In autumn when the East Asian Winter Monsoon becomes established,cold airmass(CAM)flows into low latitudes continuously,leading to cross-latitude mass and heat exchange.CAM that penetrates into low latitudes over East...In autumn when the East Asian Winter Monsoon becomes established,cold airmass(CAM)flows into low latitudes continuously,leading to cross-latitude mass and heat exchange.CAM that penetrates into low latitudes over East Asia can be linked to disturbances of tropical atmosphere-ocean system on multiple timescales,forming complex variations.Using an isentropic analysis method of CAM,we quantitatively describe the characteristics of long-term changes in CAM flux toward low-latitude East Asia in autumn and further explore its relationship with various tropical disturbances.Results show that,on interdecadal timescale,southward CAM flux experienced an increase from the 1960s to the 1970s,an obvious decrease from the1980s to early twenty-first century,and a slight recovery in recent years.The CAM flux also exhibits significant interannual variation,with an amplitude comparable to that of interdecadal variation.Unlike the southward CAM flux in winter,which is mainly governed by mid-high latitude forcing,the interannual variability of southward CAM flux in autumn is closely related to the meridionally propagating wave trains along westerlies and multiple-timescale tropical disturbances.On interannual timescale,stronger cold air activities in autumn are usually accompanied by the cold phase of El Ni?o-Southern Oscillation(ENSO).ENSO may influence the meridional flow of CAM through modulating tropical convection over the Maritime Continent and exciting poleward Rossby wave trains.Interannual variability of CAM flux is also associated with Madden-Julian oscillation(MJO)and synoptic-scale tropical cyclone activities.CAM flux in the eight phases of MJO is non-uniform,and CAM tends to move southward during the wet phases when MJO convection is active over the Maritime Continent.MJO rainfall may enhance and shift southward due to the feedback of strong CAM flux.In addition,the activities of synoptic-scale tropical cyclones over the South China Sea facilitate the southward movement of CAM and the increase of extreme CAM flux events.The interaction of CAM flux and tropical cyclones can induce tropical cyclone rainfall asymmetry.The above disturbances of tropical atmosphereocean system on three timescales together explain 44.2%of the variance in interannual variation of CAM flux in autumn.These results help to clarify the relationship and feedback between CAM and multiple-timescale tropical disturbances,further explain the connection between weather events and climate change in regional scale,and provide new insights into the mechanisms of weather and climate extremes.展开更多
Dry intrusion plays an important role in the explosive development of cyclones and the evolution of cold fronts. Characteristics of dry intrusion during a rainfall event that occurred in northern China are analyzed in...Dry intrusion plays an important role in the explosive development of cyclones and the evolution of cold fronts. Characteristics of dry intrusion during a rainfall event that occurred in northern China are analyzed in detail in this paper. The IM （ingredients-based methodology） developed by Doswell et al. in 1996 and Wetzel and Martin in 2001 is utilized. All the physical representations of dry intrusion defined in the past studies, such as low relative humidity, cold advection, and high potential vorticity （on either isobaric or isentropic surfaces）, are combined into a simple and convenient physical parameter to characterize dry intrusion. This is a new attempt to extend the IM that was primarily applied to research on heavy rainfall to the study of dry intrusion. The new dry intrusion parameter is used to analyze the isentropic evolution of dry intrusion during the rainfall event. The results show that this parameter can better quantify the intensity of dry intrusion and diagnose its evolution shown in satellite infrared and water vapor imageries. It is found that dry intrusion maintains during the rainfall period. The intensity of precipitation increases with the increasing dry intrusion, which has pushed the rainy region southeastward. From the results on the isentropic surface and the corresponding isobaric surface, it is inferred that the analyses of dry intrusion on both surfaces are consistent with each other. The isentropic analysis of dry intrusion reveals that cold and dry air at the upper level overruns that in the lower troposphere where moist and warm air is located. Thus, potential instability is built up in the vertical direction, which favors the occurrence of precipitation. In practice, we may identify dry intrusion regions by tracking strong signals of the dry intrusion parameter, and further identify the instability near the dry intrusion regions. This will aid in improving the accuracy of precipitation forecast.展开更多
基金supported by the Guangdong Major Project of Basic and Applied Basic Research(Grant No.2020B0301030004)the National Key Research and Development Program of China(Grant No.2022YFF0801700)the Project of Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai)(Grant No.SML2024SP012)。
文摘In autumn when the East Asian Winter Monsoon becomes established,cold airmass(CAM)flows into low latitudes continuously,leading to cross-latitude mass and heat exchange.CAM that penetrates into low latitudes over East Asia can be linked to disturbances of tropical atmosphere-ocean system on multiple timescales,forming complex variations.Using an isentropic analysis method of CAM,we quantitatively describe the characteristics of long-term changes in CAM flux toward low-latitude East Asia in autumn and further explore its relationship with various tropical disturbances.Results show that,on interdecadal timescale,southward CAM flux experienced an increase from the 1960s to the 1970s,an obvious decrease from the1980s to early twenty-first century,and a slight recovery in recent years.The CAM flux also exhibits significant interannual variation,with an amplitude comparable to that of interdecadal variation.Unlike the southward CAM flux in winter,which is mainly governed by mid-high latitude forcing,the interannual variability of southward CAM flux in autumn is closely related to the meridionally propagating wave trains along westerlies and multiple-timescale tropical disturbances.On interannual timescale,stronger cold air activities in autumn are usually accompanied by the cold phase of El Ni?o-Southern Oscillation(ENSO).ENSO may influence the meridional flow of CAM through modulating tropical convection over the Maritime Continent and exciting poleward Rossby wave trains.Interannual variability of CAM flux is also associated with Madden-Julian oscillation(MJO)and synoptic-scale tropical cyclone activities.CAM flux in the eight phases of MJO is non-uniform,and CAM tends to move southward during the wet phases when MJO convection is active over the Maritime Continent.MJO rainfall may enhance and shift southward due to the feedback of strong CAM flux.In addition,the activities of synoptic-scale tropical cyclones over the South China Sea facilitate the southward movement of CAM and the increase of extreme CAM flux events.The interaction of CAM flux and tropical cyclones can induce tropical cyclone rainfall asymmetry.The above disturbances of tropical atmosphereocean system on three timescales together explain 44.2%of the variance in interannual variation of CAM flux in autumn.These results help to clarify the relationship and feedback between CAM and multiple-timescale tropical disturbances,further explain the connection between weather events and climate change in regional scale,and provide new insights into the mechanisms of weather and climate extremes.
基金the National Natural Science Foundation of China under Grant Nos.40633016 and 40805001the Open Project of the State Key Laboratory of Severe Weather,Chinese Academy of Meteorological Sciences,under Grant No.2009LASWB02.
文摘Dry intrusion plays an important role in the explosive development of cyclones and the evolution of cold fronts. Characteristics of dry intrusion during a rainfall event that occurred in northern China are analyzed in detail in this paper. The IM （ingredients-based methodology） developed by Doswell et al. in 1996 and Wetzel and Martin in 2001 is utilized. All the physical representations of dry intrusion defined in the past studies, such as low relative humidity, cold advection, and high potential vorticity （on either isobaric or isentropic surfaces）, are combined into a simple and convenient physical parameter to characterize dry intrusion. This is a new attempt to extend the IM that was primarily applied to research on heavy rainfall to the study of dry intrusion. The new dry intrusion parameter is used to analyze the isentropic evolution of dry intrusion during the rainfall event. The results show that this parameter can better quantify the intensity of dry intrusion and diagnose its evolution shown in satellite infrared and water vapor imageries. It is found that dry intrusion maintains during the rainfall period. The intensity of precipitation increases with the increasing dry intrusion, which has pushed the rainy region southeastward. From the results on the isentropic surface and the corresponding isobaric surface, it is inferred that the analyses of dry intrusion on both surfaces are consistent with each other. The isentropic analysis of dry intrusion reveals that cold and dry air at the upper level overruns that in the lower troposphere where moist and warm air is located. Thus, potential instability is built up in the vertical direction, which favors the occurrence of precipitation. In practice, we may identify dry intrusion regions by tracking strong signals of the dry intrusion parameter, and further identify the instability near the dry intrusion regions. This will aid in improving the accuracy of precipitation forecast.
