In the inner core of a tropical cyclone, turbulence not only exists in the boundary layer (BL) but also can be generated above the BL by eyewall and rainband clouds. Thus, the treatment of vertical turbulent mixing must go beyond the conventional scope of the BL. The turbulence schemes formulated based on the turbulent kinetic energy (TKE) are attractive as they are applicable to both deep and shallow convection regimes in the TC inner core provided that the TKE production and dissipation can be appropriately determined. However, TKE schemes are not self-closed. They must be closed by an empirically prescribed vertical profile of mixing length. This motivates this study to investigate the sensitivity of the simulated TC intensification to the sloping curvature and asymptotic length scale of mixing length, the two parameters that determine the vertical distribution of a prescribed mixing length. To tackle the problem, both idealized and real-case TC simulations are performed. The results show that the simulated TC intensification is sensitive to the sloping curvature of mixing length but only exhibits marginal sensitivity to the asymptotic length scale. The underlying reasons for such sensitivities are explored analytically based on the Mellor and Yamada Level-2 turbulence model and the analyses of azimuthal-mean tangential wind budget. The results highlight the uncertainty and importance of mixing length in numerical prediction of TCs and suggest that future research should focus on searching for physical constraints on mixing length, particularly in the low to mid troposphere, using observations and large eddy simulations.
{"title":"Parameterization of Vertical Turbulent Transport in the Inner Core of Tropical Cyclones and Its Impact on Storm Intensification. Part I: Sensitivity to Turbulent Mixing Length","authors":"Jeremy Katz, Ping Zhu","doi":"10.1175/jas-d-23-0242.1","DOIUrl":"https://doi.org/10.1175/jas-d-23-0242.1","url":null,"abstract":"\u0000In the inner core of a tropical cyclone, turbulence not only exists in the boundary layer (BL) but also can be generated above the BL by eyewall and rainband clouds. Thus, the treatment of vertical turbulent mixing must go beyond the conventional scope of the BL. The turbulence schemes formulated based on the turbulent kinetic energy (TKE) are attractive as they are applicable to both deep and shallow convection regimes in the TC inner core provided that the TKE production and dissipation can be appropriately determined. However, TKE schemes are not self-closed. They must be closed by an empirically prescribed vertical profile of mixing length. This motivates this study to investigate the sensitivity of the simulated TC intensification to the sloping curvature and asymptotic length scale of mixing length, the two parameters that determine the vertical distribution of a prescribed mixing length. To tackle the problem, both idealized and real-case TC simulations are performed. The results show that the simulated TC intensification is sensitive to the sloping curvature of mixing length but only exhibits marginal sensitivity to the asymptotic length scale. The underlying reasons for such sensitivities are explored analytically based on the Mellor and Yamada Level-2 turbulence model and the analyses of azimuthal-mean tangential wind budget. The results highlight the uncertainty and importance of mixing length in numerical prediction of TCs and suggest that future research should focus on searching for physical constraints on mixing length, particularly in the low to mid troposphere, using observations and large eddy simulations.","PeriodicalId":508177,"journal":{"name":"Journal of the Atmospheric Sciences","volume":"35 25","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141924805","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The rapidly increasing resolution of global atmospheric reanalysis and climate model datasets necessitates finding methods for computing convective available potential energy (CAPE) both efficiently and accurately. To this end, this article compares two common methods for computing CAPE which conserve either energy or entropy. Inaccuracies in these computations arise from both physical and numerical errors. For instance, computing CAPE with entropy conserved results in physical errors from non-equilibrium phase transitions but minimizes numerical errors because solutions are analytic at each height. In contrast, computing CAPE with energy conserved avoids these physical errors, but accumulates numerical errors that are grid-resolution dependent because the numerical integration of a differential equation is required. Analysis of CAPE computed with large databases of soundings from the tropical Amazon and midlatitude storm environments shows that physical errors from the entropy method are typically 1-3 % as large as CAPE, which is comparable to the numerical errors from conserving energy with grid spacing of 25 m and 250 m using explicit first-order and second-order integration schemes respectively. Errors in entropy-based CAPE calculations are also insensitive to vertical grid spacing, in contrast with energy-based calculations whose error strongly scales with the grid spacing. It is shown that entropy-based methods are advantageous when intercomparing datasets with differing vertical resolution because they produce accurate and reasonably fast results that are insensitive to grid resolution. Whereas a second-order energy-based method is advantageous when analyzing data with a consistent vertical resolution because of its superior computational efficiency.
{"title":"Should we conserve entropy or energy when computing CAPE with mixed-phase precipitation physics?","authors":"John M. Peters","doi":"10.1175/jas-d-24-0027.1","DOIUrl":"https://doi.org/10.1175/jas-d-24-0027.1","url":null,"abstract":"\u0000The rapidly increasing resolution of global atmospheric reanalysis and climate model datasets necessitates finding methods for computing convective available potential energy (CAPE) both efficiently and accurately. To this end, this article compares two common methods for computing CAPE which conserve either energy or entropy. Inaccuracies in these computations arise from both physical and numerical errors. For instance, computing CAPE with entropy conserved results in physical errors from non-equilibrium phase transitions but minimizes numerical errors because solutions are analytic at each height. In contrast, computing CAPE with energy conserved avoids these physical errors, but accumulates numerical errors that are grid-resolution dependent because the numerical integration of a differential equation is required. Analysis of CAPE computed with large databases of soundings from the tropical Amazon and midlatitude storm environments shows that physical errors from the entropy method are typically 1-3 % as large as CAPE, which is comparable to the numerical errors from conserving energy with grid spacing of 25 m and 250 m using explicit first-order and second-order integration schemes respectively. Errors in entropy-based CAPE calculations are also insensitive to vertical grid spacing, in contrast with energy-based calculations whose error strongly scales with the grid spacing. It is shown that entropy-based methods are advantageous when intercomparing datasets with differing vertical resolution because they produce accurate and reasonably fast results that are insensitive to grid resolution. Whereas a second-order energy-based method is advantageous when analyzing data with a consistent vertical resolution because of its superior computational efficiency.","PeriodicalId":508177,"journal":{"name":"Journal of the Atmospheric Sciences","volume":"29 24","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141645707","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Flight-level airborne observations have often detected gravity waves with horizontal wavelengths λx ≲ 10 km near the tropopause. Here, in-situ and remote sensing aircraft data of these short gravity waves trapped along tropopause inversion layer and collected during a mountain wave event over southern Scandinavia are analyzed to quantify their spectral energy and energy fluxes and to identify non-stationary modes. A series of three-dimensional numerical simulations are performed to explain the origin of these transient wave modes and to investigate the parameters on which they depend. It turns out that mountain wave breaking in the middle atmosphere and the subsequent modification of the stratospheric flow are the key factors for the occurrence of trapped modes with λx ≲ 10 km. In particular, the intermittent and periodic breaking of mountain waves in the lower stratosphere forms a wave duct directly above the tropopause, in which the short gravity waves are trapped. The characteristics of the trapped, downstream-propagating waves are mainly controlled by the sharpness of the tropopause inversion layer. It could be demonstrated that different settings for optimizing the numerical solver have a significantly smaller influence on the solutions.
{"title":"Transient Tropopause Waves","authors":"Andreas Dörnbrack","doi":"10.1175/jas-d-24-0037.1","DOIUrl":"https://doi.org/10.1175/jas-d-24-0037.1","url":null,"abstract":"\u0000Flight-level airborne observations have often detected gravity waves with horizontal wavelengths λx ≲ 10 km near the tropopause. Here, in-situ and remote sensing aircraft data of these short gravity waves trapped along tropopause inversion layer and collected during a mountain wave event over southern Scandinavia are analyzed to quantify their spectral energy and energy fluxes and to identify non-stationary modes. A series of three-dimensional numerical simulations are performed to explain the origin of these transient wave modes and to investigate the parameters on which they depend. It turns out that mountain wave breaking in the middle atmosphere and the subsequent modification of the stratospheric flow are the key factors for the occurrence of trapped modes with λx ≲ 10 km. In particular, the intermittent and periodic breaking of mountain waves in the lower stratosphere forms a wave duct directly above the tropopause, in which the short gravity waves are trapped. The characteristics of the trapped, downstream-propagating waves are mainly controlled by the sharpness of the tropopause inversion layer. It could be demonstrated that different settings for optimizing the numerical solver have a significantly smaller influence on the solutions.","PeriodicalId":508177,"journal":{"name":"Journal of the Atmospheric Sciences","volume":"73 7","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141658368","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Previous observational studies have shown that the intensification rate (IR) of a tropical cyclone (TC) is often correlated with its real-time size. However, no any size parameter explicitly appears in the recent time-dependent theory of TC intensification, while the theory can still well capture the intensity evolution of simulated TCs. This study provides a detailed analysis to address how TC real-time size affects its intensification and why no size parameter explicitly appears in the theory based on results from axisymmetric numerical simulations. Results show that the overall correlation between TC IR and real-time size as reported in previous observational studies, in terms of both the radius of maximum wind (RMW) and the radius of 17-m s−1 wind (R17), is largely related to the correlation between IR and intensity because size and intensity are highly interrelated. As a result, the correlation between TC IR and size for a given intensity is rather weak. Diagnostic analysis shows that TC real-time size (RMW and R17) has two opposing effects on intensification. A larger TC size tends to result in a higher steady-state intensity, but reduce the conversion efficiency of thermodynamic energy to inner-core kinetic energy or the degree of moist neutrality of the eyewall ascent for a given intensity. The former is favorable while the latter is unfavorable for intensification. The two effects are implicitly included in the theory and largely offset, resulting in the weak dependence of IR on TC size for a given intensity.
以往的观测研究表明，热带气旋（TC）的增强率（IR）通常与其实时大小相关。然而，在最近的TC强度随时间变化的理论中，没有明确出现任何大小参数，而该理论仍能很好地捕捉模拟TC的强度演变。本研究根据轴对称数值模拟结果，详细分析了TC实时大小如何影响其强度增强，以及为什么理论中没有明确出现大小参数。结果表明，在最大风半径（RMW）和17 m s-1风半径（R17）方面，以往观测研究中报告的TC红外和实时大小之间的总体相关性在很大程度上与红外和强度之间的相关性有关，因为大小和强度是高度相关的。因此，在给定强度下，热气旋红外和大小之间的相关性很弱。诊断分析表明，热带气旋实时大小（RMW 和 R17）对强度有两种相反的影响。较大的热气旋尺寸往往会导致较高的稳态强度，但会降低热力学能量向内核动能的转换效率，或降低给定强度下的眼球上升的湿中性程度。前者对加强有利，后者对加强不利。这两种效应被隐含在理论中，并在很大程度上被抵消，从而导致在给定强度下，红外对热气旋大小的依赖性很弱。
{"title":"On the size-dependence in the recent time-dependent theory of tropical cyclone intensification","authors":"Yuanlong Li, Yuqing Wang, Zhemin Tan","doi":"10.1175/jas-d-24-0015.1","DOIUrl":"https://doi.org/10.1175/jas-d-24-0015.1","url":null,"abstract":"\u0000Previous observational studies have shown that the intensification rate (IR) of a tropical cyclone (TC) is often correlated with its real-time size. However, no any size parameter explicitly appears in the recent time-dependent theory of TC intensification, while the theory can still well capture the intensity evolution of simulated TCs. This study provides a detailed analysis to address how TC real-time size affects its intensification and why no size parameter explicitly appears in the theory based on results from axisymmetric numerical simulations. Results show that the overall correlation between TC IR and real-time size as reported in previous observational studies, in terms of both the radius of maximum wind (RMW) and the radius of 17-m s−1 wind (R17), is largely related to the correlation between IR and intensity because size and intensity are highly interrelated. As a result, the correlation between TC IR and size for a given intensity is rather weak. Diagnostic analysis shows that TC real-time size (RMW and R17) has two opposing effects on intensification. A larger TC size tends to result in a higher steady-state intensity, but reduce the conversion efficiency of thermodynamic energy to inner-core kinetic energy or the degree of moist neutrality of the eyewall ascent for a given intensity. The former is favorable while the latter is unfavorable for intensification. The two effects are implicitly included in the theory and largely offset, resulting in the weak dependence of IR on TC size for a given intensity.","PeriodicalId":508177,"journal":{"name":"Journal of the Atmospheric Sciences","volume":"74 23","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141664511","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
It has been widely reported that an increase in aerosol concentration in non-precipitating clouds leads to a decrease in their liquid water path. Here we examine the physical mechanisms that drive this response in both subtropical and Arctic stratocumulus clouds using large eddy simulations and mechanism suppression tests. Three processes have been previously identified to contribute to the decrease, namely, the size-dependency of evaporation, sedimentation, and radiation and all act to modulate the rate of entrainment of warm, dry air at the boundary layer top. We find that the liquid water path decrease is correlated with the increased entrainment, as expected, but that the decrease is enhanced by a reduction in cloud radiative cooling. The reduced cloud radiative cooling can occur even though locally at cloud top the radiative cooling rates are stronger and helping to enhance entrainment. We find that slower droplet sedimentation contributes to the increased entrainment and decreased liquid water in both cases. Faster evaporation caused directly by smaller, more numerous droplets decreases the liquid water path but does not necessarily increase the entrainment rate. On the other hand, stronger radiative cloud top cooling caused directly by smaller droplets increases the entrainment as much as slower sedimentation does but does not decrease the liquid water path as much. In general, processes that either directly or indirectly increase radiative cooling at cloud top are more important in the Arctic case and processes that increase the evaporation rate are more important in the subtropical case.
{"title":"Processes Controlling the Entrainment and Liquid Water Response to Aerosol Perturbations in Non-Precipitating Stratocumulus Clouds","authors":"A. Igel","doi":"10.1175/jas-d-23-0238.1","DOIUrl":"https://doi.org/10.1175/jas-d-23-0238.1","url":null,"abstract":"\u0000It has been widely reported that an increase in aerosol concentration in non-precipitating clouds leads to a decrease in their liquid water path. Here we examine the physical mechanisms that drive this response in both subtropical and Arctic stratocumulus clouds using large eddy simulations and mechanism suppression tests. Three processes have been previously identified to contribute to the decrease, namely, the size-dependency of evaporation, sedimentation, and radiation and all act to modulate the rate of entrainment of warm, dry air at the boundary layer top. We find that the liquid water path decrease is correlated with the increased entrainment, as expected, but that the decrease is enhanced by a reduction in cloud radiative cooling. The reduced cloud radiative cooling can occur even though locally at cloud top the radiative cooling rates are stronger and helping to enhance entrainment. We find that slower droplet sedimentation contributes to the increased entrainment and decreased liquid water in both cases. Faster evaporation caused directly by smaller, more numerous droplets decreases the liquid water path but does not necessarily increase the entrainment rate. On the other hand, stronger radiative cloud top cooling caused directly by smaller droplets increases the entrainment as much as slower sedimentation does but does not decrease the liquid water path as much. In general, processes that either directly or indirectly increase radiative cooling at cloud top are more important in the Arctic case and processes that increase the evaporation rate are more important in the subtropical case.","PeriodicalId":508177,"journal":{"name":"Journal of the Atmospheric Sciences","volume":"119 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141667719","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This study investigates the in-situ generation of planetary waves (PWs) by zonally asymmetric gravity wave drag (GWD) in the mesosphere using a fully nonlinear general circulation model extending to the lower thermosphere. To isolate the effects of GWD, we establish a highly idealized but efficient framework that excludes stationary PWs propagating from the troposphere and in-situ PWs generated by barotropic and baroclinic instabilities. The GWD is prescribed in a zonally sinusoidal form with a zonal wavenumber (ZWN) of either 1 or 2 in the lower mesosphere of the Northern Hemisphere mid-latitude. Our idealized simulations clearly show that zonally asymmetric GWD generates PWs by serving as a nonconservative source (Z′) of linearized disturbance quasi-geostrophic potential vorticity (q′). While Z′ initially amplifies PWs through enhancing q′ tendency, the subsequent zonal advection of q′ gradually balances with Z′, thereby attaining steady-state PWs. The GWD-induced PWs predominantly have the same ZWN as the applied GWD with minor contributions from higher ZWN components attributed to nonlinear processes. The amplitude of the induced PWs increases in proportion with the magnitude of the peak GWD, while it decreases in proportion to the square of ZWN. Moreover, the amplitude of PWs increases as the meridional range of GWD expands and as GWD shifts toward lower latitudes. These PWs deposit substantial positive Eliassen-Palm flux divergences (EPFD) of ∼30 m/s/day at their origin and negative EPFD of 5–10 m/s/day during propagation. In addition, the in situ PWs exhibit interhemispheric propagation following westerlies that extend into the Southern Hemisphere.
{"title":"In-Situ Generation of Planetary Waves in the Mesosphere by Zonally Asymmetric Gravity Wave Drag: A Revisit","authors":"Jinho Yoo, Hye‐Yeong Chun, I. Song","doi":"10.1175/jas-d-24-0026.1","DOIUrl":"https://doi.org/10.1175/jas-d-24-0026.1","url":null,"abstract":"\u0000This study investigates the in-situ generation of planetary waves (PWs) by zonally asymmetric gravity wave drag (GWD) in the mesosphere using a fully nonlinear general circulation model extending to the lower thermosphere. To isolate the effects of GWD, we establish a highly idealized but efficient framework that excludes stationary PWs propagating from the troposphere and in-situ PWs generated by barotropic and baroclinic instabilities. The GWD is prescribed in a zonally sinusoidal form with a zonal wavenumber (ZWN) of either 1 or 2 in the lower mesosphere of the Northern Hemisphere mid-latitude. Our idealized simulations clearly show that zonally asymmetric GWD generates PWs by serving as a nonconservative source (Z′) of linearized disturbance quasi-geostrophic potential vorticity (q′). While Z′ initially amplifies PWs through enhancing q′ tendency, the subsequent zonal advection of q′ gradually balances with Z′, thereby attaining steady-state PWs. The GWD-induced PWs predominantly have the same ZWN as the applied GWD with minor contributions from higher ZWN components attributed to nonlinear processes. The amplitude of the induced PWs increases in proportion with the magnitude of the peak GWD, while it decreases in proportion to the square of ZWN. Moreover, the amplitude of PWs increases as the meridional range of GWD expands and as GWD shifts toward lower latitudes. These PWs deposit substantial positive Eliassen-Palm flux divergences (EPFD) of ∼30 m/s/day at their origin and negative EPFD of 5–10 m/s/day during propagation. In addition, the in situ PWs exhibit interhemispheric propagation following westerlies that extend into the Southern Hemisphere.","PeriodicalId":508177,"journal":{"name":"Journal of the Atmospheric Sciences","volume":"60 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141688334","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Xin Xu, Yuanyuan Ju, Qiqing Liu, Kun Zhao, Ming Xue, Shushi Zhang, Ang Zhou, Yuan Wang, Ying Tang
Using radar observation and convection-permitting simulation, this work studies the storm-scale dynamics governing the generation of two episodes of high winds by an unusually long-lived quasi-linear convective system (QLCS) in South China on 21 April 2017. The first episode of high winds occurred at the apex of a bowing segment in the southern QLCS due to the downward transport of high momentum by a descending rear-inflow jet (RIJ). The RIJ was initially elevated, generated as low-frequency gravity wave response to the thermal forcing in the QLCS leading convective line. It descended to the surface owing to the enhancement of low-level diabatic cooling which strengthened the downdrafts at the RIJ leading edge. Vertical momentum budget revealed that the downdrafts were initiated by the negative buoyancy of cold pool and strengthened by the weakened buoyancy-induced upward pressure gradient force in the boundary layer and enhanced hydrometeor loading above. The second episode of high winds occurred in the decaying stage of the QLCS which, however, redeveloped as its northern part interacted with an intensifying large-scale shear line to the east. A zonal convective line developed along the shear line and finally merged with the QLCS. The merger greatly enhanced the low-level convergence, leading to downward development of the line-end vortex via vertical stretching of vertical vorticity. The area of high winds was notably increased by the superposition of the ambient translational wind with the vortex rotational flow. The findings provide new insights into the generation of high winds by QLCS-MCS merger, highlighting the importance of low-level vortices in addition to the RIJ.
{"title":"Dynamics of two episodes of high winds produced by an unusually long-lived quasi-linear convective system in South China","authors":"Xin Xu, Yuanyuan Ju, Qiqing Liu, Kun Zhao, Ming Xue, Shushi Zhang, Ang Zhou, Yuan Wang, Ying Tang","doi":"10.1175/jas-d-23-0047.1","DOIUrl":"https://doi.org/10.1175/jas-d-23-0047.1","url":null,"abstract":"\u0000Using radar observation and convection-permitting simulation, this work studies the storm-scale dynamics governing the generation of two episodes of high winds by an unusually long-lived quasi-linear convective system (QLCS) in South China on 21 April 2017. The first episode of high winds occurred at the apex of a bowing segment in the southern QLCS due to the downward transport of high momentum by a descending rear-inflow jet (RIJ). The RIJ was initially elevated, generated as low-frequency gravity wave response to the thermal forcing in the QLCS leading convective line. It descended to the surface owing to the enhancement of low-level diabatic cooling which strengthened the downdrafts at the RIJ leading edge. Vertical momentum budget revealed that the downdrafts were initiated by the negative buoyancy of cold pool and strengthened by the weakened buoyancy-induced upward pressure gradient force in the boundary layer and enhanced hydrometeor loading above. The second episode of high winds occurred in the decaying stage of the QLCS which, however, redeveloped as its northern part interacted with an intensifying large-scale shear line to the east. A zonal convective line developed along the shear line and finally merged with the QLCS. The merger greatly enhanced the low-level convergence, leading to downward development of the line-end vortex via vertical stretching of vertical vorticity. The area of high winds was notably increased by the superposition of the ambient translational wind with the vortex rotational flow. The findings provide new insights into the generation of high winds by QLCS-MCS merger, highlighting the importance of low-level vortices in addition to the RIJ.","PeriodicalId":508177,"journal":{"name":"Journal of the Atmospheric Sciences","volume":"5 12","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141698420","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
B. Kärcher, F. Hoffmann, A. Podglajen, A. Hertzog, R. Pluogonven, R. Atlas, M. Corcos, W. Grabowski, B. Gasparini
Effects of turbulence on ice supersaturation at cirrus heights (> 8 km) remain unexplored. Small-scale mixing processes become important for high Reynolds number flows, which may develop below the buoyancy length scale (10–100 m). The current study couples a stochastic turbulent mixing model with reduced dimensionality to an entraining parcel model to investigate, in large-ensemble simulations, how supersaturation evolves due to homogeneous turbulence in the stably stratified, cloud-free upper troposphere. The rising parcel is forced by a mesoscale updraft. The perturbation of an initially homogeneous vertical distribution of supersaturation is studied after a 36 m ascent in a baseline case and several sensitivity scenarios. Turbulent mixing and associated temperature fluctuations alone lead to changes in ensemble-mean distributions with standard deviations in the range 0.001 – 0.006, while mean values are hardly affected. Large case-to-case variability in the supersaturation field is predicted with fluctuation amplitudes of up to 0.03, although such large values are rare. A vertical gradient of supersaturation (≈ 10−3 m−1) is generated for high turbulence intensities due to the development of a dry adiabatic lapse rate. Entrainment of slightly warmer (less than 0.1 K) environmental air into the parcel decreases the mean supersaturation by less than 0.01. Supersaturation fluctuations are substantially larger after entrainment events with an additional small offset in absolute humidity (by ±3.5%) between parcel and environmental air. The predicted perturbations of ice supersaturation are significant enough to motivate studies of turbulence-ice nucleation interactions during cirrus formation that abandon the assumption of instantaneous mixing inherent to traditional parcel models.
{"title":"Effects of turbulence on upper tropospheric ice supersaturation","authors":"B. Kärcher, F. Hoffmann, A. Podglajen, A. Hertzog, R. Pluogonven, R. Atlas, M. Corcos, W. Grabowski, B. Gasparini","doi":"10.1175/jas-d-23-0217.1","DOIUrl":"https://doi.org/10.1175/jas-d-23-0217.1","url":null,"abstract":"\u0000Effects of turbulence on ice supersaturation at cirrus heights (> 8 km) remain unexplored. Small-scale mixing processes become important for high Reynolds number flows, which may develop below the buoyancy length scale (10–100 m). The current study couples a stochastic turbulent mixing model with reduced dimensionality to an entraining parcel model to investigate, in large-ensemble simulations, how supersaturation evolves due to homogeneous turbulence in the stably stratified, cloud-free upper troposphere. The rising parcel is forced by a mesoscale updraft. The perturbation of an initially homogeneous vertical distribution of supersaturation is studied after a 36 m ascent in a baseline case and several sensitivity scenarios. Turbulent mixing and associated temperature fluctuations alone lead to changes in ensemble-mean distributions with standard deviations in the range 0.001 – 0.006, while mean values are hardly affected. Large case-to-case variability in the supersaturation field is predicted with fluctuation amplitudes of up to 0.03, although such large values are rare. A vertical gradient of supersaturation (≈ 10−3 m−1) is generated for high turbulence intensities due to the development of a dry adiabatic lapse rate. Entrainment of slightly warmer (less than 0.1 K) environmental air into the parcel decreases the mean supersaturation by less than 0.01. Supersaturation fluctuations are substantially larger after entrainment events with an additional small offset in absolute humidity (by ±3.5%) between parcel and environmental air. The predicted perturbations of ice supersaturation are significant enough to motivate studies of turbulence-ice nucleation interactions during cirrus formation that abandon the assumption of instantaneous mixing inherent to traditional parcel models.","PeriodicalId":508177,"journal":{"name":"Journal of the Atmospheric Sciences","volume":"7 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141700427","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The physics of the surface drag (or surface stress) boundary condition is explored in the context of semi-idealized flows past realistic terrain. Numerical experiments are presented to explore the impact of the drag condition on flows past a region of complex topography, with a particular focus on the dependence on terrain geometry. Arguments are presented to show that the drag condition depends on the geometry of the terrain in two respects: (i) a dependence on terrain slope, as represented by a normal gradient term; and (ii) a dependence on the curvature, which appears in the drag condition as a Dirichlet term. The dependence on the geometry is illustrated through a series of numerical experiments in which simulations using the full form of the drag condition are compared to companion simulations using one of two widely used approximations: (a) the normal gradient condition, which accounts for the terrain slope but neglects curvature; and (b) the flat boundary assumption, which neglects both slope and curvature. The results show that the role of the terrain geometry in the drag condition is strongly dependent on grid spacing, with more highly resolved topography leading to a stronger dependence on the slope and curvature. For sufficiently high resolutions, the dependence on the geometry becomes significant, to the extent that simulations using the approximate drag conditions fail to capture important aspects of the flow. Some basic implications of these results for the problem of high resolution wind energy forecasting are discussed.
{"title":"Surface drag on deformed topographic boundaries: Tests using a semi-idealized model","authors":"Yi Li, C. Epifanio","doi":"10.1175/jas-d-22-0235.1","DOIUrl":"https://doi.org/10.1175/jas-d-22-0235.1","url":null,"abstract":"\u0000The physics of the surface drag (or surface stress) boundary condition is explored\u0000in the context of semi-idealized flows past realistic terrain. Numerical experiments are presented to explore the impact of the drag condition on flows past a region of complex topography, with a particular focus on the dependence on terrain geometry. Arguments are presented to show that the drag condition depends on the geometry of the terrain in two respects: (i) a dependence on terrain slope, as represented by a normal gradient term; and (ii) a dependence on the curvature, which appears in the drag condition as a Dirichlet term. The dependence on the geometry is illustrated through a series of numerical experiments in which simulations using the full form of the drag condition are compared to companion simulations using one of two widely used approximations: (a) the normal gradient condition, which accounts for the terrain slope but neglects curvature; and (b) the flat boundary assumption, which neglects both slope and curvature. The results show that the role of the terrain geometry in the drag condition is strongly dependent on grid spacing, with more highly resolved topography leading to a stronger dependence on the slope and curvature. For sufficiently high resolutions, the dependence on the geometry becomes significant, to the extent that simulations using the approximate drag conditions fail to capture important aspects of the flow. Some basic implications of these results for the problem of high resolution wind energy forecasting are discussed.","PeriodicalId":508177,"journal":{"name":"Journal of the Atmospheric Sciences","volume":"6 5","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-05-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140963530","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Sofia Menemenlis, Gabriel A. Vecchi, Kun Gao, James A Smith, Kai-Yuan Cheng
The extratropical stage of Hurricane Ida (2021) brought extreme sub-daily rainfall and devastating flooding to parts of eastern Pennsylvania, New Jersey, and New York. We investigate the predictability and character of this event using 31-member ensembles of perturbed-initial condition hindcasts with T-SHiELD, a ∼13 km global weather forecast model with a ∼3 km nested grid. At lead times of up to four days, the ensembles are able to capture the most extreme observed hourly and daily rainfall accumulations, but are negatively biased in the spatial extent of heavy precipitation. Large intra-ensemble differences in the magnitudes and locations of simulated extremes suggest that although impacts were highly localized, risks were widespread. In Ida’s tropical stage, inter-ensemble spread in extreme hourly rainfall is well predicted by large-scale moisture convergence; by contrast, in Ida’s extratropical stage, the most extreme rainfall is governed by mesoscale processes that exhibit chaotic and diverse forms across the ensembles. Our results are relevant to forecasting and communication in advance of extratropical transition, and imply that flood preparedness efforts should account for the widespread possibility of severe localized impacts.
{"title":"Extreme Rainfall Risk in Hurricane Ida’s Extratropical Stage: An Analysis with Convection-Permitting Ensemble Hindcasts","authors":"Sofia Menemenlis, Gabriel A. Vecchi, Kun Gao, James A Smith, Kai-Yuan Cheng","doi":"10.1175/jas-d-23-0160.1","DOIUrl":"https://doi.org/10.1175/jas-d-23-0160.1","url":null,"abstract":"\u0000The extratropical stage of Hurricane Ida (2021) brought extreme sub-daily rainfall and devastating flooding to parts of eastern Pennsylvania, New Jersey, and New York. We investigate the predictability and character of this event using 31-member ensembles of perturbed-initial condition hindcasts with T-SHiELD, a ∼13 km global weather forecast model with a ∼3 km nested grid. At lead times of up to four days, the ensembles are able to capture the most extreme observed hourly and daily rainfall accumulations, but are negatively biased in the spatial extent of heavy precipitation. Large intra-ensemble differences in the magnitudes and locations of simulated extremes suggest that although impacts were highly localized, risks were widespread. In Ida’s tropical stage, inter-ensemble spread in extreme hourly rainfall is well predicted by large-scale moisture convergence; by contrast, in Ida’s extratropical stage, the most extreme rainfall is governed by mesoscale processes that exhibit chaotic and diverse forms across the ensembles. Our results are relevant to forecasting and communication in advance of extratropical transition, and imply that flood preparedness efforts should account for the widespread possibility of severe localized impacts.","PeriodicalId":508177,"journal":{"name":"Journal of the Atmospheric Sciences","volume":"40 9","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-05-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140979339","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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