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- Author or Editor: Robert W. Pasken x
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Abstract
Dual-Doppler data collected during the Taiwan Area Mesoscale Experiment (TAMEX) are used to study the kinetic energy balance of a subtropical squall line over the Taiwan Straits. Values of each term in the kinetic energy budget equation are assessed from the Doppler-derived winds and retrieved thermodynamic variables using 1-km grid spacing in all three directions. The budget domain has horizontal dimensions of 23 × 40 km2,/, covering the convective region of the squall line. Results show that vertical totals of the horizontal generation and total buoyancy production terms act as a source of kinetic energy, while vertical totals of dissipation and vertical generation provide the main sinks. The horizontal flux divergence (convergence) of kinetic energy is nearly balanced by the vertical flux convergence (divergence) at most levels. The computed tendencies agree well with the observed tendencies in the lower and upper layers. The vertical total of kinetic energy change is negative for both the observed and computed values, showing the decrease of total kinetic energy of the squall system as it approached the west coast of Taiwan.
Abstract
Dual-Doppler data collected during the Taiwan Area Mesoscale Experiment (TAMEX) are used to study the kinetic energy balance of a subtropical squall line over the Taiwan Straits. Values of each term in the kinetic energy budget equation are assessed from the Doppler-derived winds and retrieved thermodynamic variables using 1-km grid spacing in all three directions. The budget domain has horizontal dimensions of 23 × 40 km2,/, covering the convective region of the squall line. Results show that vertical totals of the horizontal generation and total buoyancy production terms act as a source of kinetic energy, while vertical totals of dissipation and vertical generation provide the main sinks. The horizontal flux divergence (convergence) of kinetic energy is nearly balanced by the vertical flux convergence (divergence) at most levels. The computed tendencies agree well with the observed tendencies in the lower and upper layers. The vertical total of kinetic energy change is negative for both the observed and computed values, showing the decrease of total kinetic energy of the squall system as it approached the west coast of Taiwan.
Abstract
A thermodynamic retrieval method was employed to investigate the dynamic and thermodynamic structures of a subtropical prefrontal convective rainband associated with the Mei-Yu front on 25 June 1987 over north-western Taiwan. Three-dimensional wind fields were derived from the dual-Doppler data based on the methodology developed at Saint Louis University. Subsequently, fields of perturbation pressure and temperature were retrieved from the detailed wind field using the three momentum equations.
Results show that the maintenance of this long-lived rainband at the times of dual-Doppler analysis is caused by the combined effects of a gust front arising from the convective downdrafts ahead of the front and developments of new convection along the gust front. In the lowest layer, high pressure occurs behind the cold front with low pressure to its southeast. A buoyancy-induced low pressure area lies beneath the frontal updraft corresponding to the rising warm environmental air. The precipitation core associated with the frontal updraft is elongated toward the southeast side with the environmental shear in the mid-and upper troposphere forming the convective downdraft on the warm side of the surface front. This precipitation-induced downdraft transports cooler air downward producing a high pressure area underneath the convective downdraft. This high is accompanied by a temperature deficit resulting in cold horizontal outflows in the boundary layer. Part of these cold outflows interacts with the high-θe, southwest monsoonal flow to form a gust front ahead of the surface front. New convection develops along a gust front and then merges with the old convection, thereby prolonging the lifetime of the rainband. The vertical flux convergences and divergences of horizontal momentum by organized convection are largely responsible for forming a midlevel jet and weakening a low-level jet. The momentum budget calculation shows that the horizontal and vertical flux convergences and divergences of horizontal momentum by the mean and eddy motions are the major contributor to maintain the mean momentum.
Abstract
A thermodynamic retrieval method was employed to investigate the dynamic and thermodynamic structures of a subtropical prefrontal convective rainband associated with the Mei-Yu front on 25 June 1987 over north-western Taiwan. Three-dimensional wind fields were derived from the dual-Doppler data based on the methodology developed at Saint Louis University. Subsequently, fields of perturbation pressure and temperature were retrieved from the detailed wind field using the three momentum equations.
Results show that the maintenance of this long-lived rainband at the times of dual-Doppler analysis is caused by the combined effects of a gust front arising from the convective downdrafts ahead of the front and developments of new convection along the gust front. In the lowest layer, high pressure occurs behind the cold front with low pressure to its southeast. A buoyancy-induced low pressure area lies beneath the frontal updraft corresponding to the rising warm environmental air. The precipitation core associated with the frontal updraft is elongated toward the southeast side with the environmental shear in the mid-and upper troposphere forming the convective downdraft on the warm side of the surface front. This precipitation-induced downdraft transports cooler air downward producing a high pressure area underneath the convective downdraft. This high is accompanied by a temperature deficit resulting in cold horizontal outflows in the boundary layer. Part of these cold outflows interacts with the high-θe, southwest monsoonal flow to form a gust front ahead of the surface front. New convection develops along a gust front and then merges with the old convection, thereby prolonging the lifetime of the rainband. The vertical flux convergences and divergences of horizontal momentum by organized convection are largely responsible for forming a midlevel jet and weakening a low-level jet. The momentum budget calculation shows that the horizontal and vertical flux convergences and divergences of horizontal momentum by the mean and eddy motions are the major contributor to maintain the mean momentum.
Abstract
In Part I of this study, structural features of a convective rainband associated with the Mei-Yu front on 25 June 1987 over northwestern Taiwan are investigated. The dynamic and thermodynamic structures and momentum budgets of the convective rainband will be reported in Part II. Dual-Doppler data were collected from CP-4 and TOGA Doppler radars positioned along the northwest coast of Taiwan during the Taiwan Area Mesoscale Experiment (TAMEX). Fields of the system-relative wind and reflectivity were derived in a horizontal domain of 40 km × 36 km using the objective analysis scheme with a 1-km grid spacing in all three directions. There were ten analysis levels in the vertical ranging from 0.8 to 9.8 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment.
Results show that the low-level jet (LLJ) in the boundary layer provides large vertical shear ahead of the cold front. The orientation of the jet is in the direction almost parallel to the cold front. The depth of the cold air associated with the Mei-Yu front is rather shallow (1–2 km), in agreement with other TAMEX case studies reported in the literature. To the south of the front, winds are from the southwest in the lower troposphere and veer with height in the middle and upper troposphere. The southwest monsoon flow at low levels transports high-θ e , environmental air toward the wind-shift line. Behind the front, the northwesterly flow dominates, carrying much cooler air from northern China. The low-level convergence enhances lifting, resulting in a narrow band of convection on the warm side of the front. The length of the rainband analyzed is 5–10 km wide and 50 km long and is composed of many cells. Each cell is accompanied by the moderate convective updraft (6–8 m s−1) and weak downdraft (2–4 m s−1). The precipitation is deep but not intense on the warm side of the front. The maximum reflectivity within the rainband is less than 42 dBZ. The precipitation cores are elongated southeastward in the direction parallel to the environmental shear vector between the lower and middle layers. By contrast, the precipitation behind the front is weak and shallow, showing stratiform precipitation. Because the system traveled very slowly within the domain of interest, up to 200 mm of rainfall in 24 h occurred on the west coast of central Taiwan.
Abstract
In Part I of this study, structural features of a convective rainband associated with the Mei-Yu front on 25 June 1987 over northwestern Taiwan are investigated. The dynamic and thermodynamic structures and momentum budgets of the convective rainband will be reported in Part II. Dual-Doppler data were collected from CP-4 and TOGA Doppler radars positioned along the northwest coast of Taiwan during the Taiwan Area Mesoscale Experiment (TAMEX). Fields of the system-relative wind and reflectivity were derived in a horizontal domain of 40 km × 36 km using the objective analysis scheme with a 1-km grid spacing in all three directions. There were ten analysis levels in the vertical ranging from 0.8 to 9.8 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment.
Results show that the low-level jet (LLJ) in the boundary layer provides large vertical shear ahead of the cold front. The orientation of the jet is in the direction almost parallel to the cold front. The depth of the cold air associated with the Mei-Yu front is rather shallow (1–2 km), in agreement with other TAMEX case studies reported in the literature. To the south of the front, winds are from the southwest in the lower troposphere and veer with height in the middle and upper troposphere. The southwest monsoon flow at low levels transports high-θ e , environmental air toward the wind-shift line. Behind the front, the northwesterly flow dominates, carrying much cooler air from northern China. The low-level convergence enhances lifting, resulting in a narrow band of convection on the warm side of the front. The length of the rainband analyzed is 5–10 km wide and 50 km long and is composed of many cells. Each cell is accompanied by the moderate convective updraft (6–8 m s−1) and weak downdraft (2–4 m s−1). The precipitation is deep but not intense on the warm side of the front. The maximum reflectivity within the rainband is less than 42 dBZ. The precipitation cores are elongated southeastward in the direction parallel to the environmental shear vector between the lower and middle layers. By contrast, the precipitation behind the front is weak and shallow, showing stratiform precipitation. Because the system traveled very slowly within the domain of interest, up to 200 mm of rainfall in 24 h occurred on the west coast of central Taiwan.
Abstract
A thermodynamic retrieval method was used to study the dynamical and thermodynamical structure of a subtropical squall line, which occurred on 17 May 1987 over the Taiwan Straits. Three-dimensional wind fields were derived from the dual-Doppler data based on the methodology presented in Part I of this paper. Subsequently, fields of perturbation pressure and temperature were retrieved from the detailed wind field using the three momentum equations.
Results show that the overall structural features of this subtropical squall line are similar to those of a tropical squall line. The orientation of the squall line is almost in a north–south direction. In the lowest layer, the gust front is located to the immediate east of the main convective updrafts. High pressure occurs behind the gust front with low pressure to its east. A buoyancy-induced low pressure area lies beneath the convective updraft corresponding to the ascending warm environmental air. In the middle and upper layers, high pressure forms on the upshear side with low pressure on the downshear at the leading edge. The orientation of horizontal pressure gradients is approximately in the direction of the average shear vector in the domain. The retrieved temperature field agrees well with the updraft–downdraft structure. The convective updrafts are warmed by the release of latent heat by condensation. Conversely, cooling prevails in the convective downdrafts due, in part, to evaporation. Precipitation loading further decreases buoyancy of the downdraft air in the high reflectivity areas. To the rear of the main (old) cells the rear-to-front air is negatively buoyant resulting in a sloping downdraft. As the cool descending midtropospheric air approaches the surface, it spreads out to form a cold outflow behind the gust front. Part of the descending air moves forward, colliding with the advancing environmental warm air at the leading edge to form new cells ahead of the old cells. The interplay between a cell's cold surface outflow and the low-level shear within the system contributes to the maintenance of the subtropical squall line. The momentum budget calculation shows that the horizontal and vertical flux convergences/divergences of horizontal momentum by the mean and eddy motions are the major contributor to maintain the mean momentum.
Abstract
A thermodynamic retrieval method was used to study the dynamical and thermodynamical structure of a subtropical squall line, which occurred on 17 May 1987 over the Taiwan Straits. Three-dimensional wind fields were derived from the dual-Doppler data based on the methodology presented in Part I of this paper. Subsequently, fields of perturbation pressure and temperature were retrieved from the detailed wind field using the three momentum equations.
Results show that the overall structural features of this subtropical squall line are similar to those of a tropical squall line. The orientation of the squall line is almost in a north–south direction. In the lowest layer, the gust front is located to the immediate east of the main convective updrafts. High pressure occurs behind the gust front with low pressure to its east. A buoyancy-induced low pressure area lies beneath the convective updraft corresponding to the ascending warm environmental air. In the middle and upper layers, high pressure forms on the upshear side with low pressure on the downshear at the leading edge. The orientation of horizontal pressure gradients is approximately in the direction of the average shear vector in the domain. The retrieved temperature field agrees well with the updraft–downdraft structure. The convective updrafts are warmed by the release of latent heat by condensation. Conversely, cooling prevails in the convective downdrafts due, in part, to evaporation. Precipitation loading further decreases buoyancy of the downdraft air in the high reflectivity areas. To the rear of the main (old) cells the rear-to-front air is negatively buoyant resulting in a sloping downdraft. As the cool descending midtropospheric air approaches the surface, it spreads out to form a cold outflow behind the gust front. Part of the descending air moves forward, colliding with the advancing environmental warm air at the leading edge to form new cells ahead of the old cells. The interplay between a cell's cold surface outflow and the low-level shear within the system contributes to the maintenance of the subtropical squall line. The momentum budget calculation shows that the horizontal and vertical flux convergences/divergences of horizontal momentum by the mean and eddy motions are the major contributor to maintain the mean momentum.
Abstract
In this study, structural features of a subtropical squall line that occurred on 17 May 1987 over the Taiwan Straits, were investigated using dual-Doppler data collected during the Taiwan Area Mesoscale Experiment (TAMEX). Fields of the storm-relative wind and reflectivity were derived in a horizontal domain of 45 km × 25 km using an objective analysis scheme with 1 km grid spacing in all three directions. There were ten analysis levels in the vertical ranging from 0.3 to 8.8 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment.
Results show that many structural features of a subtropical squall line are similar to those for a fast-moving tropical squall line. A low-level jet (LLJ) associated with the frontal system provides the necessary strong shear at lower levels. On the front side of the squall line front-to-rear flow prevails at all levels and is accompanied by shallow rear-to-front flow on the back of the line. There are many individual cells embedded within the squall line. Relatively weak convective downdrafts occur between the cells and behind the main cells. Convective downdrafts on the rear of the main convective updrafts are essential to transport cooler midtropospheric air into the lower layer. Part of the negatively buoyant air from the rear continues to move forward colliding with the advancing high θ e air in the boundary layer. As a result, new convective cells form ahead of the old cells, thereby prolonging the life time of the squall line. In the convective region the low-level front-to-rear inflow is lifted at the leading edge to form the main updrafts. The lifted air continues to flow west in the middle and upper levels heading toward the trailing stratiform region. The interaction between the convective updraft and downdraft plays an important role in maintaining the three-dimensional circulation within the squall line.
Abstract
In this study, structural features of a subtropical squall line that occurred on 17 May 1987 over the Taiwan Straits, were investigated using dual-Doppler data collected during the Taiwan Area Mesoscale Experiment (TAMEX). Fields of the storm-relative wind and reflectivity were derived in a horizontal domain of 45 km × 25 km using an objective analysis scheme with 1 km grid spacing in all three directions. There were ten analysis levels in the vertical ranging from 0.3 to 8.8 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment.
Results show that many structural features of a subtropical squall line are similar to those for a fast-moving tropical squall line. A low-level jet (LLJ) associated with the frontal system provides the necessary strong shear at lower levels. On the front side of the squall line front-to-rear flow prevails at all levels and is accompanied by shallow rear-to-front flow on the back of the line. There are many individual cells embedded within the squall line. Relatively weak convective downdrafts occur between the cells and behind the main cells. Convective downdrafts on the rear of the main convective updrafts are essential to transport cooler midtropospheric air into the lower layer. Part of the negatively buoyant air from the rear continues to move forward colliding with the advancing high θ e air in the boundary layer. As a result, new convective cells form ahead of the old cells, thereby prolonging the life time of the squall line. In the convective region the low-level front-to-rear inflow is lifted at the leading edge to form the main updrafts. The lifted air continues to flow west in the middle and upper levels heading toward the trailing stratiform region. The interaction between the convective updraft and downdraft plays an important role in maintaining the three-dimensional circulation within the squall line.
