMEs generated with particular model, or a consequence of the algorithm used to select and perturb the values of the parameters. As under-dispersion is a potentially more serious issue when using ensembles to make projections, we recommend the application of rank histograms to assess reliability when designing and running perturbed physics SMEs.
The global distribution, sources, and propagation of atmospheric waves in the equatorial upper troposphere and lower stratosphere were investigated using an atmospheric general circulation model with T106L60 resolution (120-km horizontal and 550-m vertical resolution). The quasibiennial oscillation (QBO) with a period of -1.5-2 years was simulated well without gravity wave drag parameterization. The zonal wave number versus the frequency spectra of simulated precipitation represent realistic signals of convectively coupled equatorial trapped waves (EQWs). The temperature spectra in the stratosphere also indicate dominant signals of EQWs. EQWs with equivalent depths in the range of 8-90 m from the n = -1 mode to n = 2 mode were extracted separately. Each EQW in the stratosphere generally corresponded well with the source of each convectively coupled EQW activity in the troposphere. The propagations of Kelvin waves and n = 0 eastward/westward propagating EQWs are strongly influenced by the Walker circulation and the phase of the QBO. Potential energy associated with EQWs is generally larger in the westerly than in the easterly shear phase of the QBO. EQWs with vertical wavelengths ≤ 7 km contribute up to -30% of total potential energy ≤ 7 km over the equator at an altitude of 20-30 km. Gravity waves generated by cumulus convection with periods ≤ 24 h are clearly visible over areas of Africa, the Amazon, and around Indonesia, and result in localized PE distributions in areas short distances from the source region. Comparisons of the model results and recent satellite observations are discussed.
The roles of equatorial trapped waves (EQWs) and internal inertia-gravity waves in driving the quasi-biennial oscillation (QBO) are investigated using a high-resolution atmospheric general circulation model with T213L256 resolution (60-km horizontal and 300-m vertical resolution) integrated for 3 years. The model, which does not use a gravity-wave drag parameterization, simulates a QBO. Although the simulated QBO has a shorter period than that of the real atmosphere, its amplitudes and structure in the lower stratosphere are fairly realistic. The zonal wavenumber / frequency spectra of simulated outgoing longwave radiation represent realistic signals of convectively coupled EQWs. Clear signals of EQWs are also seen in the stratospheric wind components. In the eastward wind shear of the QBO, eastward EQWs including Kelvin waves contribute up to ~25-50% to the driving of the QBO. The peaks of eastward wave forcing associated with EQWs and internal inertia-gravity waves occur at nearly the same time at the same altitude. On the other hand, westward EQWs contribute up to ~10% to driving the QBO during the weak westward wind phase but make almost zero contribution during the relatively strong westward wind phase. Extratropical Rossby waves propagating into the equatorial region contribute ~10-25%, whereas internal inertia-gravity waves with zonal wavelength ≤~1000 km are the main contributors to the westward wind shear phase of the simulated QBO.
Three-dimensional wave forcing of simulated quasi-biennial oscillation (QBO) is investigated using a high-resolution atmospheric general circulation model with T213L256 resolution (60-km horizontal and 300-m vertical resolution). In both the eastward and westward wind shear phases of the QBO, nearly all Eliassen-Palm flux (EP flux) divergence due to internal inertia-gravity waves (defined as fluctuations with zonal wavenumber ≥12) results from the divergence of the vertical component of the flux. On the other hand, EP flux divergence due to equatorial trapped waves (EQWs) results from both the meridional and vertical components of the flux in regions of strong vertical wind shear. Longitudinal dependence of wave forcing is also investigated by three-dimensional wave activity flux applicable to gravity waves. Near the top of the Walker circulation, strong eastward (westward) wave forcing occurs in the Eastern (Western) Hemisphere due to internal inertia-gravity waves with small horizontal phase speed. In the eastward wind shear zone associated with the QBO, the eastward wave forcing due to internal inertia-gravity waves in the Eastern Hemisphere is much larger than that in the Western Hemisphere, whereas in the westward wind shear zone, westward wave forcing does not vary much in the zonal direction. Zonal variation of wave forcing in the stratosphere results from (i) zonal variation of wave sources, (ii) the vertically sheared zonal winds associated with the Walker circulation, and (iii) the phase of the QBO.
This paper proposes an improved method for converting a fine-resolution flow direction map into a coarse-resolution river network map for use in global river routing models. The proposed method attempts to preserve the river network structure of an original fine-resolution map in the upscaling procedure, as this has not been achieved with previous upscaling methods. We describe an improved method in which a downstream cell can be flexibly located on any cell in the river network map. The improved method preserves the river network structure of the original flow direction map and allows automated construction of river network maps at any resolution. Automated construction of a river network map is helpful for attaching sub-grid topographic information, such as realistic river meanderings and drainage boundaries, onto the upscaled river network map. The advantages of the proposed method are expected to enhance the ability of global river routing models by providing ways to more precisely represent surface water storage and movement.
Formation processes of the hydrographic structures and meridional overturning circulations in the equatorial Pacific are investigated using an ocean general circulation model where a high accuracy tracer advection scheme, the second-order moment scheme, is implemented. It is also addressed why the equatorial thermocline tends to be diffuse in conventional ocean models. The upper equatorial thermocline water is fed by the subtropical cell of the northern hemisphere, which lies over the lower thermocline water fed by the subtropical cell of the southern hemisphere. Additional experiments reveal that even slight numerical diffusion induced by a third-order tracer advection scheme disturbs the thermodynamic balance of the Pacific equatorial thermocline and makes it significantly diffuse. Magnitude of vertical numerical diffusion is of primary importance in properly reproducing the Pacific equatorial thermocline. Smaller vertical diffusion makes the isopycnals around 25.5¦Ò¦È shallower and consequently intensifies the equatorward transport by the lower part of the southern hemisphere subtropical cell. This cold water transport from the southern hemisphere is also important in further tightening the equatorial thermocline, and it is significantly reduced by horizontal numerical diffusion under less accurate advection schemes.