Attribution of Biases of Interhemispheric Temperature Contrast in CMIP6 Models

One of the basic characteristics of Earth's modern climate is that the Northern Hemisphere(NH) is climatologically warmer than the Southern Hemisphere(SH). Here, model performances of this basic state are examined using simulation results from 26 CMIP6 models. Results show that the CMIP6 models underestimate the contrast in interhemispheric surface temperatures on average(0.8 K for CMIP6 mean versus 1.4 K for reanalysis data mean), and that there is a large intermodel spread, ranging from -0.7 K to 2.3 K. A box model energy budget analysis shows that the contrast in interhemispheric shortwave absorption at the top of the atmosphere, the contrast in interhemispheric greenhouse trapping, and the crossequatorial northward ocean heat transport, are all underestimated in the multimodel mean. By examining the intermodel spread, we find intermodel biases can be tracked back to biases in midlatitude shortwave cloud forcing in AGCMs. Models with a weaker interhemispheric temperature contrast underestimate the shortwave cloud reflection in the SH but overestimate the shortwave cloud reflection in the NH, which are respectively due to underestimation of the cloud fraction over the SH extratropical ocean and overestimation of the cloud liquid water content over the NH extratropical continents.Models that underestimate the interhemispheric temperature contrast exhibit larger double ITCZ biases, characterized by excessive precipitation in the SH tropics. Although this intermodel spread does not account for the multimodel ensemble mean biases, it highlights that improving cloud simulation in AGCMs is essential for simulating the climate realistically in coupled models.

Water Vapor and Cloud Radiative Forcings over the Pacific Ocean Simulated by the LASG/IAP AGCM:Sensitivity to Convection Schemes

Characteristics of the total clear-sky greenhouse effect （GA） and cloud radiative forcings （CRFs）, along with the radiative-related water vapor and cloud properties simulated by the Spectral Atmospheric Model developed by LASGIAP （SAMIL） are evaluated. Impacts of the convection scheme on the simulation of CRFs are discussed by using two AMIP （Atmospheric Model Inter-comparison Project） type simulations employing different convection schemes： the new Zhang-McFarlane （NZH） and Tiedtke （TDK） convection schemes. It shows that both the climatological GA and its response to El Nio warming are simulated well, both in terms of spatial pattern and magnitude. The impact of the convection scheme on GA is not significant. The climatological longwave CRF （LWCRF） and its response to El Nio warming are simulated well, but with a prominently weaker magnitude. The simulation of the climatology （response） of LWCRF in the NZH （TDK） run is slightly more realistic than in the TDK （NZH） simulation, indicating significant impacts of the convection scheme. The shortwave CRF （SWCRF） shows large biases in both spatial pattern and magnitude, and the results from the TDK run are better than those from the NZH run. A spuriously excessive negative climatological SWCRF over the southeastern Pacific and an insufficient response of SWCRF to El Nio warming over the tropical Pacific are seen in the NZH run. These two biases are alleviated in the TDK run, since it produces vigorous convection, which is related to the low threshold for convection to take place. Also, impacts of the convection scheme on the cloud profile are discussed.

Shortwave Cloud Radiative Forcing on Major Stratus Cloud Regions in AMIP-type Simulations of CMIP3 and CMIP5 Models

Cloud and its radiative effects are major sources of uncertainty that lead to simulation discrepancies in climate models. In this study, shortwave cloud radiative forcing （SWCF） over major stratus regions is evaluated for Atmospheric Models Intercomparison Project （AMIP）-type simulations of models involved in the third and fifth phases of the Coupled Models Intercomparison Project （CMIP3 and CMIP5）. Over stratus regions, large deviations in both climatological mean and seasonal cycle of SWCF are found among the models. An ambient field sorted by dynamic （vertical motion） and thermodynamic （inversion strength or stability） regimes is constructed and used to measure the response of SWCF to large-scale controls. In marine boundary layer regions, despite both CMIP3 and CMIP5 models being able to capture well the center and range of occurrence frequency for the ambient field, most of the models fail to simulate the dependence of SWCF on boundary layer inversion and the insensitivity of SWCF to vertical motion. For eastern China, there are large differences even in the simulated ambient fields. Moreover, almost no model can reproduce intense SWCF in rising motion and high stability regimes. It is also found that models with a finer grid resolution have no evident superiority than their lower resolution versions. The uncertainties relating to SWCF in state-of-the-art models may limit their performance in IPCC experiments.

Cloud-Aerosol-Radiation (CAR) Ensemble Modeling System: Overall Accuracy and Efficiency

The Cloud Aerosol- Radiation （CAR） ensemble modeling system has recently been built to better un- derstand cloud/aerosol/radiation processes and determine the uncertainties caused by different treatments of cloud/aerosol/radiation in climate models. The CAR system comprises a large scheme collection of cloud, aerosol, and radiation processes available in the literature, including those commonly used by the world＇s leading GCMs. In this study, detailed analyses of the overall accuracy and efficiency of the CAR system were performed. Despite the different observations used, the overall accuracies of the CAR ensemble means were found to be very good for both shortwave （SW） and longwave （LW） radiation calculations. Taking tile percentage errors for July 2004 compared to ISCCP （International Satellite Cloud Climatology Project） data over （60~N, 60~S） as an example, even among the 448 CAR members selected here, those errors of the CAR ensemble means were only about -0.67% （-0.6 W m-2） and -0.82% （-2.0 W m-2） for SW and LW upward fluxes at the top of atmosphere, and 0.06% （0.1 W m-2） and -2.12% （-7.8 W m 2） for SW and LW downward fluxes at the surface, respectively. Furthermore, model SW frequency distributions in July 2004 covered the observational ranges entirely, with ensemble means located in the middle of the ranges. Moreover, it was found that the accuracy of radiative transfer calculations can be significantly enhanced by＂ using certain combinations of cloud schemes for the cloud cover fraction, particle effective size, water path, and optical properties, along with better explicit treatments for unresolved cloud structures.

Evaluation of Cloud Vertical Structure Simulated by Recent BCC_AGCM Versions through Comparison With CALIPSO-GOCCP Data

ABSTRACT The abilities of BCC-AGCM2.1 and BCC_AGCM2.2 to simulate the annual-mean cloud vertical structure （CVS） were evaluated through comparison with GCM-Oriented CALIPSO Cloud Product （CALIPSO-GOCCP） data. BCC-AGCM2.2 has a dynamical core and physical processes that are consistent with BCC-AGCM2.1, but has a higher horizontal resolution. Results showed that both BCC-AGCM versions underestimated the global-mean total cloud cover （TCC）, middle cloud cover （MCC） and low cloud cover （LCC）, and that BCC_AGCM2.2 underestimated the global-mean high cloud cover （HCC）. The global-mean cloud cover shows a systematic decrease from BCCA-GCM2.1 to BCC_AGCM2.2, especially for HCC. Geographically, HCC is significantly overestimated in the tropics, particularly by BCC_AGCM2,1, while LCC is generally overestimated over extra-tropical lands, but significantly underestimated over most of the oceans, especially for subtropical marine stratocumulus clouds. The leading EOF modes of CVS were extracted. The BCC_AGCMs perform well in reproducing EOF1, but with a larger variance explained. The two models also capture the basic features of EOF3, except an obvious deficiency in eigen- vector peaks. EOF2 has the largest simulation biases in both position and strength of eigenvector peaks. Furthermore, we investigated the effects of CVS on relative shortwave and longwave cloud radiative forcing （RSCRF and RLCRF）. Both BCC_AGCM versions successfully reproduce the sign of regression coefficients, except for RLCRF in PC1. However, the RSCRF relative contributions from PC1 and PC2 are overestimated, while the relative contribution from PC3 is underes timated in both BCC_AGCM versions. The RLCRF relative contribution is underestimated for PC2 and overestimated for PC3.

Analytical Studies of the Cloud Droplet Spectral Dispersion Influence on the First Indirect Aerosol Effect

Atmospheric aerosols （acting as cloud condensation nuclei） can enhance the cloud droplet number concentration and reduce the cloud droplet size, and in turn affect the cloud optical depth, as well as the cloud albedo, and thereby exert a radiative influence on climate （the first indirect aerosol effect）. In this paper, based on various relationships between cloud droplet spectral dispersion （c） and cloud droplet number concentration （Nc）, we analytically derive the corresponding expressions of the cloud radiative forcing induced by changes in the cloud droplet number concentration. Further quantitative evaluation indicates that the cloud radiative forcing induced by aerosols for the different ^-Nc relationships varies from -29.1% to 25.2%, compared to the case without considering spectral dispersion （e = 0）. Our results suggest that an accurate description of e - Nc relationships helps to reduce the uncertainty of the first indirect aerosol effect and advances our scientific understanding of aerosol-cloud-radiation interactions.

Cloud Radiative Forcing in Asian Monsoon Region Simulated by IPCC AR4 AMIP Models

This study examines cloud radiative forcing （CRF） in the Asian monsoon region （0° 50°N, 60° 150°E） simulated by Intergovernmental Panel on Climate Change （IPCC） Fourth Assessment Report （AR4） AMIP models. During boreal winter, no model realistically reproduces the larger long-wave cloud radiative forcing （LWCF） over the Tibet Plateau （TP） and only a couple of models reasonably capture the larger short-wave CRF （SWCF） to the east of the TP. During boreal summer, there are larger biases for central location and intensity of simulated CRF in active convective regions. The CRF biases are closely related to the rainfall biases in the models. Quantitative analysis further indicates that the correlation between simulated CRF and observations are not high, and that the biases and diversity in SWCF are larger than that in LWCF. The annual cycle of simulated CRF over East Asia （0°-50°N, 100°-145°E） is also examined. Though many models capture the basic annual cycle in tropics, strong LWCF and SWCF to the east of the TP beginning in early spring are underestimated by most models. As a whole, GFDL-CM2.1, MPI-ECHAM5, UKMO-HadGAM1, and MIROC3.2 （medres） perform well for CRF simulation in the Asian monsoon region, and the multi-model ensemble （MME） has improved results over the individual simulations. It is suggested that strengthening the physical parameterizations involved over the TP, and improving cumulus convection processes and model experiment design are crucial to CRF simulation in the Asian monsoon region.

Cloud Variations under Subtropical High Conditions

The cloud variations under subtropical high(STH) conditions during summers over a ten-year period are studied using combined data from the International Satellite Cloud Climatology Project and the National Centers for Environmental Prediction.The results reveal that clouds mainly experience an isolated evolution in the STHs,which is designated in this study by the 1540 gpm geopotential lines at 850 hPa.In the STH domain throughout the Northern Hemisphere,the average amount of total clouds exceeds 30%.Low clouds dominate in the STH domain,contributing over 60%of total cloud amount within the Pacific subtropical high and over 40%within the Atlantic subtropical high.The prevalence of low clouds in above regions is determined by the circulation pattern around 150°-180°E and 850 hPa,which suppresses both the upward development of the cloud tops and the water vapor divergences near the surface.Furthermore,clouds present great geographical incoherence within the STH domain.In the eastern STHs,the amount of middle and low clouds increases to peak in the early morning and decreases to a trough in the afternoon,while the amount of high clouds remains stable throughout the day.Conversely,in the western STHs,the diurnal amplitude of low and middle clouds is less than three,while high clouds dramatically reach the maximum in the afternoon and drop to the minimum in the evening.Among the nine cloud categories,stratocumulus clouds with greater optical thickness account for the most under STH conditions,no matter their occurrence or amount,causing more shortwave cloud radiative forcing to cool the local atmosphere and surface as a consequence.

Decomposition of Fast and Slow Cloud Responses to Quadrupled CO_(2)Forcing in BCC–AGCM2.0 over East Asia

In this study,the decomposed fast and slow responses of clouds to an abruptly quadrupled CO_(2)concentration(approximately 1139 ppmv)in East Asia(EA)are obtained quantitatively by using a general circulation model,BCC–AGCM2.0.Our results show that in the total response,the total cloud cover(TCC),low cloud cover(LCC),and high cloud cover(HCC)all increased north of 40°N and decreased south of 40°N except in the Tibetan Plateau(TP).The mean changes of the TCC,LCC,and HCC in EA were–0.74%,0.38%,and–0.38%in the total response,respectively;1.05%,–0.03%,and 1.63%in the fast response,respectively;and–1.79%,0.41%,and–2.01%in the slow response,respectively.By comparison,we found that changes in cloud cover were dominated by the slow response in most areas in EA due to the changes in atmospheric temperature,circulation,and water vapor supply together.Overall,the changes in the cloud forcing over EA related to the fast and slow responses were opposite to each other,and the final cloud forcing was dominated by the slow response.The mean net cloud forcing(NCF)in the total response over EA was–1.80 W m^(–2),indicating a cooling effect which partially offset the warming effect caused by the quadrupled CO_(2).The total responses of NCF in the TP,south China(SC),and northeast China(NE)were–6.74 W m^(–2),6.11 W m^(–2),and–7.49 W m^(–2),respectively.Thus,the local effects of offsetting or amplifying warming were particularly obvious.

Impact of Arctic Oscillation on cloud radiative forcing and September sea ice retreat

The Arctic Oscillation(AO)has important effects on the sea ice change in terms of the dynamic and thermodynamic processes.However,while the dynamic processes of AO have been widely explored,the thermodynamic processes of AO need to be further discussed.In this paper,we use the fifth state-of-the-art reanalysis at European Centre for Medium-Range Weather Forecasts(ERA5)from 1979 to 2020 to investigate the relationship between AO and the surface springtime longwave(LW)cloud radiative forcing(CRF),summertime shortwave(SW)CRF in the Arctic region(65°-90°N).In addition,the contribution of CRF induced by AO to the sea ice change is also discussed.Results indicate that the positive(negative)anomalies of springtime LW CRF and summertime SW CRF are generally detected over the Arctic Ocean during the enhanced positive(negative)AO phase in spring and summer,respectively.Meanwhile,while the LW(SW)CRF generally has a positive correlation with AO index(AOI)in spring(summer)over the entire Arctic Ocean,this correlation is statistically significant over 70°-85°N and 120°W-90°E(i.e.,region of interest(ROI))in both seasons.Moreover,the response of CRF to the atmospheric conditions varies in spring and summer.We also find that the positive springtime(summertime)AOI tends to decrease(increase)the sea ice in September,and this phenomenon is especially prominent over the ROI.The sensitivity study among sea ice extent,CRF and AOI further reveals that decreases(increases)in September sea ice over the ROI are partly attributed to the springtime LW(summertime SW)CRF during the positive AOI.The present study provides a new pattern of AO affecting sea ice change via cloud radiative effects,which might benefit the sea ice forecast improvement.

Evaluating the Impacts of Cloud Microphysical and Overlap Parameters on Simulated Clouds in Global Climate Models

The improvement of the accuracy of simulated cloud-related variables,such as the cloud fraction,in global climate models(GCMs)is still a challenging problem in climate modeling.In this study,the influence of cloud microphysics schemes(one-moment versus two-moment schemes)and cloud overlap methods(observation-based versus a fixed vertical decorrelation length)on the simulated cloud fraction was assessed in the BCC_AGCM2.0_CUACE/Aero.Compared with the fixed decorrelation length method,the observation-based approach produced a significantly improved cloud fraction both globally and for four representative regions.The utilization of a two-moment cloud microphysics scheme,on the other hand,notably improved the simulated cloud fraction compared with the one-moment scheme;specifically,the relative bias in the global mean total cloud fraction decreased by 42.9%–84.8%.Furthermore,the total cloud fraction bias decreased by 6.6%in the boreal winter(DJF)and 1.64%in the boreal summer(JJA).Cloud radiative forcing globally and in the four regions improved by 0.3%−1.2% and 0.2%−2.0%,respectively.Thus,our results showed that the interaction between clouds and climate through microphysical and radiation processes is a key contributor to simulation uncertainty.

Conditions Supporting Funnel Cloud Development in Alaska

The characteristics and climatology of funnel clouds in Alaska were examined using operational radiosondes, surface meteorological observations, and reanalysis data. Funnel clouds occurred under weak synoptic forcing between May and September between 11 am and 6 pm Alaska Daylight Time with a maximum occurrence in July. They occurred under Convective Available Potential Energy >500 J·kg-1 and strong low-level wind shear. Characteristic atmospheric profiles during funnel cloud events served to develop a retrieval algorithm based on similarity testing. Out of more than 129,000 soundings between 1971 and 2014, 2724, 442, and 744 profiles were similar to the profiles of observed funnel cloud events in the Interior, Alaska West Coast, and Anchorage regions. While the number of reported funnel clouds has increased since 2000, the frequency of synoptic situations favorable for such events has decreased.

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 ant hropogenic- forcing-dominant period.

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 integrations 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.

Cloud Radiative Forcing Induced by Layered Clouds and Associated Impact on the Atmospheric Heating Rate

A quantitative analysis of cloud fraction, cloud radiative forcing, and cloud radiative heating rate （CRH） of the single-layered cloud （SLC） and the multi-layered cloud （MLC）, and their differences is presented, based on the 2B-CLDCLASS-LIDAR and 2B-FLXHR-LIDAR products on the global scale. The CRH at a given atmospheric level is defined as the cloudy minus clear-sky radiative heating rate. The statistical results show that the globally averaged cloud fraction of the MLC （24.9%）, which is primarily prevalent in equatorial regions, is smaller than that of the SLC （46.6%）. The globally averaged net radiative forcings （NET CRFs） induced by the SLC （MLC） at the top and bottom of the atmosphere （TOA and BOA） and in the atmosphere （ATM） are -60.8 （40.9）, 67.5 （49.6）, and 6.6 （8.7） W m-2, respectively, where the MLC contributes approximately 40.2%, 42.4%, and 57% to the NET CRF at the TOA, BOA, and in the ATM, respectively. The MLC exhibits distinct differences to the SLC in terms of CRH. The shortwave CRH of the SLC （MLC） reaches a heating peak at 9.75 （7.5） km, with a value of 0.35 （0.60） K day-1, and the differences between SLC and MLC transform from positive to negative with increasing altitude. However, the longwave CRH of the SLC （MLC） reaches a cooling peak at 2 （8） km, with a value of -0.45 （-0.42） K day-1, and the differences transform from negative to positive with increasing altitude. In general, the NET CRH differences between SLC and MLC are negative below 7.5 km. These results provide an observational basis for the assessment and improvement of the cloud parameterization schemes in global models.

Influences of Two Convective Schemes on the Radiative Energy Budget in GAMIL1.0

The Grid-point Atmospheric Model of IAP LASG version 1.0（GAMIL1.0） is used to investigate the impacts of different convective schemes on the radiative energy budget.The two convective schemes are Zhang and McFarlance（1995）/Hack（1994）（ZM） and Tiedtke（1989）/Nordeng（1994）（TN）.Two simulations are performed：one with the ZM scheme（EX_ZM） and the other with the TN scheme（EX_TN）.The results indicate that during the convective process,more water vapor consumption and temperature increment are found in the EX_ZM,especially in the lower model layer,its environment is therefore very dry.In contrast, there is a moister atmosphere in the EX_TN,which favors low cloud formation and large-scale condensation, and hence more low cloud fraction,higher cloud water mixing ratio,and deeper cloud extinction optical depth are simulated,reflecting more solar radiative flux in the EX_TN.This explains why the TN scheme underestimates the net shortwave radiative flux at the top of the atmosphere and at surface.In addition, convection influences longwave radiation,surface sensible and latent heat fluxes through changes in cloud emissivity and precipitation.