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- Author or Editor: Hsiu-Wu Chang x
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Abstract
Meso β-scale radiosonde data at 75 km spacings and 3 or 1.5 h intervals from the fifth day of AVE-SESAME 1979 (20–21 May) are employed to investigate moisture budgets in thunderstorm environments. Budget values are computed at nine times prior to, during, and after a convective outbreak over Oklahoma. The domain under investigation includes both convective and nonconvective areas, thereby allowing budget comparisons between the two regions.
Findings show that the convective region is characterized by strong horizontal moisture flux convergence in the low levels and weak divergence aloft. Vertical motion carries moisture into the middle and upper troposphere. Magnitudes of the moisture fluxes are directly proportional to storm intensity. The vertically integrated source/sink term also is closely related to the presence and intensity of convective activity. When converted into equivalent precipitation amounts, values correspond closely with those from a rain gage network.
Moisture budgets also are obtained from routine National Weather Service rawinsonde soundings. A comparison of results for similar locations, but derived from the two different resolutions, reveals several common processes. However, magnitudes from the mesoscale data are sometimes an order of magnitude greater than those at the synoptic scale, especially in the convective areas.
Abstract
Meso β-scale radiosonde data at 75 km spacings and 3 or 1.5 h intervals from the fifth day of AVE-SESAME 1979 (20–21 May) are employed to investigate moisture budgets in thunderstorm environments. Budget values are computed at nine times prior to, during, and after a convective outbreak over Oklahoma. The domain under investigation includes both convective and nonconvective areas, thereby allowing budget comparisons between the two regions.
Findings show that the convective region is characterized by strong horizontal moisture flux convergence in the low levels and weak divergence aloft. Vertical motion carries moisture into the middle and upper troposphere. Magnitudes of the moisture fluxes are directly proportional to storm intensity. The vertically integrated source/sink term also is closely related to the presence and intensity of convective activity. When converted into equivalent precipitation amounts, values correspond closely with those from a rain gage network.
Moisture budgets also are obtained from routine National Weather Service rawinsonde soundings. A comparison of results for similar locations, but derived from the two different resolutions, reveals several common processes. However, magnitudes from the mesoscale data are sometimes an order of magnitude greater than those at the synoptic scale, especially in the convective areas.
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.
