A destructive extreme heat attacked Northeast Asia(NEA)in the midsummer of 2018,characterized by the average midsummer Tmax(daily maximum air temperature at 2 m)ranking first during the study period.The current study ...A destructive extreme heat attacked Northeast Asia(NEA)in the midsummer of 2018,characterized by the average midsummer Tmax(daily maximum air temperature at 2 m)ranking first during the study period.The current study indicates that the cyclonic anomaly over the western North Pacific(WNP)was an important cause,which presents an anomaly of two standard deviations.The cyclonic anomaly over the WNP was accompanied by anomalous convection,which favored descending and anticyclonic anomalies over NEA through a local meridional cell.The anticyclonic anomaly over NEA corresponds to the northwestward extension of the WNP subtropical high and facilitated the occurrence of extreme heat.The tropical sea surface temperature anomaly(SSTA)presents a La Ni?a decaying episode,but the SSTA over the tropical Pacific and North Indian Ocean was weak in the summer.In contrast,the southeastern tropical Indian Ocean(SETIO)was obviously cool,which was the coolest after detrending.The SETIO cooling triggered a low-level southeasterly anomaly,which turned into a southwesterly after crossing the equator,due to the Coriolis force.The southwesterly anomaly extended eastwards and favored the cyclonic anomaly over the WNP.Meanwhile,the circulation anomalies over the SETIO and WNP were connected via a local meridional cell,with the ascending branch over the WNP.Moreover,the above mechanism also operates for the climate statistics,verifying the robust in?uence of the SETIO SSTA.Considering the consistency of the SETIO SSTA,it could be a potential predictor for the climate over the WNP and NEA.展开更多
By using the data from observation on the Chinese research vessel Xiang Yang Hong No.5 and other sources during AMEX phase II, the kinetic energy budget and circulation characteristics of the tropical storm Irma were ...By using the data from observation on the Chinese research vessel Xiang Yang Hong No.5 and other sources during AMEX phase II, the kinetic energy budget and circulation characteristics of the tropical storm Irma were analyzed.Irma formed on the ITCZ of the Southern Hemisphere. During the formative stage of the storm, the SE trades and monsoon westerlies on both sides of the ITCZ strengthened, and more importantly, there was a strong divergent flow in upper troposphere. These contributed to the intensification of Irma. At the time when Irma formed, the Richardson number (Ri) in middle and lower troposphere was much smaller than that prior to and post the formation.When Irma intensified rapidly, the area-averaged kinetic energy in the general flow increased in the whole troposphere . The largest contribution came from kinetic energy generation term, -[v.(?)(?)] .indicates that there existed a strong ageostrophic accetration. As to the generation term , the conversion of available potential energy to kinetic energy, - |ωα|, made the largest contribution. This illustrates the importance of internal sources and of the ensemble effect of cumulus convection to the kinetic energy.To the increase of area-averaged eddy kinetic energy during the rapid intensification of Irma, the most impor tant source in the whole troposphere was the dissipation term - [E'], that should be interpreted as the. feeding of eddy kinetic energy from smaller to larger scale disturbances. Another important source was generation term, - [v' (?)(?)'], in the lower troposphere. Rather small contribution came from the energy conversion from the kinetic energy of area-mean flow to eddy kinetic energy. Therefore, the eddy kinetic energy of the developing tropical disturbance extracted both from smaller an, .arger scale motions. The former was much more important than the latter In addition, the disturbance acting as a generator and exporter, generated and exported eddy kinetic energy to the environmental atmosphere.展开更多
Both 1981 and 2013 were weak La Nifia years with a similar sea surface temperature （SST） anomaly in the tropical Pacific, yet the westem Pacific subtropical high （WPSH） during August exhibited an opposite anomaly ...Both 1981 and 2013 were weak La Nifia years with a similar sea surface temperature （SST） anomaly in the tropical Pacific, yet the westem Pacific subtropical high （WPSH） during August exhibited an opposite anomaly in the two years. A comparison indicates that, in the absence of a strong SST anomaly in the tropics, the cold advection from Eurasian high latitudes and the convection of the western Pacific warm pool play important roles in influencing the strength and position of the WPSH in August. In August 1981, the spatial pattern of 500 hPa geopotential height was characterized by a meridional circulation with a strong ridge in the Ural Mountains and a deep trough in Siberia, which provided favorable conditions for cold air invading into the lower latitudes. Accordingly, the geopotential height to the north of the WPSH was reduced by the cold advection anomaly from high latitudes, resulting in an eastward retreat of the WPSH. Moreover, an anomalous cyclonic circulation in the subtropical western Pacific, excited by enhanced warm pool convection, also contributed to the eastward retreat of the WPSH. By contrast, the influence from high latitudes was relatively weak in August 2013 due to a zonal circulation pattern over Eurasia, and the anomalous anticyclonic circulation induced by suppressed warm pool convection also facilitated the westward extension of the WPSH. Therefore, the combined effects of the high latitude and tropical circulations may contribute a persistent anomaly of the WPSH in late summer, despite the tropical SST anomaly being weak.展开更多
The tropical Hadley circulation （HC） plays an important role in influencing the climate in the tropics and extra-tropics. The realism of the climatological characteristics, spatial structure, and temporal evolution ...The tropical Hadley circulation （HC） plays an important role in influencing the climate in the tropics and extra-tropics. The realism of the climatological characteristics, spatial structure, and temporal evolution of the long-term variation of the principal mode of the annual mean HC （i.e., the equatorially asymmetric mode, EAM） was examined in model simulations from the Coupled Model Intercomparison Project Phase 5 （CMIP5）. The results showed that all the models are moderately successful in capturing the HC＇s climatological features, including the spatial pattern, meridional extent, and intensity, but not the spatial or temporal variation of the EAM. The possible reasons for the poor simulation of the long-term variability of the EAM were explored. None of the models can successfully capture the differences in the warming rate between the tropical Southern Hemisphere （SH） and Northern Hemisphere （NH）, which is considered to be an important driver for the variation of the AM. Most of the models produce a faster warming in the NH than in the SH, which is the reverse of the observed trend. This leads to a reversed trend in the meridional gradient between the SH and NH, and contributes to the poor simulation of EAM variability. Thus, this aspect of the models should be improved to provide better simulations of the variability of the HC. This study suggests a possible reason for the poor simulation of the HC, which may be helpful for improving the skill of the CMIP5 models in the future.展开更多
The effect of a vertical diffusion scheme over a stratocumulus topped boundary layer （STBL） was investigated using the YONU AGCM （Yonsei University Atmospheric General Circulation Model）. To consider the impact of...The effect of a vertical diffusion scheme over a stratocumulus topped boundary layer （STBL） was investigated using the YONU AGCM （Yonsei University Atmospheric General Circulation Model）. To consider the impact of clouds on the turbulence production, the turbulence mixing term, driven by radiative cooling at the cloud top, is implemented as an extended non-local diffusion scheme. In the model with this new scheme, the STBL parameterization significantly influences the lower atmosphere over the tropical and subtropical regions. Consideration of the turbulent mixing within the cloud layer leads to continuous stratocumulus formation. The cloud-top radiative cooling tends to favor more rapid entrainment and produces top-down turbulent mixing. This cooling develops a mixed layer without initiation of deep convection by surface fluxes. Variations in thermodynamical and dynamical features are produced by planetary boundary layer （PBL） cloud development. The simulated stratocumulus induces more mixing of heat and moisture due to the cloud forcing. Over STBL regions, the lower boundary layer becomes warmer and drier. It also weakens vertical motion and zonal trade winds in the eastern Pacific, which indicates that stratocumulus cloud cover plays a role in weakening the Walker circulation; that is, cloud cover damps the tropical circulation.展开更多
A linear steady model is constructed to investigate the response of the tropical atmosphere to diabatic heating.The basic equations are similar to those used by Gill(1980),but the long-wave approximation is removed an...A linear steady model is constructed to investigate the response of the tropical atmosphere to diabatic heating.The basic equations are similar to those used by Gill(1980),but the long-wave approximation is removed and periodic boundary conditions are taken in longitude.According to the features of the underlying surface temperature(including oceans and land),the heat sources(sinks)are given.Using this analytical model,we have simulated the climatological fields of wind and air pressure in the lower layers of the tropical and subtropical atmosphere in summer(June—August)and winter(December—February). The main features of observations are depicted in simulated fields.展开更多
Daily mean outputs for 12 yr （1978-1989） from two general circulation models （SAMIL-R42L9 and CAM2.0.2） are analyzed and compared with the corresponding NCEP/NCAR reanalysis dataset, and results in two models show...Daily mean outputs for 12 yr （1978-1989） from two general circulation models （SAMIL-R42L9 and CAM2.0.2） are analyzed and compared with the corresponding NCEP/NCAR reanalysis dataset, and results in two models show clearly that the root-mean square errors （RMSEs） from the simulation of intraseasonal oscillation can take 30-40 percent of the total RMSE, particularly, the distributions of the RMSE in simulating intraseasonal oscillation are almost identical with that of the total RMSE. The maximum RMSE of intraseasonal oscillation height at 500 hPa is shown in the middle latitude regions, but there are also large RMSEs of intraseasonal oscillation wind over the tropical western Pacific and tropical Indian Oceans. The simulated ISO energy in the tropic has very large difference from the result of the NCEP/NCAR reanalysis dataset which means the simulation of tropical atmospheric ISO still possesses serious insufficiency. Therefore, intraseasonal oscillation in the weather and climate numerical simulation is very important, and thus, how to improve the ability of the GCM to simulate the intraseasonal oscillation becomes very significant.展开更多
基金supported by the National Key R&D Program of China [grant number 2016YFA0600601]the National Natural Science Foundation of China [grant numbers41605027,41530530,and 41721004]the Fundamental Research Funds for the Central Universities
文摘A destructive extreme heat attacked Northeast Asia(NEA)in the midsummer of 2018,characterized by the average midsummer Tmax(daily maximum air temperature at 2 m)ranking first during the study period.The current study indicates that the cyclonic anomaly over the western North Pacific(WNP)was an important cause,which presents an anomaly of two standard deviations.The cyclonic anomaly over the WNP was accompanied by anomalous convection,which favored descending and anticyclonic anomalies over NEA through a local meridional cell.The anticyclonic anomaly over NEA corresponds to the northwestward extension of the WNP subtropical high and facilitated the occurrence of extreme heat.The tropical sea surface temperature anomaly(SSTA)presents a La Ni?a decaying episode,but the SSTA over the tropical Pacific and North Indian Ocean was weak in the summer.In contrast,the southeastern tropical Indian Ocean(SETIO)was obviously cool,which was the coolest after detrending.The SETIO cooling triggered a low-level southeasterly anomaly,which turned into a southwesterly after crossing the equator,due to the Coriolis force.The southwesterly anomaly extended eastwards and favored the cyclonic anomaly over the WNP.Meanwhile,the circulation anomalies over the SETIO and WNP were connected via a local meridional cell,with the ascending branch over the WNP.Moreover,the above mechanism also operates for the climate statistics,verifying the robust in?uence of the SETIO SSTA.Considering the consistency of the SETIO SSTA,it could be a potential predictor for the climate over the WNP and NEA.
文摘By using the data from observation on the Chinese research vessel Xiang Yang Hong No.5 and other sources during AMEX phase II, the kinetic energy budget and circulation characteristics of the tropical storm Irma were analyzed.Irma formed on the ITCZ of the Southern Hemisphere. During the formative stage of the storm, the SE trades and monsoon westerlies on both sides of the ITCZ strengthened, and more importantly, there was a strong divergent flow in upper troposphere. These contributed to the intensification of Irma. At the time when Irma formed, the Richardson number (Ri) in middle and lower troposphere was much smaller than that prior to and post the formation.When Irma intensified rapidly, the area-averaged kinetic energy in the general flow increased in the whole troposphere . The largest contribution came from kinetic energy generation term, -[v.(?)(?)] .indicates that there existed a strong ageostrophic accetration. As to the generation term , the conversion of available potential energy to kinetic energy, - |ωα|, made the largest contribution. This illustrates the importance of internal sources and of the ensemble effect of cumulus convection to the kinetic energy.To the increase of area-averaged eddy kinetic energy during the rapid intensification of Irma, the most impor tant source in the whole troposphere was the dissipation term - [E'], that should be interpreted as the. feeding of eddy kinetic energy from smaller to larger scale disturbances. Another important source was generation term, - [v' (?)(?)'], in the lower troposphere. Rather small contribution came from the energy conversion from the kinetic energy of area-mean flow to eddy kinetic energy. Therefore, the eddy kinetic energy of the developing tropical disturbance extracted both from smaller an, .arger scale motions. The former was much more important than the latter In addition, the disturbance acting as a generator and exporter, generated and exported eddy kinetic energy to the environmental atmosphere.
基金supported by the National Science Foundation of China(Grant Nos.41475052 and 41405058)
文摘Both 1981 and 2013 were weak La Nifia years with a similar sea surface temperature （SST） anomaly in the tropical Pacific, yet the westem Pacific subtropical high （WPSH） during August exhibited an opposite anomaly in the two years. A comparison indicates that, in the absence of a strong SST anomaly in the tropics, the cold advection from Eurasian high latitudes and the convection of the western Pacific warm pool play important roles in influencing the strength and position of the WPSH in August. In August 1981, the spatial pattern of 500 hPa geopotential height was characterized by a meridional circulation with a strong ridge in the Ural Mountains and a deep trough in Siberia, which provided favorable conditions for cold air invading into the lower latitudes. Accordingly, the geopotential height to the north of the WPSH was reduced by the cold advection anomaly from high latitudes, resulting in an eastward retreat of the WPSH. Moreover, an anomalous cyclonic circulation in the subtropical western Pacific, excited by enhanced warm pool convection, also contributed to the eastward retreat of the WPSH. By contrast, the influence from high latitudes was relatively weak in August 2013 due to a zonal circulation pattern over Eurasia, and the anomalous anticyclonic circulation induced by suppressed warm pool convection also facilitated the westward extension of the WPSH. Therefore, the combined effects of the high latitude and tropical circulations may contribute a persistent anomaly of the WPSH in late summer, despite the tropical SST anomaly being weak.
基金supported by the National Natural Science Foundation of China (Grant Nos. 41205046 and 41475076)the 973 Program (Grant No. 2013CB 430203)
文摘The tropical Hadley circulation （HC） plays an important role in influencing the climate in the tropics and extra-tropics. The realism of the climatological characteristics, spatial structure, and temporal evolution of the long-term variation of the principal mode of the annual mean HC （i.e., the equatorially asymmetric mode, EAM） was examined in model simulations from the Coupled Model Intercomparison Project Phase 5 （CMIP5）. The results showed that all the models are moderately successful in capturing the HC＇s climatological features, including the spatial pattern, meridional extent, and intensity, but not the spatial or temporal variation of the EAM. The possible reasons for the poor simulation of the long-term variability of the EAM were explored. None of the models can successfully capture the differences in the warming rate between the tropical Southern Hemisphere （SH） and Northern Hemisphere （NH）, which is considered to be an important driver for the variation of the AM. Most of the models produce a faster warming in the NH than in the SH, which is the reverse of the observed trend. This leads to a reversed trend in the meridional gradient between the SH and NH, and contributes to the poor simulation of EAM variability. Thus, this aspect of the models should be improved to provide better simulations of the variability of the HC. This study suggests a possible reason for the poor simulation of the HC, which may be helpful for improving the skill of the CMIP5 models in the future.
基金supported by Ministry of Environment of Korea as "The Eco-technopia 21 Project"supported by Pusan National University under the "Post-Doc. 2008 Program"
文摘The effect of a vertical diffusion scheme over a stratocumulus topped boundary layer （STBL） was investigated using the YONU AGCM （Yonsei University Atmospheric General Circulation Model）. To consider the impact of clouds on the turbulence production, the turbulence mixing term, driven by radiative cooling at the cloud top, is implemented as an extended non-local diffusion scheme. In the model with this new scheme, the STBL parameterization significantly influences the lower atmosphere over the tropical and subtropical regions. Consideration of the turbulent mixing within the cloud layer leads to continuous stratocumulus formation. The cloud-top radiative cooling tends to favor more rapid entrainment and produces top-down turbulent mixing. This cooling develops a mixed layer without initiation of deep convection by surface fluxes. Variations in thermodynamical and dynamical features are produced by planetary boundary layer （PBL） cloud development. The simulated stratocumulus induces more mixing of heat and moisture due to the cloud forcing. Over STBL regions, the lower boundary layer becomes warmer and drier. It also weakens vertical motion and zonal trade winds in the eastern Pacific, which indicates that stratocumulus cloud cover plays a role in weakening the Walker circulation; that is, cloud cover damps the tropical circulation.
文摘A linear steady model is constructed to investigate the response of the tropical atmosphere to diabatic heating.The basic equations are similar to those used by Gill(1980),but the long-wave approximation is removed and periodic boundary conditions are taken in longitude.According to the features of the underlying surface temperature(including oceans and land),the heat sources(sinks)are given.Using this analytical model,we have simulated the climatological fields of wind and air pressure in the lower layers of the tropical and subtropical atmosphere in summer(June—August)and winter(December—February). The main features of observations are depicted in simulated fields.
基金Supported jointly by the National Natural Science Foundation of China (Grant No. 40575027)the Chinese Academy of Sclences(ZKCX-SW-226).
文摘Daily mean outputs for 12 yr （1978-1989） from two general circulation models （SAMIL-R42L9 and CAM2.0.2） are analyzed and compared with the corresponding NCEP/NCAR reanalysis dataset, and results in two models show clearly that the root-mean square errors （RMSEs） from the simulation of intraseasonal oscillation can take 30-40 percent of the total RMSE, particularly, the distributions of the RMSE in simulating intraseasonal oscillation are almost identical with that of the total RMSE. The maximum RMSE of intraseasonal oscillation height at 500 hPa is shown in the middle latitude regions, but there are also large RMSEs of intraseasonal oscillation wind over the tropical western Pacific and tropical Indian Oceans. The simulated ISO energy in the tropic has very large difference from the result of the NCEP/NCAR reanalysis dataset which means the simulation of tropical atmospheric ISO still possesses serious insufficiency. Therefore, intraseasonal oscillation in the weather and climate numerical simulation is very important, and thus, how to improve the ability of the GCM to simulate the intraseasonal oscillation becomes very significant.
