It has been proposed that air pollution increases the updraft speeds of warm-phase convective clouds by reducing their supersaturation and, thereby, enhancing their buoyancy. Observations from the GoAmazon field campaign, sampled using subjective criteria, have been offered as evidence for this warm-phase invigoration. Here, we reexamine those GoAmazon observations using objective criteria and find no indication that air pollution increases warm-phase updraft speeds. In addition, the observations yield no statistically significant relationship between aerosol concentrations and either moist-convective vertical velocity or reflectivity in either the lower or upper troposphere.
{"title":"No warm-phase invigoration of convection detected during GoAmazon","authors":"R. Öktem, D. Romps, A. Varble","doi":"10.1175/jas-d-22-0241.1","DOIUrl":"https://doi.org/10.1175/jas-d-22-0241.1","url":null,"abstract":"\u0000It has been proposed that air pollution increases the updraft speeds of warm-phase convective clouds by reducing their supersaturation and, thereby, enhancing their buoyancy. Observations from the GoAmazon field campaign, sampled using subjective criteria, have been offered as evidence for this warm-phase invigoration. Here, we reexamine those GoAmazon observations using objective criteria and find no indication that air pollution increases warm-phase updraft speeds. In addition, the observations yield no statistically significant relationship between aerosol concentrations and either moist-convective vertical velocity or reflectivity in either the lower or upper troposphere.","PeriodicalId":17231,"journal":{"name":"Journal of the Atmospheric Sciences","volume":" ","pages":""},"PeriodicalIF":3.1,"publicationDate":"2023-04-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41755747","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Akshaya C. Nikumbh, A. Thakur, A. Chakraborty, G. S. Bhat, J. Sukhatme
Large-scale extreme rainfall events (LEREs) over central India are produced by monsoon low-pressure systems (LPSs) when assisted by a secondary cyclonic vortex (SCV). Both the LPS and the SCV are embedded in a monsoon trough and form mainly during the positive phase of the boreal summer intraseasonal oscillation. Here, we observe that tropical-extratropical interactions exist during LEREs. Using ray tracing, we show that extratropical Rossby waves propagate to the Indian subcontinent during the summer monsoon season. Stationary Rossby wave rays originating over the north Atlantic ocean reach India following approximately a great circle path at mid-tropospheric levels. This pathway appears to play an important role in tropical-extratropical interactions during LEREs. 77%of LEREs are preceded by a north Atlantic blocking high and 90 % by a quasi-stationary central Asian high. The Atlantic blocking high triggers a quasi-stationary Rossby wave response and strengthens the downstream central Asian high. In turn, the quasi-stationary central Asian high facilitates Rossby wave breaking, transporting high PV streamers and cut-offs equatorward. The central Asian high is in close proximity to the monsoon trough in the mid and lower troposphere. It interacts with the monsoon trough over the northwest Indian subcontinent. The equatorial monsoon trough is strengthened due to the supply of dynamic forcing and static instabilities from the extratropics. This additional forcing from the extratropics creates an environment that is conducive for LEREs.
{"title":"The role of North Atlantic blocking high during large-scale heavy rainfall events over central India","authors":"Akshaya C. Nikumbh, A. Thakur, A. Chakraborty, G. S. Bhat, J. Sukhatme","doi":"10.1175/jas-d-22-0185.1","DOIUrl":"https://doi.org/10.1175/jas-d-22-0185.1","url":null,"abstract":"\u0000Large-scale extreme rainfall events (LEREs) over central India are produced by monsoon low-pressure systems (LPSs) when assisted by a secondary cyclonic vortex (SCV). Both the LPS and the SCV are embedded in a monsoon trough and form mainly during the positive phase of the boreal summer intraseasonal oscillation. Here, we observe that tropical-extratropical interactions exist during LEREs. Using ray tracing, we show that extratropical Rossby waves propagate to the Indian subcontinent during the summer monsoon season. Stationary Rossby wave rays originating over the north Atlantic ocean reach India following approximately a great circle path at mid-tropospheric levels. This pathway appears to play an important role in tropical-extratropical interactions during LEREs. 77%of LEREs are preceded by a north Atlantic blocking high and 90 % by a quasi-stationary central Asian high. The Atlantic blocking high triggers a quasi-stationary Rossby wave response and strengthens the downstream central Asian high. In turn, the quasi-stationary central Asian high facilitates Rossby wave breaking, transporting high PV streamers and cut-offs equatorward. The central Asian high is in close proximity to the monsoon trough in the mid and lower troposphere. It interacts with the monsoon trough over the northwest Indian subcontinent. The equatorial monsoon trough is strengthened due to the supply of dynamic forcing and static instabilities from the extratropics. This additional forcing from the extratropics creates an environment that is conducive for LEREs.","PeriodicalId":17231,"journal":{"name":"Journal of the Atmospheric Sciences","volume":" ","pages":""},"PeriodicalIF":3.1,"publicationDate":"2023-04-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44280942","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Xiping Zeng, Z. Ulanowski, A. Heymsfield, Yansen Wang, Xiaowen Li
The stability of ice crystal orientation is studied by modeling the air flow around ice crystals at moderate Reynolds number, where an ice crystal is approximated by a cylinder with three parameters: diameter D, length L, and zenith angle of the axis θ. In this paper the torque acting on ice crystals is simulated at different θ first and then a special θ with zero horizontal torque, denoted as θe, is sought as an equilibrium of ice crystal orientation. The equilibrium is classified into two kinds: stable and unstable. Ice crystals rotate to θe of stable equilibriums while deviating from θe of unstable ones once they are released into quiet air. Multiple equilibriums of ice crystal orientation are found via numerical simulations. A cylinder with D/L close to one has three equilibriums, two of which are stable (i.e., θe = 0 and 90°). A cylinder with D/L away from one has only two equilibriums, one of which is stable (i.e., either θe = 0 or 90°). In addition, an asymmetric cylinder has two, three or five equilibriums, and their θ e is sensitive to the distance between its geometrical center and its center of gravity. The sensitivity of θe to crystal asymmetry suggests large symmetric ice crystals tend to become asymmetric (or irregular) and subsequently oriented randomly.
{"title":"Stability Analysis of Ice Crystal Orientation","authors":"Xiping Zeng, Z. Ulanowski, A. Heymsfield, Yansen Wang, Xiaowen Li","doi":"10.1175/jas-d-22-0223.1","DOIUrl":"https://doi.org/10.1175/jas-d-22-0223.1","url":null,"abstract":"\u0000The stability of ice crystal orientation is studied by modeling the air flow around ice crystals at moderate Reynolds number, where an ice crystal is approximated by a cylinder with three parameters: diameter D, length L, and zenith angle of the axis θ. In this paper the torque acting on ice crystals is simulated at different θ first and then a special θ with zero horizontal torque, denoted as θe, is sought as an equilibrium of ice crystal orientation. The equilibrium is classified into two kinds: stable and unstable. Ice crystals rotate to θe of stable equilibriums while deviating from θe of unstable ones once they are released into quiet air.\u0000Multiple equilibriums of ice crystal orientation are found via numerical simulations. A cylinder with D/L close to one has three equilibriums, two of which are stable (i.e., θe = 0 and 90°). A cylinder with D/L away from one has only two equilibriums, one of which is stable (i.e., either θe = 0 or 90°). In addition, an asymmetric cylinder has two, three or five equilibriums, and their θ e is sensitive to the distance between its geometrical center and its center of gravity. The sensitivity of θe to crystal asymmetry suggests large symmetric ice crystals tend to become asymmetric (or irregular) and subsequently oriented randomly.","PeriodicalId":17231,"journal":{"name":"Journal of the Atmospheric Sciences","volume":" ","pages":""},"PeriodicalIF":3.1,"publicationDate":"2023-04-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45104477","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Recently, Nakamura and Huang proposed a theory of blocking onset based on the budget of finite-amplitude local wave activity on the mid-latitude waveguide. Blocks form in their idealized model due to a mechanism that also describes the emergence of traffic jams in traffic theory. The currentwork investigates the development of a winter European block in terms of finite-amplitude local wave activity to evaluate the possible relevance of the “traffic jam” mechanism for the flowtransition. 200 members of a medium-range ensemble forecast of the blocking onset period are analyzed with correlation- and cluster-based sensitivity techniques. Diagnostic evidence points to a traffic jam onset on 17 December 2016. Block development is sensitive to upstream Rossby wave activity up to 1.5 days prior to its initiation and consistent with expectations from the idealized theory. Eastward transport of finite-amplitude local wave activity in the southern part of the block is suppressed by non-linear flux modification from the large-amplitude blocking pattern, consistent with the expected obstruction in the traffic jam model. The relationship of finite-amplitude local wave activity and its zonal flux as mapped by the ensemble exhibits established characteristics of a traffic jam. This study suggests that the traffic jam mechanism may play an important role in some cases of blocking onset and more generally that applying finite-amplitude local wave activity diagnostics to ensemble data is a promising approach for the further examination of individual onset events in light of the Nakamura and Huang theory.
{"title":"The Onset of a Blocking Event as a “Traffic Jam”: Characterization with Ensemble Sensitivity Analysis","authors":"Christopher Polster, V. Wirth","doi":"10.1175/jas-d-21-0312.1","DOIUrl":"https://doi.org/10.1175/jas-d-21-0312.1","url":null,"abstract":"\u0000Recently, Nakamura and Huang proposed a theory of blocking onset based on the budget of finite-amplitude local wave activity on the mid-latitude waveguide. Blocks form in their idealized model due to a mechanism that also describes the emergence of traffic jams in traffic theory. The currentwork investigates the development of a winter European block in terms of finite-amplitude local wave activity to evaluate the possible relevance of the “traffic jam” mechanism for the flowtransition. 200 members of a medium-range ensemble forecast of the blocking onset period are analyzed with correlation- and cluster-based sensitivity techniques. Diagnostic evidence points to a traffic jam onset on 17 December 2016. Block development is sensitive to upstream Rossby wave activity up to 1.5 days prior to its initiation and consistent with expectations from the idealized theory. Eastward transport of finite-amplitude local wave activity in the southern part of the block is suppressed by non-linear flux modification from the large-amplitude blocking pattern, consistent with the expected obstruction in the traffic jam model. The relationship of finite-amplitude local wave activity and its zonal flux as mapped by the ensemble exhibits established characteristics of a traffic jam. This study suggests that the traffic jam mechanism may play an important role in some cases of blocking onset and more generally that applying finite-amplitude local wave activity diagnostics to ensemble data is a promising approach for the further examination of individual onset events in light of the Nakamura and Huang theory.","PeriodicalId":17231,"journal":{"name":"Journal of the Atmospheric Sciences","volume":" ","pages":""},"PeriodicalIF":3.1,"publicationDate":"2023-04-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43177736","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jun Peng, Zongheng Li, Lifeng Zhang, Yuanshuen Wang, Hanyan Wu
A new formulation of the spectral budget of vertical vorticity and horizontal divergence suitable for the mesoscale atmosphere on an f plane is derived. Compared to previous formulation in large-scale studies, there are three main improvements: (i) both the squared vorticity (SV; i.e., enstrophy as usual) and squared divergence (SD) spectra are taken into account, (ii) the spectral transfers of SV and SD between scales are exactly constructed under the nonlinear advection of the full horizontal velocity, and (iii) the general relationship between spectral energy and SV/SD transfers is derived. With this new formulation, the atmospheric spectra of divergent and rotational motion components are investigated through numerical simulation of idealized dry baroclinic waves. Spectral budget analysis shows that, in the present dry simulation, the upper troposphere is almost completely dominated by the downscale SV transfer at all scales, while the lower stratosphere is dominated by the downscale SV transfer at synoptic scales and by the downscale SD transfer at mesoscales. The pressure-related term is largely cancelled out by the conversion term between SV and SD at both levels, but at the small-scale end of lower-stratospheric mesoscales there exists a significant net positive forcing, accounting for the distinct spectral transition of the total spectrum there. An explicit association between spectral energy and SV/SD transfers is further made. In the upper troposphere, the downscale enegy cascade is mainly governed by the downscale SV transfer; while in the lower stratosphere, it is mainly governed by the residual term related to non-uniformly distributed vertical velocity.
{"title":"Mesoscale Spectra of Vertical Vorticity and Horizontal Divergence in an Idealized Baroclinic Wave Simulation","authors":"Jun Peng, Zongheng Li, Lifeng Zhang, Yuanshuen Wang, Hanyan Wu","doi":"10.1175/jas-d-22-0213.1","DOIUrl":"https://doi.org/10.1175/jas-d-22-0213.1","url":null,"abstract":"A new formulation of the spectral budget of vertical vorticity and horizontal divergence suitable for the mesoscale atmosphere on an f plane is derived. Compared to previous formulation in large-scale studies, there are three main improvements: (i) both the squared vorticity (SV; i.e., enstrophy as usual) and squared divergence (SD) spectra are taken into account, (ii) the spectral transfers of SV and SD between scales are exactly constructed under the nonlinear advection of the full horizontal velocity, and (iii) the general relationship between spectral energy and SV/SD transfers is derived. With this new formulation, the atmospheric spectra of divergent and rotational motion components are investigated through numerical simulation of idealized dry baroclinic waves. Spectral budget analysis shows that, in the present dry simulation, the upper troposphere is almost completely dominated by the downscale SV transfer at all scales, while the lower stratosphere is dominated by the downscale SV transfer at synoptic scales and by the downscale SD transfer at mesoscales. The pressure-related term is largely cancelled out by the conversion term between SV and SD at both levels, but at the small-scale end of lower-stratospheric mesoscales there exists a significant net positive forcing, accounting for the distinct spectral transition of the total spectrum there. An explicit association between spectral energy and SV/SD transfers is further made. In the upper troposphere, the downscale enegy cascade is mainly governed by the downscale SV transfer; while in the lower stratosphere, it is mainly governed by the residual term related to non-uniformly distributed vertical velocity.","PeriodicalId":17231,"journal":{"name":"Journal of the Atmospheric Sciences","volume":" ","pages":""},"PeriodicalIF":3.1,"publicationDate":"2023-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47041259","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Z. Prochazkova, C. Kruse, M. Alexander, L. Hoffmann, J. Bacmeister, L. Holt, C. Wright, Kaoru Sato, S. Gisinger, M. Ern, M. Geldenhuys, P. Preusse, P. Šácha
Internal gravity waves (GWs) are ubiquitous in the atmosphere, making significant contributions to the mesoscale motions. Since the majority of their spectrum is unresolved in global circulation models, their effects need to be parameterized. In recent decades GWs have been increasingly studied in high-resolution simulations, which, unlike direct observations, allow us to explore full spatio-temporal variations of the resolved wave field. In our study we analyze and refine a traditional method for GW analysis in a high-resolution simulation on a regional domain around the Drake Passage. We show that GW momentum drag estimates based on the Gaussian high-pass filter method applied to separate GW perturbations from the background are sensitive to the choice of a cutoff parameter. The impact of the cutoff parameter is higher for horizontal fluxes of horizontal momentum, which indicates higher sensitivity for horizontally propagating waves. Two modified methods, which choose the parameter value from spectral information, are proposed. The dynamically determined cutoff is mostly higher than the traditional cutoff values around 500 km, leading to larger GW fluxes and drag, and varies with time and altitude. The differences between the traditional and the modified methods are especially pronounced during events with significant drag contributions from horizontal momentum fluxes.
{"title":"Sensitivity of mountain wave drag estimates on separation methods and proposed improvements","authors":"Z. Prochazkova, C. Kruse, M. Alexander, L. Hoffmann, J. Bacmeister, L. Holt, C. Wright, Kaoru Sato, S. Gisinger, M. Ern, M. Geldenhuys, P. Preusse, P. Šácha","doi":"10.1175/jas-d-22-0151.1","DOIUrl":"https://doi.org/10.1175/jas-d-22-0151.1","url":null,"abstract":"\u0000Internal gravity waves (GWs) are ubiquitous in the atmosphere, making significant contributions to the mesoscale motions. Since the majority of their spectrum is unresolved in global circulation models, their effects need to be parameterized. In recent decades GWs have been increasingly studied in high-resolution simulations, which, unlike direct observations, allow us to explore full spatio-temporal variations of the resolved wave field. In our study we analyze and refine a traditional method for GW analysis in a high-resolution simulation on a regional domain around the Drake Passage. We show that GW momentum drag estimates based on the Gaussian high-pass filter method applied to separate GW perturbations from the background are sensitive to the choice of a cutoff parameter. The impact of the cutoff parameter is higher for horizontal fluxes of horizontal momentum, which indicates higher sensitivity for horizontally propagating waves. Two modified methods, which choose the parameter value from spectral information, are proposed. The dynamically determined cutoff is mostly higher than the traditional cutoff values around 500 km, leading to larger GW fluxes and drag, and varies with time and altitude. The differences between the traditional and the modified methods are especially pronounced during events with significant drag contributions from horizontal momentum fluxes.","PeriodicalId":17231,"journal":{"name":"Journal of the Atmospheric Sciences","volume":" ","pages":""},"PeriodicalIF":3.1,"publicationDate":"2023-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41553752","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jiawenjing Lan, Jun Yang, Yongyun Hu, Xiang Li, Jiaqi Guo, Qifan Lin, Jing Han, Jian Zhang, Shuang Wang, Ji Nie
Abstract For modern Earth, the annual-mean equatorial winds in the upper troposphere are flowing from east to west (i.e., easterly winds). This is mainly due to the deceleration effect of the seasonal cross-equatorial Hadley cells, against the relatively weaker acceleration effect of coupled Rossby and Kelvin waves excited from tropical convection and latent heat release. In this work, we examine the evolution of equatorial winds during the past 250 million years using one global Earth system model, the Community Earth System Model version 1.2.2 (CESM1.2.2). Three climatic factors different from the modern Earth—solar constant, atmospheric CO 2 concentration, and land–sea configuration—are considered in the simulations. We find that the upper-tropospheric equatorial winds change sign to westerly flows (called equatorial superrotation) in certain eras, such as 250–230 and 150–50 Ma. The strength of the superrotation is below 4 m s −1 , comparable to the magnitude of the present-day easterly winds. In general, this phenomenon occurs in a warmer climate within which the tropical atmospheric circulation shifts upward in altitude, stationary and/or transient eddies are relatively stronger, and/or the Hadley cells are relatively weaker, which in turn are due to the changes of the three factors, especially CO 2 concentration and land–sea configuration.
对现代地球来说，对流层上层的年平均赤道风是自东向西流动的(即东风)。这主要是由于季节性跨赤道哈德利单体的减速作用，而热带对流和潜热释放激发的耦合罗斯比波和开尔文波的加速作用相对较弱。在这项工作中，我们使用一个全球地球系统模型，即社区地球系统模型1.2.2版(CESM1.2.2)，研究了过去2.5亿年赤道风的演变。模拟中考虑了三个不同于现代地球的气候因子——太阳常数、大气co2浓度和海陆格局。在250 ~ 230 Ma和150 ~ 50 Ma等特定时期，对流层上层赤道风向西风转变(称为赤道超旋)。超级旋转的强度低于4 m s - 1，与今天的东风强度相当。一般来说，这种现象发生在热带大气环流在高度上向上移动，静止和/或瞬变涡流相对较强，和/或Hadley环流相对较弱的温暖气候中，这反过来是由于三个因素的变化，特别是CO 2浓度和陆海配置。
{"title":"Weak Equatorial Superrotation during the Past 250 Million Years","authors":"Jiawenjing Lan, Jun Yang, Yongyun Hu, Xiang Li, Jiaqi Guo, Qifan Lin, Jing Han, Jian Zhang, Shuang Wang, Ji Nie","doi":"10.1175/jas-d-22-0072.1","DOIUrl":"https://doi.org/10.1175/jas-d-22-0072.1","url":null,"abstract":"Abstract For modern Earth, the annual-mean equatorial winds in the upper troposphere are flowing from east to west (i.e., easterly winds). This is mainly due to the deceleration effect of the seasonal cross-equatorial Hadley cells, against the relatively weaker acceleration effect of coupled Rossby and Kelvin waves excited from tropical convection and latent heat release. In this work, we examine the evolution of equatorial winds during the past 250 million years using one global Earth system model, the Community Earth System Model version 1.2.2 (CESM1.2.2). Three climatic factors different from the modern Earth—solar constant, atmospheric CO 2 concentration, and land–sea configuration—are considered in the simulations. We find that the upper-tropospheric equatorial winds change sign to westerly flows (called equatorial superrotation) in certain eras, such as 250–230 and 150–50 Ma. The strength of the superrotation is below 4 m s −1 , comparable to the magnitude of the present-day easterly winds. In general, this phenomenon occurs in a warmer climate within which the tropical atmospheric circulation shifts upward in altitude, stationary and/or transient eddies are relatively stronger, and/or the Hadley cells are relatively weaker, which in turn are due to the changes of the three factors, especially CO 2 concentration and land–sea configuration.","PeriodicalId":17231,"journal":{"name":"Journal of the Atmospheric Sciences","volume":"48 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135067124","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A novel high-resolution regional reanalysis is used to investigate the mesoscale processes that preceded the formation of tropical cyclone (TC) Mora (2017). Both satellite observations and the regional reanalysis show early morning mesoscale convective systems (MCSs) persistently initiated and organized in the downshear quadrant of the preexisting tropical disturbance a few days prior to the genesis of TC Mora. The diurnal MCSs gradually enhanced the meso-α-scale vortex near the center of the preexisting tropical disturbance through vortex stretching, providing a vorticity-rich and moist environment for the following burst of deep convection and enhancement of the meso-β-scale vortex. The regional reanalysis shows that the gravity waves that radiated from afternoon convection over the northern coast of the Bay of Bengal might play an important role in modulating the diurnal cycle of pregenesis MCSs. The diurnal convectively forced gravity waves increased the tropospheric stability, reduced the column saturation fraction, and suppressed deep convection within the preexisting tropical disturbance from noon to evening. Similar quasi-diurnal cycle of organized deep convection prior to TC genesis has also been observed over other basins. However, modeling studies are needed to conclusively demonstrate the relationships between the gravity waves and pregenesis diurnal MCSs. Also, whether diurnal gravity waves play a similar role in modulating the pregenesis deep convection in other TCs is worth future investigations.
{"title":"Diurnal MCSs precede the genesis of tropical cyclone Mora (2017): the role of convectively forced gravity waves","authors":"Xingchao Chen, L. Leung, Zhe Feng, Qiu Yang","doi":"10.1175/jas-d-22-0203.1","DOIUrl":"https://doi.org/10.1175/jas-d-22-0203.1","url":null,"abstract":"\u0000A novel high-resolution regional reanalysis is used to investigate the mesoscale processes that preceded the formation of tropical cyclone (TC) Mora (2017). Both satellite observations and the regional reanalysis show early morning mesoscale convective systems (MCSs) persistently initiated and organized in the downshear quadrant of the preexisting tropical disturbance a few days prior to the genesis of TC Mora. The diurnal MCSs gradually enhanced the meso-α-scale vortex near the center of the preexisting tropical disturbance through vortex stretching, providing a vorticity-rich and moist environment for the following burst of deep convection and enhancement of the meso-β-scale vortex. The regional reanalysis shows that the gravity waves that radiated from afternoon convection over the northern coast of the Bay of Bengal might play an important role in modulating the diurnal cycle of pregenesis MCSs. The diurnal convectively forced gravity waves increased the tropospheric stability, reduced the column saturation fraction, and suppressed deep convection within the preexisting tropical disturbance from noon to evening. Similar quasi-diurnal cycle of organized deep convection prior to TC genesis has also been observed over other basins. However, modeling studies are needed to conclusively demonstrate the relationships between the gravity waves and pregenesis diurnal MCSs. Also, whether diurnal gravity waves play a similar role in modulating the pregenesis deep convection in other TCs is worth future investigations.","PeriodicalId":17231,"journal":{"name":"Journal of the Atmospheric Sciences","volume":" ","pages":""},"PeriodicalIF":3.1,"publicationDate":"2023-03-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42106415","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This study revisits the issue of why tropical cyclones (TCs) develop more rapidly at lower latitudes, using ensemble axisymmetric numerical simulations and energy diagnostics based on the isentropic analysis, with the focus on the relative importance of the outflow-layer and boundary-layer inertial stabilities to TC intensification and energy cycle. Results show that although lowering the outflow-layer Coriolis parameter and thus inertial stability can slightly strengthen the outflow, it does not affect the simulated TC development, whereas lowering the boundary-layer Coriolis parameter largely enhances the secondary circulation and TC intensification as in the experiment with a reduced Coriolis parameter throughout the model atmosphere. This suggests that TC outflow is more likely a passive result of the convergent inflow in the boundary layer and convective updraft in the eyewall. The boundary-layer inertial stability is found to control the convergent inflow in the boundary layer and depth of convection in the eyewall and thus the temperature of energy sink in the TC heat engine, which determines the efficiency and overall mechanical output of heat engine and thus TC intensification. It is also shown that the hypothesized isothermal and adiabatic compression legs at the downstream end of the outflow in the classical Carnot cycle is not supported in the thermodynamic cycle of the simulated TCs, implying that the assumed TC Carnot cycle is not closed. It is the theoretical maximum work of heat engine, not the energy expenditure following the outflow downstream, that determines the mechanical work used to intensify a TC.
{"title":"Is the outflow-layer inertial stability crucial to the energy cycle and development of tropical cyclones?","authors":"Yuanlong Li, Yuqing Wang, Z. Tan","doi":"10.1175/jas-d-22-0186.1","DOIUrl":"https://doi.org/10.1175/jas-d-22-0186.1","url":null,"abstract":"\u0000This study revisits the issue of why tropical cyclones (TCs) develop more rapidly at lower latitudes, using ensemble axisymmetric numerical simulations and energy diagnostics based on the isentropic analysis, with the focus on the relative importance of the outflow-layer and boundary-layer inertial stabilities to TC intensification and energy cycle. Results show that although lowering the outflow-layer Coriolis parameter and thus inertial stability can slightly strengthen the outflow, it does not affect the simulated TC development, whereas lowering the boundary-layer Coriolis parameter largely enhances the secondary circulation and TC intensification as in the experiment with a reduced Coriolis parameter throughout the model atmosphere. This suggests that TC outflow is more likely a passive result of the convergent inflow in the boundary layer and convective updraft in the eyewall.\u0000The boundary-layer inertial stability is found to control the convergent inflow in the boundary layer and depth of convection in the eyewall and thus the temperature of energy sink in the TC heat engine, which determines the efficiency and overall mechanical output of heat engine and thus TC intensification. It is also shown that the hypothesized isothermal and adiabatic compression legs at the downstream end of the outflow in the classical Carnot cycle is not supported in the thermodynamic cycle of the simulated TCs, implying that the assumed TC Carnot cycle is not closed. It is the theoretical maximum work of heat engine, not the energy expenditure following the outflow downstream, that determines the mechanical work used to intensify a TC.","PeriodicalId":17231,"journal":{"name":"Journal of the Atmospheric Sciences","volume":" ","pages":""},"PeriodicalIF":3.1,"publicationDate":"2023-03-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48252764","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We explore the effect of aerosol perturbations on stratocumulus clouds in the context of marine cloud brightening (MCB) using high-resolution large-eddy simulations. We use a Lagrangian cloud microphysical model with very detailed treatment of aerosol activation and droplet growth. The aerosol forcing is represented as a finite-width rectangular pulse in time (uniform in space). We analyze three stratocumulus cloud systems differing in their surface precipitation rate, namely – non-precipitating, intermediate, and precipitating. We report on the diurnal evolution of these cloud systems subjected to a range of perturbations characterized by varying the amplitude and duration of the aerosol forcing pulse. Our simulations show that in the non-precipitating system, the clouds are relatively insensitive to duration and amplitude, and are sensitive only to the total number concentration of the injected aerosol. In contrast, the precipitating cloud system is affected by the duration and the amplitude of the forcing, with the sensitivity conditional on the state of the cloud system before the injection of aerosol particles. We use these case studies to assess the efficacy of potential MCB spraying strategies. Our analysis shows that negative LWP adjustments offset a substantial fraction of the Twomey induced brightening in all three cloud systems. This is countered by substantial cloud brightening obtained through precipitation suppression induced cloud-fraction adjustments.
{"title":"Evaluation of Pulse Aerosol Forcing on Marine Stratocumulus Clouds in the Context of Marine Cloud Brightening","authors":"Prasanth Prabhakaran, F. Hoffmann, G. Feingold","doi":"10.1175/jas-d-22-0207.1","DOIUrl":"https://doi.org/10.1175/jas-d-22-0207.1","url":null,"abstract":"\u0000We explore the effect of aerosol perturbations on stratocumulus clouds in the context of marine cloud brightening (MCB) using high-resolution large-eddy simulations. We use a Lagrangian cloud microphysical model with very detailed treatment of aerosol activation and droplet growth. The aerosol forcing is represented as a finite-width rectangular pulse in time (uniform in space). We analyze three stratocumulus cloud systems differing in their surface precipitation rate, namely – non-precipitating, intermediate, and precipitating. We report on the diurnal evolution of these cloud systems subjected to a range of perturbations characterized by varying the amplitude and duration of the aerosol forcing pulse. Our simulations show that in the non-precipitating system, the clouds are relatively insensitive to duration and amplitude, and are sensitive only to the total number concentration of the injected aerosol. In contrast, the precipitating cloud system is affected by the duration and the amplitude of the forcing, with the sensitivity conditional on the state of the cloud system before the injection of aerosol particles. We use these case studies to assess the efficacy of potential MCB spraying strategies. Our analysis shows that negative LWP adjustments offset a substantial fraction of the Twomey induced brightening in all three cloud systems. This is countered by substantial cloud brightening obtained through precipitation suppression induced cloud-fraction adjustments.","PeriodicalId":17231,"journal":{"name":"Journal of the Atmospheric Sciences","volume":" ","pages":""},"PeriodicalIF":3.1,"publicationDate":"2023-03-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42608978","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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