This study used ground-based dual-Doppler observations to explore an understanding of kinematic characteristics of the southwesterly flow associated with the southwest (SW) and ordinary (OR) typhoons immediately off the southwestern coast of Taiwan. The SW (OR) typhoon stated herein is referred to as a typhoon with (without) an obvious combination of its outer circulations and the summer southwesterly monsoon active over the South China Sea. Six typhoon events [Mindulle (2004), Kalmaegi (2008), Morakot (2009), Talim (2005), Jangmi (2008), and Fungwong (2008)] were chosen for analysis; the first (latter) three listed belong to the family of the SW (OR) typhoons. The vertical profiles generated from hourly synthesized winds for these typhoons indicate that intense orographic rainfall tended to occur during the prevalence of the west-southwesterly (WSW) flow that was more perpendicular to the south–north orientation of the topography in southern Taiwan. A unique, consistent feature of the WSW flow associated with the SW typhoon was its persistently increasing intensity with decreasing height in the low to midtroposphere, in contrast to a minor vertical variation in the intensity of the WSW flow for the OR typhoon. A relatively large (small) ratio of the radial and tangential velocities was evident for the SW (OR) typhoon, and the mean inflow angle of the SW typhoon was significantly larger than the typical near-surface inflow angle of previously documented hurricanes over the open ocean. In addition to the typhoon background precipitation, the observed characteristics of the SW- and OR-typhoon-induced WSW flow were shown to be closely related to the degree of orographic enhancement of precipitation.
The interactions of tropical cyclones with topographic features have been long recognized as one of the most important and frequent processes resulting in continuous, torrential rains and severe flooding over land (Brunt 1968; Hope 1975; Parrish et al. 1982; Wu and Kuo 1999). The tropical cyclone environment is typically characterized by intense winds at low levels, which favor the occurrence of a large Froude number flow regime (Fr = U/NH, where U is the upstream wind speed, N is the Brunt–Väisälä frequency, and H is the mountain height) and contribute to precipitation enhancement, especially over the windward side of mountains through upslope lifting (Hamuro et al. 1969; Lin et al. 2001; Wu et al. 2002; Lin 2007). Consistent with this expectation, relative spatial configurations between incident prevailing winds associated with tropical cyclone circulations and local topographic features have been observed to be a primary factor in controlling the overall patterns and intensities of enhanced precipitation over a mountainous landmass (Wang 1989; Chang et al. 1993; Lee et al. 2006; Huang et al. 2012). A limited number of recent studies have also indicated that the intensity and distribution of the precipitation over a relatively small mountain barrier may be governed by more complicated processes, such as the microphysical interactions between tropical cyclone rainbands (i.e., the background precipitation) and orographically forced precipitation (Yu and Cheng 2008, hereafter YC08; Smith et al. 2009; Yu and Cheng 2013, hereafter YC13).
Taiwan is a well-known target of typhoons originating in the western North Pacific and has suffered nearly every year from torrential rain associated with these approaching/landfalling storms (Wu and Kuo 1999). Past observations indicate that the typhoon-induced heaviest rainfall in Taiwan has been concentrated over the mountainous region (Wu et al. 2002; Lee et al. 2008; M.-J. Yang et al. 2008; YC08). In particular, the western slopes of significant topography in southwestern Taiwan are one of the three geographical regions with the climatological maximum of heavy rainfall occurring under the influence of typhoons (Fig. 1). Severe floods, destructive debris flow, and landslides caused by the typhoon’s precipitation are the most threatening, common natural disaster for this geographical region. Typhoon Morakot (2009) is a catastrophic example, bringing a record-breaking rainfall maximum of about 2500 mm in southwestern Taiwan and causing significant loss of human life (673 people killed and 26 missing) and more than USD500 million in agricultural damage (Wu 2013).
Experiences indicate that the occurrence of heavy rainfall over the mountains of southwestern Taiwan typically involves the presence of a prevailing southwesterly flow off the coast brought by the westward-moving typhoons as they pass over the central or northern portion of Taiwan. A growing number of recent typhoon studies have reported that the development of torrential rainfall over this particular region is frequently related to the persistent southwesterly flow due to the combination of typhoon circulations and the summer southwesterly monsoon active over the South China Sea (e.g., Chien et al. 2008; Liang et al. 2011). In this context, the presence of the southwesterly flow off the southwestern coast of Taiwan is not simply driven by the typhoon vortex itself, but it is a consequence of substantial interactions between typhoon circulations and their ambient monsoonal flow (i.e., the typhoon–monsoon interactions). This type of typhoon–monsoon-induced southwesterly flow, from a large-scale perspective, may provide efficient moisture transport and a confluent environment favorable for the occurrence of heavy orographic rainfall (e.g., Chien and Kuo 2011). Note that a similar typhoon–monsoon-interaction scenario has also been recognized to be an important contributor to the torrential rainfall over northern and/or northeastern Taiwan as approaching typhoons interact with the winter northeasterly monsoon during the late typhoon season and bring a strong, moist northeasterly flow impinging on the mountainous coast (Lee et al. 2007; YC08; Wu et al. 2009).
Because the distribution and intensity of orographic precipitation, despite the complexity of terrain geometries, are primarily determined by the kinematic and thermodynamic properties of upstream oncoming airflow (Smith 1979; Blumen 1990; Sinclair et al. 1997; Lin et al. 2001; Colle 2004; Lin 2007; Houze 2012), elucidation of the nature of the southwesterly flow associated with typhoons is an essential step to provide a better context of the upstream environment and its physical connection with orographically enhanced precipitation over southwestern Taiwan. However, this issue has been poorly understood because of the lack of detailed observations over the coastal water area. The possible similarities and differences in the structural characteristics between the southwesterly flow simply driven by typhoon circulations and the typhoon–monsoon-induced southwesterly flow also remain unresolved.
The primary objective of this study is to use ground-based dual-Doppler observations from two coastal radars in southern Taiwan to document the detailed kinematic profiles of the southwesterly flow associated with typhoons immediately off the southwestern coast of Taiwan. In this study, three typhoon events [Mindulle (2004), Kalmaegi (2008), and Morakot (2009)] possessing a substantial interaction with the southwesterly monsoon flow were chosen for dual-Doppler analysis. To explore the unique kinematic features for the typhoon–monsoon-induced southwesterly flow and their possible distinction to those of the southwesterly flow driven primarily by typhoon circulations, three other typhoon events [Talim (2005), Jangmi (2008), and Fungwong (2008)] without an obvious interaction with the southwesterly monsoon were also chosen as reference counterparts for analysis. For simplicity of this discussion, Typhoons Mindulle, Kalmaegi, and Morakot are referred to as the southwest (SW) typhoons, and Typhoons Talim, Jangmi, and Fungwong are referred to as the ordinary (OR) typhoons.
Note that in addition to Typhoon Morakot, which has been recently studied extensively, certain additional aspects of several of the selected typhoons have been described in the literature (e.g., Lee et al. 2008; C.-C. Yang et al. 2008; Kim et al. 2011; Wang et al. 2012). However, none of these studies has focused on the investigation of the detailed, mesoscale perspectives of the typhoon-induced (or typhoon–monsoon induced) southwesterly flow immediately upstream of the significant topography in southern Taiwan. An important characteristic of the selected typhoons is that they moved from the southeastern side to northwestern side of Taiwan with similar landing locations (Fig. 2) and brought a southwesterly flow to the southwestern coast of Taiwan. In addition, all of these selected typhoons caused intense orographic precipitation concentrated over the mountainous region of southern Taiwan (Fig. 3). Another important reason that these particular typhoon events were selected for study is because they exhibited considerable radar echoes off the southwestern coast of Taiwan during the development of the southwesterly flow and during the heavy orographic rainfall. This condition allows the retrieval of more complete wind information offshore, upstream of the significant topography of southern Taiwan via dual-Doppler analysis.
2. Data and methodology
The primary datasets used in this study were provided by the Taiwan Central Weather Bureau S-band (10 cm) Weather Surveillance Radar-1988 Doppler (WSR-88D) on Chi-Gu (CG) and Ken-Ting (KT) (Fig. 4). The CG and KT radars are located at the southwestern coast and at the southern end of Taiwan, respectively, and both radars provide volumetric distributions of reflectivity and radial velocity with a temporal interval of 7.5 min between each volume. The dual-Doppler synthesis of the multiple-view reflectivity and radial velocity data (Ray et al. 1980) from the two coastal radars provides a unique depiction of the kinematic characteristics for the upstream southwesterly flow associated with the studied typhoons. The synthesis domain, as indicated by the inset box in Fig. 4, extends from about 30 km off the southwestern coast of Taiwan to about 110 km offshore. The location and coverage of the synthesis domain determined herein were based on the consideration of synthesized geometries. Because the cross-beam angles of the two radars within the domain (red contours in Fig. 4) were mostly between 75° and 115°, they produce relatively smaller uncertainties and errors in the dual-Doppler-derived winds (Doviak and Zrnic 1993).
The National Center for Atmospheric Research (NCAR) SOLO software (Nettleton et al. 1993) was used to unfold the radial velocities and to remove sea clutter and unreasonable or incorrect values of radar reflectivity and radial velocity data. The NCAR REORDER software (Oye et al. 1995) was used to interpolate reflectivities and radial velocities from raw plan position indicator (PPI) scans to Cartesian coordinates with a horizontal grid spacing of 1 km and a vertical grid spacing of 500 m over a volume of 80 × 80 km2 in the horizontal (box in Fig. 4) and 10 km in the vertical direction, with the lowest analysis level located at 500 m MSL. Synthesis of the gridded radial velocities into horizontal wind fields was performed using the NCAR Custom Editing and Display of Reduced Information in Cartesian Space (CEDRIC) software package (Mohr and Miller 1983). The procedures and settings of the dual-Doppler analysis applied in this study generally follow those described in YC13.
In this study, a total of 177 sets of hourly synthesized winds were derived from the six studied typhoons. Table 1 summarizes the period of the dual-Doppler synthesis for each typhoon event and basic information for the studied typhoons. The duration of the synthesis time window varied from case to case and was primarily dependent on the availability of the reflectivity and velocity measurements within the synthesis domain. Because of a generally consistent trend of precipitation variations over upstream and inland regions, the synthesis time window closely corresponded to the occurrence of heavy rainfall over the mountainous region of southern Taiwan. Except for Typhoon Mindulle, whose location was already over the region immediately inland of the northwestern coast of Taiwan at the beginning of the synthesis, the tracks of all of the other selected typhoons during the synthesis period (highlighted with black lines in Fig. 2) typically started from regions near or off the coastal zone of eastern Taiwan, moved northwestward over central/northern Taiwan, and then entered the Taiwan Strait and/or the East China Sea. As indicated in Table 1, the mean distances from the storm center to the synthesis domain calculated during the period of analysis for the studied typhoons are greater than 250 km and are well beyond their corresponding radius threshold (i.e., 3 times the radius of maximum wind)—an approximate length scale used to classify the inner and outer regions of a tropical cyclone (Wang 2009; Yu and Chen 2011). Wind fields retrieved from the synthesis domain may reflect the kinematic characteristics over the outer region of typhoon circulations.
The primary focus of this study was to investigate the kinematic vertical profiles of the southwesterly flow associated with typhoons; therefore, at a given analysis level, winds at each grid point within the synthesis domain were averaged to obtain a mean value to represent the upstream oncoming flow at that altitude. Because of the inherent limitation of radar scanning, the best spatial coverage of dual-Doppler-derived winds over the synthesis domain was confined to the mid- to low troposphere. Additionally, the characteristics of upstream southwesterly flow in the lower troposphere are expected to be more relevant in the context of orographically enhanced precipitation. Therefore, the analyses of the kinematic profiles for the present study were restricted to altitudes below about 6.5 km MSL. As discussed in the following sections, these analysis levels were adequate for the demonstration of the important aspects of typhoon–monsoon interaction, including its vertical extent. In addition, because the observed southwesterly flow was embedded with offshore moist convection, its associated diabatic effects and convective-scale motions are likely to cause the smaller-scale fluctuations of wind fields, which may complicate the representation of the spatially averaging procedure applied herein. However, these convectively generated signals did not appear sufficiently strong to be problematic when interpreting the results. For example, the standard deviations of the wind speed calculated over the synthesis domain at different analysis levels and periods are generally small (<3 m s−1). The standard deviations of the wind direction are also typically less than 10°, and some of these spatial variations in wind direction are due to the inherent curvature of the cyclonic flow associated with typhoon outer circulations. These calculations support a relatively uniform horizontal nature of the southwesterly flow over the target region off the southwestern coast of Taiwan.
Other data sources used in this study are also indicated in Fig. 4, including the routine surface rain gauges in southern Taiwan and the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data. Because of the lack of rain gauges over the coastal water and their sparse distribution over the mountainous region in southern Taiwan (cf. Fig. 4), in this study, the detailed information of orographic and upstream rainfall were derived from the combination of surface rain gauges and high-resolution reflectivity measurements obtained from the Chi-Gu radar, following the analysis methods described in YC13. These radar-derived rainfall features and their possible relationship with upstream kinematics will be discussed in section 5. The NCEP gridded data available within the dual-Doppler synthesis domain were used for a comparison with the dual-Doppler-derived winds, which will be presented in the next section. In addition, the NCEP data over the South China Sea (not shown in Fig. 4) were utilized in this study to provide a basic context for the strength of the southwesterly monsoonal flow associated with the studied typhoons.
3. Overview of the kinematic characteristics
The general aspects of the upstream oncoming flow associated with the six selected typhoons are described by the time–height cross section of the mean dual-Doppler-derived winds during the synthesis period. To provide an initial context of the relationship between the upstream flow and the orographic rainfall, the temporal variation of the areal mean of the rainfall intensity over southern Taiwan (the inset box over land in Fig. 4) is superposed in Fig. 5.
Except for Typhoon Mindulle, the temporal variations of the upstream airflow for all of the other typhoons were characterized by a clear wind alternation from northwesterlies to westerlies or southwesterlies. This wind transition was generally consistent with the passage of the vortex circulations from the southeastern side to the northwestern side of Taiwan for the studied typhoons (cf. Fig. 2). Typhoon Mindulle lacked evidence of the northwesterly winds because the typhoon had already moved over northern Taiwan during the early stage of the synthesis time window, as described in section 2. Surface coastal winds over southwestern Taiwan revealed the presence of northwesterly winds on 1 July 2004 (not shown) while Typhoon Mindulle’s center was still located off the southeastern coast of Taiwan before its synthesis period (cf. Fig. 2).
For the SW typhoons, the occurrence of the strongest oncoming winds was observed during the prevalence of westerly or southwesterly flow, with maximum wind speeds near the lowest analysis level (Figs. 5a,c,e). In contrast, the maximum oncoming winds for the OR typhoons were generally associated with the northwesterly flow driven by the cyclonic circulations of typhoons during the earlier synthesis period (Figs. 5b,d,f). The strong northwesterly winds were present in a deep layer, and their vertical variations in intensity were relatively minor. A mutual characteristic for the studied typhoons is that more intense orographic rainfall tended to occur as the upstream oncoming flow became westerly or west-southwesterly and more perpendicular to the approximately south–north orientation of the topography in southern Taiwan (cf. Fig. 4). This result suggests a general importance of orographically forced lifting, for both SW and OR typhoons, for the production of heavy rainfall as the upstream westerly or west-southwesterly (WSW) flow impinged on the mountainous terrain. The temporal variation of the mean rainfall intensity over land corresponding to the change in the intensity of the upstream oncoming flow was not always evident for each typhoon, reflecting the complexity of the orographic processes leading to precipitation enhancement in the tropical cyclone environment (YC08; Smith et al. 2009; YC13).
Another independent view of the time–height cross section of the upstream airflow could also be generated by the NCEP reanalysis data available within the dual-Doppler synthesis domain, as shown in Fig. 6. The periods for the occurrence of strong WSW flow for each typhoon seen from the NCEP and dual-Doppler radar roughly coincided (Figs. 5 and 6). The intensities of the WSW and typhoon-induced northwesterly flow tended to have a frequent difference of approximately 2–4 m s−1 between the NCEP and dual-Doppler radar observations. The specific timing of the occurrence of the local maximum of the WSW flow at low levels and its vertical (temporal) variations, as evident in Fig. 5, were not well captured by the NCEP-derived cross sections, presumably because of the inherent limitation of the coarse resolution of the NCEP data in space and time. The comparison indicates that the NCEP wind information was useful to provide a gross picture of the upstream evolving airflow consistent with that shown by dual-Doppler observations, but it could not adequately resolve the detailed aspects of the typhoon-induced (or typhoon–monsoon induced) oncoming flow off the southwestern coast of Taiwan.
For a clearer depiction of the key characteristics of the upstream WSW flow associated with the studied typhoon events, we generated vertical profiles of the mean wind speed averaged over a 3-h period centered at the time when the maximum intensity of the flow occurred, as marked by thick dashed vertical lines in Fig. 5 (see Fig. 7). The analysis indicates a wide range of strength for the WSW flow associated with the studied typhoons, with magnitudes of approximately 16–33 m s−1 at the near-surface levels. The intensity of the WSW flow for the SW typhoons were generally stronger than those of the OR typhoons at low levels, except for Typhoon Kalmaegi. The weak WSW flow associated with Kalmaegi was not only related to the inherently weak intensity of the typhoon (e.g., Table 1) but also most likely owing in part to a significant weakening of the typhoon intensity as Kalmaegi made landfall on the eastern coast of Taiwan (not shown).
A notably consistent feature of the WSW flow for the SW typhoons (indicated by the black curves in Fig. 7) was its persistently increasing intensity with decreasing height in the low to midtroposphere, thus possessing a maximum at the lowest analysis level. This observed characteristic is in distinct contrast to the lack of a dramatic change in the intensity of the WSW flow observed at different heights for the OR typhoons (indicated by the gray curves in Fig. 7). The relatively minor vertical variations of the wind speed have been similarly documented over the outer-vortex regions of tropical cyclones located over the open ocean (Franklin et al. 2003). Note that the presence of low-level wind maxima, such as that associated with the SW typhoons, has been recognized as a common kinematic signature of upstream airflow favorable for the occurrence of heavy orographic rainfall in midlatitude and tropical cyclone environments (Lin et al. 2001; Neiman et al. 2002; Witcraft et al. 2005).
The contrasting feature of the wind speed profiles between the SW and OR typhoons, as shown in Fig. 7, may reflect their fundamental difference in the degree of the typhoon–monsoon interaction. Although there is no objective criterion that has been previously proposed to quantify the degree of the typhoon–monsoon interaction, several factors, such as the location of typhoons, the intensity of typhoon circulations, and the strength of the monsoon systems adjacent to the typhoon circulations, would probably influence the potential significance of such interactions (e.g., Wu et al. 2009). For the present study, the studied typhoons during the period of the strongest WSW flow exhibited similar locations with respect to the monsoon system active over the South China Sea (Fig. 8a). There was no systematic difference in the intensity of typhoon circulations (in terms of minimum pressure and maximum wind speed) between the SW and OR typhoons (Table 1). Regarding the strength of the southwesterly monsoon associated with the studied typhoons, it may be practically evaluated by calculating the mean intensity of the southwesterly flow over the South China Sea. The NCEP reanalysis data, valid within a selected domain located in the southwest of Taiwan (Fig. 8a), were used in this evaluation. To highlight the mean difference in the strength of the southwesterly monsoon between the SW and OR typhoons, two vertical profiles of the mean NCEP winds averaged during the prevalence of the WSW flow, corresponding to each type of typhoon, were generated (Fig. 8b). There was a deep layer of southwesterly flow and a decreasing trend of wind speed with height above the near-surface frictional layer for both types of typhoons. A stronger southwesterly flow associated with the SW typhoon was evident, with a maximum of 15.5 m s−1 near 750 m MSL. The intensity of the southwesterly flow associated with the OR typhoon was relatively weaker and less than 10 m s−1 at all of the analysis levels. These results suggest that the strength of the southwesterly monsoon over the South China Sea is essentially important for the interaction scenario at least for the typhoon cases studied herein and provide additional support for a strong (weak) typhoon–monsoon interaction for the SW (OR) typhoon.
4. Distinction of the WSW flow between the SW and OR typhoons
a. Radial and tangential flow
As described in the introduction, the possible kinematic distinction of the WSW flow induced by the OR and SW typhoons has been unresolved. In this subsection, we explore the fundamental differences in the nature of the WSW flow between the SW and the OR typhoons by investigating the characteristics of radial (u) and tangential (υ) flow calculated from the dual-Doppler-derived winds for these two types of typhoons. The movement of each typhoon was also considered in this calculation, resulting in a more representative value of the radial and tangential flows relative to the typhoon center. An essential hypothesis herein is that if the WSW flow is primarily driven by typhoon circulations, its associated radial and tangential wind components should bear certain similarities with those typically observed within tropical cyclones and vice versa. Perhaps the most common feature of airflow associated with tropical cyclones is the dominance of a strong cyclonically rotational flow; namely, the radial velocities tend to be much smaller than the corresponding tangential velocities. This characteristic prevails not only within the inner-core vortex but also over outer regions of tropical cyclones (Gray 1979; Frank 1977; Anthes 1982; Franklin et al. 1993; Didlake and Houze 2013). The intensity of the mean radial flow was usually observed to be only a few meters per second, whereas the tangential flow typically has magnitudes with several tens of meters per second. The intensity of both radial flow and tangential flow generally increases toward the storm center outside the radius of maximum wind (RMW) with a maximum typically found in the lower troposphere near the top of the boundary layer.
Figure 9 shows the vertical profiles of the mean radial and tangential flows associated with the studied typhoons calculated during the 3-h period of the strongest WSW flow. The radial velocities of the OR-typhoon-induced WSW flow were less than 10 m s−1 and varied slightly with height (Fig. 9a). The radial velocities were significantly smaller than their corresponding tangential velocities (~15–23 m s−1), as shown in Fig. 9b. In contrast to this typical tropical cyclone–like signature, stronger negative radial velocities (i.e., the inflow), which were comparable to the tangential velocities, were associated with the SW-typhoon-induced WSW flow and exhibited a prominent increase with decreasing height (Figs. 9a,b). Given a relatively minor variation of the tangential velocities in the vertical for both types of typhoons (Fig. 9b), the unique profile of the WSW flow associated with the SW typhoon, particularly with a maximum wind speed at the lowest analysis level (cf. Fig. 7), was primarily determined by the enhanced radial velocities at low levels, as shown in Fig. 9a. There were no obvious differences in the radial velocities between these two types of typhoons above 3 km MSL, which is consistent with the interaction between typhoon circulations and monsoonal flow confined primarily to the low levels, as described in section 3.
According to the observational characteristics evident in Fig. 9, we propose that the ratio of the radial and tangential velocities (i.e., u/υ; RRT) may be considered as a useful quantity to distinguish the WSW flow induced by the OR and SW typhoons. Physically, the RRT magnitude is equivalent to the tangent of the crossing angle or inflow angle, defined as an angle between the actual wind and a tangent to its corresponding circle about the storm center (Powell 1982; Willoughby 1990). A larger RRT represents a larger inflow angle and vice versa. The vertical profiles of the RRT associated with the studied typhoons are shown in Fig. 10a. Two clearly separate regimes of magnitudes for RRT corresponded to the OR and SW typhoons. The RRT magnitudes of the SW typhoon were generally greater than 0.7 at low levels, but the RRT magnitudes for the OR typhoon were less than 0.5 at different altitudes. This contrasting behavior of the RRT values between the OR and SW typhoons generally appears to be evident during the prevalence of the WSW flow (not shown). The difference in the RRT magnitude between the two types of typhoons became relatively minor above 3 km MSL (Fig. 10a).
Figure 10b indicates that the mean RRT values for the SW (OR) typhoon calculated over the duration of the WSW flow was equal to approximately −0.75 (−0.35) at the lowest analysis level. The inflow angle derived from the mean RRT value was approximately 37° for the SW typhoon and approximately 19° for the OR typhoon. The calculated mean inflow angle of the SW typhoon was larger than the typical near-surface inflow angle of previously documented hurricanes over the open ocean (~23°; Powell et al. 2009), which was, in turn, generally comparable to that of the OR typhoon. Two sample plots demonstrating the horizontal distribution of the dual-Doppler-derived winds and the inflow angle for Typhoon Kalmaegi (i.e., the SW typhoon) and Typhoon Fungwong (i.e., the OR typhoon) at the height of 2 km MSL at a time close to the strongest period of the WSW flow are shown in Fig. 11. The analyses show a clear trend of a relatively large (small) inflow angle associated with the SW (OR) typhoon, with predominant magnitudes of 30° and 40° for Kalmaegi and 10° and 20° for Fungwong (cf. Figs. 11b,d). The horizontal patterns of the inflow angles were generally uniform, except for certain small areas of greater inflow angles with intense precipitation (>40 dBZ) embedded within the WSW flow. This type of convectively modified signature occurred only occasionally and locally during the analysis period of primary interest; therefore, it should not play a significant role in contributing to the overall feature of the observed inflow angle.
The boundary layer characteristics and processes have been recognized as essential for the determination of the degree of the inflow angle for tropical cyclones, and the asymmetric distribution of the inflow angle and its weak dependence on radial distance have also been documented previously (Malkus and Riehl 1960; Powell 1982; Zhang and Uhlhorn 2012). In the present study, the kinematic information was retrieved from the synthesis domain located in a similar left-rear quadrant of the studied typhoons (cf. Figs. 2 and 4). Moreover, the differences in the mean radial distance from the storm center between the SW and OR typhoons, as indicated in Table 1, were not dramatic or systematic. Therefore, the possible asymmetric and/or radial variation of the inflow angle, if any, did not appear to be a reasonable contributor that can explain the observed difference of the inflow angle between the SW and OR typhoons. In addition, as shown in Fig. 10b, the larger mean RRT magnitude of the SW typhoon (compared with that of the OR typhoon) was evident in the lowest 3 km MSL—a vertical extent considerably deeper than the typical height of a hurricane boundary layer (~0.5–1 km; Zhang et al. 2011) but closely corresponding to the depth of the typhoon–monsoon interaction. The range of the mean RRT value at the 90% confidence level (shading in Fig. 10b) begins to overlay above about 3 km MSL for the two types of typhoons, indicating a minor difference in the kinematic characteristics at these higher altitudes. These analysis results may indicate that the typhoon–monsoon interaction plays a major role in causing the unique feature of the RRT magnitude observed for the SW typhoon. However, boundary layer effects cannot be eliminated as a potential factor that would also partially contribute to the maximum RRT magnitude (or inflow angle) observed near the lowest analysis level, as shown in Fig. 10b.
b. Local force balance and imbalance
The gradient wind balance is a good approximation above the frictional boundary layer for tropical cyclones (Willoughby 1990; Zhang et al. 2001). The occurrence of any obvious departure from the gradient wind balance may result in the substantial acceleration of radial flow, and this relationship can be quantitatively described by the following radial momentum equations in cylindrical coordinates (r, λ, z):
where u, υ, and w are the radial, tangential, and vertical velocities, respectively; υg is the gradient wind; f is the Coriolis parameter; and ρ is the air density. The terms on the right-hand side of (1) represent the centrifugal acceleration, the Coriolis acceleration, and the radial pressure gradient force, and when these three forces are in exact balance, the gradient wind relationship as expressed in (2) can be obtained. For simplicity, frictional dissipation is not included in (1), and this effect may become significant inside the tropical cyclone boundary layer. Using (2), the agradient force (AF) in (1) can also be written according to the following equation:
where υag is the agradient wind (υ − υg). Equation (3) shows that the outward acceleration (i.e., AF > 0) corresponds to supergradient wind (i.e., υag > 0; υ > υg) and vice versa. To provide insight into the contrasting intensity of the radial velocities between the SW and OR typhoons as described earlier, the dual-Doppler-derived winds are used to evaluate the AF by calculating the term of radial acceleration (i.e., du/dt) in (3). Because a complete depiction of the kinematic evolution for the studied typhoons is lacking, it is not our intent to investigate the causal relationship of the dynamical processes in detail as implicit in (1). It is possible, however, to gain a preliminary understanding of whether the mean nature of the local force balances and imbalances valid over the synthesis domain exhibits any dramatic difference for the OR- and SW-typhoon-induced WSW flow. The radial pressure gradient force can be practically estimated from (1) given that all of the other kinematic-related terms of the forces can be calculated from the synthesized wind information.
Figure 12a shows the vertical profiles of the mean AF values averaged over the synthesis domain and over the duration of the WSW flow for the OR and SW typhoons. The general magnitudes of the calculated AF values range from approximately 0 to 1.5 × 10−3 m s−2 and are comparable to those of previous studies of tropical cyclones based on the calculation of dual-Doppler observations (e.g., Didlake and Houze 2013). The signatures of the profiles associated with the two types of typhoons are distinctly different. For the OR typhoon, the AF values are nearly zero or small (i.e., close to the gradient wind balance) with minor vertical variations. In contrast, a clear inward acceleration or subgradient wind (i.e., AF < 0) for the SW typhoon is evident, particularly at low levels, which is consistent with the enhanced radial inflow and the large magnitudes of RRT observed for this type of typhoon (cf. Figs. 9a and 10b). The magnitude of the AF values of the SW typhoon generally decrease with height and are close to those of the OR typhoon above 3 km MSL.
As seen from the vertical variations of the individual forces on the right-hand side of (1) for both types of typhoons (Fig. 12b), the Coriolis and centrifugal forces are closely comparable for the OR and SW typhoons, and the mean magnitudes of these two forces are nearly the same for these two types of typhoons. For the OR typhoon, the radial pressure gradient force was in approximate balance with the summation of the Coriolis and centrifugal forces. The low-level pronounced AF value associated with the SW typhoon, as evident in Fig. 12a, appears primarily related to its larger radial pressure gradient forces within the layer of the typhoon–monsoon interaction (i.e., <3 km MSL) (black solid curve in Fig. 12b). To a certain extent, the results suggest that the redistribution of the mass field caused by the external force of the typhoon–monsoon interaction may be an important contributor to the observed nature in which the SW-typhoon-induced WSW flow deviates from the gradient wind balance.
To clarify the specific processes of the interaction between the typhoon circulations and the large-scale southwesterly monsoon is beyond the scope of this study, but the discussions above imply that the unbalanced dynamics implicit in the typhoon–monsoon interaction may be important for the determination of the mean kinematic characteristics of the SW-typhoon-induced WSW flow, especially in the lower troposphere. This scenario may become more significant in the presence of stronger monsoon systems that would favor a substantial interaction between the typhoons and the monsoon circulations, as discussed in section 3; however, this issue remains challenging to investigate because of sparse observations over the South China Sea and the coastal water to the southwest of Taiwan. Conversely, the kinematic nature of the OR-typhoon-induced WSW flow would be mostly determined by pure vortex dynamics in a manner similar to that of typical tropical cyclones over the open ocean. Future comprehensive observations in the outer regions of tropical cyclones as they approach or as they are under the influences of monsoonal systems over the South China Sea and detailed numerical simulations will be required for a better documentation of these episodes and their associated mesoscale aspects.
5. Discussion: Upstream kinematics versus orographic enhancement
Important kinematic characteristics of the SW- and OR-typhoon-induced WSW flow have been elaborated in the previous sections. In this section, we attempt to discern whether there is any possible connection of the observed upstream kinematics to the orographic enhancement of precipitation observed for the SW and OR typhoons. Note that previous investigations have indicated that the intensity and distribution of the orographic precipitation occurring in the typhoon environment is usually related to a mixed influence of orographically generated precipitation and typhoon background precipitation (YC08; YC13). This natural contamination may lead to difficulties in clarifying the specific effect of the upstream airflow on its contribution to precipitation enhancement observed over mountains. Because of this complexity, the analysis results require cautious interpretation.
Following the study by YC13, the quantitative measure of orographic enhancement of precipitation ΔR is defined as the difference between the observed orographic Rmt and the background rainfall rates Rbg. To illustrate how Rmt and Rbg are estimated from observations in this study, we first show the horizontal distribution of the mean intensity of radar-derived rainfall during the prevalence of the WSW flow for the studied typhoon (Fig. 13). The analyses indicate variable intensities of the background (or upstream) and mountainous precipitation for different typhoons. A striking, consistent feature of these observed precipitation distributions is the evidence of orographic enhancement of precipitation. The most enhanced precipitation tended to occur over the windward slopes and/or along the mountain crest of southern Taiwan, with a prominent reduction in precipitation intensity on the lee side. The representative amount of Rmt and Rbg for each typhoon is obtained by calculating the mean value of the rainfall rates over the two south–north elongated boxes indicated in each panel of Fig. 13. The orographic and background boxes are selected to encompass the major area of orographic rainfall and the region immediately upstream of the topography, respectively. In this calculation, the longitudinal position of the background box was adjusted slightly according to the orientation of the upstream radar-derived rainfall pattern observed during different analysis periods. This adjustment considers the spatial shift of precipitation resulting from the larger-scale precipitation that was not oriented perpendicular to the selected elongated box; therefore, it can effectively mitigate errors in estimating Rbg (YC13).
Table 2 summarizes the values of Rmt, Rbg, and ΔR averaged over the duration of the WSW flow for each typhoon. The intensity of the upstream oncoming flow U for each typhoon, as indicated in Table 2, was approximated by the dual-Doppler-derived east–west wind component (i.e., the cross-barrier flow) averaged below the mean mountain height of southern Taiwan (~2500 m MSL) and during the prevalence of the WSW flow. Based on the mean airflow and terrain height and upstream NCEP soundings, the Froude number was calculated to be approximately 1.6–3.7 for the studied typhoons, as indicated in Table 2. In these relatively large Fr flow regimes (i.e., greater than unity; Smith 1979), the oncoming WSW flow is expected to climb over the mountains, favoring precipitation enhancement by orographic lifting. Note that relative magnitudes of the Froude number for different typhoons studied herein were generally dominated by the intensity of their associated upstream oncoming flow because of a minor difference in ambient static stability and no difference in terrain height. The largest Fr found for Typhoon Mindulle was due to the combination of relatively strong oncoming flow and smaller static stability for this particular event.
The ΔR values calculated from the different typhoons ranged from 5.7 to 22 mm h−1, with a mean magnitude of 12.4 mm h−1 (Table 2). Although ΔR varied from case to case, its relative magnitude appears to be closely related to the strength of U and Rbg. For example, the first and second strongest orographic enhancement (ΔR = 22.0 and 14.5 mm h−1), as found in Typhoons Morakot and Talim, were associated with large values of U (~20–28 m s−1) and Rbg (~12–17 mm h−1). Conversely, the typhoons with weaker ΔR (e.g., Kalmaegi, Fungwong, and Jangmi) were characterized by a relatively weak upstream oncoming flow (<15 m s−1) and background precipitation (<~10 mm h−1). Moreover, as shown in YC13, for a narrower and lower mountain barrier in southern Taiwan, the intensity of the precipitation enhancement through the seeder–feeder process is approximately proportional to the intensity of the typhoon background precipitation times the oncoming wind speed (i.e., URbg). Consistent with this theoretical prediction, Typhoons Talim and Morakot, with large values of URbg, possessed a clear precipitation enhancement near and downstream of the mountain crest of the narrower barrier over the southern portion of the study domain (highlighted with thick arrows in Figs. 13b,e). This signature of orographic enhancement was not present for the other studied typhoons with relatively small values of URbg (Figs. 13a,c,d,f and Table 2). Note that for the studied typhoons, the mean magnitudes of the water vapor density and relative humidity calculated from the NCEP soundings located inside the synthesis domain (cf. Fig. 4) were overall similar; therefore, the role of upstream thermodynamics in contributing to the observed differences in ΔR, as evident in Table 2, can be reasonably assumed to be minor.
The results from Table 2 suggested that there was a more pronounced orographic enhancement for the SW typhoon than the OR typhoon, as evident in the mean intensity of the precipitation enhancement calculated for these two types of typhoons (14.6 versus 10.3 mm h−1). This characteristic may be simply interpreted as a result of the stronger mean oncoming flow and background precipitation for the SW typhoon (Table 2), which would more effectively facilitate upslope lifting and/or the seeder–feeder processes (YC13). However, another possible factor that may contribute to a larger value for the SW typhoon could be related to the nature of the vertical variations of the upstream oncoming winds.
As illustrated in the previous sections, the most important kinematic distinction between the SW- and the OR-typhoon-induced WSW flow involves the contrasting characteristic of their associated vertical variations—namely, the low-level wind maxima with a notable decreasing intensity with height (SW typhoon) versus a relatively uniform and consistent strength of winds in the low to midtroposphere (OR typhoon) (cf. Fig. 7). Theoretically, the presence of wind maxima in the low troposphere would favor the condensation of low-level moisture-rich air through orographic lifting. As these condensed hydrometeors grow and travel with the climbing air over the windward slopes of the mountains, the characteristic of relatively weaker winds aloft may prevent the fast downstream advection of precipitating water by the environmental flow. This effect will not only lead to a shorter downwind shift of precipitation particles, favoring larger rainfall accumulation at a given location and time, but also allow more time for the hydrometeors to grow over the windward slopes before encountering the leeside subsidence. Conversely, for a wind profile without an appreciably reduced wind speed at upper levels, such as that observed from the OR typhoon, the occurrence of similar processes would not be possible. Thus, it is reasonable to suspect that, when considering the two distinct wind profiles with a comparable intensity of upstream oncoming flow, the “reverse shear” type of kinematic profiles (i.e., a shear vector directed opposite to the wind direction and away from the barrier), such as the SW-typhoon-induced WSW flow, would have a higher potential to develop a stronger orographic enhancement of precipitation. Supporting evidence for the importance of this scenario is provided by previous numerical studies of orographic precipitation, which have demonstrated the potential significance of upstream vertical shear and its positive effects on the precipitation efficiency and rainfall intensity over mountains (Young 1974; Colle 2004). Future idealized and/or sensitivity modeling studies are required to better isolate the possible contribution of the vertical variations of upstream oncoming airflow associated with SW and OR typhoons to the orographic enhancement of precipitation and to further clarify the relative importance of upstream kinematics and background precipitation on the observed orographic precipitation in the typhoon environment.
This study used ground-based dual-Doppler observations to investigate kinematic characteristics of the southwesterly flow associated with the southwest (SW) and ordinary (OR) typhoons immediately off the southwestern coast of Taiwan. In this study, six typhoon events [Mindulle (2004), Kalmaegi (2008), Morakot (2009), Talim (2005), Jangmi (2008), and Fungwong (2008)] were chosen for detailed analysis; the first (latter) three typhoons with (without) a substantial interaction with the southwesterly monsoonal flow belong to the family of the SW (OR) typhoon. The mutual aspects of these selected typhoons are that they moved from the southeastern side to northwestern side of Taiwan, brought a southwesterly flow impinging on the mountainous region of southern Taiwan, and caused heavy orographic precipitation in that region. The vertical kinematic profiles generated from a total of 177 sets of hourly synthesized winds derived from these typhoons indicate that more intense orographic rainfall for each typhoon tended to occur during the prevalence of the west-southwesterly (WSW) flow that was more perpendicular to the approximately south–north orientation of the topography in southern Taiwan.
The unique kinematic features of the WSW flow associated with the SW and OR typhoons are schematically summarized in Fig. 14. The large-scale environment for the SW (OR) typhoon (Figs. 14a,b) was characterized by a relatively strong (weak) southwesterly monsoon over the South China Sea. The large-scale environmental characteristic for the SW typhoon, along with the typhoon center located northwest of Taiwan, favors stronger interactions between typhoon outer circulations and monsoonal flow over regions near the southwestern coast of Taiwan. A notably consistent feature of the SW-typhoon-induced WSW flow was its persistently increasing intensity with decreasing height in the low to midtroposphere, thus possessing a maximum at the lowest analysis level (Fig. 14c). This observed characteristic is in contrast to a relatively minor change in the intensity of the WSW flow observed at different heights for the OR typhoon. Another striking kinematic distinction was a larger ratio of the radial and tangential velocities (RRT) (i.e., a larger inflow angle) observed for the SW typhoon (Figs. 14a,b), which is consistent with calculations of the radial momentum equation showing an obvious inward acceleration (i.e., a subgradient wind) at low levels for this type of typhoon, as presented in section 4b. The mean inflow angle of the SW typhoon was significantly larger than the typical near-surface inflow angle of previously documented hurricanes over the open ocean, which was, in turn, comparable to that of the OR typhoon. These findings suggest that the RRT value (or the inflow angle) may be a useful quantity that can be used to distinguish the WSW flow induced by the SW and OR typhoons. The differences in the kinematics between the SW and OR typhoons were confined primarily to the lowest 3 km MSL, revealing the general vertical extent of the typhoon–monsoon-interaction layer (Figs. 14c,d). One should be noted that stronger typhoon–monsoon interactions for the SW typhoon contribute to a larger inflow angle at low levels. When considering typhoon centers roughly located in the vicinity of central or northern Taiwan like the present study, the occurrence of a larger inflow angle may intensify the low-level wind component perpendicular to the south–north-oriented mountain barrier of southern Taiwan, which may in turn favor upslope lifting and enhancement of orographic precipitation.
The possible connection of the upstream kinematics (i.e., the WSW flow) to the observed orographic precipitation was also explored in this study. The orographic enhancement of precipitation calculated over the major region of orographic precipitation for the studied typhoons ranged from 5.7 to 22 mm h−1 and appears to be closely related to the strength of the upstream WSW flow and the typhoon background precipitation. The analyses also indicate that the mean orographic enhancement calculated for the SW typhoon family appeared more pronounced compared with that of the OR typhoon family. This characteristic is consistent with a stronger mean oncoming flow and background precipitation for the SW typhoon that would more effectively facilitate upslope lifting and/or seeder–feeder processes (YC13). As discussed in section 5, the “reverse shear” type of wind profiles with a maximum near the surface, as evident in the observed WSW flow of the SW typhoon (cf. Fig. 14c), would also likely represent an important kinematic signature favorable for the development of precipitation enhancement. However, the degree to which the observed kinematic distinctions between the SW- and OR-typhoon-induced WSW flow contribute to the different nature of orographic precipitation is difficult to be explicitly addressed based on observations alone. This issue deserves future clarification with a complement of modeling sensitivity experiments and/or idealized simulations.
The radar and surface observations used in this study were provided by the Taiwan Central Weather Bureau. We thank three anonymous reviewers for providing helpful comments that improved the manuscript. This study was supported by the National Science Council of Taiwan under Research Grants NSC100-2628-M-034-001-MY3, NSC102-2111-M-034-005, and MOST103-2111-M-034-001-MY3.