著者
Kazuo Saito
出版者
Meteorological Society of Japan
雑誌
SOLA (ISSN:13496476)
巻号頁・発行日
vol.15, pp.222-227, 2019 (Released:2019-11-12)
参考文献数
27

It is well known that northward winds are often observed at southern coastal areas of Japan when a developed tropical cyclone is located off the south coast of Japan. These northward winds have been frequently referred to the northward emission of warm and humid air from the typhoon which cause pre-typhoon rainfalls, but their mechanism has not been clarified. In this paper, we show that the northward wind can be explained by the ageostrophic wind components dynamically induced by acceleration vector round the tropical cyclone. On 7 October 2009, when a developed typhoon (T0918 Melor) approached Japan, distinct northward winds were observed at aerological observations over western Japan. Using numerical simulations with the Japan Meteorological Agency nonhydrostatic model, we reproduced the observed northward wind and their mechanism were examined by numerical experiments. The origin of the northward winds is explained by the ageostrophic winds dynamically induced by the acceleration vectors. When a typhoon approaches a baroclinic zone from south, northeastward ageostrophic winds are induced by southeastward acceleration vectors. Other possible causes (diabatic heating and orographic effect) are examined by sensitivity experiments. Diabatic heating by moist process acts to enhance the ageostrophic winds but the role is not primary. Orography has little effect on the observed ageostrophic wind. Non-axisymmetric features of the upper level divergence flow of a tropical cyclone near a baroclinic zone can also be elucidated by the similar mechanism of the ageostrophic winds.
著者
Kazuo Saito
出版者
Meteorological Society of Japan
雑誌
SOLA (ISSN:13496476)
巻号頁・発行日
pp.2019-040, (Released:2019-10-10)

It is well known that northward winds are often observed at southern coastal areas of Japan when a developed tropical cyclone is located off the south coast of Japan. These northward winds have been frequently referred to the northward emission of warm and humid air from the typhoon which cause pre-typhoon rainfalls, but their mechanism has not been clarified. In this paper, we show that the northward wind can be explained by the ageostrophic wind components dynamically induced by acceleration vector round the tropical cyclone.On 7th October 2009, when a developed typhoon (T0918 Melor) approached Japan, distinct northward winds were observed at aerological observations over western Japan. Using numerical simulations with the Japan Meteorological Agency nonhydrostatic model, we reproduced the observed northward wind and their mechanism were examined by numerical experiments.The origin of the northward winds is explained by the ageostrophic winds dynamically induced by the acceleration vectors. When a typhoon approaches a baroclinic zone from south, northeastward ageostrophic winds are induced by southeastward acceleration vectors. Other possible causes (diabatic heating and orographic effect) are examined by sensitivity experiments. Diabatic heating by moist process acts to enhance the ageostrophic winds but the role is not primary. Orography has little effect on the observed ageostrophic wind.Non-axisymmetric features of the upper level divergence flow of a tropical cyclone near a baroclinic zone can also be elucidated by the similar mechanism of the ageostrophic winds.
著者
Kazuo Saito Takumi Matsunobu
出版者
Meteorological Society of Japan
雑誌
SOLA (ISSN:13496476)
巻号頁・発行日
pp.2020-034, (Released:2020-09-11)

Heavy rainfalls often occur when a tropical cyclone (TC) exists on the sea off the south coast. These pre-typhoon rainfalls (PRE) is associated with the northward moisture transport ahead of the TC. In this paper, we examine the northward moisture transport by the ageostrophic winds associated with typhoon T0918 (Melor) and its impact on PRE. According to a numerical simulation conducted in the previous study (Saito 2019), we analyzed the northward moisture fluxes by reproduced geostrophic and ageostrophic winds. Although the southerly ageostrophic winds are dominant mainly in the upper levels, the ageostrophic winds contribute to enhance the poleward water vapor transport for the upper and middle levels above 3 km.To see the impact of the ageostrophic moisture transport on PRE, we conducted a sensitivity experiment where the model moisture in middle and upper levels over the sea off the south coast of western Japan was reduced. Precipitation over western Japan was decreased about 30% when the contributions in moisture fluxes by ageostrophic winds were removed. This result suggests that the northward ageostrophic winds associated with a TC enhance PRE by moistening the middle and upper atmosphere.
著者
Tsutao OIZUMI Kazuo SAITO Junshi ITO Thoru KURODA Le DUC
出版者
Meteorological Society of Japan
雑誌
Journal of the Meteorological Society of Japan. Ser. II (ISSN:00261165)
巻号頁・発行日
pp.2018-006, (Released:2017-11-30)
被引用文献数
2

An intense rainband associated with Typhoon 1326 (Wipha) induced a fatal debris flow on Izu Oshima, Japan, on October 15-16, 2013. This rainband formed along a local front between the southeasterly humid warm air around the typhoon and the northeasterly cold air from the Kanto Plain. In this paper, the Japan Meteorological Agency Nonhydrostatic Model was optimized for the “K computer,” and ultra-high-resolution (500-250 m grid spacing) numerical simulations of the rainband with a large domain were conducted. Two of main factors that affect a numerical weather prediction (NWP) model, (1) grid spacing and (2) planetary boundary layer (PBL) schemes [Mellor–Yamada–Nakanishi–Niino (MYNN) and Deardorff (DD)], were investigated. Experiments with DD (Exps_DD: grid spacings of 2 km, 500 m, and 250 m) showed better reproducibility of the rainband position than experiments with MYNN (Exps_MYNN: grid spacings of 5 km, 2 km, and 500 m). Exps_DD simulated distinct convective-scale up/downdraft pairs on the southeast/northwest sides of the front, whereas those of Exps_MYNN were not clear. Exps_DD yielded stronger cold pools near the surface than did Exps_MYNN. These differences in the boundary layer structures likely had a large impact on the position of the front and the associated rainband. Exps_DD with the 500-m grid spacing showed the best precipitation performance according to the Fractions Skill Score. To check other factors of the precipitation forecast, model domain sizes, lateral boundary conditions in nesting simulations, and terrain representations were investigated. In the small domain experiments, the rainband shapes were very different from the observations. In the experiment using a nesting procedure, the deterioration of the forecast performance was acceptably reduced. The model with fine terrains better reproduced the intense rain over the island. These results demonstrate that the ultra-high-resolution NWP model with a large domain has the possibility to improve predictions of heavy rain.
著者
Hiromu Seko Kazuo Saito Masaru Kunii Masayuki Kyouda
出版者
(公社)日本気象学会
雑誌
SOLA (ISSN:13496476)
巻号頁・発行日
vol.5, pp.57-60, 2009 (Released:2009-04-15)
参考文献数
11
被引用文献数
2 6 4

Energy Helicity Index (EHI), defined by the product of Convective Available Potential Energy (CAPE) and Storm Relative Environmental Helicity (SREH), is one of potential parameters to diagnose the possibility of tornado outbreak. In this study, probabilities that EHI exceed some criteria were examined with a mesoscale ensemble prediction system, whose grid interval was 15 km, in two tornado events in Japan (Nobeoka and Saroma tornado events). High probability regions (HPR, hereafter) of large SREH existed in the northeastern quadrants of a typhoon or a low-pressure system, while HPRs of large CAPE extended along the warm humid airflow from the Pacific Ocean. In the two events, the tornados were formed near HPRs of large EHI, where HPRs of large SREH and CAPE were overlapped. This result indicates the possibility of the probability forecast of the potential parameter for tornado outbreak.
著者
Augusto José PEREIRA FILHO Felipe VEMADO Kazuo SAITO Hiromu SEKO José Luis FLORES ROJAS Hugo Abi KARAM
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
vol.96A, pp.247-263, 2018 (Released:2018-05-17)
参考文献数
54
被引用文献数
3

The Tokyo Metropolitan Area Convection Study (TOMACS) for extreme-weather-resilient cities is a research and development project (RDP) of the World Weather Research Programme (WWRP). TOMACS provided a multiplatform and high spatiotemporal resolution dataset for the present research on three episodes of deep convection in the Tokyo Metropolitan Area (TMA) under its heat island effect and sea breeze circulations. Heavy rainfall episodes of August 26, 2011, and July 23 and August 12, 2013, were simulated with (and without) the tropical town energy budget (T-TEB) model coupled with the advanced regional prediction system (ARPS). The T-TEB/ARPS system used initial and boundary conditions from the Japan Meteorological Agency (JMA) mesoscale analysis data for 24-hour integration runs at 5-km resolution over Japan and at 1-km resolution over TOMACS area. The 1-km resolution hourly rainfall field simulations were verified against the respective automated meteorological data acquisition system (AMeDAS) rain gauge network measurements. Statistics of the Contingency tables were obtained to estimate the critical success index (CSI), probability of detection (POD), and false alarm rate (FAR) as well as the root mean square error (RMSE). The T-TEB/ARPS simulations improved the south and east sea breeze circulations of TMA and its urban heat island effect. The time evolution of CSI scores improved within the advective time scale, whereas dissipation (phase) errors on precipitation RMSE increased with the integration time and were larger than the dispersion (amplitude) errors. The initial and boundary conditions of JMA greatly improved the simulations as compared to the previous ones performed with the outputs of NCEP's global forecast system as indicated by the TOMACS datasets. Thus, the results represent the temporal and spatial evolutions of the atmospheric conditions leading to the development of a deep convection within TOMACS region. Furthermore, TMA is a good testbed to evaluate the urban surface schemes, such as T-TEB in this study.
著者
Takuya Kawabata Kosuke Ito Kazuo Saito
出版者
(公社)日本気象学会
雑誌
SOLA (ISSN:13496476)
巻号頁・発行日
vol.10, pp.145-149, 2014 (Released:2014-10-04)
参考文献数
27
被引用文献数
2 4

A new 4-dimensional variational data assimilation system with 0.5-km grid spacing (NHM-4DVAR.v3) was developed by integrating the nonhydrostatic storm-scale 4D-Var (NHM-4DVAR.v2) and the Japan Meteorological Agency (JMA) nonhydrostatic model (NHM) based Variational Data Assimilation System (JNoVA). Both systems are based on the JMANHM, but horizontal resolutions, their formulations, adjoint models of physical processes, and observation operators are different. NHM-4DVAR.v3 comprises advantages of both systems: a penalty term, optimization of lateral boundary conditions, and observation operators for advanced observations. This development aimed at improving the forecast accuracy of hazardous weather at meso-γ-scales (5∼20 km). In this paper, the characteristics of NHM-4DVAR.v3 and some results, including the integrated formulations, are presented. An assimilation experiment of actual observations using NHM-4DVAR.v3 with 2-km grid spacing was found to show improvement over NHM-4DVAR.v2 at the same resolution. As a final goal, NHM-4DVAR.v3 was applied with a 0.5-km resolution. The comparison between assimilation results by NHM-4DVAR.v3 with 0.5- and 2-km horizontal resolutions indicates that analyses with super high resolutions can reproduce more detailed atmospheric features such as convective clouds.