著者

Masamichi OHBA Hiroaki UEDA

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.85, no.3, pp.335-348, 2007 (Released:2007-08-09)

参考文献数

46

被引用文献数

59 77

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One possible impact of sea surface temperature (SST) anomalies in the Indian Ocean (IO) on the ongoing El Niño is investigated using an air-sea coupled general circulation model (CGCM). A suite of CGCM experiments by imposing the IO basin-wide warming (BW) and IO dipole/zonal mode (IODZM) are conducted to assess the feedback effects of the El Niño-related SST anomalies on the Pacific. While the IODZM during boreal fall does not have a significant impact on the Pacific sector, the BW during boreal winter enhances the surface easterlies over the equatorial Western Pacific (WP) during the maturedecay phase of El Niño. The strengthened easterlies act to enhance the SST cooling over the WP, thereby the zonal gradient of SST between the IO and WP is greater than the climatology. These enhanced WP easterlies induce an advanced transition to the La Niña phase, which is caused by the upwelling ocean Kelvin waves. These results imply that the BW in the IO, to some extent, can be hastening the El Niño to La Niña transition.

著者

NAKAMURA Yuhi MIYAKAWA Tomoki SATOH Masaki

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.2020-046, (Released:2020-06-02)

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From 9 to 11 September 2015, the Kanto and Tohoku regions of Japan experienced an extremely heavy rainfall event. The synoptic-scale field was characterized by two typhoons, Etau (T1518) and Kilo (T1517). After Etau made landfall in the Tokai region and transformed into an extra-tropical cyclone over the Sea of Japan, meridionally oriented rain bands persisted over the Kanto region for about 12 hours and caused heavy rainfall, particularly over the Tochigi prefecture. During this time, Kilo approached the eastern ocean of the Kanto region. In this study, we examine the role of Kilo in this event by conducting numerical experiments using a stretched version of the Nonhydrostatic Icosahedral Atmospheric Model configured with a minimum grid interval of about 5.6 km. The control experiment reproduced intense rain bands around the same period and place as the observed event, although they were not reproduced in an experiment with a longer lead time. Sensitivity experiments were conducted in which Kilo was weakened by removing moisture in its central region with a longer lead time. In contrast to the expectation that reduced moisture would lead to a weaker typhoon and hence weaker rain, the sensitivity experiment reproduced the rain band with realistic location but 5 % less precipitation than the control experiment. Furthermore, this experiment indicated that precipitation over the outer band of Etau, which covers the Kanto region, increased by 10 % compared to the control experiment. We found that a southeasterly wind induced by a high-pressure ridge between Kilo and the Kanto region played a greater role in supplying moisture to the Kanto region than the strong easterly wind produced by the pressure gradient between Kilo and the Okhotsk high. In this case, weaker Kilo resulted in enhanced northwestward moisture flux associated with the ridge, thereby inducing heavier rainfall over the Kanto region.

著者

伊藤 耕介 Chun-Chieh Wu Kelvin T. F. Chan Ralf Toumi Chris Davis

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.98, no.1, pp.5-17, 2020 (Released:2020-03-26)

参考文献数

46

被引用文献数

1 13

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台風の移動の基礎的な理解はかなり成熟しているが、注目に値する研究の進展が近年も見られる。本論文では、単純化された順圧モデル・精緻な物理モデル・データ解析によって、主に2014年以降に得られた台風の移動に関する新しい概念や既存の概念に関する新たな知見を集約する。これには、台風の移動に関する環境場と台風の相互作用、および、予測可能性の研究を含んでいる。指向流・βジャイア・非断熱加熱といった従来の概念は依然として重要であるが、台風の進路を説明するメカニズムをより正確に理解することは、さらなる進路予報の精度向上に向けて、重要な基礎をなすであろう。

著者

AONO Kenji IWASAKI Toshiki SASAI Takahiro

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.2020-017, (Released:2019-12-24)

被引用文献数

1

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This study examined the roles of wind-evaporation feedback in the tropical cyclone (TC) intensification, with special attention devoted to the feedback in weak wind areas (domains where the 10-m wind speed is smaller than 5, 10 and 15 m s−1). This was done by setting lower limits of the 10-m wind speed in the calculation of water vapor exchange between the atmosphere and the underlying ocean in a nonhydrostatic cloud-resolving model. As a result, the surface evaporation is enhanced in outer regions of a TC where the actual wind speed is smaller than the prescribed lower limit(s). Results show that increasing the lower limit reduces the radial water vapor contrast in the lower troposphere (below 100 m) and suppresses the TC size and intensity at the mature stage by 30-33 % and 5-14 %, respectively, compared to the control run with all standard model settings. The increased evaporation enhances the outer convective activity and reduces the radial pressure gradient in the lower troposphere. As a result, the inflow and thus the inward advection of angular momentum were reduced and the enhanced convection in the outer region suppressed eyewall updraft, and thus reduced the secondary circulation and finally the TC intensity. Moreover, the outer region convection suppresses the rainband activity (within a radius of 300 km from the TC center). The contribution of the wind-evaporation feedback to the enhancement of the radial contrast of water vapor in the lower troposphere is a fundamentally important element for TC intensification, suggesting that the TC development process can be revealed more accurately by elucidating the role of the weak wind area.

著者

GENG Biao KATSUMATA Masaki TANIGUCHI Kyoko

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.2020-026, (Released:2020-02-06)

被引用文献数

4

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This study investigated the impact of mixed Rossby-gravity waves (MRGWs) on the diurnal cycle of precipitation over the southwestern coastal area of Sumatra using data captured during a pilot field campaign of the Years of the Maritime Continent (YMC) Project. The study focused on a 19-day period from 24 November to 12 December 2015, using data from intensive surface observations, radiosondes, a C-band polarimetric radar collected onboard the research vessel Mirai at 4.067°S, 101.900°E, and data from global objective analysis. The results indicated a relationship between oscillations with periods of several days in the intensity of diurnal precipitation and the wind field. Wind oscillations were attributed to several westward-propagating MRGWs traversing the study site. Diurnal convection and precipitation over the land and ocean were enhanced (suppressed) when MRGW-induced offshore (onshore) wind perturbations dominated. Large-scale low-level convergence and upper-level divergence, stronger sea-breeze flow, and colder land-breeze flow were also observed with the intensification of MRGW-induced offshore wind perturbations. However, diurnal precipitation displayed a similar well-defined phase and propagation pattern over the land and ocean, coherent with the regular evolution of sea- and land-breeze circulations, regardless of wind perturbations induced by MRGWs. The results suggest that local convergence induced by the land-sea contrast is mainly responsible for driving the diurnal cycle. Notwithstanding, MRGWs exert a significant impact on the amplitude of diurnal convection and precipitation by modulating the large-scale dynamic structure of the atmosphere and the intensity of local sea- and land-breeze circulations.

著者

FUDEYASU Hironori YOSHIDA Ryuji YAMAGUCHI Munehiko EITO Hisaki MUROI Chiashi NISHIMURA Syuji BESSHO Kotaro OIKAWA Yoshinori KOIDE Naohisa

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.2020-004, (Released:2019-10-20)

被引用文献数

5

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This study investigated the characteristics and environmental conditions of tropical cyclones (TCs) over the western North Pacific from 2009 to 2017 that dissipated before reaching tropical storm strength (TDs) under unfavorable environmental conditions; we compared these with TCs that reached tropical storm strength (TSs) in terms of modulations of relevant large-scale flow patterns. The flow patterns were categorized based on five factors: shear line, confluence region, monsoon gyre, easterly waves, and Rossby wave energy dispersion from a preexisting cyclone. Among 476 cases, 263 TDs were detected using best-track data and early stage Dvorak analysis. The TCs in the environments associated with the confluence region or Rossby wave energy dispersion (easterly waves) tended to reach tropical storm strength (remain weak) compared with the other factors. The average locations of TDs at the time of cyclogenesis in the confluence region, monsoon gyre, and easterly waves (Rossby wave energy dispersion) in the summer and autumn were farther to the west (east and north) than those of TSs that exhibited the same factors. The environments around TDs were less favorable for development than those of TSs, as there were significant differences in atmospheric (oceanic) environmental parameters between TDs and TSs in the factors of confluence region, easterly waves, and Rossby wave energy dispersion (shear line, monsoon gyre, and Rossby wave energy dispersion). The environmental conditions for reaching tropical storm strength over their developing stage, using five factors, can be summarized as follows: higher tropical cyclone heat potential in the shear line and monsoon gyre, weak vertical shear in the confluence region, wet conditions in the easterly waves, and higher sea surface temperatures and an intense preexisting cyclone in Rossby wave energy dispersion from a preexisting cyclone.

著者

MIAO Jyong-En YANG Ming-Jen

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.2020-008, (Released:2019-11-16)

被引用文献数

11

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On 14 June 2015, a severe afternoon thunderstorm event developed within the Taipei basin, which produced intense rainfall (with rainfall rate of 131 mm h−1) and urban-scale flooding. Cloud-resolving simulations using the Weather Research and Forecasting (WRF) model were performed to capture reasonably well the onset of sea breeze, the development and evolution of this afternoon thunderstorm system. The WRF model had four nested grids (with the finest grid size of 0.5 km) in the horizontal and 55 layers in the vertical to explicitly resolve the deep convection over complex terrain.  It is found that convection was initiated both by sea breeze at foothill and by upslope wind at mountain peak. Convective available potential energy (CAPE) was increased from 800 to 3200 J kg−1 with abundant moisture transport by the sea breeze from 08 to 12 LST, fueling large thermodynamic instability for the development of afternoon thunderstorm. Strong convergence between sea breeze and cold-air outflow triggered further development of intense convection, resulting in heavy rainfall over Taipei city.  Microphysics sensitivity experiments show that evaporative cooling played a major role in the propagation of cold-air outflow and the production of heavy rainfall within basin plain (terrain height < 100 m), while melting cooling played a minor role. The terrain-removal experiment indicates that the local topography of Mount Datun at coastal region may produce the channel effect through Danshui River Valley, intensify sea-breeze circulation and transport more moisture. This terrain-induced modification of sea breeze circulation made its dynamic and thermodynamic characteristics more favorable for convection development, resulting in stronger afternoon thunderstorm system with heavier rainfall within the Taipei City.

著者

SUEKI Kenta KAJIKAWA Yoshiyuki

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.2019-063, (Released:2019-09-06)

被引用文献数

7

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During the recent catastrophic heavy rainfall event in western Japan in July 2018, both the Hiroshima and Keihanshin areas were subjected to unusual total rainfall amounts in 72 hours from 1200 UTC 4 July onward. However, the number of sediment disasters was significantly larger in the Hiroshima area. Among the possible reasons for the difference in the sediment disaster occurrences between the Hiroshima and Keihanshin areas, here, we focus on the differences in the rainfall characteristics in these two areas during the heavy rainfall event. Based on the radar observations, we investigate the characteristics of precipitation systems striking the Hiroshima and Keihanshin areas and find that significantly large precipitation systems with areas equal to or larger than 104 km2 were dominant in the Hiroshima area, which caused rapid accumulation of the rainfall amount and enhanced the risk of deadly sediment disasters in this area. On the other hand, in the Keihanshin area, rainfall of moderate intensity and relatively small precipitation systems were found to be dominant. We suggest that the difference in the amount of damage between the Hiroshima and Keihanshin areas was mainly caused by the size difference of the precipitation systems striking these two areas. Statistics relating to the background atmospheric conditions for the precipitation systems in the heavy rainfall event reveal that a high vertical wind shear environment provides preferable conditions for the formation of large precipitation systems.

著者

OTSUKA Shigenori KOTSUKI Shunji OHHIGASHI Marimo MIYOSHI Takemasa

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.2019-061, (Released:2019-09-03)

被引用文献数

7

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Since January 2016, RIKEN has been running an extrapolation-based nowcasting system of global precipitation in real time. Although our previous paper reported its advantage of the use of data assimilation in a limited verification period, long-term stability of its forecast accuracy through different seasons has not been investigated. In addition, the algorithm was updated seven times between January 2016 and March 2018. Therefore, this paper aims to present how motion vectors can be derived more accurately, and how data assimilation can constrain an advection-diffusion model for extrapolation stably for the long-term operation. The Japan Aerospace Exploration Agency's Global Satellite Mapping of Precipitation (GSMaP) Near-Real-Time product is the only input to the nowcasting system. Motion vectors of precipitation areas are computed by a cross-correlation method, and the Local Ensemble Transform Kalman Filter generates a smooth, complete set of motion vectors. Precipitation areas are moved by the motion vectors up to 12 hours, and the product, called “GSMaP RIKEN Nowcast”, is disseminated on a webpage in real time. Most of the algorithmic updates were related to better estimating motion vectors, and the forecast accuracy was gradually and consistently improved by these updates. Particularly, the threat scores increased the most around 40°S and 40°N. A performance drop in the northern hemisphere winter was also reduced by reducing noise in advection. The time series of ensemble spread showed that an increase in the number of available motion vectors by a system update led to a decrease in the ensemble spread, and vice versa.

著者

Bagtasa Gerry

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.2019-052, (Released:2019-06-14)

被引用文献数

17

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The influence of tropical cyclones (TC) on the western North Pacific (WNP) summer monsoon flow and its impact on rainfall in the Philippines during the months of June to September from 1958 to 2017 were investigated. High precipitation event (HPE) days with rainfall in the upper 85th, 95th, and 99th percentile were determined using daily rainfall averaged from eight synoptic stations in northwestern Philippines. More than 90 % of HPE days coincide with TC occurrence in the WNP and landfalling TCs only account for 12.8-15.1 % of HPE days. The present study looks at the non-landfalling TCs coincident with the HPEs. The result shows that these non-landfalling TCs are critical in remotely affecting almost all local HPEs in northwestern Philippines. Analysis of the TC tracks and their influence on the southwesterly of the summer monsoon flow in Southeast Asia during HPE days show that most of the TCs moved along a line segment connecting northern Luzon and Okinawa, Japan. The composite low-level flow of all HPE days is characterized by a zonally-oriented eastward trough of the 1005-1007 hPa sea level isobar along 20°N that extends to at least 135°E longitude over the northern half of the Philippines, and a deepening of the monsoon trough in northern South China Sea. The 1005-1007 hPa trough induces an eastward shift of the southwesterly that increased the mean zonal wind along western Luzon by 1.94-4.69 times and water vapor flux by 2.67-6.92 times by way of the ‘moisture conveyor belt’. In addition, significant increasing trends of 6.0 % per decade in the mean annual number of HPE days per decade and 12.7 % per decade in the annual total HPE precipitation are found for the upper 85th percentile daily rainfall. These are attributed to the recent changes in WNP TC tracks.

著者

JINNO Takuya MIYAKAWA Tomoki SATOH Masaki

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.2019-017, (Released:2018-12-07)

被引用文献数

3

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In August 2016, a monsoon gyre persisted over the western North Pacific and was associated with the genesis of multiple devastating tropical cyclones. A series of hindcast simulations was performed using the nonhydrostatic icosahedral atmospheric model (NICAM) to reproduce the temporal evolution of this monsoon gyre. The simulations initiated at dates during the mature stage of the monsoon gyre successfully reproduced its termination and the subsequent intensification of the Bonin high, while the simulations initiated before the formation and during the developing stage of the gyre failed to reproduce subsequent gyre evolution even at a short lead time. These experiments further suggest a possibility that the development of the Bonin high is related to the termination of the monsoon gyre. High predictability of the termination is likely due to the predictable mid-latitudinal signals that intensify the Bonin high.

著者

SHUSSE Yukari MAESAKA Takeshi KIEDA Kaori IWANAMI Koyuru

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.2019-021, (Released:2018-12-13)

被引用文献数

4

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This study describes the spatial distribution of the melting layer (ML) in a winter stratiform precipitation system associated with a south-coast cyclone (SCC) on 30 January 2015 over the Kanto Plain, Japan, using an X-band polarimetric radar at Funabashi operated by the Ministry of Land, Infrastructure, Transport and Tourism. The detailed horizontal distribution of surface precipitation types based on Weather Reports from citizens provided by Weathernews Inc. (WNI reports) was also investigated in relation to the ML structure. Surface precipitation in the Kanto Region started with rain and then changed to snow around Tokyo. According to WNI reports, a large dry snow area had formed around Tokyo by 0900 Japan Standard Time (UTC + 9 hours), while surface rainfall continued in the southeast of the Kanto Plain (most part of Chiba and southern part of Kanagawa). A boundary line between the surface dry snow and rain areas became clear in the eastern part of Kanagawa and the northwestern part of Chiba. This boundary then gradually moved inland. Polarimetric ML signatures suggesting the presence of melting snow were continuously observed above the rainfall area in the southeast of the Kanto Plain. The polarimetric ML signatures, on the other hand, approached the ground near the surface dry snow-rain boundary while the surface snowfall was predominant around Tokyo. During the mature snowfall period around Tokyo, the ML vertically extended below 1 km above sea level near the surface dry snow-rain boundary, which indicates the presence of a local horizontal temperature gradient and a surrounding ~0°C near-isothermal layer. It is suggested that this vertically extending ML coincided with the edge of a cold air mass in the lower atmosphere, which often forms during snowfall associated with SCCs in the Kanto Region.

著者

TOCHIMOTO Eigo NIINO Hiroshi

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.2018-043, (Released:2018-04-27)

被引用文献数

7

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This study used the JRA-55 reanalysis dataset to analyze the structure and environment of extratropical cyclones (ECs) that spawned tornadoes (tornadic ECs: TECs) between 1961 and 2011 in Japan. Composite analysis indicated that the differences between the structure and environment of TECs and those of ECs that did not spawn tornadoes (non-tornadic ECs: NTECs) vary with the seasons. In spring (March–May), TECs are associated with stronger upper-level potential vorticity and colder mid-level temperature than NTECs. The colder air at the mid-level contributes to the increase in convective available potential energy (CAPE) of TECs. TECs in winter (December–February: DJF) and those northward of 40°N in autumn (September–November: SON) are accompanied by larger CAPE than are NTECs. The larger CAPE for TECs in DJF is caused by larger moisture and warmer temperature at low levels, and that for TECs northward of 40°N in SON (NSON) is caused by the colder mid-level temperature associated with an upper-level trough. The distribution of the energy helicity index also shows significant differences between TECs and NTECs for DJF and NSON. On the other hand, the distribution of the 0–1 km storm relative environmental helicity (SREH) shows no significant differences between TECs and NTECs in most seasons except DJF. A comparison of TECs between Japan and the United States (US) shows that SREH and CAPE are noticeably larger in the US. It is suggested that these differences occur because TECs in the US (Japan) develop over land (ocean), which exerts more (less) surface friction and diurnal heating.

著者

ITO Rui AOYAGI Toshinori HORI Naoto OH'IZUMI Mitsuo KAWASE Hiroaki DAIRAKU Koji SEINO Naoko SASAKI Hidetaka

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.2018-053, (Released:2018-08-24)

被引用文献数

7

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Accurate simulation of urban snow accumulation/melting processes is important to provide reliable information about climate change in snowy urban areas. The Japan Meteorological Agency operates a square prism urban canopy (SPUC) model within their regional model to simulate urban atmosphere. However, presently, this model takes no account of snow processes. Therefore, in this study, we enhanced the SPUC by introducing a snowpack scheme, and the simulated snow over Japanese urban areas was assessed by comparing the snow depths from the enhanced SPUC and from a simple biosphere (iSiB) model with the observations. Snowpack schemes based on two approaches were implemented. The diagnostic approach (sSPUCdgn) uses empirical factors for snow temperature and melting/freezing amounts and the Penman equation for heat fluxes, whereas the prognostic approach (sSPUCprg) calculates snow temperatures using heat fluxes estimated from bulk equations. Both snowpack schemes enabled the model to accurately reproduce the seasonal variations and peaks in snow depth, but it is necessary to use sSPUCprg if we wish to consider the physical processes in the snow layer. Compared with iSiB, sSPUCprg resulted in a good performance for the seasonal variations in snow depth, and the error fell to 20 %. While iSiB overestimated the snow depth, a cold bias of over 1°C appeared in the daily mean temperature, which can be attributed to excessive decreases in the snow surface temperature. sSPUCprg reduces the bias by a different calculation method for the snow surface temperature and by the inclusion of heated building walls without snow; consequently, the simulated snow depth is improved. sSPUCprg generated a relationship between the seasonal variations in snowfall and snow depth close to the observed relationship, with the correlation coefficient getting large. Therefore, the simulation accuracy of snowfall becomes more crucial for simulating the surface snow processes precisely by the enhanced SPUC.

著者

Akiyoshi WADA Ryo OYAMA

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.96, no.6, pp.489-509, 2018 (Released:2018-11-22)

参考文献数

52

被引用文献数

8

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Typhoon Lionrock (2016) made landfall in the Pacific side of northern Japan. One of the intriguing events was consecutive deep convections (convective bursts, CBs) occurred before making landfall on 31 August. Lionrock paused the decay of the intensity of the storm, although sea surface cooling (SSC) was induced distinctly by Lionrock along the track. To examine the influence of CBs on changes in storm intensity during the decay phase, numerical simulations were conducted with a 3 km mesh coupled atmosphere-wave-ocean model. The coupled model successfully simulated the occurrence of CBs north of the near-surface-convergence area, which was formed by the confluent of the storm's tangential winds with near-surface frictional spiral inflow from the surrounding region where the significant wave height was high. Simultaneously, the relatively fast translation and asymmetric tropical cyclone (TC) structure were maintained. Lower tropospheric horizontal moisture fluxes have enhanced around the convergence area, although SSC resulted in reduction of the air-sea latent heat fluxes within the storm's inner core. Local occurrences of upward moisture fluxes associated with CBs increased the mid-to-upper tropospheric condensational heating on the upstream side. This caused local increase in lower-tropospheric pressure gradient on the upstream side. This was favorable for pausing the decay of the simulated storm intensity even during the decay phase. Sensitivity experiments regarding the execution time of the coupled model showed that the vertical moisture fluxes and number of CBs could increase around the surface frictional convergence area ahead of the storm when the coupled model was not used. This suggests that the storm in mid-latitude could locally increase the maximum surface wind speed under favorable oceanic conditions. The number and distribution of CBs are indeed sensitive to oceanic conditions and are considered to affect the storm-track simulation and maximum surface wind speeds.

著者

YUMIMOTO Keiya TANAKA Taichu Y. YOSHIDA Mayumi KIKUCHI Maki NAGAO Takashi M. MURAKAMI Hiroshi MAKI Takashi

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.2018-035, (Released:2018-04-08)

被引用文献数

21

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The Japan Meteorological Agency (JMA) launched a next-generation geostationary meteorological satellite (GMS), Himawari-8, on October 7, 2014 and began its operation on July 7, 2015. The Advanced Himawari Imager (AHI) onboard Himawari-8 has 16 observational bands that enable the retrieval of full-disk maps of aerosol optical properties (AOPs), including aerosol optical thickness (AOT) and the Ångström exponent (AE) with unprecedented spatial and temporal resolution. In this study, we combined an aerosol transport model with the Himawari-8 AOT using the data assimilation method, and performed aerosol assimilation and forecasting experiments on smoke from an intensive wildfire that occurred over Siberia between May 15 and 18, 2016. To effectively utilize the high observational frequency of Himawari-8, we assimilated 1-h merged AOTs generated through the combination of six AOT snapshots taken over 10-min intervals, three times per day. The heavy smoke originating from the wildfire was transported eastward behind a low-pressure trough, and covered northern Japan from May 19 to 20. The southern part of the smoke plume then traveled westward, in a clockwise flow associated with high pressure. The forecast without assimilation reproduced the transport of the smoke to northern Japan; however, it underestimated AOT and the extinction coefficient compared with observed values, mainly due to errors in the emission inventory. Data assimilation with the Himawari-8 AOT compensated for the underestimation and successfully forecasted the unique C-shaped distribution of the smoke. In particular, the assimilation of the Himawari-8 AOT during May 18 greatly improved the forecast of the southern part of the smoke flow. Our results indicate that the inheritance of assimilation cycles and the assimilation of more recent observations led to better forecasting in this case of a continental smoke outflow.

著者

Keiichi ISHIOKA

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.96, no.2, pp.241-249, 2018 (Released:2018-03-27)

参考文献数

18

被引用文献数

9

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A new recurrence formula to calculate the associated Legendre functions is proposed for efficient computation of the spherical harmonic transform. This new recurrence formula makes the best use of the fused multiply–add (FMA) operations implemented in modern computers. The computational speeds in calculating the spherical harmonic transform are compared between a numerical code in which the new recurrence formula is implemented and another code using the traditional recurrence formula. This comparison shows that implementation of the new recurrence formula contributes to a faster transform. Furthermore, a scheme to maintain the accuracy of the transform, even when the truncation wavenumber is huge, is also explained.

著者

石原 正仁 藤吉 康志 田畑 明 榊原 均 赤枝 健冶 岡村 博文

出版者

社団法人日本気象学会

雑誌

Journal of the Meteorological Society of Japan. Ser. II (ISSN:00261165)

巻号頁・発行日

vol.73, no.2, pp.139-163, 1995-04-25

参考文献数

42

被引用文献数

19

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「集中豪雨のメカニズムと予測に関する研究」の一環として1988年の梅雨期に九州北部を中心として実施された特別観測期間中に、梅雨前線に沿ってメソスケール降雨帯が発生し、最大総降水量178mmの大雨が発生した。2台のドップラーレーダーによる観測結果をもとに、この降雨帯のレーダーエコーと循環の3次元構造を解析し、その構造と維持過程を中心に議論する。降雨帯は1988年7月17日に発生し、7時間維持された。発生環境を見ると、大気下層の水平温度傾度が大きくはなく、熱力学的不安定度は熱帯と中緯度の中間であった。降雨帯の長さは170kmに達し、内部は対流性領域と層状性領域から構成されていた。降雨帯の走向は北西-南東であり、大気中層と下層の間の風の鉛直シヤーとほぼ平行であった。対流性領域にある既存の対流セルは降雨帯の走向に沿って移動し、周囲の南西風が入り込む降雨帯の南西端に新しい対流セルが次々と発生した。降雨帯の中には次のような特徴的な流れが確認された。:1)降雨帯の前部にある対流規模上昇流、2)降雨が最も強い領域にある対流性下降流、3)後部中層のエコーのノッチ(切れ目)からの乾燥空気の流入、4)この後部流入に接続するメソ下降流、5)対流規模下降流の下の大気最下層の前方と後方に進む発散流。これら最下層の発散流は周囲より4℃程度低温の寒気プールを作り、この寒気プールと降雨帯前方の暖湿な南西流との間にガストフロントが作られた。降雨帯後方にあった中層の総観規模の乾燥域は、最下層の暖湿気流とともに、降雨帯を維持するために重要な役割を果たした。高層データによると、雨滴の蒸発冷却によると思われる低温域が対流規模下降流とメソ下降流の中に存在した。後部流入にともなう乾燥空気は対流性領域の最下層まで達していた。こうした熱力学特徴は、ドップラーレーダー解析から得られた運動学的構造とよく適合した。降雨帯は中緯度の前線帯に発生したとはいえ、対流圏下層に限れば降雨帯の前後の熱力学的条件の差異は非常に小さかった。この降雨帯は、西ヨーロッパや北米太平洋岸の寒帯前線にともなって観測されるメソ対流システムよりも、熱帯や中緯度のスコールラインのような「自立型対流システム」に属するであろう。

著者

TSENG Wang-Ling HONG Chi-Cherng LEE Ming-Ying HSU Huang-Hsiung CHANG Chi-Chun

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.2020-071, (Released:2020-10-07)

被引用文献数

4

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In July and August (JA) 2018, the monsoon trough in the western North Pacific (WNP) was unusually strong, the anticyclonic ridge was anomalously northward-shifted, and enhanced and northward-shifted tropical cyclone activity was observed. Studies have examined the effects of sea surface temperature (SST) anomalies on the North Atlantic (NA), the Indian Ocean (IO), and the tropical North Pacific. However, a synthetic view of SST forcings has yet to be identified. Based on a series of numerical experiments, this study demonstrated that the SST anomaly in the tropical WNP was the key forcing that formed the structure of the observed anomalous phenomena in the monsoon trough. Moreover, the combined effect of the SST anomaly in both the tropical and extratropical WNP resulted in enhanced circulation anomalies in the WNP. The NA SST anomaly also enhanced the monsoon trough in the presence of WNP SST anomaly. By contrast, the individual SST anomaly in the NA, IO, the extratropical WNP, and the subtropical eastern North Pacific could not force the enhanced monsoon trough. We proposed that the local effect of both the tropical and extratropical WNP SST anomaly as the major driver and the remote effect of NA SST anomaly as a minor contributor jointly induced the anomalous circulation and climate extremes in the WNP during JA 2018.

著者

Kei SAKAMOTO Masaaki TAKAHASHI

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.83, no.5, pp.817-834, 2005 (Released:2005-11-19)

参考文献数

16

被引用文献数

23 23

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Near the tropopause over the North Pacific in summer an isolated low pressure system (Upper Cold Low, UCL) is often generated by the deepening and cutting off of a trough in the mid-latitude westerlies. The tracks and structures of these UCLs have been investigated in previous studies, but understanding of the cut off and weakening processes remains poor. In this paper, the tracks of UCLs generated in the 1999 summer are analyzed using ECMWF data. The physical processes occurring in one ofthese systems are investigated in detail using the ECMWF data, and the meso-scale model MM5. We focus particularly on cut off and weakening processes, and on the structure of the vertical velocity in the UCL.The summer of 1999 was hot over Japan, and part of the Tibetan high pressure, around 200 hPa, was shifted northward. This allowed some UCLs to approach Japan in July and August. A UCL on August 19th is selected for detailed analysis, and was generated in the following process. Positive vorticity in a westerly wave at 200 hPa was extended by a northeast wind in the upper layer only. The positive vorticity was cut off by non-liner effects and upper level divergence, associated with convective clouds, generating the isolated UCL. The structure of the cyclonic circulation and the warm and cold cores were similar to those in previous studies. The structure of the vertical motion of the moving UCL was explained by dry dynamics and there was upward motion on the front side of the UCL, in the direction of movement. Upper level clouds in the UCL strengtJhened this upward motion. Convective clouds were seen in the system. The latent heat of these convective clouds played an important role in weakening the cold core of the UCL.