APRIL 2007 F U D E Y A S U E T A L . 1225 Meso-␤-scale Pressure Dips Associated with Typhoons HIRONORI FUDEYASU* AND SATOSHI IIZUKA National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan TAIICHI HAYASHI Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan (Manuscript received 25 May 2005, in final form 19 June 2006) ABSTRACT Using a mesoscale model, the formation process of a pressure dip, which was characterized by a rapid decrease and subsequent increase in surface pressure lasting less than an hour, was investigated. A simu- lated pressure dip accompanied by Typhoon Zeb was closely related to warm potential temperature anoma- lies in the lower troposphere. As the typhoon moved into the midlatitude westerlies, the inflow of a dry air mass into the moist region of the typhoon occurred to the west of the typhoon center. Then, a downdraft developed due to evaporation and sublimation. Below the melting levels where there were fewer hydrom- eteors, however, evaporation was insufficient to offset the adiabatic warming. As a result, warm potential temperature anomalies were created in the lower level, resulting in the formation of a pressure dip. The features of a pressure dip associated with other typhoons observed over Japan were also examined. The features were summarized as follows: 1) the pressure dip was observed only on the western side of the typhoon center at a distance of 50–300 km from the center, 2) the pressure dip was accompanied by a sudden cessation of rainfall, and 3) all typhoons with a pressure dip were found during boreal autumn when large-scale environmental conditions were characterized by westerlies with a trough and a dry air mass to the west of Japan at upper levels and fronts at lower levels. The formation process of the simulated pressure dip in other typhoons was found to be similar to that of Typhoon Zeb. The present study suggests that pressure dip is an inherent feature of the asymmetric structure of a typhoon undergoing transition to a extratropical cyclone. 1. Introduction during the passage of Typhoon Zeb. A pressure dip, consisting of a rapid decrease and subsequent increase Fujita (1952) first described the phenomenon of a in surface pressure, is distinct from the gradual de- pressure dip that is occasionally observed in association crease in surface pressure that is recorded during the with typhoons crossing the Japanese islands or moving passage of the typhoon center. Previous studies have along the southern coast of Japan (e.g., Nakajima et al. shown that pressure dips are meso-␤-scale phenomena 1980; Fujii 1992; Maeda 1994; Fudeyasu and Tsuka- with a bandlike structure and typical dimensions of moto 2000). The occurrence of a pressure dip is gener- 100–200 km by 10–50 km. The maximum observed am- ally only recognized by the analysis of barographic plitude of pressure dips is 7–9 hPa, and the duration data. Figure 1 shows the time series data of surface recorded at a single station is always less than 1 h. wind direction and speed, temperature, dewpoint tem- Pressure dips are accompanied by strong winds, occa- perature, and pressure recorded at station K (see Fig. 3) sionally causing severe damage at various locations over the Japanese islands. For example, it was reported * Current affiliation: Japan Agency for Marine-Earth Science that extensive damage resulting from instantaneous Ϫ1 and Technology (JAMSTEC), Yokosuka, Japan. wind speeds of 54.3 m s occurred during a pressure dip associated with Typhoon Mireille (Maeda 1994; Fujita 1992). Corresponding author address: H. Fudeyasu, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Nat- The formation mechanism of pressure dips is poorly sushima, Yokosuka, Kanagawa 237-0061, Japan. understood. Matsumoto and Okamura (1985) used E-mail: [email protected] Doppler radar data and surface observations to study a DOI: 10.1175/MWR3337.1 © 2007 American Meteorological Society MWR3337 1226 MONTHLY WEATHER REVIEW VOLUME 135 and Okamura 1985; Inoue et al. 1999; Fudeyasu and Tsukamoto 2000). The pressure dips that accompanied Typhoon Mireille developed on the rear of the typhoon at a distance of about 200 km from its center (Fujita 1992; Fujii 1992; Maeda 1994). In this study, we first examined the formation mecha- nism of pressure dips using a high-resolution numerical model. We focused on the pressure dips associated with Typhoon Zeb because its track was the most suitable for investigating the structure and occurrence of pres- sure dips because of the availability of comprehensive observational data. The model successfully reproduced the major features of the pressure dip associated with Zeb. We then attempted to clarify the formation mechanism of pressure dips. Another aim of this study was to examine the conditions necessary for the occur- rence of pressure dips using the available observational data. This paper is organized into the following parts. Sec- tion 2 describes the observational data and the model used in the study while section 3 compares the simu- lated pressure dips associated with Zeb with observa- tional data. Additionally, the internal structure of a pressure dip is described. Section 4 examines the for- FIG. 1. The record of the surface pressure, temperature, dew- point temperature, and wind speed and direction as observed at mation mechanism of pressure dips. Section 5 presents station K from 1100 to 1800 UTC 17 Oct 1998. See Fig. 3 for the the features found in observed and simulated pressure location of station K. dips associated with other typhoons. Section 6 discusses the conditions needed for pressure dip formation while pressure dip observed over Japan during the passage of section 7 summarizes the current study. Typhoon Gay and concluded that the pressure dip was due to the passage of an internal gravity wave excited 2. Data and model by the convection of the typhoon. Tsujimura (1993) a. Data concluded rather that the pressure dip associated with To identify past occurrences of pressure dips, we ana- Gay was due to a solitary internal gravity wave. On the lyzed the temporal variations of surface pressure as re- other hand, Inoue et al. (1999) argued that the pressure corded by the barographs at meteorological stations of dip observed within Typhoon Zeb was a low pressure the Japan Meteorological Agency (JMA) during the region that formed behind the gust front of a gravity period from 1980 to 1998. The stations are located at current caused by an intense rainband. Thus, it is ap- approximately 50-km intervals throughout the Japa- parent that the formation mechanism of pressure dips is nese islands. We used the following quantitative defi- a controversial issue. nitions to distinguish a pressure dip from other fea- Another controversial issue is the genesis location of tures, such as local changes in pressure. pressure dips with respect to the center of the accom- panying typhoons. Fujita (1952) described pressure dips 1) The maximum amplitude of the dip in surface pres- on both sides of the movement direction of Typhoon sure observed at each station had to be greater than Della at a distance of 50–600 km from the typhoon 1 hPa and had to last for less than 1 h. center. Nakajima et al. (1980) reported pressure dips 2) The pressure dip, as defined in the first definition both ahead of and behind the advancing Typhoon above, had to be associated with an individual ty- Owen at a distance of 50–100 km from its center. In this phoon, and had to be recorded by at least 10 me- case, the pressure dips ahead of the typhoon were short teorological stations. lived and of lesser amplitude than those behind the 3) The movement of the pressure dip, as detected by typhoon. The pressure dips associated with Typhoon conditions detailed in the first definition above, had Gay and Typhoon Zeb were observed to the left and to be recognized in tandem with the movement of rear of the northeast-moving typhoons (Matsumoto the accompanying typhoon. APRIL 2007 F U D E Y A S U E T A L . 1227 These quantitative definitions are based on the charac- teristics of a pressure dip described by Fujita (1952), who stated that a pressure dip is a mesoscale atmo- spheric phenomenon with a bandlike structure and that the propagation of the pressure dip must be recognized by reference to barographs. The second definition de- tailed above is required in order to depict the spatial structure of pressure dips. Therefore, in this study, we only considered those pressure dips with significant am- plitude and long persistence that were detected over the Japanese islands. Tracks of typhoons were obtained from the best- track archives of the Regional Specialized Meteorologi- cal Centers (RSMC) Tokyo-Typhoon Center. The dataset consisted of the names, positions, maximum surface pressures, and maximum wind speeds of ty- phoons, generally recorded at 6-h intervals. Estimates of the hourly positions of typhoon centers were deter- mined by linear interpolation based on the 6-h data. Hourly rainfall patterns were estimated using the Ra- dar-Automated Meteorological Data Acquisition Sys- tem (AMeDAS) data provided by the JMA. The Radar- FIG. 2. Model domains of the numerical simulations and terrain. AMeDAS data comprise an hourly averaged rainfall Regions higher than 400 m are shaded. Track (line) and 3-hourly locations (closed circles) of the simulated typhoon derived from analysis created from a composite of observations from the D2 simulation during the period from 0000 UTC 17 Oct to operational precipitation radars and the AMeDAS data 0000 UTC 18 Oct 1998. (Obayashi 1991). The Radar-AMeDAS data during the period from April 1988 to March 2001 have a horizontal resolution of 5 km.