Using high-resolution observations,mesoscale simulations,and idealized experiments,this study investigates the mechanisms governing an episode of orographic convection initiation(CI)during the North China Heavy Rainfa...Using high-resolution observations,mesoscale simulations,and idealized experiments,this study investigates the mechanisms governing an episode of orographic convection initiation(CI)during the North China Heavy Rainfall Experiment.On 4 August 2024,repeated CI occurred over the eastern slopes of the Taihang Mountains in the late afternoon,subsequently enhancing an upstream downhill convective storm.Wind profiler radar data and dense automatic weather stations reveal that CI was supported by strengthening southeasterly upslope winds.These winds primarily resulted from the migration of the mountain-plain solenoid and the mountainward-propagating outflow from a convective cold pool over the plain,with sensitivity experiments showing the latter contributed roughly 22%of the wind strength.The upslope flows gradually transported unstable air from the plain to the slope,fostering CI.Mesoscale simulations further highlight the key role of orographic waves near the mountain ridge,which generated strong downslope winds.The near-surface convergence between downslope and upslope flows,combined with wave-induced divergence aloft,produced deep ascent over the slope.Removing mountain ridges weakened wave strength and reduced downslope wind speeds by~8 m s^(-1).Without orographic heating in the idealized simulation(i.e.,no mountain-plain solenoid),only strong wave descent occurred below 2??km,inhibiting CI.These findings underscore the critical interplay among plain convection,orographic waves,and the mountain-plain solenoid,offering new insight into the processes controlling orographic CI in North China.展开更多
The springtime persistent rainfall(SPR)is the major rainy period before the onset of summer monsoon in East Asia,which profoundly affects the regional and even global hydrological cycle.Despite the great importance of...The springtime persistent rainfall(SPR)is the major rainy period before the onset of summer monsoon in East Asia,which profoundly affects the regional and even global hydrological cycle.Despite the great importance of the mechanical and thermal effects of the Tibetan Plateau(TP)large-scale orography on the formation of SPR,the impact of small-scale orography over the TP remains poorly understood.Here we show that upward-propagating orographic gravity waves(OGWs),which occur as the subtropical westerlies interact with the TP's small-scale orography,contribute importantly to the SPR.The breaking of OGWs induces a large zonal wave drag in the middle troposphere,which drives a meridional circulation across the TP.The rising branch of the meridional circulation acts to lower the pressure and increase the meridional pressure gradient to the south of the TP by dynamically pumping the lower-tropospheric air upwards.The southwesterly monsoonal flow on the southeastern flank of the TP thus intensifies and transports more water vapor to East Asia,resulting in an enhancement of the SPR.This finding helps more completely understand the impacts of TP's multiscale orography on the SPR and provides a new perspective on the westerly-monsoon synergy in East Asia.展开更多
Orographic drag formation is investigated using a numerical wave model(NWM),based on the pressure-coordinate dynamics of non-hydrostatic HIRLAM.The surface drag,wave stress(vertical flux of horizontal momentum),and wa...Orographic drag formation is investigated using a numerical wave model(NWM),based on the pressure-coordinate dynamics of non-hydrostatic HIRLAM.The surface drag,wave stress(vertical flux of horizontal momentum),and wave drag are split to the longitudinal and transverse components and presented as Fourier sums of their spectral amplitudes weighted with the power spectrum of relative orographic height.The NWM is accomplished,enabling a spectral investigation of the buoyancy wave stress,and drag generation by orography and is then applied to a cold front,characterised by low static stability of the upper troposphere,large vertical and directional wind variations,and intensive trapped wave generation downstream of obstacles.Resonances are discovered in the stress and drag spectra in the form of high narrow peaks.The stress conservation problem is revisited.Longitudinal stress conserves in unidirectional flow,2D orography conditions,but becomes convergent for rotating wind or 3D orography.Even in the convergent case the vertical momentum flux from the troposphere to stratosphere remains substantial.The transverse stress never conserves.Disappearing at the surface and on the top,it realises the main momentum exchange between lower an upper parts of the troposphere.Existence of stationary stratospheric quasi-turbulence(SQT)is established above wind minimum in the stratosphere.展开更多
基金supported by the National Key Research and Development Program of China(Grant No.2024YFC3013003)the National Natural Science Foundation of China(Grant Nos.424B2033 and 42475002)+3 种基金projects supported by the Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai)(Grant Nos.SML2024SP035,SML2024SP012,311024001)the Guangdong Project of Basic and Applied Basic Research(Grant Nos.2024A1515510005 and2025A1515011974)the Key Innovation Team of the China Meteorological Administration(Grant No.CMA2023ZD08)the State Key Laboratory of Severe Weather Meteorological Science and Technology(Grant No.2025QZA10)。
文摘Using high-resolution observations,mesoscale simulations,and idealized experiments,this study investigates the mechanisms governing an episode of orographic convection initiation(CI)during the North China Heavy Rainfall Experiment.On 4 August 2024,repeated CI occurred over the eastern slopes of the Taihang Mountains in the late afternoon,subsequently enhancing an upstream downhill convective storm.Wind profiler radar data and dense automatic weather stations reveal that CI was supported by strengthening southeasterly upslope winds.These winds primarily resulted from the migration of the mountain-plain solenoid and the mountainward-propagating outflow from a convective cold pool over the plain,with sensitivity experiments showing the latter contributed roughly 22%of the wind strength.The upslope flows gradually transported unstable air from the plain to the slope,fostering CI.Mesoscale simulations further highlight the key role of orographic waves near the mountain ridge,which generated strong downslope winds.The near-surface convergence between downslope and upslope flows,combined with wave-induced divergence aloft,produced deep ascent over the slope.Removing mountain ridges weakened wave strength and reduced downslope wind speeds by~8 m s^(-1).Without orographic heating in the idealized simulation(i.e.,no mountain-plain solenoid),only strong wave descent occurred below 2??km,inhibiting CI.These findings underscore the critical interplay among plain convection,orographic waves,and the mountain-plain solenoid,offering new insight into the processes controlling orographic CI in North China.
基金supported by the Second Tibetan Plateau Scientific Expedition and Research Program(STEP)(Grants No.2019QZKK0105)the National Natural Science Foundation of China(Grants Nos.42122036,91837207,42230607)。
文摘The springtime persistent rainfall(SPR)is the major rainy period before the onset of summer monsoon in East Asia,which profoundly affects the regional and even global hydrological cycle.Despite the great importance of the mechanical and thermal effects of the Tibetan Plateau(TP)large-scale orography on the formation of SPR,the impact of small-scale orography over the TP remains poorly understood.Here we show that upward-propagating orographic gravity waves(OGWs),which occur as the subtropical westerlies interact with the TP's small-scale orography,contribute importantly to the SPR.The breaking of OGWs induces a large zonal wave drag in the middle troposphere,which drives a meridional circulation across the TP.The rising branch of the meridional circulation acts to lower the pressure and increase the meridional pressure gradient to the south of the TP by dynamically pumping the lower-tropospheric air upwards.The southwesterly monsoonal flow on the southeastern flank of the TP thus intensifies and transports more water vapor to East Asia,resulting in an enhancement of the SPR.This finding helps more completely understand the impacts of TP's multiscale orography on the SPR and provides a new perspective on the westerly-monsoon synergy in East Asia.
基金supported by the Estonian Science Foundation(Grants 7478,9134)the Estonian Research Council(Targeted Financing Project SF0180038s08).
文摘Orographic drag formation is investigated using a numerical wave model(NWM),based on the pressure-coordinate dynamics of non-hydrostatic HIRLAM.The surface drag,wave stress(vertical flux of horizontal momentum),and wave drag are split to the longitudinal and transverse components and presented as Fourier sums of their spectral amplitudes weighted with the power spectrum of relative orographic height.The NWM is accomplished,enabling a spectral investigation of the buoyancy wave stress,and drag generation by orography and is then applied to a cold front,characterised by low static stability of the upper troposphere,large vertical and directional wind variations,and intensive trapped wave generation downstream of obstacles.Resonances are discovered in the stress and drag spectra in the form of high narrow peaks.The stress conservation problem is revisited.Longitudinal stress conserves in unidirectional flow,2D orography conditions,but becomes convergent for rotating wind or 3D orography.Even in the convergent case the vertical momentum flux from the troposphere to stratosphere remains substantial.The transverse stress never conserves.Disappearing at the surface and on the top,it realises the main momentum exchange between lower an upper parts of the troposphere.Existence of stationary stratospheric quasi-turbulence(SQT)is established above wind minimum in the stratosphere.
