著者

YAMADA Hiroyuki ITO Kosuke TSUBOKI Kazuhisa SHINODA Taro OHIGASHI Tadayasu YAMAGUCHI Munehiko NAKAZAWA Tetsuo NAGAHAMA Norio SHIMIZU Kensaku

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2021-063, (Released:2021-07-01)

被引用文献数

11

---

Upper-tropospheric aircraft reconnaissance was carried out for Typhoon Lan (2017) using a civil jet with a newly developed dropsonde system. This was the first case of a Japanese research group observing the inner core of an intense typhoon using dropsondes. This paper describes the warm-core structure in the eye and the associated thermodynamic and kinematic features of the eyewall. During two days of reconnaissance, this typhoon preserved its peak intensity in an environment with a strengthening vertical shear. Dropsondes captured a double warm-core structure with a higher perturbation temperature in the middle and upper troposphere, which persisted between the two flight missions. The two warm cores show a difference in the equivalent potential temperature (θe) of more than 10 K, suggesting different air origins. Saturation point analysis suggest that air observed in the upper warm core was entrained from the eyewall. The eyewall updraft in the left-of-shear semicircle had a two-layer structure with a higher θe and lower absolute angular momentum on the inner side of the updraft core. Analyses of the saturation point and parcel method suggest that the warmer air with a θe exceeding 370 K on the inner side of the updrafts originated from the eye boundary layer and was eventually transported into the upper warm core. These results led us to hypothesize that the vertical transport of high-θe air from the eye boundary layer contributed to the continuous eye warming in the upper troposphere against the negative effect of a strengthening environmental wind shear on the storm intensity. This study demonstrates the significance of eyewall-penetrating upper-tropospheric reconnaissance for monitoring the warm-core structure in the present situation where accurate measurements of both humidity and temperature for calculating θe can only be made with dropsonde-type expendables.

著者

MIN Kyeong-Seok TSUBOKI Kazuhisa YOSHIOKA Mayumi K. MORODA Yukie KANADA Sachie

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2021-017, (Released:2020-12-04)

被引用文献数

3

---

A stationary line-shaped precipitation system (SLPS), which is one type of mesoscale convective systems (MCSs), is a typical heavy-rain-producing weather system formed during warm seasons in Japan. Although the Kinki district, western Japan, is known as a frequent occurrence region for SLPSs, their formation mechanisms in the region have not been sufficiently clarified yet because of their complex formation processes. This study investigated a SLPS event that occurred on 1 September 2015, using observational data and high-resolution numerical experiments. We also carried out numerical sensitivity experiments with regard to the orography and initial time.  The observational data showed that the relative humidity at lower levels was high during the SLPS event. The southwesterly was dominant at middle levels over the Kinki district during the formation of the SLPS. The formation of the SLPS was associated with neither a mesoscale low-pressure system nor a synoptic-scale cold front, demonstrating that these were not necessary conditions for the formation of the SLPS.  In the numerical experiments, we found that the SLPS was formed in a low-level convergence zone of the westerly with the warm and moist south-southwesterly from the Kii Channel. New convective cells formed over the north of Awaji Island and are propagated northeastward by the middle-level southwesterly. This cell formation process was repeated and resulted in the formation of the SLPS. The sensitivity experiments for the orography around the occurrence area of the SLPS indicated that the orography was not a significant factor for the formation of the SLPS in this event. The orography can modify the location of the SLPS.

著者

WANG Chung-Chieh LIN Kuan-Yu DAVIS Christopher A. HUANG Shin-Yi LIU Stefano Chih-Shin TSUBOKI Kazuhisa JOU Ben Jong-Dao

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2020-036, (Released:2020-04-20)

被引用文献数

2

---

In this study, the impacts of Typhoon Morakot (2009)'s vortex structure on the extreme rainfall in Taiwan are investigated through an application of piecewise potential vorticity (PV) inversion. The control (CTL) experiment, starting at 0000 UTC 7 August or 15 h before landfall, reproduces the event realistically and is validated against the observations. By altering the PV perturbation inside 750 km from its center, we conduct sensitivity experiments in which the size and/or circulation strength of Morakot is reduced/weakened in the initial field in several different ways.  In the sensitivity tests, particularly those where the initial PV within the inner core (≤ 250 km) is significantly weakened, the storm makes landfall earlier, stays over land longer, and exits Taiwan later. Such track changes are accompanied by a contraction and spin-up of the inner core at early stages of the integration, caused by convection/latent heating within the inner core under large-scale low-level southwesterly flow. As a result, Taiwan receives an overall rainfall amount either comparable to, or even more than (up to +12 %), CTL in all tests. Thus, a weaker Morakot does not necessarily lead to less total rainfall over Taiwan, and the strong southwesterly flow and its moisture supply were bigger factors than the vortex structure in this event.   On the other hand, the rainfall in the southern Central Mountain Range on 8 August, which were the most-rainy area and period in reality, tended to decrease by up to 40 % with the contraction and a weaker outer circulation. Thus, the rainfall patterns and evolution in the sensitivity tests are considerably different than those in CTL, indicating that the vortex structure plays an important role in the rainfall of this region.

著者

WANG Chung-Chieh CHEN George Tai-Jen NGAI Chi-Hong TSUBOKI Kazuhisa

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2018-051, (Released:2018-07-31)

被引用文献数

5

---

There exists a minor, secondary early-morning peak in mei-yu rainfall climatology along the western coast of Taiwan, and this work investigates one such event on 8 June 2012 in southwestern Taiwan under weak synoptic conditions through both observational analysis and numerical modeling, with the main focus on the triggering mechanism of the convection. Observations show that the convection developed offshore around midnight near the leading edge of a moderate low-level southwesterly wind surge of 15-20 kts, and intensified and moved onshore to produce rainfall. The cold outflow from precipitation also led to new cell development at the backside, and the rain thus lasted for several hours till about 0700 LST. Numerical simulation using a cloud-resolving model at a grid size of 0.5 km successfully reproduced the event development with close agreement with the observations, once a time delay in the arrival of the southwesterly wind surge in initial/boundary conditions (from global analyses) is corrected. Aided by two sensitivity tests, the model results indicate that the convection breaks out between two advancing boundaries, one from the onshore surge of the prevailing southwesterly wind and the other from the offshore land/mountain breeze, when they move to about 40 km from each other. Also, both boundaries are required, as either one alone does not provide sufficient forcing to initiate deep convection in the model. These findings on the initiation of offshore convection in the mei-yu season, interestingly, are qualitatively similar to some cases in Florida with two approaching sea breeze fronts (in daytime over land).

著者

HIOKI Tomohito TSUBOKI Kazuhisa

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2021-064, (Released:2021-07-02)

被引用文献数

2

---

The central pressure fall in a typhoon is associated with the development of the warm core and mass divergence in the eye. Trajectory analyses were used to investigate the origins of air moving into the warm core and the paths of air parcels leaving the eye. First, developing Typhoon Wipha (2007) was simulated by using a high-resolution (2-km) cloud-resolving model to represent the central pressure fall and axisymmetric structures such as the warm core in the upper levels of the eye, the eyewall, and the secondary circulation. Then, using the model output data, backward trajectories were calculated from the eye; the results show that the air parcels comprising the warm core originated from the lower troposphere and the lower stratosphere. Those originating from the lower troposphere, whose equivalent potential temperature (θe) is increased by the latent heat flux from the sea, ascend through the eyewall and move inward in the upper troposphere. Those originating in the lower stratosphere, which have high potential temperature (θ), descend from the lower stratosphere to the upper troposphere. Thus, the warm core consists of high-θe air from the lower troposphere and high-θ air from the lower stratosphere. Next, forward trajectories were calculated to examine the paths of air parcels leaving the eye; the results show that air parcels leave the eye through the eyewall throughout the troposphere, particularly at heights below 2 km and between 9 and 12 km, which ultimately results in a central pressure fall.

著者

MORODA Yukie TSUBOKI Kazuhisa SATOH Shinsuke NAKAGAWA Katsuhiro USHIO Tomoo SHIMIZU Shingo

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2021-038, (Released:2021-03-16)

被引用文献数

3

---

A phased array weather radar (PAWR) can complete one volume scan in 30 seconds, thus enabling us to obtain high spatiotemporal resolution echo intensities and wind fields of storms. Using its rapid scanning capability, we investigated the evolution of a convective storm in detail. To describe evolution of convective storms, we used the following definitions. The precipitation cell is defined as a three-dimensionally contiguous region of 40 dBZ or greater. The precipitation core is defined by a threshold of positive deviation greater than 7 dBZ, which is a difference from the average reflectivity during the mature stage of the cell. An updraft core is defined as an updraft region of 1 m s−1 or stronger at a height of 2 km. An isolated convective storm was observed by two PAWRs on 7 August 2015 in the Kinki District, western Japan. The storm was judged as a single cell, according to the above definition. We identified nine precipitation cores and five updraft cores within 49 minutes in the mature stage of the cell. A long-lasting updraft core and its branches moved southwestward or southeastward. Around these updraft cores, the precipitation cores were generated successively. The updraft core with the longest duration lasted 73.5 minutes; in contrast, the lifetimes of precipitation cores were from 4.5 to 14.5 minutes. The precipitation cell was maintained by the successive generations of updraft cores which lifted humid air associated with a low-level southwesterly inflow. The total amounts of water vapor inflow supplied by all the identified updraft cores were proportional to the volumes of the precipitation cell, with a correlation coefficient of 0.75. Thus, the extremely high spatiotemporal resolution of the PAWR observations provides us with new evidence that an isolated convective storm can be formed by multiple precipitation cores and updraft cores.

著者

TAUVALE Luteru TSUBOKI Kazuhisa

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2019-042, (Released:2019-04-01)

被引用文献数

7

---

Geographic and meteorological characteristics of 479 tropical cyclones (TCs) in a study domain in the Southwest Pacific (defined by 135°E - 120°W and 5°S - 65°S) over the past 48 TC seasons from 1969–1970 to 2016–2017 were examined using the latest Southwest Pacific Enhanced Archive of Tropical Cyclones dataset. Examined metrics include the geographic distributions of TCs, numbers, intensity, length in days (TC days), accumulated cyclone energy (ACE), and power dissipation index (PDI). The results show increasing TC activities in the western, northwestern, northern and central subdomains of the nine subdomains in the study domain. The average latitudes of TC genesis and TC maximum intensity remained almost unchanged. Most of TCs took southward to southeastward paths, and the majority attained their maximum intensities in the western and central parts of the study domain. The annual number of TCs and TC days decreased over the study period, the numbers of stronger TCs slightly increased whereas stronger TC days increased. The highest annual lifetime-maximum intensity and average annual lifetime-maximum intensity also increased. The highest annual maximum intensification rates did not change much over the study period, nor did ACE and PDI. The results show correlations between highest annual lifetime-maximum intensity to the variations of average sea surface temperature (SST) as well as correlations between TC days to the variations of average SST in the region.