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- Author or Editor: Chun-Chieh Wu x
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Abstract
Numerical integrations using the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model were performed to study the evolution of Typhoon Gladys (1994) and its interaction with the Taiwan terrain. Consistent with most previous studies, the Taiwan topography results in the deceleration of Gladys’s translation speed and southward deviation as it approaches Taiwan. On the other hand, Gladys accelerates northwestward while passing Taiwan, which is likely to be related to the moist processes, and differs from the track pattern in the dry model of Lin et al. Although the GFDL hurricane model forecast underestimates Gladys’s intensity, the model can capture the evolution of Gladys’s intensity, especially its weakening during landfall, which is primarily due to the cutoff of the water vapor supply in the boundary layer as Gladys approached the Taiwan terrain. Other mesoscale phenomena, including the pattern of heavy precipitation and the formation of secondary lows, are well simulated by the model, though their locations are somewhat different from those observed. Detailed analyses indicate that the surface low pressure center to the east of the Central Mountain Range (CMR) is induced by the downslope adiabatic warming (foehn) associated with the circulation of Gladys. The quasi-stationary secondary low to the west of the CMR is mainly induced by the environmental easterly flow over the CMR, while the downslope adiabatic warming associated with the circulation of Gladys acts to enhance it as Gladys is close to Taiwan. The potential vorticity budget analysis indicates that the condensational heating plays a major role in the potential vorticity evolution around the storm, while the surface frictional dissipation of the potential vorticity becomes more significant as Gladys is over the Taiwan terrain. Finally, the experiment with a larger and stronger initial typhoon vortex indicates that different initial specification of a typhoon vortex can result in a different track pattern and thus leads to a totally different typhoon–topography interaction, suggesting the importance of typhoon initialization for storm prediction near Taiwan.
Abstract
Numerical integrations using the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model were performed to study the evolution of Typhoon Gladys (1994) and its interaction with the Taiwan terrain. Consistent with most previous studies, the Taiwan topography results in the deceleration of Gladys’s translation speed and southward deviation as it approaches Taiwan. On the other hand, Gladys accelerates northwestward while passing Taiwan, which is likely to be related to the moist processes, and differs from the track pattern in the dry model of Lin et al. Although the GFDL hurricane model forecast underestimates Gladys’s intensity, the model can capture the evolution of Gladys’s intensity, especially its weakening during landfall, which is primarily due to the cutoff of the water vapor supply in the boundary layer as Gladys approached the Taiwan terrain. Other mesoscale phenomena, including the pattern of heavy precipitation and the formation of secondary lows, are well simulated by the model, though their locations are somewhat different from those observed. Detailed analyses indicate that the surface low pressure center to the east of the Central Mountain Range (CMR) is induced by the downslope adiabatic warming (foehn) associated with the circulation of Gladys. The quasi-stationary secondary low to the west of the CMR is mainly induced by the environmental easterly flow over the CMR, while the downslope adiabatic warming associated with the circulation of Gladys acts to enhance it as Gladys is close to Taiwan. The potential vorticity budget analysis indicates that the condensational heating plays a major role in the potential vorticity evolution around the storm, while the surface frictional dissipation of the potential vorticity becomes more significant as Gladys is over the Taiwan terrain. Finally, the experiment with a larger and stronger initial typhoon vortex indicates that different initial specification of a typhoon vortex can result in a different track pattern and thus leads to a totally different typhoon–topography interaction, suggesting the importance of typhoon initialization for storm prediction near Taiwan.
Abstract
The processes leading to the rapid intensification (RI) of Typhoon Megi (2010) are explored with a convection-permitting full-physics model and a sensitivity experiment using a different microphysical scheme. It is found that the temporary active convection, gradually strengthened primary circulation, and a warm core developing at midlevels tend to serve as precursors to RI. The potential vorticity (PV) budget and Sawyer–Eliassen model are utilized to examine the causes and effects of those precursors. Results show that the secondary circulation, triggered by the latent heat associated with active convection, acts to strengthen the mid- to upper-level primary circulation by transporting the larger momentum toward the upper layers. The increased inertial stability at mid- to upper levels not only increases the heating efficiency but also prevents the warm-core structure from being disrupted by the ventilation effect. The warming above 5 km effectively lowers the surface pressure.
It is identified that the strong secondary circulation helps to accomplish the midlevel warming within the eye. The results based on potential temperature (θ) budget suggest that the mean subsidence associated with detrainment of active convection is the major process contributing to the formation of a midlevel warm core. On the possible causes triggering the inner-core active convection, it is suggested that the gradually increased vortex-scale surface enthalpy flux has a leading role in the development of vigorous convection. The results also highlight the potentially dominant role of weak to moderate convection in the onset of RI, while the convective bursts play a supporting role. Based on the aforementioned analyses, a schematic diagram is shown to describe the plausible path leading to RI.
Abstract
The processes leading to the rapid intensification (RI) of Typhoon Megi (2010) are explored with a convection-permitting full-physics model and a sensitivity experiment using a different microphysical scheme. It is found that the temporary active convection, gradually strengthened primary circulation, and a warm core developing at midlevels tend to serve as precursors to RI. The potential vorticity (PV) budget and Sawyer–Eliassen model are utilized to examine the causes and effects of those precursors. Results show that the secondary circulation, triggered by the latent heat associated with active convection, acts to strengthen the mid- to upper-level primary circulation by transporting the larger momentum toward the upper layers. The increased inertial stability at mid- to upper levels not only increases the heating efficiency but also prevents the warm-core structure from being disrupted by the ventilation effect. The warming above 5 km effectively lowers the surface pressure.
It is identified that the strong secondary circulation helps to accomplish the midlevel warming within the eye. The results based on potential temperature (θ) budget suggest that the mean subsidence associated with detrainment of active convection is the major process contributing to the formation of a midlevel warm core. On the possible causes triggering the inner-core active convection, it is suggested that the gradually increased vortex-scale surface enthalpy flux has a leading role in the development of vigorous convection. The results also highlight the potentially dominant role of weak to moderate convection in the onset of RI, while the convective bursts play a supporting role. Based on the aforementioned analyses, a schematic diagram is shown to describe the plausible path leading to RI.
Abstract
Tropical cyclones (TCs) encountering the terrain of Taiwan usually experience prominent track deflection, resulting in uncertainty in TC track forecasts. The underlying mechanisms of TC deflection are examined to better understand the pattern of TC tracks under various flow regimes. In this study, idealized experiments are carried out utilizing the Advanced Research version of the Weather Research and Forecasting (WRF) Model. This study investigates the motion of a TC that is deflected southward while moving westward toward an idealized terrain similar to Taiwan. An analysis of both the flow asymmetries and the potential vorticity tendency (PVT) demonstrates that horizontal advection contributes to the southward movement of the TC. The track deflection is examined in two separate time periods, with different mechanisms leading to the southward movement. Changes in the background flow induced by the terrain first cause the large-scale steering current to push the TC southward while the TC is still far from the terrain. As the TC approaches the idealized topography, the role of the inner-core dynamics becomes important, and the TC terrain-induced channeling effect results in further southward deflection. Asymmetries in the midlevel flow also develop during this period, in part associated with the effect of vertical momentum transport. The combination of the large-scale environmental flow, the low-level channeling effect, and asymmetries in the midlevel flow all contribute to the southward deflection of the TC track.
Abstract
Tropical cyclones (TCs) encountering the terrain of Taiwan usually experience prominent track deflection, resulting in uncertainty in TC track forecasts. The underlying mechanisms of TC deflection are examined to better understand the pattern of TC tracks under various flow regimes. In this study, idealized experiments are carried out utilizing the Advanced Research version of the Weather Research and Forecasting (WRF) Model. This study investigates the motion of a TC that is deflected southward while moving westward toward an idealized terrain similar to Taiwan. An analysis of both the flow asymmetries and the potential vorticity tendency (PVT) demonstrates that horizontal advection contributes to the southward movement of the TC. The track deflection is examined in two separate time periods, with different mechanisms leading to the southward movement. Changes in the background flow induced by the terrain first cause the large-scale steering current to push the TC southward while the TC is still far from the terrain. As the TC approaches the idealized topography, the role of the inner-core dynamics becomes important, and the TC terrain-induced channeling effect results in further southward deflection. Asymmetries in the midlevel flow also develop during this period, in part associated with the effect of vertical momentum transport. The combination of the large-scale environmental flow, the low-level channeling effect, and asymmetries in the midlevel flow all contribute to the southward deflection of the TC track.
Abstract
Numerical simulations are conducted to examine the role of the wind-induced surface heat exchange (WISHE) mechanism in secondary eyewall formation (SEF). The control experiment exhibits a coherent secondary eyewall structure as quantified by various parameters (e.g., the azimuthal-mean tangential wind and vertical velocity). Prior to SEF, an area characterized by increasing diabatic heating, enhanced inertial stability, and increasing supergradient winds at the top of the boundary layer is observed outside the eyewall. While these characteristics offer the possibility of both balanced and unbalanced dynamical pathways to SEF, the focus of this study is to evaluate the impact of WISHE. To examine the sensitivity of SEF to WISHE, the surface wind used for the calculation of surface heat fluxes is capped at several designated values and at different radial intervals. When the heat fluxes are moderately suppressed around and outside the SEF region observed in the control experiment, sensitivity experiments show that the formation of the outer eyewall is delayed, and the intensity of both eyewalls is weaker. When the heat fluxes are strongly suppressed in the same region, SEF does not occur. In contrast, suppressing the surface heat fluxes in the storm’s inner-core region has limited effect on the evolution of the outer eyewall. This study provides important physical insight into SEF, indicating that WISHE plays a crucial role in SEF and tropical cyclone evolution.
Abstract
Numerical simulations are conducted to examine the role of the wind-induced surface heat exchange (WISHE) mechanism in secondary eyewall formation (SEF). The control experiment exhibits a coherent secondary eyewall structure as quantified by various parameters (e.g., the azimuthal-mean tangential wind and vertical velocity). Prior to SEF, an area characterized by increasing diabatic heating, enhanced inertial stability, and increasing supergradient winds at the top of the boundary layer is observed outside the eyewall. While these characteristics offer the possibility of both balanced and unbalanced dynamical pathways to SEF, the focus of this study is to evaluate the impact of WISHE. To examine the sensitivity of SEF to WISHE, the surface wind used for the calculation of surface heat fluxes is capped at several designated values and at different radial intervals. When the heat fluxes are moderately suppressed around and outside the SEF region observed in the control experiment, sensitivity experiments show that the formation of the outer eyewall is delayed, and the intensity of both eyewalls is weaker. When the heat fluxes are strongly suppressed in the same region, SEF does not occur. In contrast, suppressing the surface heat fluxes in the storm’s inner-core region has limited effect on the evolution of the outer eyewall. This study provides important physical insight into SEF, indicating that WISHE plays a crucial role in SEF and tropical cyclone evolution.
Abstract
This study examines the role of surface heat fluxes, particularly in relation to the wind-induced surface heat exchange (WISHE) mechanism, in the rapid intensification (RI) of tropical cyclones (TCs). Sensitivity experiments with capped surface fluxes and thus reduced WISHE exhibit delayed RI and weaker peak intensity, while WISHE could affect the evolutions of TCs both before and after the onset of RI. Before RI, more WISHE leads to faster increase of equivalent potential temperature in the lower levels, resulting in more active and stronger convection. In addition, TCs in experiments with more WISHE reach a certain strength earlier, before the onset of RI. During the RI period, more surface heat fluxes could provide convective instability in the lower levels, and cause a consequent development in the convective activity. More efficient intensification in a TC is found with higher surface heat fluxes and larger inertial stability, leading to a stronger peak intensity, more significant and deeper warm core in TC center, and the axisymmetrization of convection in the higher levels. In both stages, different levels of WISHE alter the thermodynamic environment and convective-scale processes. In all, this study supports the crucial role of WISHE in affecting TC intensification rate for TCs with RI.
Abstract
This study examines the role of surface heat fluxes, particularly in relation to the wind-induced surface heat exchange (WISHE) mechanism, in the rapid intensification (RI) of tropical cyclones (TCs). Sensitivity experiments with capped surface fluxes and thus reduced WISHE exhibit delayed RI and weaker peak intensity, while WISHE could affect the evolutions of TCs both before and after the onset of RI. Before RI, more WISHE leads to faster increase of equivalent potential temperature in the lower levels, resulting in more active and stronger convection. In addition, TCs in experiments with more WISHE reach a certain strength earlier, before the onset of RI. During the RI period, more surface heat fluxes could provide convective instability in the lower levels, and cause a consequent development in the convective activity. More efficient intensification in a TC is found with higher surface heat fluxes and larger inertial stability, leading to a stronger peak intensity, more significant and deeper warm core in TC center, and the axisymmetrization of convection in the higher levels. In both stages, different levels of WISHE alter the thermodynamic environment and convective-scale processes. In all, this study supports the crucial role of WISHE in affecting TC intensification rate for TCs with RI.
Abstract
Issues concerning the initialization and simulation of tropical cyclones by integrating both dropwindsonde data and a bogused vortex into a mesoscale model have been studied. A method is proposed to combine dropwindsonde data with a bogused vortex for tropical cyclone initialization and to improve track and intensity prediction. A clear positive impact of this proposed method on both the tropical cyclone track and intensity forecasts in a mesoscale model is demonstrated in three cases of typhoons, including Meari (2004), Conson (2004), and Megi (2004). The effectiveness of the proposed method in improving the track and intensity forecasts is also demonstrated in the evaluation of all 10 cases of Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR) missions in 2004. This method provides a useful and practical means to improve operational tropical cyclone prediction with dropwindsonde observations.
Abstract
Issues concerning the initialization and simulation of tropical cyclones by integrating both dropwindsonde data and a bogused vortex into a mesoscale model have been studied. A method is proposed to combine dropwindsonde data with a bogused vortex for tropical cyclone initialization and to improve track and intensity prediction. A clear positive impact of this proposed method on both the tropical cyclone track and intensity forecasts in a mesoscale model is demonstrated in three cases of typhoons, including Meari (2004), Conson (2004), and Megi (2004). The effectiveness of the proposed method in improving the track and intensity forecasts is also demonstrated in the evaluation of all 10 cases of Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR) missions in 2004. This method provides a useful and practical means to improve operational tropical cyclone prediction with dropwindsonde observations.
Abstract
A series of numerical simulations are conducted using the advanced research version of the Weather Research and Forecasting model with a 4-km fine mesh to examine the physical processes responsible for the significant track deflection and looping motion before the landfall of Supertyphoon Haitang (2005) in Taiwan, which poses a unique scientific and forecasting issue. In the control experiment, a low-level northerly jet induced by the channeling effect forms in the western quadrant of the approaching storm, where the inner-core circulation is constrained by the presence of Taiwan’s terrain. Because of the channeling effect, the strongest winds of the storm are shifted to the western portion of the eyewall. The northerly advection flow (averaged asymmetric winds within 100-km radius) results in a sharp southward turn of the westward-moving storm. The time series of the advection flow shows that the advection wind vectors rotate cyclonically in time and well match the motion of the simulated vortex during the looping process. A sensitivity study of lowering the Taiwan terrain elevations to 70% or 40% of those in the control experiment reduces the southward track deflection and loop amplitude. The experiment with the reduced elevation to 10% of the control experiment does not show a looping track and thus demonstrates the key role of the terrain-induced channeling effect. Experiments applying different values of the structure parameter α illustrate that increasing the strength, size, and translation speed of the initial storm results in a smaller interaction with Taiwan’s terrain and a smaller average steering flow caused by the asymmetric circulation, which leads to a proportionally smaller southward track deflection without making a loop.
Abstract
A series of numerical simulations are conducted using the advanced research version of the Weather Research and Forecasting model with a 4-km fine mesh to examine the physical processes responsible for the significant track deflection and looping motion before the landfall of Supertyphoon Haitang (2005) in Taiwan, which poses a unique scientific and forecasting issue. In the control experiment, a low-level northerly jet induced by the channeling effect forms in the western quadrant of the approaching storm, where the inner-core circulation is constrained by the presence of Taiwan’s terrain. Because of the channeling effect, the strongest winds of the storm are shifted to the western portion of the eyewall. The northerly advection flow (averaged asymmetric winds within 100-km radius) results in a sharp southward turn of the westward-moving storm. The time series of the advection flow shows that the advection wind vectors rotate cyclonically in time and well match the motion of the simulated vortex during the looping process. A sensitivity study of lowering the Taiwan terrain elevations to 70% or 40% of those in the control experiment reduces the southward track deflection and loop amplitude. The experiment with the reduced elevation to 10% of the control experiment does not show a looping track and thus demonstrates the key role of the terrain-induced channeling effect. Experiments applying different values of the structure parameter α illustrate that increasing the strength, size, and translation speed of the initial storm results in a smaller interaction with Taiwan’s terrain and a smaller average steering flow caused by the asymmetric circulation, which leads to a proportionally smaller southward track deflection without making a loop.
Abstract
The goal of this work is to improve understanding of the mechanisms leading to a heavy rainfall event under the combined influences of the outer circulation of Typhoon Megi (2010), the Asian monsoon, and the topography of Taiwan. Megi is a case featuring high forecast uncertainty associated with its sudden recurvature, along with remote heavy rainfall over northeastern Taiwan (especially at Yilan) and its adjacent seas during 19–23 October 2010. An ensemble simulation is conducted, and characteristic ensemble members are separated into subgroups based on either track accuracy or rainfall forecast skill. Comparisons between different subgroups are made to investigate favorable processes for precipitation and how the differences between these subgroups affect the rainfall simulation.
Several mechanisms leading to this remote rainfall event are shown. The northward transport of water vapor by Megi’s outer circulation provides a moisture-laden environment over the coastal area of eastern Taiwan. Meanwhile, the outer circulation of Megi (with high 
Abstract
The goal of this work is to improve understanding of the mechanisms leading to a heavy rainfall event under the combined influences of the outer circulation of Typhoon Megi (2010), the Asian monsoon, and the topography of Taiwan. Megi is a case featuring high forecast uncertainty associated with its sudden recurvature, along with remote heavy rainfall over northeastern Taiwan (especially at Yilan) and its adjacent seas during 19–23 October 2010. An ensemble simulation is conducted, and characteristic ensemble members are separated into subgroups based on either track accuracy or rainfall forecast skill. Comparisons between different subgroups are made to investigate favorable processes for precipitation and how the differences between these subgroups affect the rainfall simulation.
Several mechanisms leading to this remote rainfall event are shown. The northward transport of water vapor by Megi’s outer circulation provides a moisture-laden environment over the coastal area of eastern Taiwan. Meanwhile, the outer circulation of Megi (with high
Abstract
The European Centre for Medium-Range Weather Forecasts Tropical Ocean–Global Atmosphere advanced analysis was used to study the mechanisms that affect the intensity of Typhoons Flo (1990) and Gene (1990). The outflow structure, eddy momentum flux convergence, and the mean vertical wind shear were examined.
The evolution of potential vorticity (PV) in the outflow layer showed low PV areas on top of both Typhoons Flo and Gene, and the low PV areas expanded as the typhoons intensified. The outflow pattern of the two typhoons was influenced by the upper-tropospheric environmental systems. The upper-level environmental features were shown to play a crucial role in the intensification of the two typhoons.
The tropical upper-tropospheric trough cell east of Flo provided the outflow channel for the typhoon. The enhanced outflow, the upper-level eddy flux convergence (EFC), the low vertical wind shear, and the warm sea surface temperature provided all favorable conditions for the development of Flo. On the other hand, the intensification of Gene was associated with its interaction with an upper-level midlatitude trough. The approach of the trough produced upper-level EFC of angular momentum outside 10° lat radius, and the EFC shifted inward with time. As the EFC shifted into the vicinity of the storm core, Gene started to intensify steadily until the midlatitude trough passed over.
The intensifying processes of the above cases indicate the importance of the upper-tropospheric systems to the intensity change of typhoons. The influence of upper-level environmental systems on the tropical cyclones is prominent in the low inertial stability outflow layer. However, results from the piecewise PV inversion of the upper-level environmental PV anomalies showed little evidence that the intensification of both typhoons were directly associated with the superposition of PV anomalies.
Abstract
The European Centre for Medium-Range Weather Forecasts Tropical Ocean–Global Atmosphere advanced analysis was used to study the mechanisms that affect the intensity of Typhoons Flo (1990) and Gene (1990). The outflow structure, eddy momentum flux convergence, and the mean vertical wind shear were examined.
The evolution of potential vorticity (PV) in the outflow layer showed low PV areas on top of both Typhoons Flo and Gene, and the low PV areas expanded as the typhoons intensified. The outflow pattern of the two typhoons was influenced by the upper-tropospheric environmental systems. The upper-level environmental features were shown to play a crucial role in the intensification of the two typhoons.
The tropical upper-tropospheric trough cell east of Flo provided the outflow channel for the typhoon. The enhanced outflow, the upper-level eddy flux convergence (EFC), the low vertical wind shear, and the warm sea surface temperature provided all favorable conditions for the development of Flo. On the other hand, the intensification of Gene was associated with its interaction with an upper-level midlatitude trough. The approach of the trough produced upper-level EFC of angular momentum outside 10° lat radius, and the EFC shifted inward with time. As the EFC shifted into the vicinity of the storm core, Gene started to intensify steadily until the midlatitude trough passed over.
The intensifying processes of the above cases indicate the importance of the upper-tropospheric systems to the intensity change of typhoons. The influence of upper-level environmental systems on the tropical cyclones is prominent in the low inertial stability outflow layer. However, results from the piecewise PV inversion of the upper-level environmental PV anomalies showed little evidence that the intensification of both typhoons were directly associated with the superposition of PV anomalies.
