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Abstract
The drop size distribution (DSD) and drop shape relation (DSR) characteristics that were observed by a ground-based 2D video disdrometer and retrieved from a C-band polarimetric radar in the typhoon systems during landfall in the western Pacific, near northern Taiwan, were analyzed. The evolution of the DSD and its relation with the vertical development of the reflectivity of two rainband cases are fully illustrated. Three different types of precipitation systems were classified—weak stratiform, stratiform, and convective—according to characteristics of the mass-weighted diameter Dm , the maximum diameter, and the vertical structure of reflectivity. Further study of the relationship between the height H of the 15-dBZ contour of the vertical reflectivity profile, surface reflectivity Z, and the mass-weighted diameter Dm showed that Dm increased with a corresponding increase in the system depth H and reflectivity Z.
An analysis of DSDs retrieved from the National Central University (NCU) C-band polarimetric radar and disdrometer in typhoon cases indicates that the DSDs from the typhoon systems on the ocean were mainly a maritime convective type. However, the DSDs collected over land tended to uniquely locate in between the continental and maritime clusters. The average mass-weighted diameter Dm was about 2 mm and the average logarithmic normalized intercept Nw was about 3.8 log10 mm−1 m−3 in typhoon cases. The unique terrain-influenced deep convective systems embedded in typhoons in northern Taiwan might be the reason for these characteristics.
The “effective DSR” of typhoon systems had an axis ratio similar to that found by E. A. Brandes et al. when the raindrops were less than 1.5 mm. Nevertheless, the axis ratio tended to be more spherical with drops greater than 1.5 mm and under higher horizontal winds (maximum wind speed less than 8 m s−1). A fourth-order fitting DSR was derived for typhoon systems and the value was also very close to the estimated DSR from the polarimetric measurements in Typhoon Saomai (2006).
Abstract
The drop size distribution (DSD) and drop shape relation (DSR) characteristics that were observed by a ground-based 2D video disdrometer and retrieved from a C-band polarimetric radar in the typhoon systems during landfall in the western Pacific, near northern Taiwan, were analyzed. The evolution of the DSD and its relation with the vertical development of the reflectivity of two rainband cases are fully illustrated. Three different types of precipitation systems were classified—weak stratiform, stratiform, and convective—according to characteristics of the mass-weighted diameter Dm , the maximum diameter, and the vertical structure of reflectivity. Further study of the relationship between the height H of the 15-dBZ contour of the vertical reflectivity profile, surface reflectivity Z, and the mass-weighted diameter Dm showed that Dm increased with a corresponding increase in the system depth H and reflectivity Z.
An analysis of DSDs retrieved from the National Central University (NCU) C-band polarimetric radar and disdrometer in typhoon cases indicates that the DSDs from the typhoon systems on the ocean were mainly a maritime convective type. However, the DSDs collected over land tended to uniquely locate in between the continental and maritime clusters. The average mass-weighted diameter Dm was about 2 mm and the average logarithmic normalized intercept Nw was about 3.8 log10 mm−1 m−3 in typhoon cases. The unique terrain-influenced deep convective systems embedded in typhoons in northern Taiwan might be the reason for these characteristics.
The “effective DSR” of typhoon systems had an axis ratio similar to that found by E. A. Brandes et al. when the raindrops were less than 1.5 mm. Nevertheless, the axis ratio tended to be more spherical with drops greater than 1.5 mm and under higher horizontal winds (maximum wind speed less than 8 m s−1). A fourth-order fitting DSR was derived for typhoon systems and the value was also very close to the estimated DSR from the polarimetric measurements in Typhoon Saomai (2006).
Abstract
A variational algorithm for estimating measurement error covariance and the attenuation of X-band polarimetric radar measurements is described. It concurrently uses both the differential reflectivity Z DR and propagation phase ΦDP. The majority of the current attenuation estimation techniques use only ΦDP. A few of the ΦDP-based methods use Z DR as a constraint for verifying estimated attenuation. In this paper, a detailed observing system simulation experiment was used for evaluating the performance of the variational algorithm. The results were compared with a single-coefficient ΦDP-based method. Retrieved attenuation from the variational method is more accurate than the results from a single coefficient ΦDP-based method. Moreover, the variational method is less sensitive to measurement noise in radar observations. The variational method requires an accurate description of error covariance matrices. Relative weights between measurements and background values (i.e., mean value based on long-term DSD measurements in the variational method) are determined by their respective error covariances. Instead of using ad hoc values, error covariance matrices of background and radar measurement are statistically estimated and their spatial characteristics are studied. The estimated error covariance shows higher values in convective regions than in stratiform regions, as expected. The practical utility of the variational attenuation correction method is demonstrated using radar field measurements from the Taiwan Experimental Atmospheric Mobile-Radar (TEAM-R) during 2008’s Southwest Monsoon Experiment/Terrain-Influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX). The accuracy of attenuation-corrected X-band radar measurements is evaluated by comparing them with collocated S-band radar measurements.
Abstract
A variational algorithm for estimating measurement error covariance and the attenuation of X-band polarimetric radar measurements is described. It concurrently uses both the differential reflectivity Z DR and propagation phase ΦDP. The majority of the current attenuation estimation techniques use only ΦDP. A few of the ΦDP-based methods use Z DR as a constraint for verifying estimated attenuation. In this paper, a detailed observing system simulation experiment was used for evaluating the performance of the variational algorithm. The results were compared with a single-coefficient ΦDP-based method. Retrieved attenuation from the variational method is more accurate than the results from a single coefficient ΦDP-based method. Moreover, the variational method is less sensitive to measurement noise in radar observations. The variational method requires an accurate description of error covariance matrices. Relative weights between measurements and background values (i.e., mean value based on long-term DSD measurements in the variational method) are determined by their respective error covariances. Instead of using ad hoc values, error covariance matrices of background and radar measurement are statistically estimated and their spatial characteristics are studied. The estimated error covariance shows higher values in convective regions than in stratiform regions, as expected. The practical utility of the variational attenuation correction method is demonstrated using radar field measurements from the Taiwan Experimental Atmospheric Mobile-Radar (TEAM-R) during 2008’s Southwest Monsoon Experiment/Terrain-Influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX). The accuracy of attenuation-corrected X-band radar measurements is evaluated by comparing them with collocated S-band radar measurements.
Abstract
A newly designed retrieval scheme based on three-dimensional variational analysis is used to extract the thermodynamic field of a weather system from Doppler wind measurements. As compared with the traditional retrieval method, with this formulation the proposed scheme is able to find a set of optimal solutions for the pressure and buoyancy perturbations that, in the least squares sense, will simultaneously satisfy three momentum equations and a simplified thermodynamic equation. Therefore, the products of the retrieval are the complete thermodynamic fields in three dimensions. To test the performance of this method in real cases, it is applied to the analysis of a subtropical squall line. The required wind data were synthesized by two C-band Doppler radars during the 1987 Taiwan Area Mesoscale Experiment (TAMEX). The emphasis of this study is devoted to an examination of the validity of the retrieved thermodynamic structure, especially along the vertical direction. The results indicate that the distributions of the retrieved thermodynamic parameters are consistent with the kinematic structure and can be reasonably explained by the conceptual model of a squall line. Evidence is collected that strongly supports the validity of the derived thermodynamic structure. Thus, the applicability of this new retrieval scheme is demonstrated.
Abstract
A newly designed retrieval scheme based on three-dimensional variational analysis is used to extract the thermodynamic field of a weather system from Doppler wind measurements. As compared with the traditional retrieval method, with this formulation the proposed scheme is able to find a set of optimal solutions for the pressure and buoyancy perturbations that, in the least squares sense, will simultaneously satisfy three momentum equations and a simplified thermodynamic equation. Therefore, the products of the retrieval are the complete thermodynamic fields in three dimensions. To test the performance of this method in real cases, it is applied to the analysis of a subtropical squall line. The required wind data were synthesized by two C-band Doppler radars during the 1987 Taiwan Area Mesoscale Experiment (TAMEX). The emphasis of this study is devoted to an examination of the validity of the retrieved thermodynamic structure, especially along the vertical direction. The results indicate that the distributions of the retrieved thermodynamic parameters are consistent with the kinematic structure and can be reasonably explained by the conceptual model of a squall line. Evidence is collected that strongly supports the validity of the derived thermodynamic structure. Thus, the applicability of this new retrieval scheme is demonstrated.
Abstract
This study documents observational changes in the eyewall of Typhoon Fanapi (2010) after landfall in Taiwan. The observations indicate that Fanapi’s eye and eyewall disappeared on the eastern side of Taiwan’s Central Mountain Range (CMR) after landfall, but reemerged on the western side of CMR. The cyclonic circulation, increasing wind speed, a low-level low pressure and high temperature zone, the associated updrafts and downdrafts, and surface pressure and rainfall measurements all support the existence of a reintensified eyewall. The storm slowed down during the redeveloping stage, thus prolonging the rainfall duration over Taiwan.
On the western side of CMR a northwest–southeast-oriented rainband formed at an earlier stage, possibly due to the large-scale interaction between Fanapi’s remnant flow and the environment. However, the subsequent reintensification might be attributed to the interaction between the circulation and topography. This is supported by the finding that adjacent to CMR, strong wind develops vertically from lower levels, indicating that the reintensification appears to be initiated through a bottom-up process. A vorticity budget analysis shows that at lower layers the stretching mechanism plays a leading role in increasing positive vorticity, followed by the contributions from tilting and horizontal advection. The horizontal advection plays a comparable role to the vertical advection in increasing low- to midlevel vorticity. The vertical advection aloft is responsible for transporting the vorticity upward. Finally, this research provides a relatively rare documentation of the vortical hot towers (VHTs) over terrain using ground-based radars, in contrast to most previous studies focusing on maritime VHTs using simulations or aircraft measurements.
Abstract
This study documents observational changes in the eyewall of Typhoon Fanapi (2010) after landfall in Taiwan. The observations indicate that Fanapi’s eye and eyewall disappeared on the eastern side of Taiwan’s Central Mountain Range (CMR) after landfall, but reemerged on the western side of CMR. The cyclonic circulation, increasing wind speed, a low-level low pressure and high temperature zone, the associated updrafts and downdrafts, and surface pressure and rainfall measurements all support the existence of a reintensified eyewall. The storm slowed down during the redeveloping stage, thus prolonging the rainfall duration over Taiwan.
On the western side of CMR a northwest–southeast-oriented rainband formed at an earlier stage, possibly due to the large-scale interaction between Fanapi’s remnant flow and the environment. However, the subsequent reintensification might be attributed to the interaction between the circulation and topography. This is supported by the finding that adjacent to CMR, strong wind develops vertically from lower levels, indicating that the reintensification appears to be initiated through a bottom-up process. A vorticity budget analysis shows that at lower layers the stretching mechanism plays a leading role in increasing positive vorticity, followed by the contributions from tilting and horizontal advection. The horizontal advection plays a comparable role to the vertical advection in increasing low- to midlevel vorticity. The vertical advection aloft is responsible for transporting the vorticity upward. Finally, this research provides a relatively rare documentation of the vortical hot towers (VHTs) over terrain using ground-based radars, in contrast to most previous studies focusing on maritime VHTs using simulations or aircraft measurements.
Abstract
A north–south-oriented, multicellular squall line during the Taiwan Area Mesoscale Experiment’s (TAMEX) intensive observation period 2 was studied, using dual-Doppler radar data, as it moved over the island of Taiwan. Over the open ocean, it moved eastward at a constant speed (∼15.5 m s−1).
As the squall line encountered Taiwan Island’s complex mountainous areas, the leading edge moved slower in the mountain ridge areas than over the valleys. As a result, its orientation became approximately parallel to the terrain contours. In the upper levels, the eastward movement of the squall line aloft was less affected by the terrain compared to the low levels. As a result, the westward tilt of the rising motion at the front became less significant. Part of the system-relative rear-to-front flow entered the rising branch of the storm’s circulation as a result of orographic lifting, as illustrated by the trajectory analysis, and was enhanced by latent heat release. The system-relative rear-to-front flow did not appear to descend to the lowest levels. Furthermore, it is unlikely that the low-level cold pool behind the leading edge would move upslope. When the squall line moved over the higher terrain areas, the midlevel system-relative westerly flow from the rear entered the storm’s updraft. The vertical motion pattern was dominated by orographic lifting and sinking. Low-level system-relative front-to-rear inflow was absent. Weak echo maxima were found, mainly associated with orographic lifting ahead of or near the mountain peaks. These changes occurred within a short time (∼40 min) after the squall line encountered the mountainous terrain.
The environmental low-level wind ahead of the squall line exhibited a weak southerly flow west of the mountain ridge areas and a weak southwesterly flow in the valley areas because of island blocking. As a result, the low-level system-relative front-to-rear inflow was weaker in the valley areas than that in ridge areas. Nevertheless, the low-level moisture inflow from the front decreased rapidly in both the former and the latter areas when the squall line encountered mountainous terrain. This was attributed to a rapid decrease in the depth of the low-level inflow layer and less moisture availability in the higher terrain areas. The drier midlevel air entered the storm from the west with the echo tops decreasing as the squall line moved farther over the mountainous terrain.
Abstract
A north–south-oriented, multicellular squall line during the Taiwan Area Mesoscale Experiment’s (TAMEX) intensive observation period 2 was studied, using dual-Doppler radar data, as it moved over the island of Taiwan. Over the open ocean, it moved eastward at a constant speed (∼15.5 m s−1).
As the squall line encountered Taiwan Island’s complex mountainous areas, the leading edge moved slower in the mountain ridge areas than over the valleys. As a result, its orientation became approximately parallel to the terrain contours. In the upper levels, the eastward movement of the squall line aloft was less affected by the terrain compared to the low levels. As a result, the westward tilt of the rising motion at the front became less significant. Part of the system-relative rear-to-front flow entered the rising branch of the storm’s circulation as a result of orographic lifting, as illustrated by the trajectory analysis, and was enhanced by latent heat release. The system-relative rear-to-front flow did not appear to descend to the lowest levels. Furthermore, it is unlikely that the low-level cold pool behind the leading edge would move upslope. When the squall line moved over the higher terrain areas, the midlevel system-relative westerly flow from the rear entered the storm’s updraft. The vertical motion pattern was dominated by orographic lifting and sinking. Low-level system-relative front-to-rear inflow was absent. Weak echo maxima were found, mainly associated with orographic lifting ahead of or near the mountain peaks. These changes occurred within a short time (∼40 min) after the squall line encountered the mountainous terrain.
The environmental low-level wind ahead of the squall line exhibited a weak southerly flow west of the mountain ridge areas and a weak southwesterly flow in the valley areas because of island blocking. As a result, the low-level system-relative front-to-rear inflow was weaker in the valley areas than that in ridge areas. Nevertheless, the low-level moisture inflow from the front decreased rapidly in both the former and the latter areas when the squall line encountered mountainous terrain. This was attributed to a rapid decrease in the depth of the low-level inflow layer and less moisture availability in the higher terrain areas. The drier midlevel air entered the storm from the west with the echo tops decreasing as the squall line moved farther over the mountainous terrain.
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.
Abstract
The present study documents the environment, initiation, and evolution of three isolated supercell storms on 19 December 2002, as the first case near Taiwan reported in the literature, mainly using radar data and manual and gridded analyses. In a subtropical environment, the supercells occurred behind a winter cold front that provided a large west-southwesterly vertical wind shear of 6.4 × 10−3 s−1 at 0–3 km. This combined with weak-to-moderate instability (CAPE = 887 J kg−1) above the shallow surface cold air to yield a favorable environment for supercells. An approaching upper-level jet (ULJ) at 200 hPa also provided strong shear through deep layers farther aloft. Prior to storm initiation, significant daytime solar heating occurred over the mountain slopes along the coast of southeastern China, leading to development of local circulation and onshore/upslope winds, resulting in convergence and uplifting. Three storms were initiated about 80 km inland around 1400 LST near the peaks of local terrain with a northeast–southwest alignment. After formation, the three storms evolved into isolated supercells and each experienced multiple splits. The right-moving storms were usually stronger than left-moving ones and traveled eastward rapidly at about 18 m s−1 across the Taiwan Strait. The storms reached their maximum strength over the strait where low-level shear intensified during the day due to cold air surge. The northern storm also registered a peak reflectivity of 72 dBZ, the strongest ever recorded by any radar in Taiwan. Eventually, the three supercell storms made landfall over Taiwan, producing swaths of rain, hail, and property damages. Before they diminished after midnight, each of the three storms had lasted for about 10 h and propagated for over 550 km.
Abstract
The present study documents the environment, initiation, and evolution of three isolated supercell storms on 19 December 2002, as the first case near Taiwan reported in the literature, mainly using radar data and manual and gridded analyses. In a subtropical environment, the supercells occurred behind a winter cold front that provided a large west-southwesterly vertical wind shear of 6.4 × 10−3 s−1 at 0–3 km. This combined with weak-to-moderate instability (CAPE = 887 J kg−1) above the shallow surface cold air to yield a favorable environment for supercells. An approaching upper-level jet (ULJ) at 200 hPa also provided strong shear through deep layers farther aloft. Prior to storm initiation, significant daytime solar heating occurred over the mountain slopes along the coast of southeastern China, leading to development of local circulation and onshore/upslope winds, resulting in convergence and uplifting. Three storms were initiated about 80 km inland around 1400 LST near the peaks of local terrain with a northeast–southwest alignment. After formation, the three storms evolved into isolated supercells and each experienced multiple splits. The right-moving storms were usually stronger than left-moving ones and traveled eastward rapidly at about 18 m s−1 across the Taiwan Strait. The storms reached their maximum strength over the strait where low-level shear intensified during the day due to cold air surge. The northern storm also registered a peak reflectivity of 72 dBZ, the strongest ever recorded by any radar in Taiwan. Eventually, the three supercell storms made landfall over Taiwan, producing swaths of rain, hail, and property damages. Before they diminished after midnight, each of the three storms had lasted for about 10 h and propagated for over 550 km.
Abstract
The ground-based velocity track display (GBVTD) technique is extended to two Doppler radars to retrieve the structure of a tropical cyclone’s (TC’s) circulation. With this extension, it is found that the asymmetric part of the TC radial wind component can be derived up to its angular wavenumber-1 structure, and the accuracy of the retrieved TC tangential wind component can be further improved. Although two radar systems are used, a comparison with the traditional dual-Doppler synthesis indicates that this extended GBVTD (EGBVTD) approach is able to estimate more of the TC circulation when there are missing data. Previous research along with this study reveals that the existence of strong asymmetric radial flows can degrade the quality of the GBVTD-derived wind fields. When a TC is observed by one radar, it is suggested that the GBVTD method be applied to TCs over a flat surface (e.g., the ocean) where the assumption of relatively smaller asymmetric radial winds than asymmetric tangential winds is more likely to be true. However, when a TC is observed by two radar systems, especially when the topographic effects are expected to be significant, the EGBVTD rather than the traditional dual-Doppler synthesis should be used.
The feasibility of the proposed EGBVTD method is demonstrated by applying it to an idealized TC circulation model as well as a real case study. Finally, the possibility of combining EGBVTD with other observational instruments, such as dropsonde or wind profilers, to recover the asymmetric TC radial flow structures with even higher wavenumbers is discussed.
Abstract
The ground-based velocity track display (GBVTD) technique is extended to two Doppler radars to retrieve the structure of a tropical cyclone’s (TC’s) circulation. With this extension, it is found that the asymmetric part of the TC radial wind component can be derived up to its angular wavenumber-1 structure, and the accuracy of the retrieved TC tangential wind component can be further improved. Although two radar systems are used, a comparison with the traditional dual-Doppler synthesis indicates that this extended GBVTD (EGBVTD) approach is able to estimate more of the TC circulation when there are missing data. Previous research along with this study reveals that the existence of strong asymmetric radial flows can degrade the quality of the GBVTD-derived wind fields. When a TC is observed by one radar, it is suggested that the GBVTD method be applied to TCs over a flat surface (e.g., the ocean) where the assumption of relatively smaller asymmetric radial winds than asymmetric tangential winds is more likely to be true. However, when a TC is observed by two radar systems, especially when the topographic effects are expected to be significant, the EGBVTD rather than the traditional dual-Doppler synthesis should be used.
The feasibility of the proposed EGBVTD method is demonstrated by applying it to an idealized TC circulation model as well as a real case study. Finally, the possibility of combining EGBVTD with other observational instruments, such as dropsonde or wind profilers, to recover the asymmetric TC radial flow structures with even higher wavenumbers is discussed.
Abstract
During the period of 21–25 June 1991, a mei-yu front, observed by the post–Taiwan Area Mesoscale Experiment, produced heavy precipitation along the western side of the Central Mountain Range of Taiwan. Several oceanic mesoscale convective systems were also generated in an area extending from Taiwan to Hong Kong. Numerical experiments using the Penn State–NCAR MM5 mesoscale model were used to understand the intensification of the low-level jet (LLJ). These processes include thermal wind adjustment and convective, inertial, and conditional symmetric instabilities.
Three particular circulations are important in the development of the mei-yu front. First, there is a northward branch of the circulation that develops across the upper-level jet and is mainly caused by the thermal wind adjustment as air parcels enter an upper-level jet streak. The upper-level divergence associated with this branch of the circulation triggers convection.
Second, the southward branch of the circulation, with its rising motion in the frontal region and equatorward sinking motion, is driven by frontal vertical deep convection. The return flow of this circulation at low levels can produce an LLJ through geostrophic adjustment. The intensification of the LLJ is sensitive to the presence of convection.
Third, there is a circulation that develops from low to middle levels that has a slantwise rising and sinking motion in the pre- and postfrontal regions, respectively. From an absolute momentum surface analysis, this slantwise circulation is maintained by conditionally symmetric instability located at low levels ahead of the front. The presence of both the LLJ and moisture is an essential ingredient in fostering this conditionally symmetric unstable environment.
Abstract
During the period of 21–25 June 1991, a mei-yu front, observed by the post–Taiwan Area Mesoscale Experiment, produced heavy precipitation along the western side of the Central Mountain Range of Taiwan. Several oceanic mesoscale convective systems were also generated in an area extending from Taiwan to Hong Kong. Numerical experiments using the Penn State–NCAR MM5 mesoscale model were used to understand the intensification of the low-level jet (LLJ). These processes include thermal wind adjustment and convective, inertial, and conditional symmetric instabilities.
Three particular circulations are important in the development of the mei-yu front. First, there is a northward branch of the circulation that develops across the upper-level jet and is mainly caused by the thermal wind adjustment as air parcels enter an upper-level jet streak. The upper-level divergence associated with this branch of the circulation triggers convection.
Second, the southward branch of the circulation, with its rising motion in the frontal region and equatorward sinking motion, is driven by frontal vertical deep convection. The return flow of this circulation at low levels can produce an LLJ through geostrophic adjustment. The intensification of the LLJ is sensitive to the presence of convection.
Third, there is a circulation that develops from low to middle levels that has a slantwise rising and sinking motion in the pre- and postfrontal regions, respectively. From an absolute momentum surface analysis, this slantwise circulation is maintained by conditionally symmetric instability located at low levels ahead of the front. The presence of both the LLJ and moisture is an essential ingredient in fostering this conditionally symmetric unstable environment.
