著者

釜江 陽一 植田 宏昭 井上 知栄 三寺 史夫

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.101, no.2, pp.125-137, 2023 (Released:2023-03-07)

参考文献数

31

被引用文献数

1

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冬季オホーツク海における海氷分布は、極東域および北太平洋域の大気と強く相互作用する。先行研究は、オホーツク海海氷面積の年々変動は広域の大気循環と対応することを指摘している。一方で、オホーツク海における海氷面積の数日から1週間程度の時間スケールでの急激な変動に対応する大気現象については明らかにされていない。本研究では、日ごとの高解像度海洋再解析データを用いることで、オホーツク海海氷密接度の急激な減少イベントをもたらす大気循環について調査した。1993年から2019年にかけて、海氷急減イベントを合計21事例抽出した。急減イベントに共通した大気循環の特徴として、オホーツク海南部における発達した温帯低気圧とベーリング海北部における高気圧偏差、およびその間の強い地表の南東風が確認された。海氷の季節的な張り出しを左右する気候学的な西風とは逆向きである強い南東風は、オホーツク海海氷密接度の急減をもたらす。オホーツク海北部と中部で起こる海氷の急減は、海氷の移流と東風に伴う海氷融解によって起こる。東へと移動する温帯低気圧は、海氷密接度の急減と北太平洋北部の海面気圧の低下をもたらし、結果としてオホーツク海海氷密接度の変動とアリューシャン低気圧の強度の変動の間には時間差が存在する。

著者

倉持 将也 植田 宏昭

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.101, no.1, pp.21-37, 2023 (Released:2023-02-07)

参考文献数

56

被引用文献数

1

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熱帯インド洋-西部太平洋域の対流活動に関連して、2020/21年冬季の前半と後半の間で東アジアの気温偏差は負から正へと転じた。平年より気温が低かった2020/21年冬季の前半では、対流圏上層のチベット高原南東部と日本上空にそれぞれ高気圧性と低気圧性の循環偏差対が発現していた。この波列パターンは、東インド洋から南シナ海上で強化された熱帯積雲対流に励起されたロスビー波の伝播を示し、本研究で新たに東南アジア-日本(Southeast Asia–Japan: SAJ)パターンと呼称する。一方、2020/21年冬季の後半では、活発な対流活動域は東方のフィリピン海へ移動し、上層高気圧偏差もまた日本の南へと位置を変えた。このような循環偏差は西太平洋(western Pacific: WP)的なパターンとして認識でき、東アジアの高温偏差をもたらした。SAJパターンに関連するチベット高原南東部の高気圧性循環偏差と南シナ海の対流強化の関係は、パターンが顕著に発現した月を抽出した合成解析でも統計的に有意であることが確認された。一方、WP的なパターンの半分の場合では、フィリピン海での対流の活発化を伴っていた。これらの異なる2つの循環偏差は、線形傾圧モデルに南シナ海とフィリピン海に熱源を与えることでそれぞれ再現された。さらに渦度収支解析から、チベット高原南東への気候学的な上層風の収束が SAJパターンの空間的な位相固定性において重要であることが示唆された。

著者

ISHIJIMA Kentaro TSUBOI Kazuhiro MATSUEDA Hidekazu TANAKA Taichu Yasumichi MAKI Takashi NAKAMURA Takashi NIWA Yosuke HIRAO Shigekazu

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.2022-017, (Released:2021-12-10)

被引用文献数

2

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Temporal variations of atmospheric radon-222 (222Rn) observed at four Japan Meteorological Agency stations in Japan by the Meteorological Research Institute were analyzed using an on-line Global Spectral Atmosphere Model–Transport Model (GSAM-TM). Monthly and diurnal variations, and a series of synoptic high-222Rn events were extracted from 5-12 years of 222Rn observations during 2007-2019. Observed seasonal patterns of winter maxima and summer minima, driven mainly by monsoons, were well reproduced by the GSAM-TM based on existing 222Rn emission inventories, but their absolute values were generally underestimated, indicating that our understanding of 222Rn emission processes in East Asia is lacking. The high-resolution model (∼ 60 km mesh) demonstrated that observed consecutive high-222Rn peaks at several-hour timescales were caused by two 222Rn streams from different regions and were not well resolved by the low-resolution model (∼ 200 km mesh). GSAM-TM simulations indicate that such cold-front-driven events are sometimes accompanied by complicated three-dimensional atmospheric structures such as stratospheric intrusion over the front, significantly affecting distributions of atmospheric components. A new calculation approach using hourly 222Rn values normalized to daily means was used to analyze the diurnal 222Rn cycle, allowing diurnal cycles in winter to be extracted from 222Rn data that are highly variable due to sporadic continental 222Rn outflows, which tend to obscure the diurnal variations. Normalized diurnal cycles of 222Rn in winter are consistent between observations and model simulations, and seem to be driven mainly by diurnal variations of planetary-boundary-layer height (PBLH). These results indicate that 222Rn in the near-surface atmosphere, transported from remote source regions, could vary diurnally by up to 10 % of the daily mean owing mainly to local PBLH variations, even without significant local 222Rn emissions.

著者

石川 高見

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.4, no.6, pp.137-146, 1926-09-04 (Released:2009-02-05)

被引用文献数

2 4

著者

村松 照男

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.64, no.6, pp.913-921, 1986 (Released:2007-10-19)

参考文献数

17

被引用文献数

33 37

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明瞭な多角形眼が台風8019のPPIエコー上で観測された。直径30kmの眼は4角形から6角形までその形を変え,台風眼中心に対して半時計回りに回転していた。回転周期は5-6角形で41-43分,4角形で47-50分と回転速度が速い(周期が短い)ほど多角形の角数が増加した。多角形眼は形状を変えながら約15時間観測され,5角形が最も頻度が高く,111分という長寿命であった。逆に,4角形は不安定で寿命は12分前後で,頻度も最もひくく,3角形や7角形は観測されなかった。多角化は眼の壁雲の最も内側の数km幅の狭い領域で起こっていた。眼の中の小エコーセルの追跡の結果,小エコーセル(眼の中の気塊)は等角速度運動をしており,多角形に変形した部分はそれよりやや速い速度で回転していた。多角形眼の現象は水平シヤーの大きい,眼の中の下降流と眼の壁雲の上昇流領域の境界で起こり,境界面の不安定を示唆する多角形の各辺の波打ち現象がしばしば見られた。この現象は発達した台風(ハリケーン)で明瞭な二重眼構造をもつ,中心気圧940mb前後,眼径が30-50kmの場合で多く見られた。

著者

平田 徳太郎

出版者

公益社団法人 日本気象学会

雑誌

氣象集誌. 第1輯 (ISSN:00261165)

巻号頁・発行日

vol.41, no.11, pp.585-592, 1922-11-10 (Released:2009-02-05)

著者

NAKAMURA Shingo KUSAKA Hiroyuki SATO Ryogo SATO Takuto

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.2022-030, (Released:2022-04-08)

被引用文献数

4

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This study assesses heatstroke risk in the near future (2031-2050) under RCP8.5 scenario. The developed model is based on a generalized linear model with the number of ambulance transport due to heatstroke (hereafter the patients with heatstroke) as the explained variable and the daily maximum temperature or Wet-Bulb Globe Temperature (WBGT) as the explanatory variable. With the model based on the daily maximum temperature, we performed the projection of the patients with heatstroke in case of considering only climate change (Case 1), climate change and population dynamics (Case 2), and climate change, population dynamics, and long-term heat acclimatization (Case 3). In Case 2, the number of patients with heatstroke in the near future will be 2.3 times higher than that in the baseline period (1981-2000) on average nationwide. The number of future patients with heatstroke in Case 2 is about 10 % larger than that in Case 1 on average nationwide despite of population decline. This is due to the increase in the number of elderly people from the baseline period to the near future. However, there were 21 prefectures where the number of patients in Case 2 is smaller compared to Case 1. Comparing the results from Cases 1 and 3 reveals that the number of patients with heatstroke could be reduced by about 60 % nationwide by acquiring heat tolerance and changing lifestyles. Notably, given the lifestyle changes represented by the widespread use of air conditioners, the number of patients with heatstroke in the near future was lower than that of the baseline period in some areas. In other words, lifestyle changes can be an important adaptation to the risk of heatstroke emergency. All of the above results were also confirmed in the prediction model with WBGT as the explanatory variable.

著者

ISLAM Md. Rezuanul SATOH Masaki TAKAGI 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.2022-024, (Released:2022-02-03)

被引用文献数

6

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This study investigated tidal records and landfall tropical cyclone (TC) best tracks from 1980 to 2019 to determine changes in storm surge heights in coastal regions of Central Japan, including Tokyo. The results indicate that annual mean storm surge heights have increased in the last 20 years (2000-2019) compared to those in 1980-1999, and that these changes are noteworthy, particularly in Tokyo Bay. TC wind intensity and size during landfall time frame have become stronger and larger, respectively, corresponding to increasing storm surge magnitudes from 1980 to 2019. The increased occurrence frequency of TCs with more northeastward tracks is another factor that may have contributed to the increased surge hazards around Tokyo. Additionally, a positive correlation between surge heights and a hazard index supports these statistical findings. Japan central coast will likely experience increasing numbers of extreme storm surge events in the future, if, the current increasing tendency continues.

著者

TAKEMURA Kazuto MUKOUGAWA Hitoshi

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.2020-061, (Released:2020-08-03)

被引用文献数

5

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To reveal a maintenance mechanism for Rossby wave breaking (RWB) east of Japan and Pacific-Japan (PJ) pattern, which are triggered due to quasi-stationary Rossby wave propagation along the Asian jet, the past 44 RWB cases east of Japan is analyzed using a reanalysis dataset. A comparison between the composites of 7 persistent and 7 non-persistent cases, which are classified based on duration of the RWB and the PJ pattern, indicates that the persistent case shows the stronger and longer-lived quasi-stationary Rossby wave propagation along the Asian jet. The subsequent stronger RWB in the persistent case causes the consequential formation of the more enhanced PJ pattern, through the stronger high potential vorticity intrusion toward the subtropical western North Pacific. The persistent case further shows a persistent northward tilting vertical structure of the anomalous anticyclone east of Japan, accompanied by the enhanced anomalous warm air advection in the lower to middle troposphere north of the anomalously extended North Pacific Subtropical High associated with the PJ pattern. The Q-vector diagnosis and partial correlation analysis indicate that the anomalous warm air advection in the middle troposphere is closely associated with dynamically induced anomalous ascent from Japan to the east by an adiabatic process. Enhanced anomalous moisture flux convergence from Japan to the east, which is due to moisture inflow along the fringe of North Pacific Subtropical High from the subtropical western North Pacific, also causes the anomalous ascent over the region by a diabatic process. A simple correlation analysis indicates nearly equivalent associations of the adiabatic and diabatic factors with the anomalous ascent. The anomalous ascent contributes to the enhanced and persistent RWB, through negative vorticity tendency due to vortex squashing in the upper troposphere, which further contributes to the enhanced and persistent PJ pattern in the persistent case.

著者

KUDO Atsushi

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.2022-011, (Released:2021-11-02)

被引用文献数

2

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The Japan Meteorological Agency (JMA) operates gridded temperature guidance to predict two-dimensional (2D) snowfall amounts and precipitation types e.g., rain and snow because surface temperature is one of the key elements to predict them. Operational temperature guidance is based on the Kalman filter, which uses temperature observation and numerical weather prediction (NWP) outputs only around observation sites. Correcting a temperature field when NWP models incorrectly predict a front's location or when observed temperatures are extremely cold or hot has been challenging.  In this study, an encoder–decoder-based convolutional neural network (CNN) has been proposed to predict gridded temperatures at the surface around the Kanto region in Japan. Verification results showed that the proposed model greatly improves the operational guidance and can correct NWP model biases, such as a positional error of fronts and extreme temperatures.

著者

Syukuro MANABE Ronald J. STOUFFER

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.85B, pp.385-403, 2007 (Released:2007-10-26)

参考文献数

54

被引用文献数

12 16

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Based upon the results obtained from coupled ocean-atmosphere models of various complexities, this review explores the role of ocean in global warming. It shows that ocean can play a major role in delaying global warming and shaping its geographical distribution. It is very encouraging that many features of simulated change of the climate system have begun to agree with observation. However, it has been difficult to confirm the apparent agreement because the density and frequency of the observation are insufficient in many oceanic region of the world, in particular, in the Circumpolar Ocean of the Southern Hemisphere. It is therefore essential to intensify our effort to monitor not only at the surface but also in the subsurface layers of oceans.

著者

Wei-Ting CHEN Chien-Ming WU Wei-Ming TSAI Peng-Jen CHEN Po-Yen CHEN

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.97, no.6, pp.1155-1171, 2019 (Released:2019-11-29)

参考文献数

61

被引用文献数

3

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In this study, the climatological characteristics of object-based precipitation systems (OPSs) and moisture development are analyzed over the South China Sea (SCS) during the sharp transition of the summer monsoon onset. The satellite-observed statistics of the OPSs showed that over the 20-day pre-onset period, OPSs of small (< 100 km) to medium size (100-300 km) are active over the lands surrounding the SCS. The pre-onset composite mean shows a basin-scale (∼ 1000 km) local circulation with anomalous subsidence over the ocean, and ocean convection is mostly suppressed. Over the 20-day post-onset period, large (> 300 km) OPSs develop over the coastal ocean and contribute to over 60 % of the total precipitation. The number of large OPSs observed significantly increases along with the sharp moisture buildup within 10 days after the onset. The moisture budget suggests that the local contribution from convective vertical mixing is the major moisture source during the first pentad after the onset. The relationship between moisture buildup and convection organization is then examined using a set of idealized cloud-resolving model (CRM) experiments, with a land–ocean configuration approximating the SCS basin. The CRM appropriately represents the observed development of coastal convection. In the no-shear environment, a strong basin-scale circulation is formed, which suppresses the ocean moisture development. When large-scale vertical wind shear is imposed to represent the changes of large-scale circulation during the onset pentad, organized convection systems are increased over the coastal ocean and propagate toward the open ocean, accompanied by fast ocean moistening within 5-10 days.

著者

UNUMA Takashi TAKEMI Tetsuya

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.2021-009, (Released:2020-11-30)

被引用文献数

8

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In July of 2017 and 2018, heavy rainfall events occurred, leading to significant damage in Japan. This study investigated the rainfall characteristics and environmental conditions for these heavy rainfall events using rain intensity data from operational weather radars and mesoscale analysis data. An automatic algorithm was developed to categorize precipitating cloud systems into five types, one with weaker rainfall (less than 10 mm h−1) and four with stronger rainfall (greater than or equal to 10 mm h−1), i.e., quasi-stationary convective clusters (QSCCs), propagating convective clusters (PCCs), short-lived convective clusters (SLCC), and other convective but unorganized rainfall. The rainfall amount due to the weaker rainfall was found to dominate the total rainfall in most of the analysis region; however, the contribution from the stronger rainfall types became larger than that from the weaker rainfall type in regions that experienced heavy rainfall. Among the stronger rain types, SLCCs dominate over the rainfall contributions from QSCCs or PCCs, whereas rainfalls from convective but unorganized systems are very minor. It was emphasized that the contribution from stronger rains due to organized systems with areas of 200 km2 plays a major role in regions with significant amounts of rainfall during the heavy rainfall events examined here. The examination of the environmental conditions for the development of each system demonstrated that, from the viewpoint of moisture content, the stability conditions were more unstable in 2018 than in 2017. There is also a clear linkage in the time series between rainfall types and the environmental properties of precipitable water and vertical shear. It was found that both the column moisture content and the middle-to-upper-level relative humidity characterize the environmental conditions for the occurrence of the present heavy rainfall events. Features of the rainfall types and their environmental conditions were compared with the QSCC climatology.

著者

MIYAMOTO Yoshiaki SATO Yousuke NISHIZAWA Seiya YASHIRO Hisashi SEIKI Tatsuya NODA Akira T.

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.2020-051, (Released:2020-07-09)

被引用文献数

1

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This study proposes a new energy balance model to determine the cloud fraction of low-level clouds. It is assumed that the horizontal cloud field consists of several individual cloud cells having a similar structure. Using a high–resolution simulation dataset with a wide numerical domain, we conducted an energy budget analysis. It is shown that the energy injected into the domain by surface flux is approximately balanced with the energy loss due to radiation and advection due to large–scale motion. The analysis of cloud cells within the simulated cloud field showed that the cloud field consists of a number of cloud cells with similar structures. We developed a simple model for the cloud fraction from the energy conservation equation. The cloud fraction diagnosed using the model developed in this study was able to quantitatively capture the simulated cloud fraction.

著者

OHNO Tomoki NODA Akira T. 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.2020-054, (Released:2020-07-20)

被引用文献数

7

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The impacts of the saturation adjustment type approach to sub-grid-scale (SGS) ice clouds in a turbulent closure scheme on the high clouds and their response to global warming were investigated based on the radiative–convective equilibrium experiments (RCEs). This was motivated by the fact that the time scale of ice condensation is several orders of magnitude longer than that for liquid water. The RCEs were conducted with uniform sea surface temperatures over the spherical domain for the Earth's radius without rotation using an explicit cloud microphysics and a non-hydrostatic icosahedral atmospheric model. This study revealed that suppressing the phase change effect associated with the SGS ice condensation on the buoyancy of the SGS turbulence could cause approximately a 20 % reduction of the total high cloud covers and a significantly different response of high cloud amounts to global warming due to the change in static stability near high clouds, which leads to weaker vertical heat transport at a sub-grid scale there. Since the typical value of the time scale of the ice-phase cloud is much longer than that for liquid water and the ice supersaturation is in general, using the saturation adjustment type approach for SGS ice clouds could lead to an overestimation of the effect of ice condensation for the turbulent mixing and model biases in simulations with both cloud resolving models and general circulation models. The present result underlines the critical nature of the treatment of SGS ice clouds in turbulence schemes which reflects a realistic ice condensation time scale not only for a better representation of high clouds in the current climate but for an improved projection of changes of high clouds due to global warming.

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

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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

NAOE Hiroaki MATSUMOTO Takanori UENO Keisuke MAKI Takashi DEUSHI Makoto TAKEUCHI Ayako

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.2020-019, (Released:2020-02-03)

被引用文献数

1

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This study constructs a merged total column ozone (TCO) dataset using 20 available satellite Level 2 TCO (L2SAT) datasets over 40 years from 1978 to 2017. The individual 20 datasets and the merged TCO dataset are corrected against ground-based Dobson and Brewer spectrophotometer TCO (GD) measurements. Two bias correction methods are used: simple linear regression (SLR) as a function of time and multiple linear regression (MLR) as a function of time, solar zenith angle, and effective ozone temperature. All of the satellite datasets are consistent with GD within ±2-3%, except for some degraded data from the Total Ozone Mapping Spectrometer/Earth Probe during a period of degraded calibration and from the Ozone Mapping and Profiling Suite (OMPS) provided from NOAA at an early stage of measurements. OMPS data provided from NASA show fairly stable L2SAT-GD differences. The Global Ozone Monitoring Experiment/MetOp-A and -B datasets show abrupt changes of approximately 8 DU coincident with the change of retrieval algorithm. For the TCO merged datasets created by averaging all coincident data located within a grid cell from the 20 satellite-borne TCO datasets, the differences between corrected and uncorrected TCOs by MLR are generally positive at lower latitudes where the bias correction increases TCO because of low effective ozone temperature. In the trend analysis, the difference between corrected and uncorrected TCO trends by MLR shows clear seasonal and latitudinal dependency, whereas such seasonal and latitudinal dependency is lost by SLR. The root mean square difference of L2SAT-GD for the uncorrected merged datasets, 8.6 DU, is reduced to 8.4 DU after correction using SLR and MLR. Therefore, the empirically corrected merged TCO datasets that are converted into time-series homogenization with high temporal-resolution are suitable as a data source for trend analyses as well as assimilation for long-term reanalysis.

著者

CHEN Wei GUAN Zhaoyong YANG Huadong XU Qi

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.2020-014, (Released:2019-12-08)

被引用文献数

6

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The East Asian summer monsoon (EASM) and the Australian winter monsoon (AWM) are two important components of the Asian-Australian monsoon system during boreal summer. The simultaneous variations of these two monsoons would have remarkable impacts on climate in the Asian-Australian region. Using the reanalysis datasets, we investigate the mechanisms of variation and impacts of East Asian-Australian Monsoons (EAAMs). The singular value decomposition (SVD) is performed of the June-July-August (JJA) mean anomalous zonal wind for AWM as left field and JJA mean anomalous meridional wind for EASM as the right field after both El Niño-Southern Oscillation (ENSO) and India Ocean Dipole (IOD) signals are filtered out. Our results demonstrate that AWM and EASM are closely related to each other as revealed by the first leading SVD mode. The anomalously strong (weak) EAAMs correspond to anomalously strong (weak) AWM and EASM to the south of 30°N. When EAAMs are anomalously strong, cold sea surface temperature anomaly (SSTA) appears in regions near northern and northeastern coasts of Australia whereas the warmer SSTA appears in the northwestern tropical Pacific and South China Sea. The colder SSTA is associated with the upwelling of cold water from below induced by equatorial easterly anomalies, reinforcing the anticyclonic circulation over Australia through the Matsuno/Gill-type response whereas warm SSTA appears in the northwestern tropical Pacific and South China Sea as a result of oceanic response to the intensified northwest Pacific subtropical anticyclonic circulation. The EASM couples with AWM via the anomalous easterlies near equator in the Maritime Continent (MC) region and the slanted vertical anomalous circulations. In the years with strong EAAMs, precipitation decreases in northern Australia and over areas from the western Pacific to Bohai Sea and Yellow Sea of China. Meanwhile, the western MC and the southeastern China experience more than normal precipitation.

著者

YOKOYAMA Chie TSUJI Hiroki TAKAYABU Yukari N.

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.2020-013, (Released:2019-11-30)

被引用文献数

31

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In this study, we examined the characteristics of a rainfall system that brought heavy rainfall to a broad portion of western Japan on July 5-8, 2018 and the role played by an upper-tropospheric trough which stayed to the rear of the extensive rainfall area during the event. The Dual-frequency Precipitation Radar onboard the core satellite of the Global Precipitation Measurement revealed the significant contribution of rainfall with its top below 10 km, the broad spatial extent covered by stratiform rainfall, and the presence of convective rainfall embedded in the large stratiform rainfall area. These features are characteristic of well-organized rainfall systems. Based on the analysis of meteorological data, large-scale environmental conditions related to the event were found to be relatively stable and very humid throughout most of the troposphere, compared to the climatology. This large-scale environment, which is consistent with previous statistical results for extreme rainfall events, was present across an extensive area of Japan. We found that the trough played an important role in maintaining an environment favorable for the organization of rainfall. Dynamical ascent associated with the trough acted to produce vertical moisture flux convergence in the mid-troposphere and upper troposphere, and moistened most of the troposphere in conjunction with horizontal moisture flux convergence. Humid conditions in the mid- to lower troposphere enhanced the development of deep convection when the lower troposphere was convectively unstable. Once deep convection was promoted in this way, convection itself could moisten the mid- to upper troposphere further through diabatic ascent, thereby loading the free troposphere with moisture. This synergy between the dynamical effect and the diabatic effect enhanced the conditions that allowed for a well-organized rainfall system that produced very heavy rainfall over a large portion of Japan.

著者

AWAZU Taeka OTSUKA Shigenori 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.2019-066, (Released:2019-09-22)

被引用文献数

1

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This paper proposes a new verification metric that can evaluate location errors and shapes of rainfall areas simultaneously: the Pattern Similarity Index (PSI). Pixel-by-pixel verification methods such as the threat score and root mean squared error have difficulties in evaluating location errors and shapes of rainfall areas, and in evaluating small rainfall areas. To address these difficulties, various object-based methods have been developed. However, object-based methods tend to be complicated and computationally expensive. Therefore, PSI adopts a simpler, computationally more efficient algorithm as follows. First, bounding rectangles of individual rainfall areas are computed, and neighboring rectangles are combined so that they are treated as a single precipitation system to mimic the human recognition. Next, shape parameters are computed for each integrated bounding rectangle. For each pair of the observed and forecasted rainfall areas, the location error weighted by the differences of the shape parameters is used as the verification score. If no observed rainfall area with a similar size exists near a forecasted rainfall area, this distance- based score of the forecasted area is set to a large value. The integration method of the bounding rectangle and the precipitation threshold are the only tunable parameters in this method, and we repeat computing the verification score by varying these parameters. The best value is used as the final verification score. Idealized cases showed the ability of PSI to evaluate location errors and differences in the shape parameters. A real case with global precipitation nowcasting showed that the proposed evaluation value increased almost linearly with the forecast time, whereas the threat score and root mean squared error tended to saturate as the forecast time increases, showing a potential advantage of PSI. Comparison with another object-based method revealed the advantage of PSI in its computational efficiency while providing similar verification scores.