Journal of the Meteorological Society of Japan, Vol. 87, No. 1, pp. 101--117, 2009. 101 DOI:10.2151/jmsj.87.101 A Mechanism for Heavy Precipitation over the Kii Peninsula Accompanying Typhoon 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 hÀ1. 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 rainband 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)-Tokyo, was character- Ten tropical cyclones, that had a maximum wind ized by heavy rains accompanying the storm. A speed of more than 17 m sÀ1, made landfall 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 landfalls a year). The 100 mm hÀ1. 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 rainbands. 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 tropical cyclone vestigation of physical processes involving heavy and contains active convection. 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