Search Results
You are looking at 1 - 10 of 16 items for :
- Author or Editor: Wen Chen x
- Monthly Weather Review x
- Refine by Access: All Content x
Abstract
A fully compressible, three-dimensional, nonhydrostatic model is developed using a semi-implicit scheme to avoid an extremely small time step. As a result of applying the implicit scheme to high-frequency waves, an elliptic partial differential equation (EPDE) has been introduced. A multigrid solver is applied to solve the EPDEs, which include cross-derivative terms due to terrain-following coordinate transformation.
Several experiments have been performed to evaluate the model as well as the performance of the scheme with respect to tolerance number, relaxation choice, sweeps of prerelaxation and postrelaxation, and a flexible hybrid coordinate (FHC).
An FHC with two functions (base and deviation functions) is introduced. The basic function provides constant vertical grid spacing required in the multigrid solver, while the deviation function helps to adjust the vertical resolution.
Abstract
A fully compressible, three-dimensional, nonhydrostatic model is developed using a semi-implicit scheme to avoid an extremely small time step. As a result of applying the implicit scheme to high-frequency waves, an elliptic partial differential equation (EPDE) has been introduced. A multigrid solver is applied to solve the EPDEs, which include cross-derivative terms due to terrain-following coordinate transformation.
Several experiments have been performed to evaluate the model as well as the performance of the scheme with respect to tolerance number, relaxation choice, sweeps of prerelaxation and postrelaxation, and a flexible hybrid coordinate (FHC).
An FHC with two functions (base and deviation functions) is introduced. The basic function provides constant vertical grid spacing required in the multigrid solver, while the deviation function helps to adjust the vertical resolution.
Abstract
A method of computing low-level trajectories from observed sea level pressure is shown to be capable of distinguishing cases of blocked and non-blocked flow around the Alps. A flow-splitting parameter (S), derived from the trajectories, is found to be a reasonable criterion for distinguishing blocked from nonblocked cases, with a value of S = 1.5 serving as a useful threshold for classification. In considering eight cases of postfrontal cold flow against the Alps, only a few can be clearly identified as blocking or noblocking, and even these identifications may not pertain to all levels and all segments of the Alpine chain.
The trajectory-based classification scheme compares well with sounding and flight level data from the NOAA P-3 research aircraft. The degree of blocking is shown to be related to the upstream Froude number and the strength of the upstream pressure nose.
A remarkable result is that the observed surface wind patterns appear blocked in all cases considered here. We refer to this as “boundary layer blocking”
Unsteady effects tend to stretch out streaklines east to west along the northern foothhills of the Alps.
Abstract
A method of computing low-level trajectories from observed sea level pressure is shown to be capable of distinguishing cases of blocked and non-blocked flow around the Alps. A flow-splitting parameter (S), derived from the trajectories, is found to be a reasonable criterion for distinguishing blocked from nonblocked cases, with a value of S = 1.5 serving as a useful threshold for classification. In considering eight cases of postfrontal cold flow against the Alps, only a few can be clearly identified as blocking or noblocking, and even these identifications may not pertain to all levels and all segments of the Alpine chain.
The trajectory-based classification scheme compares well with sounding and flight level data from the NOAA P-3 research aircraft. The degree of blocking is shown to be related to the upstream Froude number and the strength of the upstream pressure nose.
A remarkable result is that the observed surface wind patterns appear blocked in all cases considered here. We refer to this as “boundary layer blocking”
Unsteady effects tend to stretch out streaklines east to west along the northern foothhills of the Alps.
Abstract
The El Niño–Southern Oscillation (ENSO) is regarded as one of the most important factors for onset of the South China Sea summer monsoon (SCSSM). Previous studies generally indicated that an El Niño event tends to result in a late onset of the SCSSM monsoon. However, this relationship has not been true in recent years, particularly when an extremely early SCSSM onset (1 May 2019) occurred following the 2018/19 El Niño event in the preceding winter. The processes of the second earliest SCSSM onset in the past 41 years were investigated using NCEP–DOE reanalysis, OLR data, and ERSST. A negative sea surface temperature and associated anticyclonic anomalies were absent over the western North Pacific in the late spring of 2019 following an El Niño event in the preceding winter. Thus, the mean circulation in the late spring of 2019 does not prevent SCSSM onset, which is in sharp contrast to the composited spring of the El Niño decaying years. The convective active and westerly phases of a 30–60-day oscillation originating from the Indian Ocean provided a favorable background for the SCSSM onset in 2019. In addition, the monsoon onset vortex over the Bay of Bengal and the cold front associated with a midlatitude trough over East Asia also played important roles in triggering the early onset of the SCSSM in 2019. No tropical cyclone appeared over the western North Pacific during April and May, and the enhancement of quasi-biweekly oscillation mainly occurs after the SCSSM onset; thus, these two factors contribute little to the SCSSM onset in 2019.
Abstract
The El Niño–Southern Oscillation (ENSO) is regarded as one of the most important factors for onset of the South China Sea summer monsoon (SCSSM). Previous studies generally indicated that an El Niño event tends to result in a late onset of the SCSSM monsoon. However, this relationship has not been true in recent years, particularly when an extremely early SCSSM onset (1 May 2019) occurred following the 2018/19 El Niño event in the preceding winter. The processes of the second earliest SCSSM onset in the past 41 years were investigated using NCEP–DOE reanalysis, OLR data, and ERSST. A negative sea surface temperature and associated anticyclonic anomalies were absent over the western North Pacific in the late spring of 2019 following an El Niño event in the preceding winter. Thus, the mean circulation in the late spring of 2019 does not prevent SCSSM onset, which is in sharp contrast to the composited spring of the El Niño decaying years. The convective active and westerly phases of a 30–60-day oscillation originating from the Indian Ocean provided a favorable background for the SCSSM onset in 2019. In addition, the monsoon onset vortex over the Bay of Bengal and the cold front associated with a midlatitude trough over East Asia also played important roles in triggering the early onset of the SCSSM in 2019. No tropical cyclone appeared over the western North Pacific during April and May, and the enhancement of quasi-biweekly oscillation mainly occurs after the SCSSM onset; thus, these two factors contribute little to the SCSSM onset in 2019.
Abstract
The field program TAMEX (Taiwan Area Mesoscale Experiment) was held during May and June 1987. One of its objectives was to study the cited of terrain on precipitation systems. On 7 June 1987 a band of radar echo, orientated from north to south, developed during the afternoon along the western slope and mountainous area of Taiwan island. Before this system moved eastward toward the Pacific Ocean in the late afternoon, it dumped more than 100 mm of precipitation at a few stations in only a few hours. The analysis of radar data from CAA radar revealed that the precipitation occurred over western-sloped terrain and a mountain plateau in the early afternoon. The system was wider than 60 km in the east-west direction, and the echo top was higher than 10 km. The maximum reflectivity was over 50 dBZ along the steep slope and near the mountain peak. The precipitation system over the mountain area extended eastward with the passage of time; meanwhile, new echoes continually formed along the western-sloped area and moved eastward. They intensified as they moved toward the mountain peak merging with the precipitation system. Through this mechanism the precipitation system could maintain itself for several hours and produce a large amount of rainfall.
A two-dimensional numerical cloud model with a terrain-following coordinate system, similar to the one developed by Durran and Klemp, was used to investigate the topographic effect on the precipitation system. A smoother terrain feature was used for the lower boundary, with a 30-km-wide mountain plateau (of less than 1 km in height) and sloped terrain on the western and eastern sides. Surface heating and boundary-layer moisture supply were parameterized in the model. Simulation results indicated that during the early simulation a cell formed near the foothills of the west slope and moved eastward. As it climbed up the sloping terrain it intensified. Its speed decreased and its high intensity was maintained over the slope and the mountain plateau. At the same time, a new cell formed west of the older cell and moved eastward. Finally this new cell merged into the western side of the older one near the mountain peak to form one precipitation system and moved eastward slowly. Thus, the intensity of the merged system was enhanced over the mountain plateau. While this system maintained its high intensity and moved eastward, new cells continually formed along the western slope and moved eastward to merge into the western side of the precipitation system over the mountainous area. The intensity of the precipitation system was enhanced for a few hours over the mountain itself and became a long-lasting system. Toward the end of the simulation, this long-lasting system had moved near the eastern slope and had still maintained its intensity. At the same time, the low-level temperature decreased over the mountainous area as a result of precipitation evaporation. When new cells, forming over the western slope, moved toward the mountain plateau, they entered their decaying stage 45 min after their occurrence. They did not merge into the existing system on the eastern part of the mountain; therefore, the precipitation over the mountain plateau became weaker.
Several sensitivity tests have been made to study the effect of varying the magnitude of surface heating, the boundary-layer moisture supply, the height of the terrain, and the temperature, moisture, and wind profiles on the simulation result. The result indicated that low-level and midlevel moisture were important for the formation of new cells over the western slope and a long-lasting system over the mountain area, respectively. The initial wind speed of 7 m s−1 below 4 km and calm wind above 4 km was used in the model; then a long-lasting precipitation system over the mountainous area appeared. If the wind speed was reduced to 3.5 m s−1, only new cells formed over the western slope. If the maximum height of the terrain was decreased from 1 to 0.5 km, then only new cells formed over the slope area. Hence, sensitivity tests indicated that the combination of the adequate thermodynamic structure, the westerly wind pattern, and the correct size of the mountain could help form both the new cells over the sloped terrain and a long-lasting system over mountain areas as in northern Taiwan on 7 June 1987 during TAMEX. The surface heating effect played the role of creating the upslope wind and augmentation of this precipitation system.
Abstract
The field program TAMEX (Taiwan Area Mesoscale Experiment) was held during May and June 1987. One of its objectives was to study the cited of terrain on precipitation systems. On 7 June 1987 a band of radar echo, orientated from north to south, developed during the afternoon along the western slope and mountainous area of Taiwan island. Before this system moved eastward toward the Pacific Ocean in the late afternoon, it dumped more than 100 mm of precipitation at a few stations in only a few hours. The analysis of radar data from CAA radar revealed that the precipitation occurred over western-sloped terrain and a mountain plateau in the early afternoon. The system was wider than 60 km in the east-west direction, and the echo top was higher than 10 km. The maximum reflectivity was over 50 dBZ along the steep slope and near the mountain peak. The precipitation system over the mountain area extended eastward with the passage of time; meanwhile, new echoes continually formed along the western-sloped area and moved eastward. They intensified as they moved toward the mountain peak merging with the precipitation system. Through this mechanism the precipitation system could maintain itself for several hours and produce a large amount of rainfall.
A two-dimensional numerical cloud model with a terrain-following coordinate system, similar to the one developed by Durran and Klemp, was used to investigate the topographic effect on the precipitation system. A smoother terrain feature was used for the lower boundary, with a 30-km-wide mountain plateau (of less than 1 km in height) and sloped terrain on the western and eastern sides. Surface heating and boundary-layer moisture supply were parameterized in the model. Simulation results indicated that during the early simulation a cell formed near the foothills of the west slope and moved eastward. As it climbed up the sloping terrain it intensified. Its speed decreased and its high intensity was maintained over the slope and the mountain plateau. At the same time, a new cell formed west of the older cell and moved eastward. Finally this new cell merged into the western side of the older one near the mountain peak to form one precipitation system and moved eastward slowly. Thus, the intensity of the merged system was enhanced over the mountain plateau. While this system maintained its high intensity and moved eastward, new cells continually formed along the western slope and moved eastward to merge into the western side of the precipitation system over the mountainous area. The intensity of the precipitation system was enhanced for a few hours over the mountain itself and became a long-lasting system. Toward the end of the simulation, this long-lasting system had moved near the eastern slope and had still maintained its intensity. At the same time, the low-level temperature decreased over the mountainous area as a result of precipitation evaporation. When new cells, forming over the western slope, moved toward the mountain plateau, they entered their decaying stage 45 min after their occurrence. They did not merge into the existing system on the eastern part of the mountain; therefore, the precipitation over the mountain plateau became weaker.
Several sensitivity tests have been made to study the effect of varying the magnitude of surface heating, the boundary-layer moisture supply, the height of the terrain, and the temperature, moisture, and wind profiles on the simulation result. The result indicated that low-level and midlevel moisture were important for the formation of new cells over the western slope and a long-lasting system over the mountain area, respectively. The initial wind speed of 7 m s−1 below 4 km and calm wind above 4 km was used in the model; then a long-lasting precipitation system over the mountainous area appeared. If the wind speed was reduced to 3.5 m s−1, only new cells formed over the western slope. If the maximum height of the terrain was decreased from 1 to 0.5 km, then only new cells formed over the slope area. Hence, sensitivity tests indicated that the combination of the adequate thermodynamic structure, the westerly wind pattern, and the correct size of the mountain could help form both the new cells over the sloped terrain and a long-lasting system over mountain areas as in northern Taiwan on 7 June 1987 during TAMEX. The surface heating effect played the role of creating the upslope wind and augmentation of this precipitation system.
Abstract
A heavy rainfall event during the Taiwan Area Mesoscale Experiment intensive observing period 13 has been studied using upper-air, surface mesonet, and dual-Doppler radar data. The heavy rainfall (≥231 mm day−1) occurred over northwestern Taiwan with the maximum rainfall along the northwestern coast and was caused by a long-lived, convective rainband in the prefrontal atmosphere. It occurred in an upper-level divergence region and along the axis of the maximum equivalent potential temperature at the 850-hPa level.
As a Mei-Yu front advanced southeastward, the postfrontal cold air in the lowest levels was retarded by the hilly terrain along the southeastern China coast. As a result, a low-level wind-shift line associated with a pressure trough at the 850-hPa level moved over the Taiwan Strait before the arrival of the surface front. The westerly flow behind the trough interacted with a barrier jet along the northwestern coast of Taiwan. The barrier jet is caused by the interaction between the prefrontal southwest monsoon flow and the island obstacle. A low-level convergence zone (∼3 km deep) was observed along the wind-shift line between the westerly flow coming off the southeastern China coast and the barrier jet. A long-lived rainband developed within the low-level convergence zone and moved southeastward toward the northwestern Taiwan coast with the wind-shift line.
There were several long-lived (>2 h) reflectivity maxima embedded in the rainband. They often had several individual cells with a much shorter lifetime. The reflectivity maxima formed on the southwestern tip of the rainband and along the low-level wind-shift line. They intensified during their movement from the southwest to the northeast along the rainband. The continuous generation of the reflectivity maxima along the wind-shift line and the intensification of them over the low-level convergence zone maintained the long lifetime of the rainband and produced persistent heavy rainfall along the northwestern coast as these reflectivity maxima moved toward the coast. During the early stage of their lifetime, the reflectivity maxima were observed along the wind-shift line with upward motion in the lower troposphere. As they moved toward the northeastern part of the rainband and matured, the reflectivity maxima were observed southeast of the convergence zone with sinking motion in the lower troposphere. The upward motion was rooted along the wind-shift line and tilted southeastward with height. The reflectivity maxima dissipated as they moved inland. During the early stage of the rainband, the reflectivity maxima on the northeastern part of the rainband also merged with the convective line associated with the land-breeze front offshore of the northwestern coast.
The Mei-Yu front was shallow (<1 km) and moved slowly southward along the western coast. Convection associated with the front was weak with echo tops (∼10 dBZ) below 6 km.
Abstract
A heavy rainfall event during the Taiwan Area Mesoscale Experiment intensive observing period 13 has been studied using upper-air, surface mesonet, and dual-Doppler radar data. The heavy rainfall (≥231 mm day−1) occurred over northwestern Taiwan with the maximum rainfall along the northwestern coast and was caused by a long-lived, convective rainband in the prefrontal atmosphere. It occurred in an upper-level divergence region and along the axis of the maximum equivalent potential temperature at the 850-hPa level.
As a Mei-Yu front advanced southeastward, the postfrontal cold air in the lowest levels was retarded by the hilly terrain along the southeastern China coast. As a result, a low-level wind-shift line associated with a pressure trough at the 850-hPa level moved over the Taiwan Strait before the arrival of the surface front. The westerly flow behind the trough interacted with a barrier jet along the northwestern coast of Taiwan. The barrier jet is caused by the interaction between the prefrontal southwest monsoon flow and the island obstacle. A low-level convergence zone (∼3 km deep) was observed along the wind-shift line between the westerly flow coming off the southeastern China coast and the barrier jet. A long-lived rainband developed within the low-level convergence zone and moved southeastward toward the northwestern Taiwan coast with the wind-shift line.
There were several long-lived (>2 h) reflectivity maxima embedded in the rainband. They often had several individual cells with a much shorter lifetime. The reflectivity maxima formed on the southwestern tip of the rainband and along the low-level wind-shift line. They intensified during their movement from the southwest to the northeast along the rainband. The continuous generation of the reflectivity maxima along the wind-shift line and the intensification of them over the low-level convergence zone maintained the long lifetime of the rainband and produced persistent heavy rainfall along the northwestern coast as these reflectivity maxima moved toward the coast. During the early stage of their lifetime, the reflectivity maxima were observed along the wind-shift line with upward motion in the lower troposphere. As they moved toward the northeastern part of the rainband and matured, the reflectivity maxima were observed southeast of the convergence zone with sinking motion in the lower troposphere. The upward motion was rooted along the wind-shift line and tilted southeastward with height. The reflectivity maxima dissipated as they moved inland. During the early stage of the rainband, the reflectivity maxima on the northeastern part of the rainband also merged with the convective line associated with the land-breeze front offshore of the northwestern coast.
The Mei-Yu front was shallow (<1 km) and moved slowly southward along the western coast. Convection associated with the front was weak with echo tops (∼10 dBZ) below 6 km.
Abstract
Moist convection occurred repeatedly in the midnight-to-morning hours of 11–16 June 1998 and yielded excessive rainfall in a narrow latitudinal corridor over East Asia, causing severe flood. Numerical experiments and composite analyses of a 5-day period are performed to examine the mechanisms governing nocturnal convection. Both simulations and observations show that a train of MCSs concurrently developed along a quasi-stationary mei-yu front and coincided with the impact of a monsoon surge on a frontogenetic zone at night. This process was regulated primarily by a nocturnal low-level jet (NLLJ) in the southwesterly monsoon that formed over southern China and extended to central China. In particular, the NLLJ acted as a mechanism of moisture transport over the plains. At its northern terminus, the NLLJ led to a zonal band of elevated conditionally unstable air where strong low-level ascent overcame small convective inhibition, triggering new convection in three preferred plains. An analysis of convective instability shows that the low-tropospheric intrusion of moist monsoon air generated CAPE of ~1000 J kg−1 prior to convection initiation, whereas free-atmospheric forcing was much weaker. The NLLJ-related horizontal advection accounted for most of the instability precondition at 100–175 J kg−1 h−1. At the convective stage, instability generation by the upward transport of moisture increased to ~100 J kg−1 h−1, suggesting that ascending inflow caused feedback in convection growth. The convection dissipated in late morning with decaying NLLJ and moisture at elevated layers. It is concluded that the diurnally varying summer monsoon acted as an effective discharge of available moist energy from southern to central China, generating the morning-peak heavy rainfall corridor.
Abstract
Moist convection occurred repeatedly in the midnight-to-morning hours of 11–16 June 1998 and yielded excessive rainfall in a narrow latitudinal corridor over East Asia, causing severe flood. Numerical experiments and composite analyses of a 5-day period are performed to examine the mechanisms governing nocturnal convection. Both simulations and observations show that a train of MCSs concurrently developed along a quasi-stationary mei-yu front and coincided with the impact of a monsoon surge on a frontogenetic zone at night. This process was regulated primarily by a nocturnal low-level jet (NLLJ) in the southwesterly monsoon that formed over southern China and extended to central China. In particular, the NLLJ acted as a mechanism of moisture transport over the plains. At its northern terminus, the NLLJ led to a zonal band of elevated conditionally unstable air where strong low-level ascent overcame small convective inhibition, triggering new convection in three preferred plains. An analysis of convective instability shows that the low-tropospheric intrusion of moist monsoon air generated CAPE of ~1000 J kg−1 prior to convection initiation, whereas free-atmospheric forcing was much weaker. The NLLJ-related horizontal advection accounted for most of the instability precondition at 100–175 J kg−1 h−1. At the convective stage, instability generation by the upward transport of moisture increased to ~100 J kg−1 h−1, suggesting that ascending inflow caused feedback in convection growth. The convection dissipated in late morning with decaying NLLJ and moisture at elevated layers. It is concluded that the diurnally varying summer monsoon acted as an effective discharge of available moist energy from southern to central China, generating the morning-peak heavy rainfall corridor.
Abstract
A new three-dimensional (3D) turbulent kinetic energy (TKE) subgrid mixing scheme is developed using the Advanced Research version of the Weather Research and Forecasting (WRF) Model (WRF-ARW) to address the gray-zone problem in the parameterization of subgrid turbulent mixing. The new scheme combines the horizontal and vertical subgrid turbulent mixing into a single energetically consistent framework, in contrast to the conventionally separate treatment of the vertical and horizontal mixing. The new scheme is self-adaptive to the grid-size change between the large-eddy simulation (LES) and mesoscale limits. A series of dry convective boundary layer (CBL) idealized simulations are carried out to compare the performance of the new scheme and the conventional treatment of subgrid mixing to the WRF-ARW LES dataset. The importance of including the nonlocal component in the vertical buoyancy specification in the newly developed general TKE-based scheme is illustrated in the comparison. The improvements of the new scheme with the conventional treatment of subgrid mixing across the gray-zone model resolutions are demonstrated through the partitioning of the total vertical flux profiles. Results from real-case simulations show the feasibility of using the new scheme in the WRF Model in lieu of the conventional treatment of subgrid mixing.
Abstract
A new three-dimensional (3D) turbulent kinetic energy (TKE) subgrid mixing scheme is developed using the Advanced Research version of the Weather Research and Forecasting (WRF) Model (WRF-ARW) to address the gray-zone problem in the parameterization of subgrid turbulent mixing. The new scheme combines the horizontal and vertical subgrid turbulent mixing into a single energetically consistent framework, in contrast to the conventionally separate treatment of the vertical and horizontal mixing. The new scheme is self-adaptive to the grid-size change between the large-eddy simulation (LES) and mesoscale limits. A series of dry convective boundary layer (CBL) idealized simulations are carried out to compare the performance of the new scheme and the conventional treatment of subgrid mixing to the WRF-ARW LES dataset. The importance of including the nonlocal component in the vertical buoyancy specification in the newly developed general TKE-based scheme is illustrated in the comparison. The improvements of the new scheme with the conventional treatment of subgrid mixing across the gray-zone model resolutions are demonstrated through the partitioning of the total vertical flux profiles. Results from real-case simulations show the feasibility of using the new scheme in the WRF Model in lieu of the conventional treatment of subgrid mixing.
Abstract
Coarse-grained results from a large-eddy simulation (LES) using the Weather Research and Forecasting (WRF) Model were compared in this study with the WRF simulations at a typical convection-permitting horizontal grid spacing of 3 km for an idealized case of deep moist convection. The purpose of this comparison is to identify major differences at the subgrid process level between two widely used deep convection parameterization schemes in the WRF Model. It is shown that there are considerable differences in subgrid process representations between the two schemes due to different parameterization formulations and underlying assumptions. The two schemes not only differ in trigger function, subgrid cloud model, and closure assumptions but also disagree with the coarse-grained LES results in terms of vertical mass flux profiles. Thus, it is difficult to discern which scheme is more advantageous over the other at the subgrid process level. The conclusions from this study highlight the importance of establishing benchmarks using observations and LES to develop and evaluate convection parameterization schemes suitable for models at convection-permitting resolution.
Abstract
Coarse-grained results from a large-eddy simulation (LES) using the Weather Research and Forecasting (WRF) Model were compared in this study with the WRF simulations at a typical convection-permitting horizontal grid spacing of 3 km for an idealized case of deep moist convection. The purpose of this comparison is to identify major differences at the subgrid process level between two widely used deep convection parameterization schemes in the WRF Model. It is shown that there are considerable differences in subgrid process representations between the two schemes due to different parameterization formulations and underlying assumptions. The two schemes not only differ in trigger function, subgrid cloud model, and closure assumptions but also disagree with the coarse-grained LES results in terms of vertical mass flux profiles. Thus, it is difficult to discern which scheme is more advantageous over the other at the subgrid process level. The conclusions from this study highlight the importance of establishing benchmarks using observations and LES to develop and evaluate convection parameterization schemes suitable for models at convection-permitting resolution.
Abstract
A succession of MCSs developed during the last week of October 2016 and produced extreme heavy rainfall in central China. The event underwent an evident shift from a mei-yu-like warm scenario to an autumn cold scenario. Diurnal cycles of rainfall and low-level winds may be modulated by the shifting of large-scale atmospheric conditions. We conducted observational analyses and numerical experiments to examine how large-scale circulations influenced rainfall systems through diurnally varying processes. The results show that, in the first half (warm) period of the event, intense rainfall mostly occurred in eastern-central China with an early morning peak. It was closely related to a nocturnal southwesterly low-level jet (NLLJ) on the flank of the western Pacific subtropical high. The NLLJ formed near midnight in southern China where ageostrophic wind rotated clockwise due to Blackadar’s inertial oscillation. The NLLJ extended downstream to central China during the predawn hours due to the horizontal advection of momentum. Both the formation and extension of the NLLJ were supported by an enhanced subtropical high that provided relatively warm conditions with surface heating for boundary layer inertial oscillation and strong background southwesterly winds for momentum transport. The NLLJ induced MCSs at its northern terminus where the low-level ascent, moisture flux convergence, and convective instability were enhanced during the predawn hours. In the second half period with an intrusion of cold air, the diurnal amplitude of low-level winds became small under relatively cold and cloudy conditions. Moderate rainfall tended to occur in western-central China with a peak after midnight, most likely due to frontogenetic processes, upslope lifting, and nighttime cloud-top cooling.
Abstract
A succession of MCSs developed during the last week of October 2016 and produced extreme heavy rainfall in central China. The event underwent an evident shift from a mei-yu-like warm scenario to an autumn cold scenario. Diurnal cycles of rainfall and low-level winds may be modulated by the shifting of large-scale atmospheric conditions. We conducted observational analyses and numerical experiments to examine how large-scale circulations influenced rainfall systems through diurnally varying processes. The results show that, in the first half (warm) period of the event, intense rainfall mostly occurred in eastern-central China with an early morning peak. It was closely related to a nocturnal southwesterly low-level jet (NLLJ) on the flank of the western Pacific subtropical high. The NLLJ formed near midnight in southern China where ageostrophic wind rotated clockwise due to Blackadar’s inertial oscillation. The NLLJ extended downstream to central China during the predawn hours due to the horizontal advection of momentum. Both the formation and extension of the NLLJ were supported by an enhanced subtropical high that provided relatively warm conditions with surface heating for boundary layer inertial oscillation and strong background southwesterly winds for momentum transport. The NLLJ induced MCSs at its northern terminus where the low-level ascent, moisture flux convergence, and convective instability were enhanced during the predawn hours. In the second half period with an intrusion of cold air, the diurnal amplitude of low-level winds became small under relatively cold and cloudy conditions. Moderate rainfall tended to occur in western-central China with a peak after midnight, most likely due to frontogenetic processes, upslope lifting, and nighttime cloud-top cooling.
Abstract
Multiscale processes from synoptic disturbances to diurnal cycles during the record-breaking heavy rainfall in summer 2020 were examined in this study. The heavy rainfall consisted of eight episodes, each lasting about 5 days, and were associated with two types of synoptic disturbances. The type-1 episodes featured a northwestward extending western Pacific subtropical high (WPSH), while the type-2 episodes had approaching midlatitude troughs with southward retreat in the WPSH. Each heavy rainfall episode had 2–3 occurrences of nocturnal low-level jets (NLLJs), in close association with intense rainfall in the early morning. The NLLJs formed partly due to the geostrophic wind by increased pressure gradients under both types of synoptic disturbances. The NLLJs were also driven by the ageostrophic wind that veered to maximum southerlies at late night due to the boundary layer inertial oscillation. The diurnal amplitudes of low-level southerlies increased remarkably after the onset of type-1 episodes, in which the extending WPSH provided strong daytime heating from solar radiation. By contrast, the wind diurnal amplitudes were less changed after the onset of type-2 episodes. The NLLJs strengthened the mesoscale low-level ascent, net moisture flux convergence, and convective instability in elevated warm moist air, which led to the upscale growth of MCSs at the northern terminus of the LLJ after midnight. The MCSs-induced mei-yu rainband was reestablished in Central China during the type-1 episodes with the increased diurnal variations. The findings highlight that the regional diurnal cycles of low-level winds in response to synoptic disturbances can strongly regulate mesoscale convective activities in a downscaling manner, and thus produce heavy rainfall.
Abstract
Multiscale processes from synoptic disturbances to diurnal cycles during the record-breaking heavy rainfall in summer 2020 were examined in this study. The heavy rainfall consisted of eight episodes, each lasting about 5 days, and were associated with two types of synoptic disturbances. The type-1 episodes featured a northwestward extending western Pacific subtropical high (WPSH), while the type-2 episodes had approaching midlatitude troughs with southward retreat in the WPSH. Each heavy rainfall episode had 2–3 occurrences of nocturnal low-level jets (NLLJs), in close association with intense rainfall in the early morning. The NLLJs formed partly due to the geostrophic wind by increased pressure gradients under both types of synoptic disturbances. The NLLJs were also driven by the ageostrophic wind that veered to maximum southerlies at late night due to the boundary layer inertial oscillation. The diurnal amplitudes of low-level southerlies increased remarkably after the onset of type-1 episodes, in which the extending WPSH provided strong daytime heating from solar radiation. By contrast, the wind diurnal amplitudes were less changed after the onset of type-2 episodes. The NLLJs strengthened the mesoscale low-level ascent, net moisture flux convergence, and convective instability in elevated warm moist air, which led to the upscale growth of MCSs at the northern terminus of the LLJ after midnight. The MCSs-induced mei-yu rainband was reestablished in Central China during the type-1 episodes with the increased diurnal variations. The findings highlight that the regional diurnal cycles of low-level winds in response to synoptic disturbances can strongly regulate mesoscale convective activities in a downscaling manner, and thus produce heavy rainfall.
