Fine Structure of an Active Monsoon Trough Observed in the Western North Pacific

SOLA, 2021, Vol. 17, 81−87, doi:10.2151/sola.2021-013 81

Biao Geng and Ryuichi Shirooka Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan

a deep layer to the synoptic-scale cyclonic circulation (Holland Abstract 1995; Gray 1998; Houze et al. 2009), though the detailed processes are disputed. Therefore, knowledge of the internal characteristics The internal structure and evolution of a monsoon trough (MT) of the MT and MCSs and their interaction with large-scale circula- and associated mesoscale convective systems (MCSs) in the west- tions is necessary for understanding TCs (Harr et al. 1996; Ritchie ern North Pacific were investigated, based mainly on radiosonde and Holland 1999; Harr and Chan 2005). So far, however, studies and a Doppler radar observations in Palau. The MT was observed examining the fine structure of the MT and associated MCSs con- on 15−16 June 2013, with the pre-existing disturbance of Typhoon ductive to TC genesis in the WNP are lacking. Leepi (2013) being embedded in it. The large-scale circulation On 15−16 June 2013, an MT in the WNP was captured from around the MT featured a pattern representing an active MT. the intensive observations of radiosondes and an X-band Doppler Deep convection developed ahead and at the leading edge of the radar conducted in Palau. During this period, the pre-existing downward-sloping monsoonal flow, where intense low-level con- disturbance of a typhoon was embedded in the MT. The data vergence was observed. Stratiform precipitation broadened rear- obtained in Palau provided us with an opportunity to investigate ward over the MT axis. A deep and wide layer of warm and moist the fine structure of the MT and associated convective activities air over the MT axis was undercut by a layer of cold air sloping facilitating typhoon formation in the WNP for the first time. downward from the trailing stratiform region to the leading convective region. An intense low-pressure zone formed in the interface between the warm layer above and cold layer below, with 2. Data and analysis methods the westerly monsoonal and easterly trade flows being enhanced on its west and east sides, respectively, from the low to middle Synoptic-scale characteristics were ascertained from grid troposphere. The results suggest that a strengthening of the large- point values of global objective analysis data from the Japan Me- scale cyclonic circulation in response to the internal processes of teorological Agency (Nakagawa 2009) and the infrared brightness the MCSs triggered by the MT is important for typhoon genesis. temperature (Tb) data of the Multifunctional Transport Satellite. In (Citation: Geng, B., and R. Shirooka, 2021: Fine structure of addition, the vertical structure of the atmosphere during the MT an active monsoon trough observed in the western North Pacific. passage over Palau was investigated using the data observed by SOLA, 17, 81−87, doi:10.2151/sola.2021-013.) radiosondes in 6-hour intervals. Further, the internal structure and evolution of the MT and associated precipitation were analyzed based on the volumetric data obtained using the Doppler radar in 1. Introduction 7.5-minute intervals. The velocity-azimuth display (VAD) method of Mapes and Lin (2005) was used to retrieve the profiles of meso A monsoon trough (MT) is defined as a line that has westerly scale winds and divergence within the area determined by a 40-km monsoonal winds on the equatorward side and easterly trade radius from the radar. The vertical air motion was then calculated winds on the poleward side (Sadler 1964). In the western North via the integration of the anelastic continuity equation and sub- Pacific (WNP), the MT has a significant impact not only on the jected to the variational adjustment described by O’Brien (1970). WNP summer monsoon onset (Murakami and Matsumoto 1994; Ueda et al. 1995; Wu and Wang 2001; Wu 2002), but also on the formation and track of typhoons or tropical cyclones (TCs) (Lander 3. Results 1996; Chia and Ropelewski 2002; Harr and Chan 2005; Harr and Wu 2011). 3.1 Synoptic-scale overview The MT region is characterized by large-scale cyclonic vortic- Figure 1 shows the evolution of synoptic-scale winds at ity and is the place where deep convection tends to occur. These 700 hPa. A southeast-northwest-oriented MT existed south of Palau features make the WNP MT region a favorable zone for the de- at 0000 UTC June 15 (Fig. 1a). It was propagating northeastward velopment of TCs (Gray 1968; Ramage 1974; Zehr 1992; Ritchie and extended westward over time (Figs. 1b−1h). The MT’s posi- and Holland 1999; Molinari and Vollaro 2013). Harr and Elsberry tion was similar to the long-term mean position of the WNP MT (1995) illustrated that an active (inactive) TC period in the WNP in June (Lander 1996). Notably, both the westerly monsoonal and is related to the large-scale circulation associated with an active easterly trade flows were enhanced in the vicinity of the MT (Fig. (inactive) MT. The active MT is characterized by increased cy- 1), with the monsoonal flow extending upward to and broadening clonic circulation, whereas the inactive MT exhibits the opposite in the middle troposphere (Fig. 2). A cyclonic vortex was centered feature. Enhanced convection develops throughout the active MT near the western end of the MT. Coherent with the intensification region, leading to TC genesis. of the monsoonal and trade flows, values of stream function Long-lived and intense cloud clusters with embedded meso- around the MT increased over time (Figs. 1 and 2), indicating scale convective systems (MCSs) tend to occur in the WNP MT the strengthening of large-scale cyclonic vorticity in a deep layer region (Gray 1998). The spin-up of TCs from MCSs is observed during the study period. The cyclonic vortex eventually developed in different geographic regions (Harr et al. 1996; Simpson et al. into Typhoon Leepi (2013) on 18 June (not shown). As indicated 1997; Ritchie and Holland 1999; Reasor et al. 2005; Houze et al. by regions with Tb values of < −60°C (Fig. 2), convective activ- 2009). It is hypothesized that MCSs facilitate TC development ities were enhanced in a wide area around the MT. These facts by generating cyclonic vorticity and accumulating it throughout indicate that the MT can be classified as an active MT (Harr and Elsberry 1995). Evidently, a segment of the MT and associated Corresponding author: Biao Geng, Global Ocean Observation Research convective activities had propagated over the radiosonde station Center, Japan Agency for Marine-Earth Science and Technology, 2-15 and Doppler radar in Palau. Natsushima, Yokosuka, 237-0061, Japan. E-mail: [email protected] ©The Author(s) 2021. This is an open access article published by the Meteorological Society of Japan under a Creative Commons Attribution 4.0 International (CC BY 4.0) license (http://creativecommons.org/license/by/4.0). 82 Geng and Shirooka, Fine Structure of an Active Monsoon Trough

Fig. 1. Evolution of 700-hPa stream function (106 m2 s−1, red contours) and horizontal wind speed (m s−1, shaded). Horizontal wind bars (half barb = 2.5 m s−1, full barb = 5 m s−1, and flag = 20 m s−1) are superimposed. The position of the monsoon trough is outlined with a bold black line in each figure. The location of the radiosonde station is indicated by alphabet “K”. The black circle indicates the radar domain.

Fig. 2. Same as Fig. 1, but for the evolution of 500-hPa stream function (106 m2 s−1, red contours) and infrared brightness temperature (°C, shaded) observed by the Multifunctional Transport Satellite. The satellite data were obtained from Kochi University’s archives (http://weather.is.kochi-u.ac.jp/). SOLA, 2021, Vol. 17, 81−87, doi:10.2151/sola.2021-013 83

3.2 Radiosonde analysis The MT was passing over Palau after 0600 UTC 15 June (Fig. 3a). The MT axis sloped leftward (i.e., rearward) with height and extended upward to approximately 3 km by 1800 UTC 15 June. It deepened into the middle troposphere and reached approximately 7.5 km by 0000 UTC 16 June. Figure 3a indicates that the westerly monsoonal and easterly trade flows were enhanced behind and ahead of, respectively, the MT axis. The enhanced monsoonal flow (> 4 m s−1) sloped downward and forward from the middle troposphere of the trailing portion to the low troposphere of the leading part of the MT. Simultaneously, the enhanced trade flow (< −6 m s−1) sloped upward and rearward over the MT axis. Warm and moist air rose up from the low troposphere ahead of the surface MT and had a rearward extension in the middle and upper troposphere over the MT axis (Figs. 3b and 3c). Simul taneously, a layer of relatively intense cold-air perturbations (< −0.6 K) existed below the deep and wide layer of warm and moist air, sloping downward and forward in the vicinity of the MT axis from the middle to low troposphere. Notably, the downward sloping of the intense monsoonal flow coincided with the cooling near the MT axis (Figs. 3a and 3b). A zone of relatively intense low-pressure perturbations (< −0.2 hPa) sloped rearward over the MT axis from the low to middle troposphere (Fig. 3d). The intense low-pressure zone was sand- wiched by warm air above and cold air below (Fig. 3b). Notably, the enhancement of the monsoonal and trade flows in the low and middle troposphere (Fig. 3a) occurred on the west and east sides of the intense low-pressure zone, respectively.

3.3 Doppler radar analysis At 0630 UTC 15 June (Fig. 4a), cellular echoes of > 30 dBZ developed approximately within a south-north-oriented zone. The length of the complex of the cellular echoes exceeded 100 km, which meets the definition of an MCS (Houze 2004). The MCS had developed ahead of the surface MT (Figs. 1b and 2b). An east- west vertical cross section cut through the radar for 0630 UTC is shown in Fig. 4b. Convective cells were developing beyond a distance approximately 50 km east of the radar. Mature and decaying convective cells with echo tops of approximately 10 km were found west of the developing cells. Stratiform precipitation, which was characterized by enhanced reflectivity near the melting level (~5 km), existed farther to the west and had a width of only a few tens of kilometers. In Fig. 4b, the Doppler velocities were qualitatively linked to westerly or easterly winds. The MCS was characterized by overall easterly winds. Only a small region of westerly winds centered around 3 km was observed at the rear edge of the MCS. At 0930 UTC 15 June (Fig. 4c), a new MCS emerged to the west of the radar. As indicated by the zero isopleth of the Doppler velocity (Fig. 4d), the new MCS had developed at the leading edge of the MT. Similar to the result derived from the radiosonde observation (Fig. 3a), the MT axis sloped rearward with height and extended to approximately 3.5 km. The westerly flow behind the MT axis, which corresponded to the monsoonal flow, was weak at this time. The contours of the Doppler velocity suggested that low-level easterly flow rose up ahead of the surface MT and sloped rearward over the MT axis. Figure 4d indicates that the MCS west of the radar was organized as new convection formed at the leading edge, while old convection decayed and stratiform Fig. 3. Time-height diagrams of (a) zonal winds (m s−1, shaded), (b) anom- precipitation formed toward the rear of the MT. alous temperature (K, shaded), (c) anomalous specific humidity (g kg−1, By 1230 UTC June 15 (Figs. 5a and 5b), the leading edge of shaded), and (d) anomalous pressure (hPa, shaded) observed by radio- the MT had moved to the east of the radar. At this time, stratiform sondes. Horizontal wind bars (plotted same as in Fig. 1) are superimposed precipitation spanned some 100 km behind the surface MT (Fig. in each figure. An anomaly is defined as a deviation from the mean value 5b), much wider than that in the rear portion of the MCS devel- of the study period. A 1-day running mean was applied to the time-height oped ahead of the surface MT (Fig. 4b). The maximum depth of diagram of anomalous pressure to eliminate diurnal pressure fluctuations. the MT axis remained approximately 3.5 km, but the intensity of The monsoon trough axis determined from radiosonde-observed winds is −1 outlined with a white curve in each figure. The horizontal black line indi- the westerly monsoonal flow increased (> 3 ms ). The down- cates the height of the melting level. ward-sloping structure of the monsoonal flow penetrating into the leading edge of the MT was evident. Accompanying the intensification of the monsoonal flow, convection at the leading edge of the MT developed more deeply. The trailing portion of the MT was observed by the Doppler 84 Geng and Shirooka, Fine Structure of an Active Monsoon Trough

100 km

Fig. 4. (a) Horizontal distribution of reflectivity (dBZ, shaded) at an elevation angle of 2.5° observed by the Doppler radar at 0630 UTC 15 June 2013. Gray sectors indicate the regions suffering from beam blockage caused by obstructions on the ground. The location of the radar is indicated by alphabet “R”. (b) Vertical cross section of reflectivity (dBZ, shaded) and Doppler velocity (m s−1, contours) along the bold east-west line in (a). Positive and negative velocities are moving away from and toward the radar located at 0 km, respectively. Horizontal arrows indicate the directions of Doppler velocities. Gray regions indicate where data are unavailable. (c) Same as (a), but for 0930 UTC 15 June 2013. (d) Same as (b), but for 0930 UTC 15 June 2013.

100 km

Fig. 5. Same as Fig. 4, but for (a, b) 1230 UTC and (c, d) 1800 UTC 15 June 2013. SOLA, 2021, Vol. 17, 81−87, doi:10.2151/sola.2021-013 85

100 km

Fig. 6. Same as Fig. 4, but for (a, b) 0000 UTC and (c, d) 0230 UTC 16 June 2013.

radar at 1800 UTC 15 June (Figs. 5c and 5d). By this time, strat- flow (Fig. 7d). Comparing Figs. 7b and 7c with Fig. 3b reveals iform precipitation had broadened to more than 200 km in the that both intense low-level downdrafts in the convective region trailing portion of the MT. The intensity of the downward-sloping and intense mid-level downdrafts in the stratiform region had a monsoonal flow was further enhanced (> 6 m −1s ). Simultaneously, strong correlation with the intense cooling below the warm and the monsoonal flow also developed at middle levels in the rear moist layer. portion of the stratiform region. As a result, the depth of the MT The intense low-pressure zone as observed by radiosondes had exceeded 6 km. The trailing portion of the MT and broad strat- sloped over the MT axis from the leading convection region to the iform precipitation could still be observed in the early hours of trailing stratiform region (Figs. 3d and 7b). A zone of intense con- June 16 (Fig. 6). vergence (< −5 × 10−5 s−1) was observed over the MT axis between Figure 7 shows the results derived by utilizing the VAD approximately 1700 and 2200 UTC 15 June (Fig. 7d), with the method. Those kinematic characteristics of the MT as shown vertical extension of the MT into the middle troposphere taking before (Figs. 3a, 4d, 5b, 5d, 6b, and 6d), including the rearward place at its rear edge (Fig. 7a). Notably, this intense convergence sloping and deepening of the MT axis, the intensification of the zone coincided with the intense low-pressure zone (Fig. 3d), downward-sloping monsoonal flow behind the MT axis, and the indicating that the formation of the intense convergence zone over intensification of the upward-sloping trade flow over the MT axis, the MT axis is closely related to the development of the intense were also revealed quite well by VAD-derived winds (Fig. 7a). low-pressure zone. Strong updrafts (> 18 cm s−1) and deep convection formed ahead and in the leading part of the MT (Figs. 7b and 7c), in conjunction with intense low-level convergence and upper-level 4. Summary and discussion divergence in these regions (Fig. 7d). Updrafts also sloped and extended rearward over the MT axis, coinciding with the upward An MT in the WNP was investigated using the data obtained and rearward sloping of the easterly trade flow over the MT axis from the intensive observations of radiosondes and an X-band and the broadening of stratiform precipitation behind the surface Doppler radar in Palau, as well as the global objective analysis MT (Figs. 7a, 7b, and 7c). Notably, the deep and wide layer of and geostationary satellite image data. The MT was observed on warm and moist air over the MT axis as observed by radiosondes 15−16 June 2013 and had the pre-existing disturbance of Typhoon (Fig. 3b) was coincident with updrafts within either convective or Leepi (2013) embedded in it. The large-scale circulation around stratiform precipitation observed by the Doppler radar (Figs. 7b the MT featured a pattern representing an active MT. The south- and 7c). east-northwest-oriented MT moved northeastward, with the MT Intense downdrafts (< −9 cm s−1) appeared from an altitude of axis inclining rearward with height and extending upward to at 3 km to the surface in the convective region just behind the MT least the middle troposphere. axis and near the melting level in the stratiform region (Figs. 7b Enhanced convective activities occurred in a wide area around and 7c). Notably, the downward sloping and intensifying of the the MT. Strong updrafts and deep convection occurred ahead and westerly monsoonal flow from the trailing stratiform region to the in the leading part of the MT, where intense low-level conver- leading convective region (Fig. 7a) coincided with downdrafts gence formed ahead of the intense downward-sloping westerly behind the surface MT, with the intense low-level convergence monsoonal flow. Simultaneously, updrafts and the easterly trade taking place ahead of the intense downward-sloping monsoonal flow sloped upward and rearward over the MT axis and stratiform 86 Geng and Shirooka, Fine Structure of an Active Monsoon Trough

precipitation broadened behind the surface MT. Apparently, the intense low-level convergence associated with the monsoonal flow facilitates the development and maintenance of convection ahead and in the leading part of the MT. In addition, the rearward sloping of the MT helps the formation of upper-level mesoscale updrafts and the trade flow ascending over the MT axis helps transport warm air and condensate from the convective region rearward (Houze 2004), thus facilitating the genesis and maintenance of broad stratiform precipitation in the trailing portion of the MT. Consequently, the MT is effective in promoting the development and organization of MCSs. A deep and wide layer of warm and moist air formed over the MT axis, coherent with updrafts within either convective or stratiform precipitation. Simultaneously, a layer of cold air sloping downward from the middle to low troposphere appeared below the warm and moist layer, coincident with intense downdrafts and the downward sloping of the intense monsoonal flow behind the surface MT. These results indicate that deep and wide warming occurred over the MT axis by the process of latent heat release, while cooling occurred below the warming layer due to the melting and evaporation of precipitation (Houze 2004). As a result, the thermodynamic structure of the atmosphere over the MT region has been modulated significantly by the internal processes of the MT-triggered MCSs. An interesting finding is that a zone of intense low-pressure perturbations sloped rearward over the MT axis from the low troposphere of the leading convective region to the middle troposphere of the trailing stratiform region. The intense low-pressure zone formed in the interface between the MCS-induced warm layer above and cold layer below, with the westerly monsoonal and easterly trade flows being enhanced on its west and east sides, respectively, from the low to middle troposphere. Evidently, the intense low-pressure zone has developed in response to variations in the buoyancy distribution induced by the MT-triggered MCSs, with the development of the intense low-pressure zone facilitating the convergence and acceleration of environmental flows around the MT in the low and middle troposphere (LeMone 1983; Smull and Houze 1987; Fovell and Ogura 1988; Lafore and Moncrieff 1989; Weisman 1992). The coherence of the descending and intensifying of the monsoonal flow with intense downdrafts indicates that the monsoonal flow is further enhanced by the latent cooling process shown by Yang and Houze (1995). Therefore, by modifying the thermodynamic structure of the atmosphere, the MT-triggered MCSs have exerted a profound dynamical influence on the atmosphere in a deep layer over the MT region, which is consistent with the findings of simulation experiments (Choudhury and Krishnan 2011). Previous studies have illustrated that the enhancement of cyclonic vorticity in a deep layer facilitates the development of TCs (Holland 1995; Gray 1998; Houze et al. 2009). Notably, it was observed that the cyclonic vorticity of the pre-existing disturbance was strengthening with the intensification of the westerly monsoonal and easterly trade flows around the MT. Therefore, the enhancement of the monsoonal and trade flows from the low to middle troposphere by the MT-triggered MCSs, as discussed above, is effective in promoting the development of cyclonic vorticity in a deep layer over the MT region, which is coherent with the progressive intensification of the pre-existing disturbance Fig. 7. Time-height diagrams of (a) zonal winds (m s−1, shaded), (b) mean of Typhoon Leepi (2013) during the study period. Consequently, reflectivity (dBZ, shaded), (c) vertical velocity (cm −1s , shaded), and (d) the results of this study highlight that a large-scale dynamical horizontal divergence (10−5 s−1, shaded) derived from the velocity-azimuth response to the internal processes of the MT-triggered MCSs display (VAD) method. Horizontal wind bars (plotted same as in Fig. 1) is important for enhancing the monsoonal and trade flows and are superimposed in each figure. All variables have been filtered with cyclonic vorticity in a deep layer, thus promoting TC genesis over a 1-hour running mean to dampen short-term variations. The monsoon the MT region. trough axis determined from VAD-derived winds is outlined with a white Comparing the active MT investigated in this study and an curve in each figure. The horizontal black line indicates the height of the inactive MT of the WNP studied by Geng et al. (2014) reveals melting level. that a major difference between two MTs is that precipitation systems were weak and scattered near the inactive MT, where no TC developed. Such difference further illustrates the role of the MT-triggered MCSs in promoting TC genesis, which is worth more investigation. SOLA, 2021, Vol. 17, 81−87, doi:10.2151/sola.2021-013 87

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