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A Study of Two Propagating Heavy-Rainfall Episodes near Taiwan during SoWMEX/TiMREX IOP-8 in June 2008. Part I: Synoptic Evolution, Episode Propagation, and Model Control Simulation [Monthly Weather Review]
[August 15, 2014]

A Study of Two Propagating Heavy-Rainfall Episodes near Taiwan during SoWMEX/TiMREX IOP-8 in June 2008. Part I: Synoptic Evolution, Episode Propagation, and Model Control Simulation [Monthly Weather Review]


(Monthly Weather Review Via Acquire Media NewsEdge) ABSTRACT This paper is the first of a two-part study to investigate two rain-producing episodes in the longitude-time (Hovmöller) space upstream from Taiwan during the eighth intensive observing period (IOP-8, 12-17 June 2008) of the Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX), with a goal to better understand the mechanism and controlling factors for their organization and propagation. Both in a prefrontal environment, the first episode moved eastward and the second was a rare westward-moving event, and each caused heavy rainfall in Taiwan, on 14 and 16 June, respectively. In Part II, the roles played by synoptic conditions and terrain effects are further examined through sensitivity tests.



With the aid from a successful simulation with a grid spacing of 2.5 km, the structure and organization of convection embedded in the two episodes are shown to be different. With stronger low-level vertical wind shear in its environment, the first episode consisted of well-organized squall-line-type convective systems and propagated eastward mainly through cold-pool dynamics. However, the convection of the second episode was scattered and less organized with weaker vertical shear, and individual cells traveled with background flow toward the north-northeast. Throughout the 6-day case period, the southwesterly low-level jet (LLJ) is found to have much control over the general region of convection, and thus dictates the overall rainfall pattern in the Hovmöller space at the regional scale. The rapid development of the mei-yu front and LLJ over southeastern China during 16-17 June, to the west of the previous location of the jet, is found to result in the westward movement of the second episode.

1. Introduction a. Literature review During the past several decades, the skills of quantita- tive precipitation forecasts (QPFs) at major operational centers around the world have improved gradually over time (e.g., Olson et al. 1995). For instance, the mean threat score for 0-24-h rainfall at the threshold of 1 in. (25.4 mm) at the National Oceanic and Atmospheric Administration (NOAA) Hydrometeorological Prediction Center (HPC) increased from about 0.17 in the 1960s to 0.24 near 2000 (Fritsch and Carbone 2004). Most of this improvement, however, came from the cold season while the scores in the warm season show little increase. To meet the needs for more accurate weather forecasts, the improvement in predictive skills for summer rainfall, when thermal effects and local circulations are often pronounced, is hence an urgent and challenging task (Shapiro and Thorpe 2002; Fritsch and Carbone 2004).


Carbone et al. (2002) reported on the coherent be- havior of warm-season ''precipitation episodes,'' de- fined as clusters of rainfall systems in the longitude-time (Hovmöller) space that propagate eastward to the lee of the Rockies in the continental United States using radar- derived rain rates. In their diurnal cycle, these episodes exhibit close ties to the elevated terrain due to solar heating (e.g., Dai et al. 1999; Carbone and Tuttle 2008), and tend to develop over the eastern slopes of the Rockies in the afternoon then propagate across the Great Plains overnight at a preferred speed range (e.g., Maddox 1980; Laing and Fritsch 1997; Ahijevych et al. 2004). This coherent behavior and the longevity of some episodes (up to 3000 km in space and 60 h in time) to- gether suggest the existence of intrinsic predictability and therefore a potential to improve warm-season QPFs (Carbone et al. 2002).

Since Carbone et al. (2002), the coherency and prop- agation of rain-producing episodes have been inves- tigated in many regions around the world (e.g., Laing et al. 2008; Keenan and Carbone 2008; Levizzani et al. 2010; Pereira Filho et al. 2010; He and Zhang 2010), with particular similarities to their U.S. counterparts in be- havior for those near and to the lee of the Tibetan Plateau (TP) in East Asia (e.g., Asai et al. 1998; Wang et al. 2004, 2005a; Baoet al. 2011). The climatologyof the episodes in East Asia was documented by Wang et al. (2004, 2005a) using satellite infrared (IR) brightness temperature (TB), showing frequent development of convection over east- ern TP in late afternoon and subsequent eastward prop- agation along the Yangtze River valley (YRV). Soon afterward, the phenomenon was confirmed and explored by studies employing rain gauge records (Yu et al. 2007; Zhou et al. 2008; Chen et al. 2010) and satellite derived rain rates such as the Tropical Rainfall Measuring Mis- sion (TRMM) dataset (Johnson 2011; Wang et al. 2011b; Xu and Zipser 2011; Bao et al. 2011; Chen et al. 2012). Note, however, that spanning roughly over 278-408N, the TP is situated at a lower latitudinal range than the U.S. Great Plains. As a consequence, the propagation of epi- sodes downstream from the TP almost ceases in mid- summer when the upper-level westerly flow shifts to the north (Wang et al. 2004, 2005a; Xu and Zipser 2011).

The roles played by the mountain-plain solenoid (MPS) associated with elevated terrain in triggering deep convection have been studied quite extensively in the past (e.g., Holton 1967; Wallace 1975; Tripoli and Cotton 1989a,b). In response to stronger solar heating over the terrain during the day in summer, the MPS develops and low-level flow climbs upslope to frequently initiate convection over the mountains in local after- noon (e.g., Carbone and Tuttle 2008; He and Zhang 2010; Bao et al. 2011). After initiation, convective cells often grow upscale and organize into mesoscale con- vective systems (MCSs), and propagate downstream through the well-known cold-pool dynamics in a sheared environment when upper-level steering winds are present (e.g., Rotunno et al. 1988; Tuttle and Carbone 2004; Trier et al. 2006; Bao and Zhang 2013) or other mechanisms such as gravity waves (e.g., Yang and Houze 1995; Fovell et al. 2006; Pritchard et al. 2011). At night, the pattern of the MPS reverses and widespread ascent occurs over the Great Plains (Carbone and Tuttle 2008), and the role of such a reversal in the maintenance of episodes and modulation of their propagation near eastern TP in a case has been examined by Huang et al. (2010). Over the lee side, the development of the noc- turnal low-level jet (LLJ; Zhong et al. 1996) also helps maintain the MCSs through the supply of warm and moist air in both regions (Tuttle and Davis 2006; Trier et al. 2010; Chen et al. 2009).

In the central United States, eastward-propagating episodes often develop in successive days and evolve in a similar fashion under relatively weak synoptic forcing (Carbone et al. 2002; Carbone and Tuttle 2008). Thus, they are highly controlled by topographic effects and local diurnal disturbances such as the nocturnal LLJ and the associated low-level frontogenesis at night (Trier et al. 2006). From a 10-day simulation driven only by repeated mean diurnal variations on top of an averaged background in midsummer, Trier et al. (2010) concluded that transient tropospheric disturbances (e.g., mobile fronts and short waves) are not crucial in the process, since many features in precipitation similar to those observed in the climatology (such as afternoon maxi- mum over the Rockies and eastward progression over- night) are successfully reproduced in the model.

Over southern and eastern China, however, the syn- optic environments within which the rain-producing episodes form and evolve may play a more significant role than in the United States, since the southwesterly monsoon and mei-yu front in early summer are prom- inent features that often dictate rainfall distribution (Lau and Li 1984; Tao and Chen 1987; Ding 1992; Chen 1994; Ding and Chan 2005) as demonstrated in many previous studies (e.g., Wang and Orlanski 1987; Chen and Yu 1988; Wang et al. 1993; Chen 2004). Through compositing, Wang et al. (2012a) identified several synoptic patterns favorable and unfavorable to the de- velopment and propagation of episodes to the lee of the TP based on the flow at 500 hPa. Although strong upper- level forcing (from approaching trough, short waves, or large-scale confluence) is indeed beneficial to overall event organization and strength, their results also point to the importance of the LLJ in the maintenance and propagation of episodes, as it is the one necessary in- gredient that differs the most between long-lived and short-lived events (Wang et al. 2012a).

Using a strategy similar to Trier et al. (2010), Sun and Zhang (2012) performed model simulation and tested the impact of the MPS on rainfall diurnal cycle along the mei-yu front over eastern China for early July 2007. In the mean cycle, the propagation of episodes are rea- sonably captured by the model in runs forced only with 10-day-averaged diurnal variations at lateral bound- aries, or even with fixed boundary conditions (BCs) without temporal variations (since diurnal forcing still exists in the model interior). Note, however, that larger differences in rainfall patterns among successive days as seen in the observations are only reproduced in experi- ments using real analyses (i.e., those containing synoptic evolutions) as BCs (Sun and Zhang 2012, cf. their Figs. 5 and 6).

b. Motivation and study objectives The studies reviewed above have greatly improved our understanding about the behavior and diurnal variations of rainfall episodes and MCSs, and their re- lationships with specific elevated terrain in general. Compared to climatological studies, however, case studies are relatively few and how exactly some of the factors influence the organization and propagation of rain-producing episodes in real cases remain not fully explored, especially in East Asia. One such factor is the LLJ, which is recognized as crucial for system organi- zation, maintenance, and propagation (e.g., Jou and Deng 1992; Trier et al. 2006, 2010; Chen et al. 2009; Sun and Zhang 2012) and controlled largely by synoptic- scale features in East Asia (e.g., Chen 2004; Wang et al. 2012a). However, few studies have addressed the roles of the environmental conditions in real cases of rainfall episodes, at least not from the perspectives of their or- ganization and propagation.

The present study focuses on the region near and upstream from Taiwan, with a hope to improve the warm-season QPFs there through a better under- standing about the phenomena involved. As shown in Fig. 1a, the Hovmöller domain (208-268N, 112.58- 122.58E) covers the southeastern coast of China, the Taiwan Strait, and Taiwan. The topography of the Nanling Mountains, Wuyi Mountains, and coastal mountains in southeastern China (SEC) is on the order of 500-750 m with isolated peaks over 1.5 km (Fig. 1a). While eastward movement of rainfall signals across the Taiwan Strait overnight are visible in some earlier studies (e.g., Fig. 14a of Wang et al. 2005a;Fig.8of Johnson 2011), long-term rainfall climatology over the coastal plains in western Taiwan also reveals a second- ary early-morning peak in addition to the primary af- ternoon peak (e.g., Chen et al. 1999; Chen et al. 2005; Kerns et al. 2010). Link to the steep topography (cf. Fig. 1a), the large diurnal rainfall variation in Taiwan in the mei-yu season is recently shown by Ruppert et al. (2013) to be persistent and pronounced even during disturbed periods.

The event during the eighth intensive observing period (IOP-8, 14-17 June) of the Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Ex- periment (SoWMEX/TiMREX) field campaign held in the 2008 mei-yu season (May-June) in Taiwan (Jou et al. 2011) was selected as our target in this study. As will be shown shortly, two rainfall episodes occurred during IOP-8, one moving eastward and the other westward, and each produced heavy rainfall in Taiwan. Under a prevailing south-southwesterly flow, the westward-moving event is very rare, and to our knowledge such a case has neither been documented nor examined around Southeast Asia (or Taiwan) during the mei-yu season. Thus, this event provides a unique opportunity for us to understand the controlling factor(s) on the organization and propagation of episodes. This is our first objective of the study. Another objective of the study is to examine the relative importance of, or the roles played by, the synoptic conditions and terrain- associated local circulations in the SoWMEX/TiMREX IOP-8 case in greater detail. For heavy-rainfall cases like the present one, synoptic conditions are most likely quite favorable. Different aspects of the IOP-8 case have been studied by Tai et al. (2011), Davis and Lee (2012), and Xu et al. (2012), and the readers are referred to them for relevant details.

This study is the first part (hereafter Part I) of a two- part paper and mainly addresses our first objective stated above. Through an analysis and discussion on the synoptic evolution and our control simulation, the controlling factors on convection organization and episode propagation are identified with an emphasis on the role of the LLJ. In Wang et al. (2014, hereafter Part II), our second objective is addressed through a comparison among the control simulation and three sensitivity tests designed to isolate the effects of syn- optic conditions and diurnal variations induced by land-sea distribution and topography. The roles of to- pography examined include those in SEC and in Tai- wan (cf. Fig. 1a), and both the thermodynamic and dynamic aspects.

In later sections, this Part I paper is organized as fol- lows. The data and methodology are described in section 2, and section 3 gives an analysis on the evolution of the synoptic environment and the overview of the IOP-8 case. In section 4, the numerical model and the control experiment (CTL) are described, and the model results are presented in section 5. Section 6 provides further discussion on the characteristics, organization, and propagation of the two episodes. Finally, section 7 gives the summary and conclusion of this Part I paper.

2. Data and methodology For synoptic description and discussion, the data used in this study include the gridded final (FNL) analyses from the National Centers for Environmental Pre- diction (NCEP) Global Forecast System (GFS), with a horizontal resolution of 1831 8 latitude-longitude at 26 pressure ( p) levels (from 1000 to 10 hPa, surface level included) every 6h (at 0000, 0600, 1200,and 1800UTC, while LST 5 UTC 1 8 h) covering the case period over 12-17 June 2008. Surface and upper-level weather maps from the Central Weather Bureau (CWB) of Taiwan and the Japan Meteorological Agency (JMA) are also reviewed, but will not be presented in this paper. The NCEP GFS FNL analyses are also used as the initial and boundary conditions (IC/BCs) for the model control simulation in this two-part series, and further details will be given in section 4b.

To document the overall distribution of convection and rainfall in our case, the infrared cloud imageries from the Multifunctional Transport Satellite (MTSAT) at 1-h intervals and the TRMM 3B42 rain rates (Huffman et al. 2007) at 0.258 latitude-longitude every 3 h are employed. To further examine the details of convection near Taiwan, reflectivity composites of ver- tical maximum-echo indicator (VMI) from the opera- tional radars in Taiwan at 30-min intervals are also used. In addition, for rainfall distribution over Taiwan, hourly accumulation data from a network of nearly 400 auto- matic gauges (Hsu 1998) are utilized. Where needed, the above observations are compared with model results to validate the CTL simulation.

3. Synoptic evolution and case overview a. Synoptic evolution In this section, the synoptic environment and evolu- tion associated with the two heavy-rainfall episodes during SoWMEX IOP-8 is first reviewed, with special attention on the factors controlling the development and organization of the convection and rainfall. The NCEP 18318 FNL gridded analyses of geopotential height and horizontal winds at 0000 UTC of 12-17 June 2008 are presented in Fig. 2. At 0000 UTC 12 June, the surface mei-yu front extended from the migratory low near southwestern Japan, through the ocean north of Taiwan, into SEC and the low near 248N, 1068E(Fig. 2a). With the subtropical high farther southeast, southerly to southwesterly flow prevailed south of the front, with stronger winds near Hainan Island over the western South China Sea (SCS). North of the front, weak east- erly to northerly winds existed over land (Fig. 2a). On 13 June, the low over southwestern China deepened and extended rapidly toward the northeast (Fig. 2b). In re- sponse, the height gradients and surface winds over SEC and the SCS strengthened, and the mei-yu front was pushed northward. The surface pressure over eastern China continued to fall and a closed low appeared near 308N, 1208E on 14 June, as the front moved toward the coast of SEC (Fig. 2c). To the east of this developing low, the winds near Taiwan and over eastern SCS and the ECS also intensified.

On 15 June (Fig. 2d), while the surface front was along the coastline and the frontal low moved to 318N, 1288E, another weak low formed near 288N, 1228E and the zone with strong southwesterly winds moved slightly east of Taiwan. On 16 June, the front remained stationary and the surface winds to its south weakened slightly, but the pressure behind the front over SEC started to fall again (Fig. 2e). The mei-yu front strengthened and retreated back over land in response to the pressure fall, and the area with stronger winds (and height gradients) moved westward to just offshore of the SEC on 17 June (Fig. 2f) and weakened on 18 June (not shown).

In Fig. 3, the NCEP FNL analyses of geopotential height and winds (streamline and isotach) at 850 hPa and TRMM rain rates every 24 h over 12-17 June are pre- sented. The basic evolution is very close to that shown in Fig. 2. During 12-15 June (Figs. 3a-d), rapid devel- opment of the cyclone and its migration along the front (or wind-shift line) across SEC were evident at 850 hPa, and resulted in the formation and subsequent eastward to northeastward migration and extension of the LLJ ($12 m s21). On the contrary, the deepening of the 850-hPa trough over SEC later, during 16-17 June, caused the LLJ to move westward (Figs. 3e,f). In both instances, the LLJ at 850 hPa reached a peak of over 20 m s21 near Taiwan (Figs. 3c,f). As revealed by the TRMM rain rates, most of the precipitation during 12- 17 June occurred either inside the region of the LLJ or immediate downstream from wind maxima (where speed convergence existed), except for that near the low center (Fig. 3). Farther aloft at 700 hPa, synoptic evo- lution was similar to that at 850 hPa and the LLJ moved eastward during the first episode but westward during the second one (not shown).

At 500 hPa, a major cyclone (328N, 1128E) was ap- proaching from the west on 13 June (Fig. 4a), and much of the SEC was located ahead of its associated trough and in the area of positive vorticity advection (PVA; e.g., Uccellini 1990). A shortwave trough also appeared over southern China (near 258N, 1138E). The cyclone- trough system moved over eastern China on 14 June (Fig. 4b) and toward Japan on 15 June (not shown). With a deep structure that tilted northwestward with height (cf. Figs. 2-4), this upper-level cyclone-trough system clearly led to the development and northeastward mi- gration of the low and LLJ at lower levels. Over 16-17 June, another 500-hPa trough moved into SEC from the west and amplified (Figs. 4c,d). This second trough sys- tem was also deep and exhibited a northwestward tilt with height, and its significant development at 500 hPa and below on 17 June resulted in the westward retreat of the LLJ (cf. Figs. 2 and 3). In both cases, a major trough was also present at 200 hPa near the tropopause with a diffluent flow pattern over SEC (Fig. 5), but the one on 13-14 June seemed to be slightly downstream of the 500-hPa trough. During our case period, the upper-level anticyclone developed northward into the Taiwan area and the 500-hPa subtropical high also intensified and intruded westward and northward (cf. Fig. 4), in asso- ciation with significant tropospheric drying immediately afterward (cf. Fig. 3 of Ruppert et al. 2013). The synoptic analysis presented above is also consistent with Davis and Lee (2012) and Xu et al. (2012), and these studies estimate an ample amount of convective available po- tential energy (CAPE) in the flow upstream from Tai- wan, about 1500-2500 J kg21.

b. Case overview As depicted in Fig. 1b and described earlier, two major rainfall episodes passed through the area of focus during SoWMEX/TiMREX IOP-8. The first episode propagated from about 113.98 to 122.28E during 0300 UTC 13 June-0400 UTC 14 June, at a zonal speed of roughly 9.4 m s21. On the other hand, the second episode was westward moving at approximately 7.3 m s21 from 122.18 to 113.68E during the period of 2100 UTC 15 June- 0600 UTC 17 June (Fig. 1b). Over SEC and near Taiwan, these two episodes were in close association with the LLJ ahead of the mei-yu front-wind-shift line (cf. Fig. 3), and in the Hovmöller domain, such an intimate connection between TRMM rain rates and 850-hPa wind speed is further shown in Fig. 6. The rainfall was in- phase with the LLJ during episode 1 but slightly ahead (and to the west) of the jet axis during the second episode (Fig. 6). As discussed, the LLJ and the rainfall area moved eastward in response to the development of the baroclinic cyclone-trough system through deep tro- posphere during 12-15 June, then retreated backward (westward) over 16-18 June when another amplifying cyclone-trough system moved across SEC (cf. Figs. 2-6).

The daily rainfall distributions over Taiwan from the rain gauge network (Fig. 7) indicate that two days, 14 and 16 June, received the most rainfall during IOP-8 (Davis and Lee 2012). The rainfall on 14 June was more widespread and covered the western half of the island south of 24.58N, with a peak amount over 200 mm (Fig. 7c). Significant rainfall also took place over southwest- ern Taiwan on 16 June, especially along the coast (.200 mm), with a secondary maximum in northern Taiwan (Fig. 7e). It is evident that these two heavy- rainfall events on the island were associated with the two episodes, respectively (cf. Figs. 1 and 6). On 15 June, the rainfall pattern over Taiwan was very similar to that on 16 June but with less amounts (Fig. 7d), while the rain was more localized on other days (Figs. 7a,b,f).

To further document the structure and evolution of the convection surrounding Taiwan, composites of VMI radar reflectivity at 6-h intervals from 1800 UTC 12 June to 1200 UTC 17 June are shown, with the propagation direction of nearby echoes marked (Fig. 8). Note that the range of detection from land-based radars in Taiwan is limited not only by the radars (about 450 km) but also by the earth's curvature. Prior to 0600 UTC 13 June, a convective line gradually appeared along the coast of SEC with a northeast-southwest alignment (Figs. 8a-c). During the subsequent hours, this line became better organized and moved into the Taiwan Strait, while scat- tered convection developed near Taiwan (Figs. 8d,e). The convective line quickly evolved into a well-defined squall line, and reached its peak strength with bulged segments (i.e., bow echoes) near 0000 UTC 14 June, as it ap- proached the coastline of Taiwan (Fig. 8f). At this time, the squall line also became more north-south oriented, while other narrow convective lines appeared about 200 km downstream, just off the eastern coast of Taiwan. The squall line was clearly the major constituent of the first rainfall episode, and the embedded echoes mostly traveled northeastward at the early stage, at the direction of the low-level flow (cf. Figs. 8a-d and 3a,b). However, the squall line became more north-south aligned and propagated more eastward as a system when it intensified and matured (Figs. 8e,f), coincided with likely develop- ment of the rear inflow jet that descended from midlevels with stronger westerly component (cf. Fig. 4b, e.g., Weisman 1992; Braun and Houze 1997). Later in section 5b, the structure of this squall line will be examined in more detail using model results.

After making landfall in Taiwan, the squall line weak- ened gradually (Fig. 8g). During the following hours on 14 June, on the other hand, convective cells continued to move into the western and southwestern Taiwan from the southern Taiwan Strait and the northern SCS (Figs. 8g-i). While the region of active convection moved gradually southward and eastward, cells often organized into linear shape and propagated northeastward, or arc shape and moved more eastward. On 15 June (Figs. 8j-m), the convective cells appeared mostly south of Taiwan, over the Bashi Channel and southeastern Taiwan Strait, just offshore from southwestern Taiwan. While apparently becoming less organized, the general propagation di- rection of the cells also shifted from toward the northeast to the north-northeast, and those moving onshore over southwestern Taiwan obviously led to the coastal rainfall on this day (cf. Fig. 7d). On 16 June, another better or- ganized, arc-shaped convective system moved onshore near 0600 UTC from the south-southwest and caused the second heavy-rainfall event in Taiwan (Figs. 8n-p and 7e). Afterward, the region of active convection moved westward and northward, gradually out of the detection range of radars in Taiwan (cf. Figs. 8q-t and 3e,f). These cells over northern SCS propagated mostly toward the north or north-northeast, also consistent with the 850-hPa flow direction.

4. Numerical model and configuration a. The CReSS model The Cloud-Resolving Storm Simulator (CReSS) used in this study is a nonhydrostatic and compressible cloud model developed at Nagoya University, Japan (Tsuboki and Sakakibara 2002, 2007). This model employs a terrain- following vertical coordinate, with prognostic equations for 3D momentum (u, y, w), p, potential temperature (u), and mixing ratios of water vapor (qy) and other hydro- meteors (qx,wherex denotes a species). To properly simulate clouds at high resolution, an explicit bulk cold- rain scheme based on Lin et al. (1983), Cotton et al. (1986), Murakami (1990), Ikawa and Saito (1991),and Murakami et al. (1994) are used without any cumulus parameterization. In this scheme, a total of six species (water vapor, cloud water, cloud ice, rain, snow, and graupel) are included with microphysical processes of nucleation (condensation), sublimation, evaporation, deposition, freezing, melting, falling, conversion, collec- tion, aggregation, and liquid shedding. The subgrid-scale turbulent mixing is parameterized using a 1.5-order clo- sure with turbulent kinetic energy (TKE) prediction (Tsuboki and Sakakibara 2002). Momentum and energy fluxes and radiation at the surface are also considered with a substrate model (Kondo 1976; Louis et al. 1981; Segami et al. 1989), but cloud radiation is neglected.

For computational efficiency, a time-splitting scheme (Klemp and Wilhelmson 1978) is adopted in the CReSS model to separately integrate fast and slow waves. At large time steps (Dt), the filtered leapfrog scheme (Asselin 1972) is used, while the forward-backward scheme (implicit Crank-Nicolson scheme) is employed in the horizontal (vertical) at small time steps (Dt). The message passing interface (MPI) and/or Open MP are used for parallel computing. In Table 1, the main fea- tures of the CReSS model are summarized, and the readers are referred to Tsuboki and Sakakibara (2007) for a more complete description. The CReSS model has been used to study various types of convective systems (e.g., Tsuboki and Sakakibara 2002; Liu et al. 2004; Akter and Tsuboki 2012; Wang et al. 2012b), including those in the mei-yu seasons near Taiwan (e.g., Wang et al. 2005b, 2011a; Wang and Huang 2009).

b. The control experiment The control simulation in this study adopted a hori- zontal grid spacing of 2.5 km at cloud-resolving scale and a grid dimension of 600 3 480 with 40 layers (vertically stretched, Fig. 1a and Table 1). Only a single high- resolution domain was used, as the CReSS model has no nesting function. As mentioned, the NCEP 18318 FNL gridded analyses (with 26 levels) at 6-h intervals were used as IC/BCs, and real terrain (at roughly 1-km reso- lution) and observed weekly mean sea surface temper- ature (SST; Reynolds et al. 2002) were provided at the lower boundary. The simulation starts from 0000 UTC 12 June 2008 for 144h (6days,until 0000 UTC 18June) using 6 (3) s for Dt (Dt). Such an integration length of almost a week is comparatively long to typical model experiments of heavy-rainfall events, but even longer ranges have been used to study organized propagating episodes (e.g., Trier et al. 2006, 2010; Sun and Zhang 2012). The major aspects in the model configuration and experiment are summarized in Table 1, and this particular simulation serves as our CTL run both for more detailed investigation in later sections of this pa- per, as well as for comparison with other sensitivity tests in Part II.

5. Model results of CTL a. Description of model results and validation Using the configuration described in section 4b, model results from the CTL simulation at 1594 m, the closest model (output) level to 850 hPa, every 24 h during the case period are shown in Fig. 9. At this level, southerly to southwesterly flow prevailed over SEC and Taiwan during much of IOP-8, with generally higher (lower) pressure to the southeast (northwest). With the gradual approach of the trough from the northwest, the tightening of the northeast-southwest oriented isobars and the develop- ment of the LLJ are evident (Figs. 9a-d). During 12-15 June, the LLJ moved eastward from the SEC coast into the Taiwan Strait then farther passing Taiwan, with good agreement with NCEP analyses (cf. Figs. 3a-d). Later, the low-level trough (or wind-shift line) over eastern China retreated northward and westward during 16-17 June, and the LLJ along the SEC coast reintensified (Figs. 9e,f). This dramatic intensification of the jet and the shift of wind direction from southwesterly to southerly on 17 June in CTL are both consistent with the analyses (Figs. 3 and 9).

Results of surface wind and convection from the CTL simulation are shown in Fig. 10, also at 0000 UTC for each day except 12 June where 0600 UTC is used in- stead. By 0600 UTC 12 June (i.e., 6 h into the inte- gration), convection along and to the south of the front had fully developed (Fig. 10a). A squall line appeared just south of the front near the SEC coast (at about 1158E) by 13 June (Fig. 10b), over and immediately down- stream from the LLJ at 1594 m (cf. Fig. 9b). This squall line then propagated eastward across the Taiwan Strait, and made landfall in Taiwan around 0000 UTC 14 June (Fig. 10c). On 15 June, the convection moved into the ocean east and south of Taiwan and become somewhat less well organized (Fig. 10d). On 16 June, the surface winds near Taiwan started to turn more southerly and the region of active convection shifted back into the Taiwan Strait (Fig. 10e). At this time, convective cells appeared along the western and southwestern coast of Taiwan. Later on 17 June, they shifted farther westward to the coastal area of SEC and the northern SCS (Fig. 10f). Note that during the entire case period, the surface front never reached Taiwan, in agreement with Fig. 2, although its local structure and evolution were also clearly affected by nearby convection due to the fine resolution of the model (Fig. 10). On the other hand, except along and near the front, convective development appeared to tie closely to the LLJ, when one compares Figs. 9 and 10,as also seen earlier in the NCEP analyses and TRMM data (cf. Figs. 2, 3,and6).

For regions near Taiwan, model-simulated maximum total mixing ratios of precipitation (rain, snow, plus graupel) in the vertical column are shown every 6 h in Fig. 11, and can be compared with the radar VMI composites in Fig. 8. During 12-13 June, deep con- vection developed along the coast of SEC, gradually organized into a squall line, and moved eastward to- ward Taiwan in CTL (Figs. 11a-e). This squall line made landfall over Taiwan near 0000 UTC 14 June, then weakened after passing through the island (Figs. 11f,g). With cells moving onshore into southwestern Taiwan, the convection gradually migrated southeastward on 14 June (Figs. 11h-j) and into the Bashi Channel and northern SCS after 0600 UTC 15 June (Figs. 11k,l). Since about 1800 UTC 15 June, the scattered convec- tion over the northern SCS started to track toward the north-northeast, into southwestern Taiwan once again on 16 June, before moving farther north and back over land in the SEC (Figs. 11m-s). Although a one-to-one relationship is not possible among the cells, a close comparison between Figs. 8 and 11 confirms that the model reproduced the evolution of the convection near Taiwan during the entire case period in close agreement with the observation, including the region, structure, organization, and general movement of the convection.

As shown by the daily 24-h rainfall distributions over Taiwan (Fig. 12), the CTL experiment captured both heavy-rainfall events in IOP-8 nicely, with a large rain area over the windward slopes of Taiwan's Central Mountain Range (CMR; cf. Fig. 1a)southofabout 24.58N and peak values exceeding 300 mm on 14 June, and in the southwestern plains with maximum just over 300 mm on 16 June (Figs. 12c,e). On 15 June, significant rainfall (peaking at almost 200 mm) was also produced along the southwestern coast of Taiwan. A comparison between Figs. 7 and 12 again confirms the close agreement in rainfall between the observations and the CTL, although small differences inevitably exist between the two. For example, the model slightly over- predicts the amounts in both events, and does not capture the rainfall over northern Taiwan on 16 June. Overall, judging from results presented herein, the model is con- cluded to reproduce the atmospheric processes and convection-rainfall during IOP-8 very well, even on 16 and 17 June, which are the fifth and sixth days into the sim- ulation (cf. Figs. 3n-t, 7e,f, 11n-t, and 12e,f). Therefore, the CTL experiment is highly successful and can be employed to further study the behavior of rainfall episodes in section 6, as well as to investigate the roles played by synoptic conditions and local topography through sensi- tivity tests in Part II.

b. Structure and organization of heavy-rainfall episodes The Hovmöller diagram of rain rates from the CTL, using north-south averaging over the same domain (208-268N, 112.58-122.58E), is presented in Fig. 13a. Although the general rainfall distribution and the two episodes are captured, a direct comparison with Figs. 1b and 6 is not straightforward due to the much higher spatial and temporal resolutions of model out- put (at 2.5 km and 15 min). Thus, in Fig. 13b the model rain rates are aggregated (smoothed) into resolutions comparable to the TRMM dataset, at 25 km and 3 h, overlaid with the wind speed at 1594 m every 6 h for easy comparison.

Atthefirstglance,thesimilaritybetweenFigs. 6 and 13b is striking, apart from generally higher peak rain rates in the CTL experiment. While both episodes in the model are closely associated with the LLJ as in the observation, episode 1 in Fig. 13b propagated roughly from113.28 to1218Eduring1800UTC12June-0100UTC 14 June, at a zonal speed of about 7.2 m s21, slightly slower than that in Figs. 1b (and Fig. 6). The second, westward- moving episode traveled from about 120.98 to 114.88E during 2200 UTC 15 June-1200 UTC 17 June, at about 4.6 m s21 (Fig. 13b) and was also slower than the obser- vation. Between the two major episodes, eastward- moving rainfall streaks are also often found in the TRMMdata,aswellasinCTL.Inthemodel,suchstreaks travel at zonal speeds comparable to that of episode 1, especially evident in the full-resolution plot (cf. Fig. 13a).

6. Discussion a. Characteristics of the two episodes To illustrate the finescale characteristics of convection and organization of the first episode, vertical cross sec- tions of model-simulated winds, u, and total mixing ratio of precipitation in CTL, along line A-A0 in Fig. 10b, are shown in Fig. 14 for the period over 0400- 1000 UTC 13 June. The section is aligned northwest- southeast, and intersects the convective line at large angle. The squall line moved slowly forward on the section plane at a mean speed of ;2.8 m s21, while in- dividual cells moved mostly toward the northeast both in the observation and the model (cf. Figs. 8c,d and 11c,d). Shown here as an example, the MCS clearly has typical characteristics of a squall line (Xu et al. 2012) with tilted updraft and downdraft and both leading and trailing stratiform regions (e.g., Smull and Houze 1985; Parker and Johnson 2000). The low-level front-to-rear flow carries relatively warm/moist air from the south into the updraft (cf. Fig. 9b) and rises and condenses to form hydrometeors, which are then transported toward the rear to produce the trailing stratiform precipitation aloft (Fig. 14, e.g., Biggerstaff and Houze 1991). On the other hand, the midlevel rear-to-front flow in the downdraft can reach about 18 m s21 at times near the height of 2- 3 km, while the leading stratiform is produced through advection by the upper-level westerlies (cf. Fig. 5a). Evidently, this squall line embedded in episode 1 is well organized and exhibits a strong leading updraft, and is therefore reminiscent to the cases studied by Trier et al. (2006; cf. their Fig. 7) in the United States. During this period, there are also decaying cells trailing behind the updraft, most visible at 0600 UTC (Fig. 14b), and thus the MCS possesses a multicellular structure.

For the second episode, similar plots along the cross section B-B0 (cf. Fig. 10e), oriented from the south- southwest to north-northeast and parallel to the storm moving direction (cf. Figs. 8m,n and 11m,n), over a 3-h period from 2200 UTC 15 June to 0100 UTC 16 June are shown in Fig. 15 for comparison. In section B-B0 , the scattered convection exists over the area upstream from southwestern Taiwan, and multiple cells at various stages of development appear in the vertical cross sec- tions (Fig. 15) in an environment of onshore flow through the deep troposphere, especially at lower to midlevels (cf. Figs. 2e, 3e, 4c, and 9e). Such character- istics remain unchanged over the period shown in Fig. 15, as well as for hours afterward (not shown, see Part II). Thus, compared to 13-14 June, the convection on 16 June near southwestern Taiwan was much less orga- nized in both the observation and the model, and could be classified as the ''broken areal'' type (Bluestein and Jain 1985) at least at its early stage.

b. Episode organization and storm propagation As discussed in section 3a, the surface and low-level front/wind-shift line were in SEC and never reached Taiwan during the entire period of 12-17 June 2008, and our Hovmöller domain was largely located in a pre- frontal environment (cf. Figs. 1a, 2, and 3). Figure 16 presents the time-height section of areal-mean hori- zontal winds, vertical wind shear, u, and equivalent po- tential temperature (ue) over the Hovmöller domain from the NCEP gridded analyses. Southerly to south- westerly flow prevailed in the lower to middle tropo- sphere and turned gradually into westerly flow at upper levels during 13-14 June, and southwesterly to south- southwesterly winds dominated through deep layers over 16-17 June (Fig. 16). Using data from the intensive sounding network in SoWMEX/TiMREX (Ciesielski et al. 2010; Jou et al. 2011), similar changes in the characteristics of LLJ (and stability) during IOP-8 are also depicted from a local perspective in Davis and Lee (2012, their Figs. 13a and 19) and Xu et al. (2012, their Fig. 8). While convective cells generally moved in the direction of low- to midlevel winds (Figs. 8 and 11), their structural characteristics and organization were distinctly different during the two episodes, as seen in Figs. 14 and 15. Evidently, one environmental factor that also differed is the low-level vertical wind shear, which reached about 7.5 m s21 per 100 hPa toward the north- northeast at 850 hPa on 13 June but was much weaker [#2.5 ms21 (100hPa)21]on16June(Fig. 16)before theLLJmovedbackintothedomainon17June(cf. Figs. 3e,f). Thus, while the LLJ supplied the warm and moist air toward the convection and contributed to the development and maintenance of rainfall systems, its associated strong vertical shear was clearly the key factor controlling the organization of the storm, in agreement with previous studies (Rotunno et al. 1988; LeMone et al. 1998; Parker and Johnson 2000)aswell as Davis and Lee (2012) and Xu et al. (2012).Forepi- sode 1, Davis and Lee (2012) conclude that the squall line propagated through cold-pool dynamics on top of advection, so its evolution was also subject to much mesoscale control.

Beside dynamic conditions, thermodynamic and sta- bility conditions are also revealed in Fig. 16. The areal- averaged u values decreased monotonically with height, while ue had minimum values (about 335-345 K) near 550-600 hPa and increased both upward and toward the surface, where maxima were at least about 355 K. Thus, potential and convective instabilities existed during our case period, as the low-level flow generally came from the moisture-laden ocean to the southwest or south- southwest (cf. Figs. 2 and 3) and in agreement with the ample CAPE estimated by Davis and Lee (2012) and Xu et al. (2012). Note that both distributions of u and ue show relatively small variations in time (Fig. 16), sug- gesting that the magnitude of instability is not a crucial factor in determining convection organization and prop- agation near Taiwan in our case, in agreement with Wang et al. (2012a), although the instability does need to exist for deep convection to occur (e.g., Doswell 2001).

c. The westward propagation of the second episode From the analysis and discussion in earlier sections, it is clear that the low-level vertical wind shear was the most important factor in the organization of the con- vection, while the southwesterly to south-southwesterly LLJ also played a role in system development and main- tenance in our IOP-8 case. Also, the westward propagation of the second episode as a whole in the Hovmöller space over 16-17 June (cf. Fig. 1b) was largely controlled by the location of the LLJ (cf. Figs. 3 and 6) and thus the evolu- tion of surrounding synoptic environment. As discussed, the LLJ played the crucial role to determine the general area of convection (in strong-wind area and near its lead- ing edge) in the prefrontal environment (cf. Figs. 3, 6,and 8), and therefore dictated the pattern of rainfall signals and resulted in westward movement in the Hovmöller diagram in our case. Obviously, in response to the upper-level forcing, the significant development of the mei-yu front and trough in the lower troposphere over SEC around 17 June resulted in the westward retreat of the LLJ (section 3,cf.Figs. 2, 3,and6), and was required to pro- duce this westward-moving episode. This important role of synoptic conditions in this process will further be dis- cussed and confirmed in Part II. Here, it is worthy to note that the results from the SoWMEX/TiMREX IOP-8 case near Taiwan are in contrast with and complimentary to those obtained by Trier et al. (2010) for the United States and, to a lesser extent, by Sun and Zhang (2012) for the YRV in eastern China. Both studies emphasize the co- herent behavior of the episodes through successive days during active periods and the role played by the topo- graphically induced MPS and local diurnal disturbances.

7. Summary and conclusions In this study, the period of 12-17 June 2008, covering much of IOP-8 during the SoWMEX/TiMREX field campaign, was selected to investigate the mechanism and controlling factors for the organization and propa- gation of convection, in the context of rain-producing episodes in the Hovmöller (longitude-time) space near and upstream from Taiwan (208-268N, 112.58-122.58E). Two episodes occurred during IOP-8 in a prefrontal en- vironment: the first moved eastward during 13-14 June while the second moved westward on 16 June, and each produced heavy rainfall in excess of 200 mm in Taiwan (on 14 and 16 June, respectively). The present paper (Part I) is the first of a two-part series, and its objective is to un- derstand the controlling factor(s) on the organization and propagation of episodes, partially through the examination of a rare westward-moving event (the second episode) that has never been documented around Southeast Asia or Taiwan during the mei-yu season.

With the aid of a cloud-resolving numerical simula- tion at 2.5-km grid spacing (the CTL experiment) that highly agrees with the observations, the structural and organizational characteristics of convective storms in the two episodes are shown to be quite different. The first episode on 13-14 June consisted of well-organized MCSs resembling squall lines, and the storms moved to- ward the northeast or east-northeast largely through cold- pool dynamics and by advection with the presence of southwesterly LLJ and strong vertical wind shear near 850 hPa (Davis and Lee 2012). In contrast, though also south of the mei-yu front, the convection associated with the second episode on 16 June was scattered and less or- ganized and individual cells traveled north-northeastward to northward in the direction of the LLJ with much weaker vertical shear (e.g., Xu et al. 2012). While the magnitude of vertical shear was essential for the organization of con- vection, the movement of both episodes as a whole was largely controlled by the LLJ, which developed and evolved together with the mei-yu front-trough system mainly in response to the upper-level dynamical forcing. In the prefrontal region, convection tended to concentrate in areas near LLJ or at its leading edge during the entire case period. After the LLJ associated with the first episode moved south and east of Taiwan, rapid development of the mei-yu front over SEC during 16-17 June caused the strong wind area to move westward, and resulted in a shift in the general region of convection and consequently the second, westward-moving episode.

Even though the LLJ and thus the synoptic environ- ment exhibit much control over the general organization and propagation of the two rainfall episodes during IOP-8 at the regional scale, the development and evolution of convection at the local scale are still clearly subject to the control of mesoscale processes and terrain in- fluences, and the role played by local circulations such as the MPS in our case is yet to be examined. Therefore, in Part II of the study, the local aspects and the relative importance of synoptic conditions and terrain-associated circulations in the IOP-8 case will be further investigated in greater detail using model sen- sitivity tests (Part II).

Acknowledgments. The authors thank Prof. Richard H. Johnson of Colorado State University, Prof. Ben Jong-Dao Jou of National Taiwan University, Taiwan, and the anonymous reviewers for their constructive comments and valuable suggestions. This study was supported jointly by the National Science Council of Taiwan under Grants NSC-99-2111-M-003-004-MY3 and NSC-102-2119-M-003-003, the Korea Meteorologi- cal Administration Research and Development Pro- gram under Grant CATER 2012-2071, and by the National Research Foundation of Korea through a grant provided by the Korean Ministry of Education, Science, and Technology in 2014 (Grant 200603874).

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CHUNG-CHIEH WANG AND JASON CHIEH-SHENG HSU Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan GEORGE TAI-JEN CHEN Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan DONG-IN LEE Department of Environmental Atmospheric Sciences, Pukyong National University, Busan, South Korea (Manuscript received 9 October 2013, in final form 8 March 2014) Corresponding author address: Prof. Chung-Chieh Wang, De- partment of Earth Sciences, National Taiwan Normal University, No. 88, Sec. 4, Ting-Chou Rd., Taipei, 11677 Taiwan.

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