著者

Hiroshige TSUGUTI Teruyuki KATO

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.92, no.2, pp.163-183, 2014 (Released:2014-05-16)

参考文献数

32

被引用文献数

4 22

---

On 20 October 2010, a heavy rainfall event occurred on Amami-Oshima Island, Japan, delivering a record 622 mm of rainfall in one day. To clarify the factors underlying this event, the supply mechanism and formation process of low-level humid air and the formation and maintenance mechanisms of the precipitation systems causing the heavy rainfall were examined using observation data, objective analysis data, and numerical simulation results. These investigations showed that low-level humid air, carried to Amami-Oshima Island during the rainfall event by strong east-northeasterly winds, originated more than 500 km to the east-northeast as low-level dry air on the northern side of a stationary front. This dry air was transformed into humid air on the way to the island by receiving large latent heat flux from the sea surface (air-parcel transformation). Warm sea surface temperatures around Amami-Oshima Island, about 2°C higher than the annual mean, contributed to this air-parcel transformation. At Amami-Oshima Island, the collision of the humid flows with a cold pool formed under earlier precipitation systems contributed significantly to the formation and maintenance of the precipitation systems, supplemented by topographic effects of the island.

著者

DUC Le SAWADA Yohei

出版者

公益社団法人 日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2024-003, (Released:2023-09-19)

---

It is well-known in rainfall ensemble forecasts that ensemble means suffer substantially from the diffusion effect resulting from the averaging operator. Therefore, ensemble means are rarely used in practice. The use of the arithmetic average to compute ensemble means is equivalent to the definition of ensemble means as centers of mass or barycenters of all ensemble members where each ensemble member is considered as a point in a high-dimensional Euclidean space. This study uses the limitation of ensemble means as evidence to support the viewpoint that the geometry of rainfall distributions is not the familiar Euclidean space, but a different space. The rigorously mathematical theory underlying this space has already been developed in the theory of optimal transport (OT) with various applications in data science. In the theory of OT, all distributions are required to have the same total mass. This requirement is rarely satisfied in rainfall ensemble forecasts. We, therefore, develop the geometry of rainfall distributions from an extension of OT called unbalanced OT. This geometry is associated with the Gaussian-Hellinger (GH) distance, defined as the optimal cost to push a source distribution to a destination distribution with penalties on the mass discrepancy between mass transportation and original mass distributions. Applications of the new geometry of rainfall distributions in practice are enabled by the fast and scalable Sinkhorn-Knopp algorithms, in which GH distances or GH barycenters can be approximated in real-time. In the new geometry, ensemble means are identified with GH barycenters, and the diffusion effect, as in the case of arithmetic means, is avoided. New ensemble means being placed side-by-side with deterministic forecasts provide useful information for forecasters in decision-making.

著者

Yoshiharu IWASA Takashi ARAKAWA Akimasa SUMI

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.90, no.1, pp.11-33, 2012-02-29 (Released:2012-02-29)

参考文献数

31

被引用文献数

1 1

---

Time integration using the Regional Atmospheric Modeling System (RAMS), a non-hydrostatic cloud-resolving model, was performed for 12 days over a low-latitude band (45°S–45°N) circling an aqua planet with 5-km horizontal grid-point intervals. Tropical and subtropical regions with active precipitation and clear sky, respectively, were clearly divided at 10° latitudes. The numerical experiments derived obvious tropospheric mid-level detrainment (TMD) flows near the 0°C level (z ∼ 5 km) out of the tropics into the subtropics. The TMD flows became largest near the border (10° latitude). In this paper, the time-longitudinal mean field was spotlighted and the atmospheric structure accompanying the TMD flow was investigated. When averaged over time and longitude, the subtropical mid-level troposphere, into which the TMD flows move, is approximately in a state of local thermodynamic equilibrium sustained mainly by the balance between the net radiative cooling and adiabatic heating due to mean subsidence flow. Considering the heat balance, a thermodynamic diagnosis of the mean subsidence flow field suggests the following mechanisms for the mean TMD flow: (1) The mean atmosphere near the melting level has stronger radiative cooling and a larger temperature lapse rate than the atmosphere above it. (2) Free subsidence in the mean subtropical mid-level troposphere, which is consistent with the vertical variation of thermal structure and suffers from no direct dynamic forcings, such as buoyancy, involves a vertically mass-divergent layer just above the melting level. (3) The steady poleward mean TMD flow out of the convective tropic atmosphere exists so as to compensate for the vertical mass divergence in the subtropical atmosphere. Because net meridional transports of sensible heat and water vapor in the middle troposphere are influenced by the mean TMD flow, the existence and the maintaining mechanisms of the mean TMD flow could be important elements of the climate system.

著者

Gerald STANHILL Shabtai COHEN

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.86, no.1, pp.57-67, 2008 (Released:2008-05-12)

参考文献数

28

被引用文献数

44 47

---

Annual values of sunshine duration (SS) measured in Japan between 1890 and 2002 were used as a proxy for global irradiance (Eg↓) to study trends and changes in solar forcing at the Earth's surface. Proxy relationships established for the two SS recorders used in the JMA network both yielded estimates of mean annual values of Eg↓ with RMS &1t; 6%. A first order integrated moving average model (ARIMA) adequately described the time course of SS and Eg↓, which indicated a small, irregular but significant annual increase in solar forcing during the 20th century averaging 0.08 W m-2 or 2.3 hours of Jordan SS recorder sunshine, equivalent to 0.5% per decade. The rate of increase was four times the average in the first four and last three decades of the century reaching a maximum after 1980. The negative effect of the five major volcanic eruptions on Eg↓ was shown to yield a significant linear negative forcing of -41 W m-2 per unit AOD stratospheric aerosol optical depth). The degree of negative solar forcing was related to latitude: between 25° and 44°N each degree shift to the North was associated with an annual increase in Eg↓averaging 0.02%. The time course of changes in solar radiation in Japan during the 20th century resembled that measured in air temperature; correlations between annual values of Eg↓and those in the air temperature of the Northern Hemisphere were very highly significant (P < 0.001) both for the concurrent and preceding year.

著者

Kenneth Sassen

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.58, no.5, pp.422-429, 1980 (Released:2007-10-19)

参考文献数

14

被引用文献数

64 69

---

大気中を自然落下している板状氷晶の落下姿勢と氷晶の大きさとの関係を知るため,氷晶によって生ずる光学現象である光柱(light pillars)の拡がり角および散乱光強度の分布を光柱写真の解析から求めた。その結果,レイノルズ数(Re)にして1.0<Re<100の範囲氷晶は,基底面(basal plane)を落下方向に対して垂直,すなわち水平方向に保つような落下姿勢が卓越し,特にRe=10前後ではこの姿勢が最も安定な落下姿勢であることが判った。落下中の板状結晶の基底面の水平方向からの傾き角は,大気中の乱れのため一般的には,水平方向を中心にガウス分布をしている。これら観測結果と,大気光学現象との関係や雲構成要素の性質のアクチブリモートセンシングについて議論を行った。

著者

Kensuke NAKAJIMA Eizi TOYODA Masaki ISHIWATARI Shin-ichi TAKEHIRO Yoshi-Yuki HAYASHI

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.82, no.6, pp.1483-1504, 2004 (Released:2005-03-02)

参考文献数

23

被引用文献数

6 9 7

---

For the purpose of examining the initial development of the atmospheric response to a warm SST anomaly placed at the equator, an ensemble switch-on experiment is conducted with an aqua-planet GCM. An ensemble average of the size of 128 significantly reduces the transient noises caused by both small scale convective activity and large scale intraseasonal variability.In the first three days after the switch-on of the SST anomaly, a convection center develops above the warm SST area. As a barotropic response to the heating of convection center, a global increase of surface pressure occurs outside the low pressure region around the warm SST area. The response after the emergence of the high pressure anomaly is consistent with Gill (1980); a warm Kelvin wave-like anomaly is emitted to the east of the convection center, while a warm Rossby wave-like anomaly is emitted to the west.The Kelvin wave-like signal propagates at a speed slower than that of free Kelvin wave expected from its vertical wavelength, suggesting that the signal is a “moist” Kelvin wave. Transient decrease of precipitation occurs at the moist Kelvin wave front; a decrease of convection associated with the downward motion at the wave front is consistent with its slow propagation. After several days, precipitation recovers and is even intensified because of the surface frictional convergence associated with the Kelvin wave-like equatorial low pressure anomaly. To the west of the warm SST area, on the other hand, precipitation decreases monotonically. The continuous reduction of precipitation is caused by the equatorial surface frictional divergence associated with the relatively high pressure anomaly at the equator of the Rossby wave structure.Finally, there appears a slow zonally symmetric response within the Hadley cell characterized with surface pressure rise in the tropics and westerly wind anomaly in the troposphere. The change of eddy zonal momentum transport, together with the transport toward the lower level by the Hadley circulation and the geostrophic adjustment to the resulting low level westerly acceleration, seems to be responsible for the response.

著者

Takeshi ENOMOTO Hirokazu ENDO Yayoi HARADA Wataru OHFUCHI

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.87, no.1, pp.139-156, 2009 (Released:2009-03-07)

参考文献数

43

被引用文献数

13 14

---

In July 2004, torrential rainfalls caused significant damages in parts of Japan, followed by heat waves. Our data analysis shows that both rainfall and heat wave events in late Baiu season were caused by the intensification of the subtropical anticyclone near Japan (Bonin high) and that intensity of the Bonin high was significantly influenced by propagation of Rossby waves along the subtropical jet. Hindcast experiments from 15 July were conducted to study the mechanisms and predictability of these high-impact weather events. On 17-18 July, localized rainfalls at a few locations along the coast of the Sea of Japan including Sakata and Fukui were successfully simulated in a high-resolution (21-km mesh) global hindcast simulation. These rainfall events were found to occur near the leading edge of a filament of moist and warm air advected clockwise. On 20 July, anomalously high temperature was reproduced in the high-resolution hindcast simulation. With a moderate resolution of 83 km, the intensification of the subtropical anticyclone was reproduced although the föhn was much weaker. This result indicates that temperature distribution associated with föhn requires a resolution high enough to resolve major mountains. In order to investigate the predictability of propagation of Rossby waves and intensification of the Bonin high, 25-member ensemble experiments from 1 July 2004 were conducted using the moderate-resolution model. It is shown that the region along the Asian jet has twice as long predictability as the entire Northern Hemisphere. This case study suggests that the intensification of the Bonin high associated with the propagation of Rossby waves along the Asian jet could be predicted a few weeks in advance with an ensemble forecast at a moderate resolution.

著者

和田 章義 柳瀬 亘 岡本 幸三

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.100, no.2, pp.387-414, 2022 (Released:2022-04-07)

参考文献数

48

被引用文献数

1 4

---

2018年台風第12号(ジョンダリ)は7月29日の日本上陸前に、対流圏上層寒冷低気圧の円周に沿った異常な経路をとった。大気海洋相互作用および対流圏上層寒冷低気圧とジョンダリの相互作用が台風経路に及ぼす影響を調べるため、3kmメッシュ非静力学大気モデルと大気波浪海洋結合モデル及び異なる初期時間を採用して作成した初期条件に基づく数値シミュレーションを実施した。シミュレーションされた対流圏上層寒冷低気圧は355K等温線上の高ポテンシャル渦度、低い気圧、低い相対湿度の特徴をもつ。7月25日から27日にかけて対流圏上層寒冷低気圧はジョンダリの北側を南西方向に移動し、この期間にシミュレーションされたジョンダリは対流圏上層寒冷低気圧の円周に沿って反時計回りに移動した。ジョンダリが西に移動し始めてから、大気波浪海洋結合モデルによるシミュレーション結果において、経路に沿って海面冷却が生じていた。日本上陸後にジョンダリは勢力を弱めると、対流圏上層寒冷低気圧も日本の南側で勢力を弱めた。特に潜熱フラックスと対流による対流圏上部における加湿が勢力の弱化に影響を与えていた。ジョンダリが九州の南海上で再び発達したとき、台風域では渦位は柱状に高くなり、一方で対流圏上層寒冷低気圧付近の対流圏上層渦位は相対的に低い値であったことから、台風域の渦は対流圏上層寒冷低気圧と合体する様子がシミュレーションされた。大気波浪海洋結合モデルのシミュレーション結果では、寒冷低気圧付近の対流圏上層部における高渦位は維持される一方、柱状の台風域の渦位はその高さを下げつつ弱まり、台風中心からの対流圏上層への外出流が弱まった。この結果、対流圏上層寒冷低気圧から変形した高渦位の折り返し位置に影響を与える様子が見られた。ジョンダリの経路に影響を及ぼす指向流は、対流圏上層寒冷低気圧下の地衡風の影響を受けていたため、実際は上記海洋結合の効果よりも大気初期条件の違いがジョンダリと対流圏上層寒冷低気圧両方の経路と強度により強い影響を与えていた。

著者

REN Suling FANG Xiang NIU Ning SONG Wanjiao

出版者

公益社団法人 日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2023-021, (Released:2023-05-16)

---

Based on the vertical atmospheric sounding system carried by the FY-3D meteorological satellite (FY-3D/VASS) and the new wind radar instrument carried by the FY-3E meteorological satellite (FY-3E/WindRAD), a study of the potential application of research on the changes of temperature, humidity, and ocean wind vector (OWV) during the onset of the South China Sea summer monsoon (SCSSM) was carried out. The applications of these satellite datasets in SCSSM monitoring was evaluated, and the SCSSM onset process in 2022 was analyzed. The results showed that the mean bias of the FY-3D/VASS temperature and specific humidity at 850 hPa, compared with that of the fifth-generation ECMWF reanalysis, were −0.6 K and −0.53 g kg−1, respectively, and the pseudo-equivalent potential temperature (θse) was slightly lower, by 1-2 K; the distribution of θse was consistent with the seasonal advancement of the SCSSM. Compared with Metop-C/ASCAT, the mean bias of FY-3E/WindRAD zonal wind was positive and that of meridional wind was negative. The correlation coefficient, mean bias, mean absolute error, and root-mean-square error of the wind speed were 0.79, −0.45 m s−1, 1.56 m s−1, and 2.03 m s−1, respectively. The distributions of OWV were consistent, and the region and intensity of strong wind speed were close to each other. The temperature, humidity, and wind reversal during the onset of the SCSSM in 2022 were well-monitored by the FY-3D/E-derived θse and OWV dual indices, which are consistent with the SCSSM onset date, the third pentad in May, issued officially by the National Climate Center, China Meteorological Administration. Before the SCSSM onset in 2022, the tropical storms' pumping effect in early May increased the westerly wind over the tropical ocean north of the equator. After the storm weakened, the southwesterly wind passed across the Indochina Peninsula and reached South China Sea, causing the SCSSM onset.

著者

直井 萌香 釜江 陽一 植田 宏昭 Wei MEI

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.3, pp.655-668, 2020 (Released:2020-06-20)

参考文献数

48

被引用文献数

11 15

---

中緯度の細い水蒸気輸送帯は大気の川と呼ばれ、東アジアにしばしば重大な社会・経済的影響をもたらす。夏季東アジアにおける大気の川の活動は、先行する冬季エルニーニョの発達に大きく左右される一方で、冬季から夏季にかけてのENSOの季節的な遷移が大気の川の活動にどの程度影響するのかは、明らかになっていない。本研究では、大気再解析と高解像度大気大循環モデルによるアンサンブル実験の結果を用いて、ENSOの季節的な遷移と夏季東アジアにおける大気の川の活動の関係を調査する。先行する冬季のエルニーニョから夏季のラニーニャへと早く遷移した年には、エルニーニョが持続または衰退した年に比べ、西部北太平洋の下層の高気圧偏差がより北へ拡大することにより、東アジア北部でより多くの大気の川が通過する。この高気圧の北への拡大は、海洋大陸と赤道太平洋上の凝結熱加熱偏差に対する大気の定常応答と整合する。再解析と大気大循環モデル実験とでは、中緯度の大気の川と循環の偏差が生じる位置が南北にずれており、これにはサンプル数が限られることとモデルバイアスが影響している可能性があり、東アジアにおける大気の川に関連した地域ごとの自然災害リスクの季節的な予測には課題が残されていることを示唆している。

著者

ISHIOKA Keiichi

出版者

公益社団法人 日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2023-008, (Released:2022-11-17)

被引用文献数

1

---

Inspired by the detection of the Pekeris mode of atmospheric free oscillations by a recent study, high-accuracy numerical calculations of the problem of determining the equivalent depth of atmospheric free oscillations are performed. Here, the computational method is largely based on a previous study, but with modifications to improve the accuracy of the calculation. Two equivalent depths are found, with values of 9.9 km and 6.6 km. The former corresponds to the Lamb mode and the latter corresponds to the Pekeris mode. These values deviate from those obtained in the previous study, especially for the Pekeris mode. The causes of this discrepancy is discussed, as well as the correspondence between the equivalent depths obtained in this study and that of the Pekeris mode detected in the recent study.

著者

NAKANO Masuo CHEN Ying-Wen SATOH Masaki

出版者

公益社団法人 日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2023-013, (Released:2023-02-24)

---

Typhoon Krosa (2019) formed in the eastern part of the Philippine Sea and ∼ 1400 km east of another typhoon Lekima on 6 August and made landfall in the western part of Japan's mainland on 15 August. The operational global model forecasts, which were initialized just after Krosa's formation, showed a very large uncertainty and totally failed to predict the actual track of Krosa. In this study, we investigated the causes of this large uncertainty through 101-member ensemble forecast experiments by using a 28-km mesh global nonhydrostatic model. The experiments initialized at 1200 UTC 6 August, showed a large uncertainty. An ensemble-based lagged correlation analysis indicated that the western North Pacific Subtropical High (WNPSH) retreated further east in the members with large track forecast errors than in the members with small errors. For the members with a large track forecast error for Krosa, Krosa and Lekima approached each other by 250 km and Krosa moved northward faster than the observation in 36 hours from the initialization time. For the members with a small track forecast error for Krosa, two typhoons approached each other by only 50 km, and the northward moving speed was comparable with that of the observation. The typhoon-center relative composite analysis exhibited that at the initialization time, the members with a large Krosa track forecast error had a larger horizontal size of Krosa and the difference in Krosa's size was kept during the forecast period. This difference in size led to a stronger interaction between the two typhoons and retreatment of the WNPSH, thus resulting in a fast northward moving speed for the members with a large Krosa track error.

著者

Yousuke YAMASHITA Hideharu AKIYOSHI Theodore G. SHEPHERD 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.93, no.6, pp.629-644, 2015 (Released:2016-01-13)

参考文献数

52

被引用文献数

7 9

---

The combined influences of the westerly phase of the quasi-biennial oscillation (QBO-W) and solar maximum (Smax) conditions on the Northern Hemisphere extratropical winter circulation are investigated using reanalysis data and Center for Climate System Research/National Institute for Environmental Studies chemistry climate model (CCM) simulations. The composite analysis for the reanalysis data indicates strengthened polar vortex in December followed by weakened polar vortex in February-March for QBO-W during Smax (QBO-W/Smax) conditions. This relationship need not be specific to QBO-W/Smax conditions but may just require strengthened vortex in December, which is more likely under QBO-W/Smax. Both the reanalysis data and CCM simulations suggest that dynamical processes of planetary wave propagation and meridional circulation related to QBO-W around polar vortex in December are similar in character to those related to Smax; furthermore, both processes may work in concert to maintain stronger vortex during QBO-W/Smax. In the reanalysis data, the strengthened polar vortex in December is associated with the development of north-south dipole tropospheric anomaly in the Atlantic sector similar to the North Atlantic oscillation (NAO) during December-January. The structure of the north-south dipole anomaly has zonal wavenumber 1 (WN1) component, where the longitude of anomalous ridge overlaps with that of climatological ridge in the North Atlantic in January. This implies amplification of the WN1 wave and results in the enhancement of the upward WN1 propagation from troposphere into stratosphere in January, leading to the weakened polar vortex in February-March. Although WN2 waves do not play a direct role in forcing the stratospheric vortex evolution, their tropospheric response to QBO-W/Smax conditions appears to be related to the maintenance of the NAO-like anomaly in the high-latitude troposphere in January. These results may provide a possible explanation for the mechanisms underlying the seasonal evolution of wintertime polar vortex anomalies during QBO-W/Smax conditions and the role of troposphere in this evolution.

著者

LI Linhui LI Shuanglin

出版者

公益社団法人 日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2022-042, (Released:2022-07-07)

---

Twentieth-century atmospheric reanalysis datasets are substantially important for understanding climate in the early era of the century. This paper first compares two sets of the twentieth-century atmospheric reanalyses, the NOAA-CIRES-DOE 20th Century Reanalysis Version 3 (20CRv3) and the ECMWF 20th century reanalysis (ERA20C), as far as the summer low-level cross-equatorial flows (CEFs) over the Asian-Australian monsoon region are concerned. The results show evident regional differences in intensity of individual branches of CEFs between the two reanalyses, in spite of an overall agreement in climatological seasonal mean and variability. At interannual timescale, significant differences are seen prior to 1925 and in the 1940s. During the two periods there are often opposite variations in Somali CEF in the two datasets, along with obvious different amplitudes (variances) in the Bay of Bengal (BOB) and Australian CEFs. At interdecadal timescale, the two datasets have different periodicities in Somali CEF, and have a greater fluctuation of BOB CEF after 1925 in ERA20C than 20CRv3, as well as an opposite decadal variation in the Australian CEF prior to 1940 and in the 1960s. As for the long-term trend, both the Somali and BOB CEFs exhibit intensification in both the datasets, but the intensification amplitude is bigger in 20CRv3 than ERA20C for Somali CEF; the Australian CEF exhibits a weakening trend in both the datasets, but is less evident in 20CRv3. To figure out which of the two datasets is relatively more reliable, the observed cross-equatorial meridional gradient of sea-level pressure index and the Indian summer monsoon rainfall index, which both have longer instrumental records, are used as benchmarks to validate the CEFs in view of their close connections. The results suggest that ERA20C is more reliable, and thus more suitable for investigating decadal climate variability of the 20th century across the hemispheres.

著者

WU Jing KUROSAKI Yasunori SEKIYAMA Tsuyoshi Thomas MAKI Takashi

出版者

公益社団法人 日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2023-004, (Released:2022-10-27)

---

In drylands, the dry vegetation coverage affects dust occurrence by modulating threshold friction velocity (or wind speed) for dust emission. However, there has been little research into quantifying the effect of dry vegetation coverage on dust occurrence. This study investigated spatial and temporal variations of dust occurrence and three definitions of strong wind frequency over the Gobi Desert and surrounding regions in March and April, months when dust occurrence is frequent, during 2001-2021. We evaluated the effects of variations in dry vegetation on dust occurrence by using the threat scores of forecasted dust occurrences for each strong wind definition. Our results indicate that dry vegetation, which was derived from the MODIS Soil Tillage Index, affects dust occurrence more remarkably in April than in March. In March, land surface parameters such as soil freeze-thaw and snow cover, in addition to dry vegetation coverage, should be considered to explain dust variations in that month. However, use of the threshold wind speed estimated from dry vegetation coverage improved the prediction accuracy of dust occurrence in April. Therefore, we propose that the dry vegetation coverage is a key factor controlling dust occurrence variations in April. The findings imply that estimation of dry vegetation coverage should be applied to dust models.

著者

SATO Kazutoshi KAMEDA Takao SHIRAKAWA Tatsuo

出版者

公益社団法人 日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2022-045, (Released:2022-07-22)

---

Iwamizawa on the Sea of Japan side of Hokkaido is one of the cities in Japan that experience frequent heavy snowfall events. Warm surface-layer ocean anomalies over the Sea of Japan can induce heavy snowfall over the Sea of Japan side of Japan; however, the relationship between ocean temperature over the northern Sea of Japan and snowfall events at Iwamizawa remains uncertain. This study used reanalysis data to investigate atmospheric and oceanic circulation anomalies associated with each anomalous heavy snowfall winter month at Iwamizawa. During all anomalous snowfall winter months at Iwamizawa, a cold air anomaly with northwesterly winds existed over the Far East that was associated with a dipole pattern with anticyclone anomalies over the north coast of the Eurasian Continent and cyclonic anomalies extending zonally over the Far East and northern Pacific Ocean. The surface cold air temperature and strong wind speed anomalies are major factor for anomalous upward turbulent heat flux over the northern Sea of Japan during all anomalous snowfall winter months at Iwamizawa. Additionally, during anomalous snowfall January, warm surface-layer ocean anomaly over the northern Sea of Japan, which preceded the heavy snowfall events at Iwamizawa by two months, has an important role in upward turbulent heat flux anomaly. This preceding warm ocean temperature anomaly was associated with a strong Tsushima Warm Current anomaly. Results showed that warm surface-layer ocean anomaly over the northern Sea of Japan that precedes anomalous cold advection from the Eurasian Continent has also large impact on producing heavy snowfall events over western Hokkaido coastal regions near Iwamizawa in January.

著者

HSIEH Min-Ken CHEN Yu-Wen CHEN Yi-Chun WU Chien-Ming

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.2022-028, (Released:2022-03-10)

被引用文献数

3

---

We applied tracer transport simulations using Taiwan vector vorticity equation cloud-resolving model (TaiwanVVM) to evaluate the effects of the local circulation associated with the lee vortex and the planetary boundary layer development on the transport and accumulation of the pollutants on a diurnal time scale in central Taiwan. The wind directions of crucial synoptic northeast monsoon are idealized as the initial conditions of the simulations to examine the impact of the lee vortex on the pollutants transport. The primary local non-traffic emission sources are taken as the tracer emission sites so that the experiment results could be a good proxy of the realistic scenarios. With the local circulation over complex topography being resolved explicitly, the impact of the boundary layer development on the tracer transport of the Puli basin is discussed. The simulation results clarify the contribution of the sea breeze and the lee vortex to the tracer transport in central Taiwan. We conclude that high tracer concentration at Puli at night is due to the tracer being trapped by the thinning of the mixed layer depth in the evening. The sensitivity of the local tracer transport to the change of the synoptic wind direction shows that under northeasterly due east (due north) environment, the pollutant transports from the southern source (northern source) of central Taiwan are most likely to induce high concentration in Puli at night. This is the first study to distinguish the contribution of the sea breeze and the lee vortex in pollutants transport in Taiwan. The results obtained from idealized experiments provide the possible mechanism of pollutants transport, which could be taken as an insight to interpret the observations and guide the design of field experiment to further establish the fundamental principles of the pollution transports in central Taiwan.

著者

YAMADA Shunsuke KUSAKA Hiroyuki

出版者

公益社団法人 日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2022-039, (Released:2022-06-21)

被引用文献数

1

---

We examined the essential features and formation mechanism of the strong local “Suzuka-oroshi” winds, which are located leeward of the Suzuka Mountains in Japan. This area features a favorable topography for downslope windstorms. Climatological analysis revealed that Suzuka-oroshi mainly occurred after an extratropical cyclone with a cold front and passed the Sea of Japan (55 % of all occurrences). Additionally, inversion layers (1-5 km level) were observed in 74 % of cases. Climatological analysis using spatially dense observational data showed that the strongest winds tended to blow in the northern part of the plain on the leeward side. Numerical simulations for one case by the Weather Research and Forecasting (WRF) model with 1 km grid increment supported this finding. Simulation results with and without the Suzuka Mountains demonstrated that the strong Suzuka-oroshi in the northern part of the plain comprised downslope windstorms with transition of flow regime (internal Froude number was less than 1.0 at the windward of mountains and larger than 1.0 above the leeward slope). Additionally, differences in height of the mountains between the north and south parts results in the greater wind speed in the northern parts compared to the southern parts.

著者

YANASE Wataru ARAKI Kentaro WADA Akiyoshi SHIMADA Udai HAYASHI Masahiro HORINOUCHI Takeshi

出版者

公益社団法人 日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2022-041, (Released:2022-06-30)

被引用文献数

5

---

Torrential rain in Typhoon Hagibis caused a devastating disaster in Japan in October 2019. The precipitation was concentrated in the northern half of Hagibis during extratropical transition (ET). To elucidate the mechanisms of this asymmetric precipitation, synoptic- and meso-scale processes were analyzed mainly using the Japan Meteorological Agency Non-Hydrostatic Model. The present study demonstrates that the asymmetric processes were different depending on the ET stages. When Hagibis was close to the baroclinic zone at middle latitudes around 12 October (the frontal stage), heavy precipitation in the northeastern part of Hagibis was attributed to warm frontogenesis and a quasi-geostrophic ascent, as reported in many previous studies. In contrast, when Hagibis was moderately distant from the baroclinic zone around 11 October (the prefrontal stage), heavy precipitation in the northern part occurred in slantwise northward ascending motion in the outer region. This slantwise motion developed in a region with strong westerly vertical shear, which was enhanced between Hagibis and a westerly jet stream. Based on the analyses of potential vorticity and absolute angular momentum, this region was characterized by reduced moist symmetric stability in the lower and middle troposphere accompanied by inertial instability in the upper troposphere and conditional instability in the lower troposphere. These results provide additional insights into the time evolution of asymmetric processes during ET in the absence of a distinct upper-tropospheric trough, particularly the slantwise motion in the prefrontal stage.

著者

DATT Ishan CAMARGO Suzana J. SOBEL Adam H. MCTAGGART-COWAN Ron WANG Zhuo

出版者

公益社団法人 日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2022-037, (Released:2022-05-13)

被引用文献数

3

---

A significant fraction of tropical cyclones develop in baroclinic environments, following tropical cyclogenesis “pathways” that are characterized by dynamical processes often associated with higher latitudes. This study investigates whether such storms are more likely to undergo subsequent extratropical transition than those that develop in more typical, non-baroclinic environments. We consider tropical cyclones globally in the period 1979-2011 using best-track datasets, and define the genesis pathway of each storm using McTaggart-Cowan's classification: non-baroclinic, low-level baroclinic, trough-induced, weak and strong tropical transition. In each basin, we analyze the total number and the fraction of storms that underwent extratropical transition, their seasonality, and storm tracks, according to their genesis pathways. The relationship between the pathways and extratropical transition is statistically significant in the North Atlantic and Western North Pacific, where the strong tropical transition and the trough-induced pathways have a significantly greater extratropical fraction compared to all other pathways, respectively. Latitude, longitude and environmental factors such as sea surface temperature and vertical shear were further analyzed to explore whether storms in these pathways happen to be in environments conducive to extratropical transition, or whether a “memory” of the genesis pathway persists throughout the storm life cycle. After controlling for genesis latitude, the relationship between the strong tropical transition and trough induced pathways, and extratropical transition occurrence remains statistically significant, implying a lasting effect from the pathway on the probability of an eventual extratropical transition.