Journal of the Meteorological Society of , Vol. 87, No. 1, pp. 101--117, 2009. 101 DOI:10.2151/jmsj.87.101

A Mechanism for Heavy Precipitation over the Accompanying Meari (2004)

Akihiko MURATA

Typhoon Research Department, Meteorological Research Institute, Tsukuba, Japan

(Manuscript received 17 December 2007, in final form 31 October 2008)

Abstract

An extremely heavy precipitation event occurred in the mountainous Kii Peninsula in Japan, associated with Typhoon Meari in 2004. A marked characteristic of this heavy precipitation was its extreme rainfall rate, more than 100 mm h1. Another feature was that the area of precipitation was far from the storm center, more than 500 km. From radar and surface observations, it was found that the heavy rains were composed of a stationary precipitation system and two moving precipitation systems. In order to evince a physical mechanism for the precipitation systems, the event was analyzed in detail using data from cloud-resolving simulations and observations. The results demonstrated that the heavy precipitation was produced in a synergistic manner from the three precipitation systems. The following are identified as the key factors for the formation and maintenance of each precipitation system: a) elimination of vertical convective instability in a low-level warm and moist easterly on the eastern slope of the mountainous region, b) moisture supply due to a low-level confluent flow along the boundary between the low-level easterly and south-easterly flow, and lifting of a slightly warmer south-easterly flow, and c) low-level convergence due to cold pool, running nearly parallel to the axis and located slightly to the southwest of the band, acting as an obstacle to the low-level inflow into the system. Precipitation e‰ciency revealed that precipitation was enhanced when moving precipitation systems merged with the stationary precipitation system. The enhancement was attributed to the greater rate of conversion of cloud water to rainwater via accretion of cloud water by rain, under the condition of intense water vapor flux convergence. The moving precipitation systems provided raindrops for the accretion of cloud droplets in the sta- tionary precipitation system. Based on our findings, extremely heavy precipitation in the present case is caused by the enhancement of the accretion process due to the merger of precipitation systems in addition to precipitation in each system.

1. Introduction rial number 21 by the Regional Specialized Meteo- rological Center (RSMC)-, was character- Ten tropical cyclones, that had a maximum wind ized by heavy rains accompanying the storm. A speed of more than 17 m s1, made in Ja- marked characteristic of this heavy precipitation pan in the year 2004. The number easily broke the was its extreme rainfall rate: more than previous record in Japan (six a year). The 100 mm h1. Another characteristic was the area series of the landfall risked the lives of many people of precipitation: 500-km farther from the storm and caused widespread material damage. Of the ten center. The area covered the eastern half of the tropical cyclones, Typhoon Meari, assigned the se- mountainous Kii Peninsula, located around 34 N, 136 E. The goal of this study is to clarify the mech- Corresponding author: Akihiko Murata, Typhoon Re- anism of the characteristic rainfall. search Department, Meteorological Research Institute, Heavy precipitation accompanying tropical cy- 1-1, Nagamine, Tsukuba, Ibaraki, 305-0052, Japan. E-mail: [email protected] clones could be divided into two types. One is ac- ( 2009, Meteorological Society of Japan companying the eyewall and the other is associated 102 Journal of the Meteorological Society of Japan Vol. 87, No. 1 with relatively outer precipitation systems such as heavy precipitation in the Kii Peninsula. Saito et al. spiral . A part of the heavy rainfall event (1994) simulated airflow over the Kii Peninsula addressed in this study falls within the latter type. using a dry version of a nonhydrostatic model. They This type of precipitation could be classified into showed that heavy precipitation occurred when a several categories. On the basis of radar reflectivity low-level synoptic wind was east-southeasterly to data, Willoughby et al. (1984) divided the precipita- southerly. Low-level updrafts were found around tion systems into stationary one and moving one. the middle and southern parts of the east coast of The former was further divided into three compo- the peninsula, consistent with the rainfall distribu- nents: the principal band, the secondary bands, tion shown in previous observational studies. and the connecting band. The principal band lies Numerical simulations are powerful tools for in- along streamlines accompanying a vestigation of physical processes involving heavy and contains active . The secondary precipitation in the Kii Peninsula. More studies bands, which are located between the principal need to be performed to further explore the mecha- band and the eyewall, contain weak convection. nisms for enhancement of precipitation. However, The connecting band, which joins the principal few numerical studies have examined the structure band and the eyewall, crosses the streamlines and of precipitation systems and in particular the inter- contains either stratiform clouds or weak convec- action between the systems. Recent development of tive clouds. Shimazu (1998) classified precipitation nonhydrostatic models enables us to realistically systems accompanying tropical cyclones approach- simulate the structure of precipitation systems in ing Japan on the basis of data from a conventional the real atmosphere. The purpose of this study is radar network. Rain shields whose distance from to clarify the mechanisms for enhancement of pre- the storm center was within 150–250 km were cipitation in the present case. called inner rain shield. Outer rain shields were re- It is desirable to quantify the e‰ciency of net ferred to as the rain shields that were located far- production of rainwater in heavy precipitation. A ther from the storm center. Similarly, band-shaped measure for this purpose is called precipitation e‰- precipitation systems were divided into inner rain- ciency, defined as the ratio of the surface rainfall bands and outer rainbands. rate to moisture convergence, or to the sum of con- E¤orts have also been made to investigate heavy densation and deposition. Market et al. (2003) re- precipitation events occurring in the Kii Peninsula. viewed previous works on precipitation e‰ciency Sakakibara and Takeda (1973) made a case study and summarized the precipitation-e‰ciency values of the rainfall during the passage of typhoon 7002 derived from previous studies. over the Kii Peninsula. They found that the rainfall Many observational studies have been conducted distribution was controlled by topographic features. on precipitation e‰ciency. Rauber et al. (1996) esti- They defined an amplification factor of rainfall and mated precipitation e‰ciency of trade wind clouds found that the factor was dependent upon topo- and suggested that the estimated values in the trade graphic features and the direction and strength of wind regime were relatively low (i.e., no more than prevailing wind. The rainfall amplification was 20–30%). Hanesiak et al. (1997) investigated an also observed in Takeda et al. (1976). They ana- Arctic low pressure system passed over Canada. lyzed heavy rains occurred around the middle part Their observational and modeling analyses sug- of the Kii Peninsula and showed a substantial in- gested that the precipitation e‰ciency of the system crease in precipitation there. The results were ac- was relatively high (i.e., 60–70%). They concluded counted for by the interaction between deep cumu- that the higher values were partly attributed to sat- lus clouds and shallower clouds. Takeda et al. uration near the frontal surface. They also men- suggested that an e‰cient mechanism for rain pro- tioned that precipitation e‰ciency was highly de- duction occurred following the modification of pendent on the stage of development. Market et al. cumulonimbus due to orographic e¤ect. According (2003) calculated precipitation e‰ciency for meso- to Takeda and Takase (1980), the e‰cient mecha- scale convective systems over the central United nism was related to the direction of low-level winds. States. Their statistical analysis revealed that pre- They showed that the low-level clouds that played a cipitation e‰ciency had a correlation with the rela- role in enhancing precipitation were observed when tive humidity in the layer between the surface and the low-level wind was easterly. the lifting condensation level. They also showed Numerical studies have also provided insight into negative correlations between the precipitation e‰- February 2009 A. MURATA 103 ciency and both the convective inhibition and the equations with a map factor and employs a semi- environmental wind shear. implicit time integration scheme. The vertical coor- Few modeling studies have discussed issues re- dinate is terrain-following and contains 50 levels, lated to precipitation e‰ciency. Ferrier et al. with variable grid intervals of 40 m to 904 m. The (1996) conducted numerical simulations of convec- lowest level is located at 20 m from ground surface, tive systems in various ambient conditions using a whereas the highest level is at 22 km. 2-dimensional cloud model and investigated factors The numerical model includes bulk cloud micro- that determined precipitation e‰ciency. Their con- physics introduced by Ikawa et al. (1991). The clusion was that the vertical orientation of updrafts scheme predicts the mixing ratios of six water spe- and ambient moisture content were the major fac- cies (water vapor, cloud water, rain, cloud ice, tors that determined precipitation e‰ciency. They snow, and graupel) and the number concentration found that upright convection allowed for e¤ective of cloud ice. The size distributions of the water sub- collection of cloud condensate by precipitation, stances are assumed to be inverse exponential for whereas lower e‰ciencies in upshear storms were rain, snow, and graupel, and assumed to be mono- due to greater evaporation of cloud at middle levels disperse for cloud water and cloud ice. The treat- and evaporation of rain at lower levels. In con- ment is based on Lin et al. (1983), Murakami trast, changes in environmental moisture produced (1990), and Murakami et al. (1994). A box- smaller variation in precipitation e‰ciency. Lagrangian rain-drop scheme (Kato 1995) is incor- Several modeling studies have recently made use porated for calculating rain fall-out. of more accurate approaches that are based on A grid-nesting strategy is adopted for the lateral actual budget analyses. Li et al. (2002) investigated boundary conditions: double nested JMANHM precipitation e‰ciency in the tropical deep convec- with the horizontal grid spacing of 5 km and 1 km tive regime by analyzing a 2-dimensional cloud- (referred to 5 km-NHM and 1 km-NHM, respec- resolving model. Their results suggested that pre- tively) (Fig. 1). The nesting procedure is as follows: cipitation was more e‰cient in heavy rain regime The simulation using 5 km-NHM (719 575 50 under warm environmental condition. Sui et al. grid points) is initialized at 2100 Japan Standard (2005) estimated precipitation e‰ciency on the ba- Time (JST) (1200 UTC) 28 September 2004. The sis of data from 2-dimensional cloud-resolving sim- initial and lateral boundary data for 5 km-NHM ulation of a tropical convective system and from are obtained from the JMA mesoscale analysis 3-dimensional cloud-resolving simulation of a ty- data produced with a four-dimensional varia- phoon event. Their results showed that convergence tional assimilation technique (Koizumi et al. 2005). (divergence) of hydrometeors made precipitation The 1 km-NHM simulation (501 501 50 grid e‰ciency larger (smaller). points) is initialized at 2200 JST (1300 UTC) 28 In this paper, the concept of precipitation e‰- September 2004. The initial and lateral boundary ciency is utilized to clarify the mechanisms for the data for 1 km-NHM are obtained from forecasts enhancement of precipitation in the present case produced by 5 km-NHM. The time-step intervals and to quantify the degree of the enhancement. are 24 s for 5 km-NHM and 5 s for 1 km-NHM. This paper is structured as follows. In Section 2, Kain-Fritsch convection scheme (Kain and Fritsch the methods for numerical experiments using a 1990; Kain 2004) is included in 5 km-NHM in ad- nonhydrostatic model are described. Section 3 pro- dition to a bulk cloud microphysical scheme. vides a brief overview of Typhoon Meari (2004). 3. Typhoon Meari (2004) Section 4 examines the structures of precipitation systems that brought about heavy precipitation in 3.1 Track and intensity the Kii Peninsula. Section 5 estimates precipitation Typhoon Meari, assigned the serial number 21 e‰ciency and discusses the mechanisms for en- by the Regional Specialized Meteorological Center hancement of precipitation. Finally, our main con- (RSMC)-Tokyo, developed out of cloud clusters clusions are summarized in Section 6. near the Mariana Islands on 21 September 2004. Meari initially tracked northwestward as it gradu- 2. Numerical model and experimental design ally intensified and reached its lowest minimum The numerical model used is the Japan Meteoro- sea-level pressure (MSLP) of 940 hPa on 24 Sep- logical Agency Nonhydrostatic Model (JMANHM; tember. The typhoon recurved to the northeast in Saito et al. 2006). The model has fully compressible the East China Sea on 27 September. At 0830 JST 104 Journal of the Meteorological Society of Japan Vol. 87, No. 1

Fig. 1. Map of the (a) coarse-mesh (5-km NHM) and (b) fine-mesh (1-km NHM) do- mains. Terrain is shown in shading [m]. The location of the Kii Peninsula and five Fig. 2. (a) Track and (b) minimum sea level observation stations (Shiono-misaki, Mi- pressure of Meari. Symbols are drawn at hama, Owase, Kayumi, and Tsu) are also 3-h intervals starting at 2100 JST (1200 shown. UTC) 28 September. In (a), closed squares and open circles are observed and simu- lated (5-km NHM) tracks, respectively. In (b), closed square and solid line are ob- 29 September Meari made landfall in Kyushu, a served and simulated (5-km NHM) mini- mum sea level pressure, respectively. southern part of Japan, and moved almost parallel to the mountainous spine of , the main land of Japan. The typhoon was transformed into an ex- observed storm. The MSLP of the model typhoon tratropical cyclone in an area of Honshu on 30 Sep- is 960 hPa at the initial time and rises to 980 hPa tember. over 24 h (Fig. 2b). The time tendency for MSLP The track and intensity of Meari are shown in in the model storm agrees well with that in the ob- Fig. 2. The track of the model typhoon is close to served one. that of the real storm before landfall (Fig. 2a). The track errors, measured by the distance of storm cen- 3.2 Synoptic conditions and precipitation ters between the observed and simulated , The surface weather map at 2100 JST 28 Septem- are not more than 100 km. After landfall, however, ber shows that stationary front, which crosses Hon- the model typhoon tracks slightly to the west of the shu, lies between the typhoon and high pressure February 2009 A. MURATA 105

Heavy rainfall in the eastern part of the Kii Pen- insula started before the typhoon landfall. From the 12-h accumulated rainfall amounts, ranging be- tween 6 h before and 6 h after the landfall, it was found that the amounts were remarkable in the eastern part of the Kii Peninsula (Fig. 4). In partic- ular, the amounts were more than 400 mm around Owase, located in the middle part of the east coast (34 N, 136 E). Observed 1-h accumulated rainfall amounts in the eastern part of the Kii Peninsula demonstrated that the present heavy precipitation event was ex- treme on a short time scale (Fig. 4). For example, data at Owase showed 130 mm per 1 hour between 0700 and 0800 JST 29 September. 4. Individual precipitation systems 4.1 Partitioning the heavy rainfall into three precipitation systems From the observed precipitation based on radar reflectivity and rainfall amount measured by rain gauges, it is found that the precipitation systems are categorized as follows (Fig. 5): A) the station- ary system observed around Owase, located at the middle part of the east coast of the peninsula, B) the moving system to the southwest of the system A, and C) the band-like moving system to the southwest of the system B. The categorized precipi- tation systems are also observed in the time evolu- tion of 10-min accumulated rainfall amount mea- sured with rain gauges located along the coast (Fig. 4). In the time series of each observational point, a couple of peaks are found. The peaks of precipitation systems B and C move along the coast from southwest to northeast, whereas the peak of precipitation system A is observed only at Mihama and Owase. The simulation of Typhoon Meari using 1 km- NHM successfully reproduces the heavy precipita- Fig. 3. Surface weather charts for (a) 2100 tion (Fig. 6). Compared with the horizontal distri- JST (1200 UTC) 28 September and (b) butions of the observed precipitation (Fig. 5), the 0900 JST (0000 UTC) 29 September. simulated precipitation patters are consistent at least qualitatively. The simulation well represents the area of stationary rainfall that stretches along systems (Fig. 3a). The eastern part of the front is the eastern coast of the Kii Peninsula. Band-like suppressed at 0900 JST 29 September (Fig. 3b). precipitation systems, whose axes are nearly per- The horizontal gradient of sea-level pressure near pendicular to the coast line, are also well repro- the Kii Peninsula at that time becomes greater duced in the simulation. Quantitatively speaking, than that at 12 h before (2100 JST 28 September) however, the simulated rainfall amount is under- because of the approach of the typhoon and the in- estimated. In particular, the amounts along the tensification of an anticyclone to the east of Hok- northern coast of the peninsula (i.e., around Tsu) kaido. are much less than the observed ones. The less 106 Journal of the Meteorological Society of Japan Vol. 87, No. 1

amounts seem to be attributed to the initial data for the model integration, in addition to the slow bias of the simulated storm. 4.2 Precipitation system A The primary reason for the formation of the pre- cipitation system A is that convective instability is eliminated as warm and moist easterly flow in the planetary boundary layer is forced to move upward on mountainous slopes. Radiosonde observations at Shiono-misaki show that below 1.5-km altitude is highly convectively unstable (Fig. 7). Above the height, the profile has a conditionally stable layer between 1.5- and 3-km altitudes. Vertical stratification reproduced by 1 km-NHM also shows convective unstable easterly flow below 1-km altitude on the ocean to the east of Owase (Fig. 8). Above the level, between 1- and 2-km alti- tudes, is a stable layer, which is also shown in the sounding profile. Of interest is that the stable layer disappears on the slope of the east coast of the pen- insula, indicating that the vertical instability ap- pears explicitly owing to upward motion on the slope and is quickly eliminated owing to convec- tion. The warmer and moister air in the lower tropo- sphere is found in observational data, consistent with the simulation findings. At the Owase station, a sudden rise in temperature and specific humidity on the surface occurs 0400 JST 29 September (Fig. 9). The increases are 1.5 K and 1.5 g kg1, respec- tively. Simultaneously, easterly wind velocity at the surface increases rapidly. An increase of precipita- tion starts after the rapid increase in the low-level temperature, moisture, and wind velocity (cf. Fig. 4). The results suggest that the advection of warm and moist air is greatly responsible for the heavy precipitation. It might be speculated that the warm and moist air originates from that on the south- western side of the stationary front lying between the typhoon and the anticyclone to the east of . The low-level easterly flow seems to be forced by pressure gradient between the two sys- tems.

sula on 29 September. The location of each observation point is shown in Fig. 1b. The peaks corresponding to the precipitation systems A, B, and C are also shown. Ar- Fig. 4. Time series of 10-min accumulated rows represent the movement of the precip- rainfall amount observed at five stations lo- itation system B and C. cated along the east coast of the Kii Penin- February 2009 A. MURATA 107

Fig. 5. Hourly accumulated Radar- AMeDAS-analyzed rainfall at 0800, 0900, Fig. 6. Simulated (1-km grid) 1-hour accu- and 1000 JST 29 September. Letters indi- mulated rainfall amount at 0800, 0900, cate precipitation systems discussed in the and 1000 JST 29 September. Letters indi- text. cate precipitation systems discussed in the text. Arrows show the surface wind vectors. 108 Journal of the Meteorological Society of Japan Vol. 87, No. 1

Fig. 7. Vertical profiles of equivalent poten- tial temperature (solid line) and saturated equivalent potential temperature (dashed line) from Shiono-misaki station at 0900 JST (0000 UTC) 29 September.

Fig. 9. Time series of (a) temperature and specific humidity and (b) wind direction and speed at the surface measured at 1- min intervals at the Owase station on 29 September.

horizontal pressure gradient between the typhoon and the anticyclone, as mentioned before. The southeasterly flow located southwest of the shear line, seems to be a part of the typhoon circulation. The primary reason for the heavy rains accompa- nying the system B is moisture supply due to a higher speed flow in the planetary boundary layer near the east coast of the Kii Peninsula, compared Fig. 8. Vertical cross section of simulated equivalent potential temperature (shading) with the flow located o¤shore. Figure 10 shows the and wind (arrows) along the line a–b in vertical cross section of horizontal wind speed Fig. 6. Arrows show the wind vectors pro- along the shear line. It is found from the figure jected on the cross section. that air below 1-km altitude accelerates as it ap- proaches the peninsula. The results suggest that the easterly and southeasterly flow form a confluent 4.3 Precipitation system B zone near the land. The shear line corresponds to a The precipitation system B is located at the line of 18 g kg1 specific humidity (Fig. 11). To the boundary between a low-level easterly and a south- southwest of the shear line, specific humidity ex- easterly flow. The boundary forms the shear line ceeds 18 g kg1. The higher speed wind, therefore, of the horizontal wind. The easterly flow, located enables the air to carry much water vapor, gained northeast of the shear line, seems to be forced by from the ocean to the southwest of the shear line, February 2009 A. MURATA 109

Fig. 10. Vertical cross section of simulated horizontal wind velocity along the line a–c in Fig. 6.

Fig. 12. Same as in Fig. 9, but at the Siono- misaki station.

The e¤ect of the shear line is observed at a cou- ple of stations located along the east coast of the Kii Peninsula. A sudden change in wind direction is observed at the surface of Shiono-misaki just be- fore 0700 JST 29 September (Fig. 12). The direc- tion of the surface wind changes from east to south- east, consistent with the passage of the shear line. Simultaneously, a sudden rise in specific humidity is observed at the surface, which is also consistent with the moisture field. The rapid changes in wind direction and specific humidity correspond with the peak of rainfall amount. Thirty minutes later, a similar change in wind direction is observed at Fig. 11. Same as in Fig. 6, but specific hu- Singu, located at 40 km northeast of Shiono- midity at the lowest model level. misaki, consistent with the passage of the shear line (not shown). leading to the heavy rains over the east coast of the 4.4 Precipitation system C peninsula. A feature also noted is the lifting of a The precipitation system C is associated with a slightly warmer south-easterly flow over an easterly rainband generated in an outer part of the typhoon. flow, producing clouds along the shear line (not The rainbands move toward east as the typhoon shown). moves. The most eastern (outer) part of the rain- 110 Journal of the Meteorological Society of Japan Vol. 87, No. 1

Fig. 14. Vertical cross section of the simu- lated rainwater mixing ratio (shading), cloud water mixing ratio (contours), and rainband-relative wind vectors (arrows) along the line d–e in Fig. 13. Contours are drawn at 0.2 g kg1 intervals. Arrows Fig. 13. Simulated temperature (shading) show the wind vectors projected on the and mixing ratio of rainwater (contours) cross section. at the lowest model level for 0600 JST 29 (2100 UTC 28) September. Contours are drawn at 0.5 g kg1 intervals (thin solid lines) with an extra contour at 0.1 g kg1 axis. The outward tilt is clearly shown in both (thick slid lines). cloud water and rain water. The figure indicates that the outward tilt is resulted from vertical wind shear normal to the rainband. It should be also bands corresponds to the precipitation system C noted that the outer side of the rainband displays and a¤ects the Kii Peninsula. stratiform characteristics. The precipitation system C has typical features of Observations support the features seen in the si- tropical cyclone outer rainbands. One of them is mulated results. The e¤ect of the rainband is ob- cold pools formed near the surface (Powell 1990a, served at a couple of stations located along the b). Figure 13 shows the horizontal distributions of east of the Kii Peninsula. Direction change in the temperature and rain water at the surface. There surface wind is observed at Owase after 1100 JST are some noteworthy features worth mentioning 29 September (Fig. 9). Overall, the main direction in this figure. One of them is that the area of low of the wind changes from southeast (around 1000 temperature is parallel to the rainband axis, in the JST) to south (1200–1300 JST), consistent with southeast-northwest direction, and has width of the passage of the rainband, although wind changes around 30 km, which are comparable to the width in direction from southeast to northwest on a of the rain area. Another one, a more important shorter time scale. A possible reason for the change feature, is that cold pools are slightly located to on the short time scale is surface outflow from cold the southwest of the rain area. The shift in the tem- pools accompanying the rainband. Simultaneously, perature minimum suggests the influence of inflow a sudden drop and rise in the surface temperature toward the rainband axis from the outer (northeast- are observed (1030–1130 JST), suggesting cold ern) side. pools accompanying the rainband. Similar changes Another characteristic feature is the outward tilt in wind direction and temperature are also ob- of convection toward the outer (northeastern) side. served at Shiono-misaki around 0915 JST 29 Sep- Figure 14 shows the vertical cross section of cloud tember (Fig. 12). Before the rapid changes in wind water and rain water perpendicular to the band direction and temperature, precipitation starts (cf., February 2009 A. MURATA 111

Fig. 4). The results suggest the northeastward tilt of convection due to flow in middle troposphere, lead- ing to precipitation distributed at the front side of the rainband. The existence of cold pools, the tilt of convec- tion, and stratiform rainfall on the outer side are consistent with the typical structure of tropical cy- clone rainbands. Willoughby (1995) and Sakaki- bara (2000) reviewed observational studies of tropi- cal cyclone rainbands and mentioned the structure of the rainbands. For example, Powell (1990a, b) showed the results of aircraft observation of rain- bands in two hurricanes. From his results, convec- tive downdrafts and cold pools were located in the inner (concave) side of the rainband on the surface. Fig. 15. Time series of water vapor flux con- Inflow on the surface entered from the outer (con- vergence, the sum of condensation and de- vex) side and passed through the band toward the position of water vapor, and precipitation cold pools. The generated convergence fed updrafts on 29 September. The former two are verti- on the inner side. Convective and stratiform rain- cally integrated quantities. Time in abscissa represents 20-min period just before (e.g., falls tended to lie inner and outer sides of the rain- ‘‘4’’ means 0340–0400 JST). band, respectively. Another example is Ishihara et al. (1986) that showed the results of dual- Doppler radar observations of a rainband in a ty- phoon. Updrafts lay in convergence above the low- set for the budget analysis. The area extends south- level inflow layers on the inner (concave) side of the ward, including Mihama, and northward, close to band. The convergence was associated with the sur- Kayumi (cf. Fig. 1b). The time series of variables, face wind that spiraled inward cyclonically accom- regarding the water budget, horizontally averaged panying the storm. The wind was blowing into the over the Owase area are shown in Fig. 15. The band from the outer region of the typhoon and was variables include water vapor flux convergence, the entering the band through the inner edge. In the sum of condensation and deposition, and precipita- studies of both Powell and Ishihara et al., the tion, where the former two are vertically integrated updrafts leaned outward from the storm center to- variables and the latter is the variable observed at ward the outer side of the band. We also investi- the surface. The period shown in Fig. 15 is divided gated a tropical cyclone rainband using a non- into two sub-periods: 1) 0300–0800 JST and 2) hydrostatic model (Murata et al. 2003). The 0800–1300 JST. The former period (0300–0800 simulated structures of the rainband were consis- JST) corresponds to that when only the precipita- tent with observational results reported for other tion system A a¤ects the variables. In the latter pe- tropical cyclones. Among the realistic features riod (0800–1300 JST), on the other hand, the vari- were: cold pools and convergence on the inner side ables are influenced by all precipitation systems of the band; convergence above low-level inflow (i.e., A, B, and C). The figure clearly shows that layers; and the outward slope of the updraft with the values of the three quantities in the latter period height. are generally larger and reach the maximum values between 1000 and 1200 JST. 5. Discussion It is useful to calculate the e‰ciency of precipita- 5.1 Period of analysis tion for clarifying the mechanisms that amplify the It is desirable to explore the water budget of the amount of the heavy precipitation. A measure for heavy-rainfall event for elucidating its mechanisms. this purpose is called precipitation e‰ciency, de- Because extremely heavy precipitation was ob- fined as the ratio of the surface rainfall rate to served around Owase and the influence of the pre- moisture convergence, or to the sum of condensa- cipitation system A, in addition to B and C, should tion and deposition. Two kinds of precipitation e‰- be considered, a 60-km square area centered on ciency used are defined as follows: 1) Condensation Owase (hereafter referred to as the Owase area) is e‰ciency (CE): The sum of vertically accumulated 112 Journal of the Meteorological Society of Japan Vol. 87, No. 1

water vapor flux divergence, C ð>0Þ is condensa- tion (condensation onto water plus deposition onto ice for all hydrometeor species), E ð>0Þ is evapora- tion (evaporation from water plus sublimation from ice), and R1 is the residual. The residual includes contributions from di¤usion and the surface flux. All variables are vertically integrated ones. It should be noted that the calculations of F1 and D1 are made in the following manner: First, diver- gence of rqvu is calculated, where r is density, qv is specific humidity, and u is three-dimensional wind vector. Next, the sum of the negative divergence is calculated and divided by n, where n is the number of grids in the targeted area. Finally, the calculated quantity is integrated from the bottom to the top of the model, resulting in F1. Similarly, D1 is derived Fig. 16. Precipitation e‰ciency (condensa- from the calculations for positive divergence in- tion e‰ciency and rainfall e‰ciency) and stead of negative one. accretion e‰ciency for each periods. The There are several advantages to dividing the wa- index ‘‘Owase 0300–0800 JST’’ represents ter vapor flux divergence into the negative and pos- the period when only the precipitation sys- itive parts. One of them is that the e¤ect of only the tem A a¤ects precipitation in the Owase area. The index ‘‘Owase 0800–1300 JST’’ negative part of the flux divergence (i.e., water va- is for all the systems (i.e., A, B, and C) in por flux convergence) on condensation can be in- the Owase area. The index ‘‘Shionomisaki vestigated. By definition, the e¤ect of positive part 0500–1000 JST’’ is for the systems B and of the flux divergence is eliminated in CE. Another C in the Shiono-misaki area. advantage is consistency of the notation between water vapor flux divergence and condensation. That is, the phase change of water can be divided condensation and deposition (C) divided by verti- into positive and negative parts (i.e., evaporation cally accumulated water vapor flux convergence and condensation) as well as water vapor flux di- (F ), and 2) Rainfall e‰ciency (RE): The amount 1 vergence. of rainfall reaching the ground (P) divided by the CE can be obtained by rearranging Eq. (1) and sum of vertically accumulated condensation and dividing by F . The obtained equation is as follows: deposition (C). 1 The calculated precipitation e‰ciencies show C=F1 ¼ 1 D1=F1 þ E=F1 T1=F1 þ R1=F1; ð2Þ that RE in the latter period has higher value than where C=F is CE. The equation indicates that CE that in the former period (Fig. 16). The period of 1 depends on four terms. Their contributions to CE, the higher RE corresponds to that of heavier rains. for each period mentioned above, are shown in Of interest is that values of CE in the two periods Fig. 17. Equation (2) states that the sum of the are almost equal. It is probable that the results are terms in Fig. 17 plus 1 equals CE in Fig. 16. due to the structure of precipitation systems, de- It is found from the figure that the contributions pending on the periods. of the terms, D1=F1 and E=F1, are dominant, 5.2 Condensation e‰ciency whereas the last two terms of the right-hand side A Budget analysis of vertically-integrated water of Eq. (2), T1=F1 and R1=F1, are negligible. That vapor is conducted. The analysis provides a power- is, CE balances the sum of D1=F1 and E=F1. ful tool for investigation of processes involved in However, di¤erence in each term between the two determining CE. A suitable equation for water va- periods is very small, consistent with the fact that por budget is described as follows: CEs in the two periods are almost equal. T ¼ F D C þ E þ R ; ð1Þ 1 1 1 1 5.3 Rainfall e‰ciency where T1 is the time tendency for water vapor, F1 We conduct a budget analysis, similar to that ð>0Þ is water vapor flux convergence, D1 ð>0Þ is described in the preceding subsection, regarding February 2009 A. MURATA 113

Fig. 18. Contribution of each term in Eq. (4) Fig. 17. Contribution of each term in Eq. (2) to the rainfall e‰ciency. The index ‘‘0300– to the condensation e‰ciency. The index 0800 JST’’ represents the period when only ‘‘0300–0800 JST’’ represents the period the precipitation system A a¤ects precipita- when only the precipitation system A af- tion in the Owase area. The index ‘‘0800– fects precipitation in the Owase area. The 1300 JST’’ is for all the systems (i.e., A, B, index ‘‘0800–1300 JST’’ is for all the sys- and C) in the Owase area. tems (i.e., A, B, and C) in the Owase area. vertically-integrated suspended condensate (the sum It is found from the figure that the contributions of cloud water, rain water, cloud ice, snow, and of the terms, F2=C, D2=C, and E=C, are domi- graupel), in order to investigate processes involved nant, whereas the last two terms of the right-hand in determining RE. A suitable equation for the sus- side of (4), T2=C and R2=C, are negligible. That pended condensate budget is described as follows: is, RE balances the sum of F2=C, D2=C, and E=C, although the influence of E=C on the dif- T F D C E P R ; 3 2 ¼ 2 2 þ þ 2 ð Þ ference in RE between the two periods is limited. where T2 is the time tendency for the condensate, The value of F2=C becomes larger in the latter pe- F2 ð>0Þ is the flux convergence of the suspended riod, which is the same result to RE. In contrast to condensate, D2 ð>0Þ is the divergence of the sus- that, the value of D2=C becomes smaller, which pended condensate, C ð>0Þ is condensation (con- has an opposite e¤ect on RE. The di¤erence in the densation onto water plus deposition onto ice for magnitude of D2=C between the two periods is all hydrometeor species), E ð>0Þ is evaporation smaller than that of F2=C, consistent with higher (evaporation from water plus sublimation from RE in the latter period. Cause and e¤ect relation- ice), P ð>0Þ is surface rainfall, and R2 is the resid- ship, however, cannot be resolved with the results. ual, including di¤usion. All variables except for P Additional analyses are needed to help to under- are vertically integrated ones. The calculations of stand the factors controlling RE. F2 and D2 are made in the same manner as in those It might be speculated in the latter period that of F1 and D1. the much more cloud water converts into rainwater RE can be obtained by rearranging Eq. (3) and (part of which is melting snow and graupel) via ac- dividing by C. The obtained equation is as follows: cretion of cloud water by rain, leading to the en- hancement of RE. In order to assess the e¤ect of P=C 1 F =C D =C E=C T =C R =C; ¼ þ 2 2 2 þ 2 this cloud microphysical process, a new e‰ciency 4 ð Þ is introduced. The e‰ciency, referred to as accre- where P=C is RE. The equation indicates that RE tion e‰ciency (AE), is defined as the rainwater pro- depends on five terms. Their contributions to RE, duction rate for accretion of cloud water by rain for each period mentioned above, are shown in divided by the sum of vertically accumulated con- Fig. 18. Equation (4) states that the sum of the densation and deposition. The values of AE, shown terms in Fig. 18 plus 1 equals RE in Fig. 16. in Fig. 16, are nearly equal to those of RE, indicat- 114 Journal of the Meteorological Society of Japan Vol. 87, No. 1 ing that precipitation is produced mostly by accre- tion of cloud water by rain. The figure exhibits dif- ference in AE across the periods as well as in RE. Possible reasons for this di¤erence in AE are dis- cussed in detail in the next sub-section.

5.4 E¤ect of cloud microphysics The di¤erence in AE between the two periods seems to depend on the structure of the precipita- tion systems. The vertical profile of accretion of cloud water by rain is shown in Fig. 19 (a). The maximum heights of the accretion for the two peri- ods are almost equal (i.e., below 2-km altitude), al- though the maximum value is significantly larger in the latter period. As shown in the figure, the di¤er- ence in the quantity between the two periods is re- markable above the peak of the first period. The maximum height in the di¤erence is around 3-km altitude, higher than that in the first period. The increase in the accretion at higher altitudes suggests that the precipitation systems in the second period consist of the system B and C in addition to the system A. In fact, the maximum height of the accretion for the precipitation system B and C in the Shiono-misaki area (a 60-km square area cen- tered on Shiono-misaki) is higher (i.e., around 3- km altitude in Fig. 19 (b)) than that for the system A in the Owase area (i.e., around 2-km altitude in Fig. 19 (a)). Raindrops in the system B and C seem to collect cloud droplets in the system A in addition to their own droplets, bringing about higher AE. In fact, when the system B and C are located around Shiono-misaki, both AE and RE are lower, compared with those e‰ciencies for the systems when they move toward Owase and merge with the system A (Fig. 16). Fig. 19. Vertical profiles of production rate Ferrier et al. (1996) pointed out that ambient for accretion of cloud water by rain aver- moisture content was one of the major factors that aged over (a) the Owase area and (b) the determined precipitation e‰ciency in terms of rain- Shiono-misaki area. Di¤erence in the pro- fall divided by condensation, equivalent to RE. Fig. duction rate between the two periods in 20 shows the time evolution of relative humidity in the Owase area is also shown. In (a), the the lower and middle troposphere averaged over index ‘‘0300–0800 JST’’ represents the pe- the Owase area. The values of relative humidity at riod when only the precipitation system A each level ranges from 80 to 100% except for the a¤ects precipitation in the Owase area. values at 5-km altitude after 11 h. RE in the pres- The index ‘‘0800–1300 JST’’ is for all the ent moist environment is probably di¤erent from systems (i.e., A, B, and C) in the Owase that in a drier environment, such as the case of Fer- area. In (b), the index ‘‘0500–1000 JST’’ is for the systems B and C in the Shiono- rier et al. The present results also suggest that the misaki area. relatively small variation of relative humidity has only a minor impact on RE. Inverse relationship between the accretion and the vertical tilt of updrafts was discussed in Ferrier February 2009 A. MURATA 115

Fig. 20. Time series of relative humidity av- Fig. 21. Time series of vertical shear aver- erage over the Owase area on 29 Septem- aged over the Owase area on 29 Septem- ber. The values at 1-, 3-, and 5-km altitude ber. The shear is calculated by subtracting are shown. horizontal wind speed at 1-km altitude from that at 5-km altitude. et al. (1996). They found that upright convection allowed for e¤ective collection of cloud condensate gauge measurements showed that 1-h accumulated by precipitation, whereas lower e‰ciencies in tilt- rainfall amounts were extremely large: more than ing convection were due to greater evaporation of 100 mm. Another feature is that the area of precip- cloud and rain at middle and lower levels, respec- itation is far from the storm center: more than tively. The present results appear to be in contra- 500 km. The goal of this study is to clarify the diction to their results because the precipitation sys- mechanism of the characteristic rainfall. tems, particularly the moving system C, in the latter From radar and surface observations, it was period are tilted, as depicted in Fig. 14. In fact, the found that the precipitation systems were catego- values of vertical wind shear, as shown in Fig. 21, rized as follows: A) the stationary system observed are larger in the latter period than those in the for- around Owase, B) the moving system to the south- mer period. The results can be accounted for by west of the system A, and C) the band-like moving considering the di¤erence in environmental humid- system to the southwest of the system B. ity between the two cases. The e¤ect of the evapo- The numerical model used is the Japan Meteoro- ration of cloud and rain on RE in the present case logical Agency Nonhydrostatic Model with the is limited as shown in Fig. 18 because the relatively horizontal grid spacing of 5 km and 1 km. We moist environment, as depicted in Fig. 20, inhibits adopt a grid-nesting strategy for the initial and lat- the evaporation. eral boundary conditions for the 1-km grid. The track and intensity of the model typhoon, simulated 6. Summary and conclusions with the 5-km grid, are close to those of Meari. The Heavy precipitation in the mountainous Kii Pen- simulation using 1-km grid successfully reproduces insula in Japan caused by Typhoon Meari (2004) is the heavy precipitation. The simulated precipitation investigated using a cloud-resolving model and ob- patterns are favorably compared with those in the servations. The targeted area is the east coast of the observed precipitation. The simulation well repre- peninsula, located around 34 N, 136 E. Data ob- sents the three precipitation systems categorized on tained from rain gauge networks showed that 12- the basis of observations. hour accumulated rainfall amounts, ranging be- Detailed examination of the model fields and ob- tween 6 h before and 6 h after the typhoon landfall servations reveals that the predominant processes (at 0830 JST 29 September), are more than 400 mm for the formation and maintenance of each precipi- at several observation stations located along the tation system are as follows: system A) Elimination east coast of the peninsula. of vertical convective instability in the low-level A marked characteristic of this heavy precipita- warm and moist easterly on the slope of mountains tion is its extreme rainfall rate. Several surface rain located along the east coast of the Kii Peninsula, 116 Journal of the Meteorological Society of Japan Vol. 87, No. 1 system B) Moisture supply due to a higher-speed author also thanks anonymous reviewers and the flow in the planetary boundary layer along the hor- editor for their valuable comments on this paper. izontal boundary between the low-level easterly and The numerical experiments were performed using south-easterly flow, and lifting of slightly warmer the NEC SX6 computer system at Meteorological south-easterly flow, and system C) Cold pool along Research Institute. the edge of the system and a low-level inflow into the system like a tropical cyclone outer rainband. References Precipitation e‰ciency is calculated for elucidat- Ferrier, B. S., J. Simpson, and W. -K. Tao, 1996: Factors ing the mechanisms of the heavy rains. 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