Variation of surface temperature during the last millennium in a simulation with the FGOALS-gl climate system model

A reasonable past millennial climate simulation relies heavily on the specified external forcings, including both natural and anthropogenic forcing agents. In this paper, we examine the surface temperature responses to specified external forcing agents in a millennium-scale transient climate simulation with the fast version of LASG IAP Flexible Global Ocean-Atmosphere-Land System model (FGOALS-gl) developed in the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics (LASG/IAP). The model presents a reasonable performance in comparison with reconstructions of surface temperature. Differentiated from significant changes in the 20th century at the global scale, changes during the natural-forcing-dominant period are mainly manifested in the Northern Hemisphere. Seasonally, modeled significant changes are more pronounced during the wintertime at higher latitudes. This may be a manifestation of polar amplification associated with sea-ice-temperature positive feedback. The climate responses to total external forcings can explain about half of the climate variance during the whole millennium period, especially at decadal timescales. Surface temperature in the Antarctic shows heterogeneous and insignificant changes during the preindustrial period and the climate response to external forcings is undetectable due to the strong internal variability. The model response to specified external forcings is modulated by cloud radiative forcing (CRF). The CRF acts against the fluctuations of external forcings. Effects of clouds are manifested in shortwave radiation by changes in cloud water during the natural-forcing-dominant period, but mainly in longwave radiation by a decrease in cloud amount in the anthropogenic-forcing-dominant period.     

Climate Sensitivity and Feedbacks of a New Coupled Model CAMS-CSM to Idealized CO_2 Forcing: A Comparison with CMIP5 Models

Climate sensitivity and feedbacks are basic and important metrics to a climate system. They determine how large surface air temperature will increase under CO_2 forcing ultimately, which is essential for carbon reduction policies to achieve a specific warming target. In this study, these metrics are analyzed in a climate system model newly developed by the Chinese Academy of Meteorological Sciences(CAMS-CSM) and compared with multi-model results from the Coupled Model Comparison Project phase 5(CMIP5). Based on two idealized CO_2 forcing scenarios, i.e.,abruptly quadrupled CO_2 and CO_2 increasing 1% per year, the equilibrium climate sensitivity(ECS) and transient climate response(TCR) in CAMS-CSM are estimated to be about 2.27 and 1.88 K, respectively. The ECS is near the lower bound of CMIP5 models whereas the TCR is closer to the multi-model ensemble mean(MME) of CMIP5 due to compensation of a relatively low ocean heat uptake(OHU) efficiency. The low ECS is caused by an unusually negative climate feedback in CAMS-CSM, which is attributed to cloud shortwave feedback(λSWCL) over the tropical Indo-Pacific Ocean.The CMIP5 ensemble shows that more negative λSWCL is related to larger increase in low-level(925–700 hPa)cloud over the tropical Indo-Pacific under warming, which can explain about 90% of λSWCL in CAMS-CSM. Static stability of planetary boundary layer in the pre-industrial simulation is a critical factor controlling the low-cloud response and λSWCL across the CMIP5 models and CAMS-CSM. Evidently, weak stability in CAMS-CSM favors lowcloud formation under warming due to increased low-level convergence and relative humidity, with the help of enhanced evaporation from the warming tropical Pacific. Consequently, cloud liquid water increases, amplifying cloud albedo, and eventually contributing to the unusually negative λSWCL and low ECS in CAMS-CSM. Moreover, the OHU may influence climate feedbacks and then the ECS by modulating regional sea surface temperature responses.     

Simulation of the radiative effect of black carbon aerosols and the regional climate responses over China

As part of the development work of the Chinese new regional climate model (RIEMS), the radiative process of black carbon (BC) aerosols has been introduced into the original radiative procedures of RIEMS,and the transport model of BC aerosols has also been established and combined with the RIEMS model.Using the new model system, the distribution of black carbon aerosols and their radiative effect over the China region are investigated. The influences of BC aerosole on the atmospheric radiative transfer and on the air temperature, land surface temperature, and total rainfall are analyzed. It is found that BC aerosols induce a positive radiative forcing at the top of the atmosphere (TOA), which is dominated by shortwave radiative forcing. The maximum radiative forcing occurs in North China in July and in South China in April. At the same time, negative radiative forcing is observed on the surface. Based on the radiative forcing comparison between clear sky and cloudy sky, it is found that cloud can enforce the TOA positive radiative forcing and decrease the negative surface radiative forcing. The responses of the climate system in July to the radiative forcing due to BC aerosols are the decrease in the air temperature in the middle and lower reaches of the Changjiang River and Huaihe area and most areas of South China, and the weak increase or decrease in air temperature over North China. The total rainfall in the middle and lower reaches of the Changjiang River area is increased, but it decreased in North China in July.     

Empirical Estimates of Global Climate Sensitivity： An Assessment of Strategies Using a Coupled GCM

A control integration with the normal solar constant and one with it increased by 2.5% in the National Center for Atmospheric Research （NCAR） coupled atmosphere-ocean Climate System Model were conducted to see how well the actual realized global warming could be predicted just by analysis of the control results. This is a test, within a model context, of proposals that have been advanced to use knowledge of the present day climate to make ＂empirical＂ estimates of global climate sensitivity. The scaling of the top-of-the-atmosphere infrared flux and the planetary albedo as functions of surface temperature was inferred by examining four different temporal and geographical variations of the control simulations. Each of these inferences greatly overestimates the climate sensitivity of the model, largely because of the behavior of the cloud albedo. In each inference the control results suggest that cloudiness and albedo decrease with increasing surface temperature. However, the experiment with the increased solar constant actually has higher albedo and more cloudiness at most latitudes. The increased albedo is a strong negative feedback, and this helps account for the rather weak sensitivity of the climate in the NCAR model. To the extent that these model results apply to the real world, they suggest empirical evaluation of the scaling of global-mean radiative properties with surface temperature in the present day climate provides little useful guidance for estimates of the actual climate sensitivity to global changes.     

Cloud and Radiation Processes Simulated by a Coupled Atmosphere-Ocean Model

Using NCC/IAP T63 coupled atmosphere-ocean general circulation model (AOGCM),two 20-yr integra- tions were processed,and their ability to simulate cloud and radiation was analysed in detail.The results show that the model can simulate the basic distribution of cloud cover,and however,obvious differences still exist compared with ISCCP satellite data and ERA reanalysis data.The simulated cloud cover is less in general,especially the abnormal low values in some regions of ocean.By improving the cloud cover scheme, simulated cloud cover in the eastern Pacific and Atlantic,summer hemisphere's oceans from subtropical to mid-latitude is considerably improved.But in the tropical Indian Ocean and West Pacific the cloud cover difference is still evident,mainly due to the deficiency of high cloud simulation in these regions resulting from deep cumulus convection.In terms of the analysis on radiation and cloud radiative forcing,we find that simulation on long wave radiation is better than short wave radiation.The simulation error of short wave radiation is caused mostly by the simulation difference in short wave radiative forcing,sea ice,and snow cover,and also by not involving aerosol's effect.The simulation error of long wave radiation is mainly resulting from deficiency in simulating cloud cover and underlying surface temperature.Corresponding to improvement of cloud cover,the simulated radiation (especially short wave radiation) in eastern oceans, summer hemisphere's oceans from subtropical to mid-latitude is remarkably improved.This also brings obvious improvement to net radiation in these regions.     

The Impacts of Optical Properties on Radiative Forcing Due to Dust Aerosol

There are large uncertainties in the quantitative assessment of radiative effects due to atmospheric dust aerosol. The optical properties contribute much to those uncertainties. The authors perform several sensitivity experiments to estimate the impacts of optical characteristics on regional radiative forcing in this paper. The experiments involve in refractive indices, single scattering aibedo, asymmetry factor and optical depth. An updated dataset of refractive indices representing East Asian dust and the one recommended by the World Meteorology Organization （WMO） are contrastively analyzed and used. A radiative transfer code for solar and thermal infrared radiation with detailed aerosol parameterization is employed. The strongest emphasis is on the refractive indices since other optical parameters strongly depend on it, and the authors found a strong sensitivity of radiative forcing on refractive indices. Studies show stronger scattering, weaker absorption and forward scattering of the East Asian dust particles at solar wavelengths, which leads to higher negative forcing, lower positive forcing and bigger net forcing at the top of the atmosphere （TOA） than that of the WMO dust model. It is also found that the TOA forcings resulting from these two dust models have opposite signs in certain regions, which implies the importance of accurate measurements of optical properties in the quantitative estimation of radiative forcing.     

Attribution of the Present-Day Aerosol Direct Radiative Forcing to Anthropogenic Emission Sectors

In this study,a general circulation model coupled with a gas-phase module and an aerosol chemistry module was employed to investigate the impacts of anthropogenic emission sectors on aerosol direct radiative forcing at the top of atmosphere(TOA)in the present-day climate.The predictions were based on the emission inventories developed in support of the Intergovernmental Panel on Climate Change(IPCC)Fifth Assessment Report(AR5).Six emission sectors—agriculture,open biomass burning,domestic activities,industry,energy generation,and transport—were considered,with a special focus on nitrate aerosol that shows large uncertainties in current models.The results show that the energy sector accounts for the largest contribution(-222 mW m 2)to global aerosol radiative forcing,with substantial negative forcing from sulfate.Inclusion of nitrate results in the transport sector yielding a global nitrate radiative forcing of-92 mW m-2and an internally mixed aerosol radiative forcing of -85 mW m-2,which is opposite to the positive radiative forcing predicted in the past,indicating that the transport emissions could not be a potential control target to counteract climate warming as expected before.The maximum change in nitrate burden is found to be associated with agricultural emissions,which accounts for about75%of global ammonia gas(NH3)emissions.Agricultural emissions account for global nitrate radiative forcing of -186 mW m-2and internally mixed aerosols direct radiative forcing of -149 mW m-2.Such agricultural radiative forcing exceeds the radiative forcing of the industrial sector and is responsible for a large portion of negative radiative forcing over the Northern Hemisphere.     

A DIAGNOSTIC STUDY OF FEEDBACK MECHANISM IN GREENHOUSE EFFECTS SIMULATED BY NCAR CCM1

This paper describes a diagnostic study of the feedback mechanism in greenhouse effects of increased CO_2 and oth-er trace gases(CH_4,N_2O and CFCs),simulated by general circulation model.The study is based on two sensitivity exper-iments for doubled CO_2 and the inclusion of other trace gases,respectively,using version one of the community climatemodel(CCM1)developed at the National Centre for Atmospheric Research.A one-dimensional(1-D)and atwo-dimensional(2-D)radiative-convective models are used to diagnose the feedback effect.It shows that thefeedback factors in global and annual mean conditions are in the sequence of surface albedo,water vapor amount,watervapor distribution,cloud height,critical lapse rate and cloud cover,while in zonal and annual mean conditions in thetropical region the above sequence does not change except the two water vapor terms being the largest feedback compo-nents.Among the feedback components,the total water vapor feedback is the largest(about 50%).The diagnosis alsogives a very small feedback of either the cloud cover or the lapse rate,which is substantially different from the 1-Dfeedback analysis by Hansen et al.(1984).The small lapse rate feedback is considered to be partly caused by theconvective adjustment scheme adopted by CCM1 model.The feedback effect for doubled CO_2 is very different from that of the addition of other trace gases because of theirdifferent vertical distributions of radiative forcing although the non-feedback responses of surface air temperature forboth cases are almost the same.For instance,the larger forcing at surface by the addition of other trace gases can causestronger surface albedo feedback than by doubled CO_2.Besides,because of the negative forcing of doubled CO_2 in thestratosphere,cloud height feedback is more intense.The larger surface forcing in the case of other trace gases can also in-fluence atmospheric water vapor amount as well as the water vapor distribution,which will in turn have strongerfeedback effects.All these indicate that it is incorrect to use“effective CO_2”to replace other trace gases in the generalcirculation model.     

Radiative Forcing and Climate Response Due to Black Carbon in Snow and Ice

The radiative forcing and climate response due to black carbon(BC) in snow and/or ice were investigated by integrating observed effects of BC on snow/ice albedo into an atmospheric general circulation model(BCC AGCM2.0.1) developed by the National Climate Center(NCC) of the China Meteorological Administration(CMA).The results show that the global annual mean surface radiative forcing due to BC in snow/ice is +0.042 W m 2,with maximum forcing found over the Tibetan Plateau and regional mean forcing exceeding +2.8 W m 2.The global annual mean surface temperature increased 0.071 C due to BC in snow/ice.Positive surface radiative forcing was clearly shown in winter and spring and increased the surface temperature of snow/ice in the Northern Hemisphere.The surface temperatures of snow-covered areas of Eurasia and North America in winter(spring) increased by 0.83 C(0.6 C) and 0.83 C(0.46 C),respectively.Snowmelt rates also increased greatly,leading to earlier snowmelt and peak runoff times.With the rise of surface temperatures in the Arctic,more water vapor could be released into the atmosphere,allowing easier cloud formation,which could lead to higher thermal emittance in the Arctic.However,the total cloud forcing could decrease due to increasing cloud cover,which will offset some of the positive feedback mechanism of the clouds.     

Interdecadal and Interannnual Variabilities of the Antarctic Oscillation Simulated by CAM3

Based on four sets of numerical simulations prescribed with atmospheric radiative forcing and sea surface temperature（SST） forcing in the Community Atmospheric Model version 3（CAM3）, the interannual and interdecadal variabilities of the Antarctic oscillation（AAO） during austral summer were studied. It was found that the interannual variability is mainly driven by SST forcing. On the other hand, atmospheric radiative forcing plays a major role in the interdecadal variability. A cooling trend was found in the high latitudes of the Southern Hemisphere（SH） when atmospheric radiative forcing was specified in the model. This cooling trend tended to enhance the temperature gradient between the mid and high latitudes in the SH, inducing a transition of the AAO from a negative to a positive phase on the interdecadal timescale. The cooling trend was also partly weakened by the SST forcing, leading to a better simulation compared with the purely atmospheric radiative forcing run. Therefore, SST forcing cannot be ignored, although it is not as important as atmospheric radiative forcing.     

CAUSATION ANALYSIS OF THE DIRECT CLIMATE EFFECTS OF ANTHROPOGENIC AEROSOL ON EAST-ASIAN SUMMER MONSOON

A regional climate model coupled with an aerosol model is employed to numerically simulate the direct climate effects of the anthropogenic aerosol emitted in South Asia and China in the East Asian summer monsoon during 1988 to 2009. Based on the data of the numerical simulation, composite analysis and correlation analysis are used to make diagnostic study of climate dynamics. Results show that the month of maximum emission of the mean column burden of the anthropogenic aerosol in the main emission areas of South Asia is opposite in phase to that in China. Summer is the season of maximum emission amount in China, but the emission amounts are more in South Asia in spring and winter. On the whole, the mean column burden of the anthropogenic aerosol in China is relatively high compared with that in South Asia. The trend of distribution of aerosol is SW-NE in China, and Sichuan Basin is the emission center of aerosol. The effect of negative short wave radiative forcing alters the gradient of pressure between land and sea, weakening the development of East Asian summer monsoon over the northern part of Yangtze-Huaihe River Basin. We also discuss the feedback effect of East-Asian summer monsoon which is changed by the anthropogenic aerosol on the concentration and distribution of aerosol in China.     

Impact of Different East Asian Summer Monsoon Circulations on Aerosol-Induced Climatic Effects

The different spatial distributions of aerosol-induced direct radiative forcing and climatic effects in a weak(2003)and a strong(2006)East Asian summer monsoon(EASM)circulation were simulated using a high-resolution regional climate model(RegCM3).Results showed that the atmospheric circulations of summer monsoon have direct relations with transport of aerosols and their climatic effects.Both the top-of-the-atmosphere(TOA)and the surface-negative radiative forcing of aerosols were stronger in weak EASM circulations.The main difference in aerosol-induced negative forcing in two summers varied between 2 and 14 W m 2from the Sichuan Basin to North China,where a maximum in aerosol-induced negative forcing was also noticed in the EASM-dominated areas.The spatial difference in the simulated aerosol optical depth(AOD)in two summers generally showed the similar pictures.Surface cooling effects induced by aerosols were spatially more uniform in weak EASM circulations and cooler by about 1–4.5 C.A preliminary analysis here indicated that a weaker low-level wind speed not conducive to the transport and diffusion of aerosols could make more contributions to the differences in the two circulations.     

Performance of CAMS-CSM in Simulating the Shortwave Cloud Radiative Effect over Global Stratus Cloud Regions: Baseline Evaluation and Sensitivity Test

The ability of climate models to correctly reproduce clouds and the radiative effects of clouds is vitally important in climate simulations and projections. In this study, simulations of the shortwave cloud radiative effect(SWCRE) using the Chinese Academy of Meteorological Sciences Climate System Model(CAMS-CSM) are evaluated. The relationships between SWCRE and dynamic–thermodynamic regimes are examined to understand whether the model can simulate realistic processes that are responsible for the generation and maintenance of stratus clouds. Over eastern China, CAMS-CSM well simulates the SWCRE climatological state and stratus cloud distribution. The model captures the strong dependence of SWCRE on the dynamic conditions. Over the marine boundary layer regions, the simulated SWCRE magnitude is weaker than that in the observations due to the lack of low-level stratus clouds in the model. The model fails to simulate the close relationship between SWCRE and local stability over these regions. A sensitivity numerical experiment using a specifically designed parameterization scheme for the stratocumulus cloud cover confirms this assertion. Parameterization schemes that directly depict the relationship between the stratus cloud amount and stability are beneficial for improving the model performance.     

Direct Radiative Forcing and Climatic Effects of Aerosols over East Asia by RegCM3

The authors used a high-resolution regional climate model(RegCM3) coupled with a chemistry/aerosol module to simulate East Asian climate in 2006 and to test the climatic impacts of aerosols on regionalscale climate.The direct radiative forcing and climatic effects of aerosols(dust,sulfate,black carbon,and organic carbon) were discussed.The results indicated that aerosols generally produced negative radiative forcing at the top-of-the-atmosphere(TOA) over most areas of East Asia.The radiative forcing induced by aerosols exhibited significant seasonal and regional variations,with the strongest forcing occurring in summer.The aerosol feedbacks on surface air temperature and precipitation were clear.Surface cooling dominated features over the East Asian continental areas,which varied in the approximate range of-0.5 to-2°C with the maximum up to-3-C in summer over the deserts of West China.The aerosols induced complicated variations of precipitation.Except in summer,the rainfall generally varied in the range of-1 to 1 mm d-1 over most areas of China.     

The Cloud Processes of a Simulated Moderate Snowfall Event in North China

The understanding of the cloud processes of snowfall is essential to the artificial enhancement of snow and the numerical simulation of snowfall. The mesoscale model MM5 is used to simulate a moderate snowfall event in North China that occurred during 20–21 December 2002. Thirteen experiments are performed to test the sensitivity of the simulation to the cloud physics with different cumulus parameterization schemes and different options for the Goddard cloud microphysics parameterization schemes. It is shown that the cumulus parameterization scheme has little to do with the simulation result. The results also show that there are only four classes of water substances, namely the cloud water, cloud ice, snow, and vapor, in the simulation of the moderate snowfall event. The analysis of the cloud microphysics budgets in the explicit experiment shows that the condensation of supersaturated vapor, the depositional growth of cloud ice, the initiation of cloud ice, the accretion of cloud ice by snow, the accretion of cloud water by snow, the deposition growth of snow, and the Bergeron process of cloud ice are the dominant cloud microphysical processes in the simulation. The accretion of cloud water by snow and the deposition growth of the snow are equally important in the development of the snow.     

Numerical Simulation of Global Temperature Change during the 20th Century with the IAP／LASG GOALS Model

The IAP/LASG GOALS coupled model is used to simulate the climate change during the 20th century using historical greenhouse gases concentrations, the mass mixing ratio of sulfate aerosols simulated by a CTM model, and reconstruction of solar variability spanning the period 1900 to 1997. Four simulations, including a control simulation and three forcing simulations, are conducted. Comparison with the observational record for the period indicates that the three forcing experiments simulate reasonable temporal and spatial distributions of the temperature change. The global warming during the 20th century is caused mainly by increasing greenhouse gas concentration especially since the late 1980s; sulfate aerosols offset a portion of the global warming and the reduction of global temperature is up to about 0.11℃ over the century; additionally, the effect of solar variability is not negligible in the simulation of climate change over the 20th century.     

Impact of dust aerosol on glacial-interglacial climate

The temperature anomaly and dust concentrations recorded from central Antarctic ice core records display a strong negative correlation. The dust concentration recorded from an ice core in central Antarctica is 50-70 times higher during glacial periods than interglacial periods. This study investigated the impact of dust aerosol on glacial-interglacial climate, using a zonal energy balance model and dust concentration data from an Antarctica ice core. Two important effects of dust, the direct radiative effect and dust-albedo feedback, were considered. On the one hand, the direct radiative effect of dust significantly cooled the climate during the glacial period, with cooling during the last glacial maximum being as much as 2.05℃ in Antarctica. On the other hand, dust deposition onto the ice decreased the surface albedo over Antarctica, leading to increased absorption of solar radiation, inducing a positive feedback that warmed the region by as much as about 0.9℃ during the glacial period. However, cooling by the direct dust effect was found to be the controlling effect for the glacial climate and may be the major influence on the strong negative correlation between temperature and dust concentration during glacial periods.     

20世纪中国气候变暖的归因分析

Progress in the attribution of climate warming in China for the 20th century is summarized. Three sets of climate model experiments including both coupled and uncoupled runs have been used in the attribution analyses. Comparison of climate model results with the observations proves that in the 20th century, especially in the recent half century, climate warming in China is closely related to the increasing of the anthropogenic emissions of greenhouse gases, while sulfate aerosol should also have contributions. When both external forcing and natural forcing agents are prescribed, coupled climate models have better results in producing the observed variation of temperature in China. The role of oceanic forcing is also emphasized in the attribution analyses. The observed climate warming of China in the 1920s could not be reproduced in any set of climate model simulations.     

The natural oscillation of two types of ENSO events based on analyses of CMIP5 model control runs

The eastern-and central-Pacific El Ni（n）o-Southem Oscillation （EP-and CP-ENSO） have been found to be dominant in the tropical Pacific Ocean,and are characterized by interannual and decadal oscillation,respectively.In the present study,we defined the EP-and CP-ENSO modes by singular value decomposition （SVD） between SST and sea level pressure （SLP） anomalous fields.We evaluated the natural features of these two types of ENSO modes as simulated by the pre-industrial control runs of 20 models involved in phase five of the Coupled Model Intercomparison Project （CMIP5）.The results suggested that all the models show good skill in simulating the SST and SLP anomaly dipolar structures for the EP-ENSO mode,but only 12 exhibit good performance in simulating the tripolar CP-ENSO modes.Wavelet analysis suggested that the ensemble principal components in these 12 models exhibit an interannual and multi-decadal oscillation related to the EP-and CP-ENSO,respectively.Since there are no changes in external forcing in the pre-industrial control runs,such a result implies that the decadal oscillation of CP-ENSO is possibly a result of natural climate variability rather than external forcing.     

Latitude-Dependent Response of East Asian Summer Monsoon to External Forcing in the Last Millennium

Response of the East Asian summer monsoon(EASM) rainfall to external forcing(insolation,volcanic aerosol,and greenhouse gases) is investigated by analysis of a millennium simulation with the coupled climate model ECHO-G.The model reproduces reasonably realistic present-day EASM climatology.The simulated precipitation variation in East Asia over the last millennium compares favorably with the observed and proxy data.It is found that the features and sensitivity of the forced response depend on latitude.On...