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A multi-cellular storm over the Zoshigaya area of Tokyo, Japan (5 August 2008) was observed by two X-band dual-polarization radars, and this paper aims to investigate the structure of the precipitation cores within its individual precipitation cells. The precipitation cell and core are defined here on the basis of liquid water content (LWC). The storm comprised 20 precipitation cells, each with a precipitation core. Of these, 17 cells were characterized by a single precipitation core (single-core cells) and lasted for less than 30 minutes (5–25 minutes). In contrast, the other three cells consisted of several auxiliary precipitation cores (multi-core cells) that were produced in succession and descended to the ground each lasting approximately 15 minutes, the cells themselves were relatively longlived (≥40 minutes). Single-core cells developed via updrafts driven by low-level convergence that persisted for approximately 10 minutes before being converted to a downdraft by precipitation loading. Without the supporting updraft, the precipitation core fell to the ground 5–25 minutes after its first appearance. In contrast, replacement of precipitation cores in multi-core cells during their mature stage was driven by periodic strong updrafts associated with a low-level southeasterly inflow that supplied warm, moist air to the precipitation cell. The results of a statistical analysis of precipitation cell and core are presented. The rainfall amount from each precipitation cell was proportional to the cell’s lifetime, with a slope of 0.89 for the relationship and a correlation coefficient of 0.95. The average updraft and downdraft velocities of multi-core cells (7.9 and 4.7 m s–1, respectively) were stronger than in the single-core cells (5.2 and 3.7 m s–1, respectively). The average liquid water content of precipitation cores in single-core cells was 4.0 g m–3, whereas the multi-core cells averaged 5.3 g m–3. The average formation heights of singleand multi-core cells were 4.7 km and 4.6 km, respectively. The intensity and formation height of the precipitation cores are approximately proportional to each other. Kanto Plain of Japan, generating heavy rainfall that may cause urban flooding and landslides, especially when the storm cells are mature and slow moving. A multi-cellular storm is defined as a storm comprising several precipitating convective cells which may persist for several hours due to the periodic appearance of new cells. The individual embedded cells are generated from a quasi-steady updraft cell located over the leading edge of the density current (Lin et al. 1998) Corresponding author: Dong-Soon Kim, National Research Institute for Earth Science and Disaster Prevention (NIED), 3-1, Tennodai, Tsukuba, Ibaraki 305-0006, JAPAN E-mail: soony@bosai.go.jp ©2012, Meteorological Society of Japan Journal of the Meteorological Society of Japan Vol. 90, No. 5 702 and are normally short-lived, lasting about 15–30 min (Browning et al. 1976; Hobbs and Locatelli 1978; Warner et al. 1980; Takeda and Seko 1986). Although most cells are short-lived, some may persist for several hours or more. To determine the influence of vertical wind shear on the motion, structure, evolution, and longevity of convective storms, numerous workers have considered the relationship between storm type (multicelled and supercelled storms), vertical buoyancy and wind shear through empirical studies (Marwitz 1972a, b, c; Peterson 1984; Westcott 1994; Rasmussen and Blanchard 1998; Evans and Doswell 2001; Markowski et al. 2003; Shusse et al. 2005), theoretical studies (Moncrieff and Green 1972; Davies-Jones 1984; Lilly 1986a, b; Rotunno et al. 1988), and numerical simulations (Klemp and Wilhelmson 1978a, b; Weisman and Klemp 1982; Weisman and Klemp 1984; Misumi et al. 1994; McCaul and Weisman 2001). A comprehensive review of this work is outside the scope of this paper. To describe the behavior and structure of precipitating convective cells that develop in multi-celled and supercelled storms, the terms “precipitation area (or cell)” and “precipitation core” (a greater precipitation area) are often loosely interchanged with “radar reflectivity area (radar echo or cell)” and “reflectivity core” respectively, and there appears to be no clear definition of these terms. Previous studies have equated the reflectivity core to the precipitation core (Kropeli and Miller 1976; Wakimoto and Bringi 1988; Stalker and Knupp 2002; Wiens et al. 2005). Hobbs and Locatelli (1978) use radar reflectivity measurements, supported by airborne and ground observations, to define precipitation cores as small mesoscale regions of precipitation within a rainband (a large mesoscale region of precipitation within the storm). Kingsmill and Wakimoto (1991) identified individual convective cells from distinct areas of maximum radar reflectivity, and the precipitation core within each cell was defined as the region of 60 dBZ (or greater) reflectivity within the dominant cell. Wakimoto and Lew (1993) defined the precipitation core as the region of 50 dBZ reflectivity, and they showed a descending precipitation core in a waterspout. However, it may not be appropriate to define a precipitation core based solely on radar reflectivity because reflectivity does not always equate directly to precipitation intensity. At the Rayleigh limit, the radar backscatter cross-section of a raindrop is proportional to the sixth power of the drop diameter; consequently, a large number of small drops can produce the same radar reflectivity as a small number of large drops. Tuttle et al. (1989) showed that rainfall rates estimated from radar reflectivity could be as much as 200% larger than those estimated from one-way specific attenuation and differential reflectivity if the precipitation contained a low concentration of large raindrops, resulting in low rainfall rates but moderate radar reflectivity. This result demonstrates some of the inherent difficulties associated with attempting to identify precipitation cores based solely on radar reflectivity. We present here a case study of a multi-cellular storm observed over the Zoshigaya area of Tokyo, Japan on August 5, 2008. This storm produced localized, severe rainfall, and resulted in five sewer workers being swept away by a flash flood (Kato and Maki 2009; Hirano and Maki 2010). A unique aspect of this study is that the precipitation cells and cores are defined by LWC (liquid water contents) which was estimated from dual-polarization measurements made using X-band wavelength radar. The aim of this study is to investigate the precipitation core structure of the individual precipitation cells within this storm by analyzing these radar measurements. The remainder of the manuscript is organized as follows. The analytical data are described in Section 2, and the atmospheric conditions are briefly explained in Section 3. Section 4 outlines the characteristic of the precipitation cells, the three-dimensional wind structure (as obtained from dual-Doppler analysis), and the precipitation core structure, followed by a statistical summary of the precipitation cores. Finally, a summary and discussion of the results are presented in Section 5. 2. Data acquisition and processing Two X-band dual-polarization radars (at Ebina and Kisarazu), operated by the Japanese National Research Institute for Earth Science and Disaster Prevention (NIED), were used to observe a storm over Tokyo, Japan (Fig. 1). Upper air information was obtained from the Tateno Aerological Observatory of the Japanese Meteorological Agency (JMA). These radars simultaneously transmit and receive horizontally and vertically polarized signals at frequencies of 9.375 GHz (Ebina) and 9.709 GHz (Kisarazu). The polarimetric variables measured by these radars were the radar reflectivity factor at horizontal polarization (ZH), differential reflectivity (ZDR), copolar correlation coefficient (ρHV), differential phase shift (ΦDP), Doppler velocity (VD), and spectral width (WS). The polarimetric variables ZH, ZDR, and the specific differential phase shift (KDP) obtained from Ebina were used primarily to analyze the structure of the storm. Velocity data obtained from both radars were used to analyze the kinematic structure of the storm. Polarimetric variables and Doppler velocity data were recorded at 5 minute D.-S. KIM et al. October 2012 703 intervals using consecutive volume scans comprising 12 elevation angles (0.7°, 1.2°, 1.7°, 2.2°, 2.7°, 3.3°, 3.9°, 4.7°, 5.7°, 6.9°, 8.4°, and 10.4°) with a range resolution of 100 m and an azimuthal resolution of 1°. Filtering algorithms were used to process the measured polarimetric variables to eliminate high-frequency random fluctuations from gate to gate. The variables ZH and ZDR were processed using the infinite-impulse response (IIR) filter of Hubbert et al. (1993). The range profiles of the total differential phase (ΨDP) were iteratively filtered in the range using the finite-impulse response (FIR) filter proposed by Hubbert and Bringi (1995), which separates the scattering differential phase (δ) from the ΨDP profiles and then extracts the filtered differential propagation phase, ΦDP. After the filtered ΦDP range profiles had been extracted, KDP was computed using the procedures described by Maesaka et al. (2011). The ZH and ZDR biases of Ebina were estimated to be –5.6 dBZ and +1.44 dB respectively, according to the method of Gorgucci et al. (1999). The observed ZH and ZDR were also corrected for rain attenuation using the attenuation correction algorithm (SSCM: Shifted Self-Consistent Method) proposed by Kim et al. (2008, 2010), which considers variability in the optimum coefficient α (AH = αKDP) along the radar slant-range. Using the vertical incidence observations (Liu et al. 1993), the accuracy of KDP was estimated to be 0.3° km–1. The accuracy of KDP (i.e., 0.3° km–1) is the statistical error derived from analysis of the experimental data. The majori

kisho shushi. dai1shu/kisho shushi. dai2shu/journal of the meteorological society of japan

X-Band Dual-Polarization Radar Observations of Precipitation Core Development and Structure in a Multi-Cellular Storm over Zoshigaya, Japan, on August 5, 2008