P1.35 MESO-SCALE PRESSURE DIPS ACCOMPANIED BY A SEVERE CONVECTIVE STORM OF TROPICAL CYCLONES Hironori Fudeyasu∗, Satoshi Iizuka, and T. Matsuura National Research Institute for Earth Science and Disaster Prevention 1. Introduction Gay was rather due to a solitary internal gravity wave. In contrast, Inoue et al. (1999) argued that a Fujita (1952) first described the phenomenon of pressure dip observed within Typhoon Zeb was a a pressure dip that is occasionally observed in low-pressure region formed behind the gust front association with typhoons crossing the Japan of a gravity current caused by an intense rain band. Islands or moving along the southern coast of It is apparent that the formation process of Japan (Fujita 1952, 1992; Nakajima et al. 1980; pressure dips is a controversial issue. Fujii 1992; Maeda 1994; Fudeyasu and Tsukamoto 2000). The occurrence of a pressure dip is generally only recognized by analysis of barograph 990 Pressure data. Figure 1 shows the time series data of (hPa) surface wind direction and speed, temperature, 980 dew-point temperature, and pressure recorded at 25 Temperature C) station K (see Fig. 3) during the passage of º ( Dew-point temperature Typhoon Zeb. A pressure dip, consisting of a rapid 20 decrease and subsequent increase in surface pressure for 1500-1530 UTC, is distinct from the N gradual decrease in surface pressure recorded W S during the passage of typhoon center. The E previous studies show that pressure dips are direction Wind meso-β-scale phenomena with a band-like structure and typical dimensions of 150 km by 50 20 km. The maximum observed amplitude of pressure dips is 7-9 hPa, and the duration recorded at a 10 single station is always less than one hour. Pressure dips are accompanied by severe (m/s) Wind speed convection, causing sometimes the severe 011 12 13 14 15 16 17 18 damages at various locations over the Japan Time (UTC) Islands. It was reported that, for example, there was extensive damage resulting from Fig. 1. Record of surface pressure, temperature, instantaneous wind speed of 54.3 m s-1 within a dew-point temperature, and wind speed and direction as pressure dip associated with Typhoon Mireille observed at station K from 1100–1800 UTC on 17 (Maeda 1994; Fujita 1992). They appeared on the October 1998. See Fig. 3 for location of station K. left side of the typhoon center, although strong winds associated with typhoons are generally In this study, we first examine the process of observed on the right side of typhoon center. pressure dip formation using a high-resolution The formation process of pressure dips numerical model. We focus on the pressure dips remains poorly understood. Matsumoto and associated with Typhoon Zeb, because the track is Okamura (1985) used Doppler radar data and most suitable for investigating the structure and surface observations to study a pressure dip occurrence of pressure dips by using available observed over Japan during the passage of comprehensive observational data. The model Typhoon Gay, and concluded that the pressure dip successfully reproduces the major features of the was due to the passage of an internal gravity wave pressure dip with Zeb. We then attempt to clarify excited via decay of the typhoon. Tsujimura (1993) the formation process of pressure dips. concluded that the pressure dip associated with 2. Data and Model ∗Corresponding author address: Hironori Fudeyasu, National 2.1 Data Research Institute for Earth Science and Disaster Prevention , Tennodai 3-1, Tsukuba, Ibaraki 306-0006, Japan; E-mail: Tracks of typhoons were obtained from the [email protected] best-track archives of the Regional Specialized 1 Meteorological Centers (RSMC) Tokyo–Typhoon the simulation using a horizontal resolution of 3 km, Center. The dataset consists of the names, but the results are essentially similar to those with positions, maximum surface pressure, and a horizontal resolution of 5 km. maximum wind speeds of typhoons, generally The simple ice scheme (Dudhia 1989) is used recorded at 6-hour intervals. Estimates of the as cloud microphysics. In this scheme, cloud water hourly positions of typhoon centers were and rainwater below 0 ºC are treated as cloud ice determined by linear interpolation of the 6-hour and snow respectively. The convective scheme of data. Grell (1993) is used as a cumulus parameterization An hourly rainfall patterns are estimated from except D2. The model also includes the planetary the Radar–Automated Meteorological Data boundary layer (Hong and Pan 1996), a five-layer Acquisition System (AMeDAS) data provided by soil model, and a cloud-radiation interaction the JMA. The Radar–AMeDAS data is an hourly scheme (Dudhia 1989, 1993; Grell et al. 1995). averaged rainfall analysis created from a The initial and lateral boundary conditions of D1 composite of observations from operational for the simulation of Typhoon Zeb are derived from precipitation radars and the AMeDAS data the Global Asian Meteorological Experiment (Obayashi, 1991). The Radar-AMeDAS data (GAME) reanalysis dataset produced by the during the period April 1988 to March 2001 has a Meteorological Research Institute–JMA horizontal resolution of 5 km. (http://gain-hub.mri-jma.go.jp/GAME_reanal.html). Details of the large-scale atmospheric The dataset includes the 3-dimensional analyzed conditions around the typhoons with pressure dips atmospheric fields over the Asian and Pacific were reconstructed from reanalysis data, sourced regions (30ºE–180º, 80ºN–30ºS), covering the from the National Centers for Environmental period from April to October 1998. The data have a Prediction–National Center for Atmospheric horizontal resolution of 0.5° for both longitude and Research (NCEP–NCAR), with 6-hour intervals latitude, with 17 vertical levels and a 6-hour time covering the period 1980 to 1998 and a horizontal interval. Sea surface temperature is represented resolution of 2.5° (Kalnay et al. 1996). Infrared by the skin temperature of the NCEP–NCAR satellite images from the Japanese Geostationary reanalysis dataset (Kalnay et al. 1996). Meteorological Satellite (GMS) spanning the period 1980-1998 are also used. 2.2 Model 984hPa A numerical simulation was performed using 983hPa the Mesoscale Model 5 (MM5) ver. 3.5 jointly 978hPa developed by Pennsylvania State University and 976hPa the NCAR. MM5 is a non-hydrostatic model 978hPa 980hPa0 980hPa developed as a community mesoscale model 980hPa (Dudhia 1993; Grell et al. 1995). The model Latitude 981hPa incorporates multiple nested grids and includes both explicit moist physical processes and cumulus parameterization schemes. This model solves the nonlinear, primitive equations using Cartesian coordinates in the horizontal and terrain-following sigma coordinates in the vertical. Longitude Pressures at sigma levels are determined from a reference state that is estimated using the Fig. 2. Model domains of the numerical simulations and hydrostatic equation from the given sea level terrain. The regions higher than 400 m is shaded. Track pressure and temperature of standard lapse rate. (line) and 3-hourly locations (closed circles) of simulated In this study, the model contains 30 sigma typhoon derived from the domain 2 simulation during the levels of fine resolution near the surface, while the period 0000 UTC on 17 to 0000 UTC on 18 October top of the model is at 50 hPa. The simulation was 1998. conducted using two domains as shown in Fig. 2. The outer domain (D1) has 271 × 271 grid points, The simulation of D1 was performed for 42 centered at 33ºN, 133ºE, with a horizontal hours, starting at 1800 UTC on 16 October 1998, resolution of 15 km. The inner domain (D2) has after the model was integrated for 18 hours during 460 × 460 points with a horizontal resolution of 5 an initial spin up from 0000 UTC on 16 October. km. D1 is designed to produce the initial and During the initial spin-up, the continuous dynamical boundary conditions for D2, while D2 is designed assimilation where forcing function are added to to resolve explicitly the fine structures of typhoons the governing model equations to gradually and associated pressure dips. We also conducted 2 “nudge” the model state toward the reanalysis data increase in surface pressure was recorded at for only wind components (Grell et al. 1995). The several meteorological stations. Figure 4 shows relaxation boundary conditions are adopted as the time series data of surface pressure recorded by lateral boundary conditions (Anthes et al. 1987), in barographs at the meteorological stations. In which the model-predicted values are relaxed to addition to a gradual change in surface pressure those estimated from large-scale analysis data. related to the passage of Zeb, a sudden 3-8 hPa The D2 simulation began at 0000 UTC on 17 drop in surface pressure is discernable. The October 1998, and was performed for 36 hours pressure dips appear a few hours after the using the initial and boundary conditions passage of the typhoon center for all stations interpolated from the results of the D1 simulation. except J, where a pressure dip was recorded prior At the start of the D2 simulation, the center of to the arrival of the typhoon center. It should be Typhoon Zeb was located to the southwest of noted that the pressure dip exists only to the Kyushu (Fig. 2). northwest of Zeb as it tracked to the northeast (Fig. 3). There is no signal of a pressure dip at stations I and L located to the southeast of the typhoon. A 3. Pressure dip associated with Typhoon pressure dip was also not observed on the Zeb northern part of Kyushu and Chugoku distinct 3.1 Features of observed pressure dip during the period from 0800 to 1200 UTC, indicating the extent of the pressure dip axis that is Typhoon Zeb was upgraded to a tropical storm defined by isopleths of the minimum pressure (maximum 1 minute sustained wind speed in within the pressure dip.