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 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.
