著者

LIU Ying Lung TAM Chi-Yung AU-YEUNG Andie Yee Man

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.2022-008, (Released:2021-10-26)

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The role of extratropical forcing on the summertime tropical synoptic-scale disturbances (TSDs) in the western north Pacific has been investigated, by conducting parallel integrations of the Regional Climate Model (RegCM). The suite of experiments consists of a control run (CTRL) with European Centre for Medium Range Forecasts (ECMWF) Reanalysis data as boundary conditions, and an experimental run (EXPT) with the same setting, except that signals with zonal wavenumber > 6 were suppressed at the northern boundary (located at 42°N) of the model domain. Comparison between CTRL and EXPT showed that, without extratropical forcing, there is weaker TSD activity in the June-to-August season, with reduced precipitation over the TSD pathway. Associated with suppressed TSD, southeastward-directed wave activity is also reduced, leading to less active mixed Rossby gravity (MRG) waves in the equatorial western Pacific area. Further analysis revealed that extratropical forcing and associated circulation changes can modulate the TSD wavetrain and its coherence structure, in relation to low-level vorticity in far western north Pacific. In CTRL, west of about 140°E, TSD-related circulation tends to be stronger; in EXPT, vorticity signals and vertical motions are found to be slightly more coherent in the more eastern portion of the TSD wavetrain. The latter enhanced coherency of TSD east of 140°E, from the EXPT simulations, might be due to changes in wave activity transport channelled by modulated upper-level mid-latitude westerlies in EXPT. Our results serve to quantify how extratropical forcing and related general circulation features influence western north Pacific summertime TSD activities. Implications on the understanding of initiation of TSD are discussed.

著者

松尾 敬世 佐粧 純男

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.59, no.1, pp.26-32, 1981 (Released:2007-10-19)

参考文献数

2

被引用文献数

25 33

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輪島,松本,日光測候所の地上気象観測データを使用して,雪片の融解に与える湿度の影響を解析的に調べた。地上で観測される降水のタイプ(雨,雪)は,地上気温のみならず湿度に依存して変化することが明らかになった。降雪は地上気温が0°C以上のかなり高い場合にも見られ,これは低い地上の湿度と関連していた。これらの降雪は,地上の湿度がある臨界値以下であれば必ず見られた。三地点の臨界湿度と地上気温の関係を回帰分析により求めると次のようになった。RHcri=-7.5T+93,輪島 RHcri=-7.3T+96,松本 RHcri=-6.2T+91,日光得られた関係は松尾,佐粧(1981)が計算によって求めた雪片の融解開始時の温•湿度の関係式と良く一致した。

著者

Shinta Seto

出版者

Meteorological Society of Japan

雑誌

SOLA (ISSN:13496476)

巻号頁・発行日

pp.2022-009, (Released:2022-02-10)

被引用文献数

2

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For over 20 years, precipitation measurement has continued with spaceborne radars including the Precipitation Radar (PR) operating at 13.8 GHz on the Tropical Rainfall Measuring Mission and the Ku-band Precipitation Radar (KuPR) operating at 13.6 GHz on the Global Precipitation Measurement mission core satellite. PR and KuPR have essentially the same hardware designs and the same algorithm to make standard products (PRV8 and KuPRV06, respectively). The surface precipitation rate estimates (R) and related variables are statistically compared between PR and KuPR for a common observation area (within 35°N and 35°S) and period (April to September 2014). Due to the difference in sensitivity, the total precipitation amount recorded by KuPR is larger than recorded by PR by approximately 1.3%. For heavy precipitation, PR shows a smaller measured radar reflectivity factor (Zm) and a larger R than KuPR. Zm is affected by the attenuation and it is smaller for PR than KuPR, as the frequency is slightly higher. The attenuation corrected radar reflectivity factor is almost the same for PR and KuPR. However, the adjustment factor is larger for PR, which results in a larger R. Direct comparison between PR and KuPR during matchup cases demonstrates similar results.

著者

Yasutaka Hirockawa Teruyuki Kato

出版者

Meteorological Society of Japan

雑誌

SOLA (ISSN:13496476)

巻号頁・発行日

vol.18, pp.13-18, 2022 (Released:2022-02-02)

参考文献数

23

被引用文献数

4

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We propose a new application method in which radar/raingauge-analyzed precipitation amounts (RAP) produced by the Japan Meteorological Agency are spatially converted into 5km-resolution data, in addition to a three-hourly accumulation procedure, in order to statistically analyze localized heavy rainfall areas (HRAs) for a long period. A long-term trend and homogeneity in the appearance frequency of RAP with 5km-resolution converted by several methods, including the conventional method, are statistically evaluated in comparison with rain-gauge observations. The results indicate that the following application method is the most suitable to represent long-term variations in the appearance frequency of HRAs; (1) the converted value of RAP from 1 km to 5 km resolution is set to the 90th percentile value in 30 segments with 1km-resolution included in a grid with 5km-resolution, and (2) this spatial conversion is conducted after accumulating original RAP with 1km-resolution for three hours. Statistical analyses were performed for the appearance frequency of HRAs extracted from 5km-resolution RAP that were produced by the new application method, which indicates that the number of HRAs of the linear-stationary type could be increased without compromising the characteristics of HRAs extracted by the conventional method.

著者

Yuhji Kuroda Miho Toryu Hiroaki Naoe

出版者

Meteorological Society of Japan

雑誌

SOLA (ISSN:13496476)

巻号頁・発行日

pp.2022-008, (Released:2022-01-27)

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This study examined the influence of stratospheric variability on the polar winter tropospheric climate. The winter-mean tropospheric condition can be well represented by a winter-mean stratospheric index (the Polar-night Jet Oscillation (PJO) index) defined from profiles of monthly polar temperature anomalies. In winters with a positive (negative) index, the winter-mean surface pressure anomaly tends to acquire a positive (negative) pattern resembling the Arctic Oscillation (AO). This tropospheric condition tends to become a persistent polarity of the AO index throughout the winter. This tendency is also found when the PJO index for each month is used. Although the PJO index in January shows the best results, those in early winter can be used as predictors for the entire winter troposphere. Use of the PJO index for the stratospheric effect on winter troposphere is compared with that associated with the occurrence of the major stratospheric sudden warmings. The origin of the decadal variability of the index is also discussed.

著者

和達 清夫

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.3, no.8, pp.201-211, 1925-09-18 (Released:2009-02-05)

被引用文献数

1 1

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Some seismograms of the destructive earthquake occurred on May 23rd, 1925 in North Tazima district, show distinctly so-called the Mohorovi_??_i_??_ wave _??_. One of them is shown in Fig. (4) magnified from the seismograms of Wiechert's 200kg. seismograph installed at the Central Meteorological Observatory in Tokyo. (Epicenter distance about 450km.)The author of the present paper has drawn the hodograph for the longitudinal wave P from the results of seismic observations of the meteorological stations in this country as well as the time difference between P and _??_ from seismograms of these stations in case when it revealed on it. Moreover the curve of _??_ is also drawn on the same diagram and is shown in Fig. (6).Nextly, the author has assumed that, to interprete the time curve the discontinuity layer exists not so far from the surface in the earth crust. Following numerical values are obtained:-Velocity of the longitudinal waveabove the discontinuity layer, v1=5.56km. per see.Ditto under the discontinuity layer, v2=7.50km. per see.Epicenter distance of stations where Pand _??_ waves arrive simultaneously Δ0-120km.Difference of T(P)-T(_??_), shown in Fig. (6) is 6.2 sec.From these values, the depth of the discontinuity layer and that of the seismic foeus of this earthquake is determined. He has also compared the time of occurrence with those of the other two great earthquakes which have occurred on Jan. 15th, 1924 and Sept. 1st, 1923 respectively in Fig. (1); and ascertained that the geographical effect is not so great in these time-curves that the following conclusion may hold good in its main figure, even in any earthquake which occurred everywhere in this country.Followings are main results obtained in this paper:-(1) So called “initial movement” takes place with sudden change in its magnitude, and sometimes in direction, at the place about 100-200km. distant from the epicenter. Fig. (5).(2) The epicenter distance above mentioned depends chiefly upon the depth of seismic focus, therefore the depth may be estimated by this means. (3) I_??_itial movement must be very weak at the stations which lie within the epicenter distance, from 200km. to 1000km. approximately.(4) For the determination of the direction of epicenter from that of the initial movement of longitudinal wave, it is better to use _??_ phase than P and this will be done with much accuracy and easiness. (5) Depth of the discontinuity layer and that of the focus of Tazima-earthquakes are calculated and their values have been found to be about 42km. and 32km. respectively.(6) S-phase of the transverse wave begin to appear gradually o_??_ seismogram and with increasing epicenter distance it becomes more distinct.(7) Irregular form of isochronal curves may be considered as the effect of the discontinuity layer whose depth may not be definite in a part of the earth-crust of this part of the globe.(8) Well known “Omori's formula” which expresses the relation between the duration of preliminary tremor and epicenter distance, may hold good with _??_ and _??_ phases for the region of small epicenter distance and with P and L phases at distant places.The present paper is a preliminary report and the more complete studies will follow in a near future.

著者

Jiawei BAO Bjorn STEVENS

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.99, no.6, pp.1483-1499, 2021 (Released:2021-12-24)

参考文献数

34

被引用文献数

2 5

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Understanding of the tropical atmosphere is elaborated around two elementary ideas, one being that density is homogenized on isobars, which is referred to as the weak temperature gradient (WTG), and the other being that the vertical thermal structure follows a moist-adiabatic lapse rate. This study uses simulations from global storm-resolving models to investigate the accuracy of these ideas. Our results show that horizontally, the density temperature appears to be homogeneous, but only in the mid- and lower troposphere (between 400 hPa and 800 hPa). To achieve a homogeneous density temperature, the horizontal absolute temperature structure adjusts to balance the horizontal moisture difference. Thus, water vapor plays an important role in the horizontal temperature distribution. Density temperature patterns in the mid- and lower troposphere vary by about 0.3 K on the scale of individual ocean basins but differ by 1 K among basins. We use equivalent potential temperature to explore the vertical structure of the tropical atmosphere, and we compare the results assuming the temperature following pseudo-adiabat and reversible-adiabat (isentropic) with the effect of condensate loading. Our results suggest that the tropical atmosphere in saturated convective regions tends to adopt a thermal structure that is isentropic below the zero-degree isotherm and pseudo-adiabatic above it. However, the tropical mean temperature is substantially colder and is set by the bulk of convection, which is affected by entrainment in the lower troposphere.

著者

ENDO Hirokazu KITOH Akio MIZUTA Ryo OSE Tomoaki

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.2021-073, (Released:2021-09-01)

被引用文献数

10

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Future changes in East Asian summer monsoon (EASM) precipitation and the associated atmospheric circulation changes are investigated based on ensemble projections with the 60-km mesh Meteorological Research Institute atmospheric general circulation model (MRI-AGCM60). The projections at the end of the twenty-first century under the Representative Concentration Pathway 8.5 (RCP8.5) scenario indicate an overall increase in EASM precipitation, but with large sub-seasonal and regional variations. In June, the Meiyu–Baiu rainband is projected to strengthen, with its eastern part (i.e., the Baiu rainband) shifted southward relative to its present-day position. This result is robust within the ensemble simulations. In July and August, the simulations consistently project a significant increase in precipitation over the northern East Asian continent and neighboring seas; however, there is a lack of consensus on the projection of the Meiyu–Baiu rainband in July. A small change in precipitation over the Pacific is another feature in August.  Sensitivity experiments with the MRI-AGCM60 reveal that the precipitation changes in early summer are dominated by the effects of sea surface temperature (SST) warming (i.e., uniform warming and the tropical pattern change), which induce an increase in atmospheric moisture and a strengthening and southward shift of the upper-level East Asian westerly jet (EAJ), especially over the Pacific. On the other hand, the influence of land warming and successive large SST warming in the extratropics is evident in the precipitation changes in late summer. These late summer effects oppose and exceed the early summer effects through changes in the EAJ and low-level monsoon winds. These results suggest that the competition between the opposing factors makes the signal of the Meiyu–Baiu rainband response smaller in July than in June, and thus there tends to be a larger spread among simulations regarding the future tendency of the rainband in July.

著者

BAO Jiawei STEVENS Bjorn

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.2021-072, (Released:2021-09-06)

被引用文献数

5

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Understanding of the tropical atmosphere is elaborated around two elementary ideas, one being that density is homogenized on isobars, which is referred to as the weak temperature gradient (WTG), the other being that the vertical structure follows a moist-adiabatic lapse rate. This study uses simulations from global storm-resolving models to investigate the accuracy of these ideas. Our results show that horizontally the density temperature appears to be homogeneous, but only in the mid- and lower troposphere (between 400 hPa and 800 hPa). To achieve a homogeneous density temperature, the horizontal absolute temperature structure adjusts to balance the horizontal moisture difference. Thus, water vapor plays an important role in the horizontal temperature distribution. Density temperature patterns in the mid- and lower troposphere vary by about 0.3 K on the scale of individual ocean basins, but differ by 1 K among basins. We use equivalent potential temperature to explore the vertical structure of the tropical atmosphere and we compare the results assuming pseudo-adiabat and the reversible-adiabat (isentropic) with the effect of condensate loading. Our results suggest that the tropical atmosphere in saturated convective regions tends to adopt a thermal structure that is isentropic below the zero-degree isotherm and pseudo-adiabatic above. However, the tropical mean temperature is substantially colder, and is set by the bulk of convection which is affected by entrainment in the lower troposphere.

著者

真鍋 淑郎

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.36, no.4, pp.123-134, 1958 (Released:2007-10-19)

参考文献数

24

被引用文献数

39 38

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先ず毎日のデータを使って,冬の日本海から大気に供給される熱量及び水蒸気量を大気の熱収支から求めた6前の論文(12)で日本海に寒気の吹き出しが卓越するときの変質を議論したが,ここでは同じ計算を冬全体について行った結果を述べる。従って,エントロピー及び水蒸気の流入量流出量の計算,放射冷却及び凝結の潜熱の解放量等の見積り法の詳細は前の論文(12)を参照されたい。前にも述べた様に,周辺を密な観測網で囲まれた日本海はこの種の研究にはもってこいの場所であろう。得られた結果を簡単に述べる。1955年1月及び2月について顕熱供給量を得たが平均約555cal/cm2/dayで相当大きな値である。これは放射によつて逃げる熱量或いは凝結の潜熱発生量の数倍になる。尤も前に求めた典型的な吹き出しの時(1554年12月下旬)の顕熱供給量1030ca1/cm2/dayに比べればずっと小さい。この事は又気象状態に応じて供給量がかなり大きく変動する事を暗示している。一方同じ冬について平均蒸発量を計算した。即ち約5.6mm/dayでこれを熱量に換算すると約340cal/cm2/dayとなる。ここで興味ある事は,顕熱供給量が潜熱供給量を遙かに上廻つているという事である。この傾向が水温と気温との差の更に大きい典型的吹き出しの時一層顕ちょになるという事は前の論文(12)を参照すればわかる。ところがこの期間の日本海での平均ボーエン比を計算して見たが,吹き出しのときと同様ほぼ1に近い。この違いは,冬の日本海の様な不安定なところでは,恐らく対流が非常に盛んで熱と水蒸気とが必ずしも古典乱流論で仮定した,ように同じメカニズムで供給されていない事を暗示している様に思われる。次に上の計算結果を確かめるため,冬の日本海水の熱収支の計算を行った。既にW.Jacobs氏(10)及び宮崎氏(13)は,海の一年間の熱収支をもとにしてエネルギー交換量の経験式を出したが,ここでは冬期について収支計算を行った。従って冬期の水温変化のデーターを用いたが,この点が彼等の場合と違う点である。その結果大気の熱収支から得られた全エネルギー供給量に近い値を得る事ができた(約880cal/cm2/day)。ここで気付いたのは,エネルギー供給係数が安定度によつてかなり違うらしく,従ってJacobs氏或は宮崎氏が年平均のエネルギー供給係数を使って求めた値は,冬の日本海の様な不安定な場所では,かなり実際より小さいらしいという事である。

著者

青梨 和正 田島 知子 久保田 拓志 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.99, no.5, pp.1201-1230, 2021 (Released:2021-10-30)

参考文献数

35

被引用文献数

2

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全天候マイクロ波イメージャ輝度温度を雲解像モデルの降水物理量へ同化するため、2スケール近傍法を使うアンサンブルに基づく変分同化法スキーム(EnVar)に、降水の非正規型確率分布関数(PDF)と、PDFの代用レジームを使った新しい位置ずれ補正法を導入した。 多くの事例について降水のアンサンブル予報摂動の既存の非正規PDFモデルへの適合性を評価した。これをもとに、降水強度のPDFとして混合対数正規分布を選び、EnVarに降水なし、降水ありの2つのPDFレジームを導入した。次に、EnVarで非降水、降水、強雨の代用レジームを導入し、そのPDFを対象地点の周囲のPDFの平均で近似する、降水の位置ずれ補正法を開発した。この平均の水平スケールは、アンサンブル予報摂動の相似性に基づいて推定した。上記手法が、マイクロ波イメージャ輝度温度の観測値と第一推定値の差のバイアスと正規性を向上させた。 台風1518について全天候マイクロ波イメージャ輝度温度観測データを同化する実験を行なった。その結果、本研究のEnVarは、従来の、降水の単一の正規分布PDFレジームを使うEnVarに比べて、衛星全球降水マップ(GSMaP)に近い降水解析値を与えた。降水の混合対数正規分布の導入は、台風や前線付近の強雨域で解析降水量を強め、代用レジームの使用は、解析値の降水の位置ずれ誤差を大幅に減らした。本研究のEnVarは、雲解像モデルの12時間予報までの降水予報を改善し、1日以上の台風中心位置や中心気圧の予報を改善した。さらにEnVarの予報解析サイクルは、1時刻の輝度温度同化よりも、台風周辺の強雨の短期予報と台風に付随した降水帯の予報を改善した。

著者

HIOKI Tomohito TSUBOKI Kazuhisa

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

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

被引用文献数

2

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

著者

NGUYEN T. Hanh ISHIJIMA Kentaro SUGAWARA Satoshi HASEBE Fumio

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.2021-056, (Released:2021-05-20)

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Stratospheric profiles of the mean age of air estimated from cryogenic air samples acquired during a field campaign over Indonesia, the Coordinated Upper-Troposphere-to-Stratosphere Balloon Experiment in Biak (CUBE/Biak), are investigated by employing the boundary impulse evolving response (BIER) method and Lagrangian backward trajectories, with the aid of an atmospheric general circulation model-based chemistry transport model (ACTM). The ACTM provides realistic meteorological fields at one-hour intervals by nudging toward the European Centre for Medium-Range Weather Forecasts Reanalysis-Interim (ERA-Interim). Since the BIER method is capable of taking unresolved diffusive processes into account, while the Lagrangian method can distinguish the pathways the air parcels took before reaching the sample site, the application of the two methods to the common transport field simulated by the ACTM is useful in assessing the CO2- and SF6-derived mean ages. The reliability of the simulated transport field has been verified by the reproducibility of the observed CO2, SF6, and water vapor profiles using the Lagrangian method. The profile of CO2 age is reproduced reasonably well by the Lagrangian method with a small young bias being consistent with the termination of trajectories in finite length of time, whereas the BIER method overestimates the CO2 age above 25 km altitude possibly due to high diffusivity in the transport model. In contrast, the SF6 age is only reproducible in the lower stratosphere, and far exceeds the estimates from the Lagrangian method above 25 km altitude. As air parcels of mesospheric origin are excluded in the Lagrangian age estimation, this discrepancy, together with the fact that the observed SF6 mole fractions are much lower than the trajectory-derived values in this height region, supports the idea that the stratospheric air samples are mixed with SF6-depleted mesospheric air, leading to overestimation of the mean age.

著者

A GETTELMAN P.M. de F FORSTER

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.80, no.4B, pp.911-924, 2002 (Released:2002-10-10)

参考文献数

28

被引用文献数

205 243

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The tropical tropopause layer (TTL) is a transition region between the troposphere and the stratosphere. In this study the vertical extent of the TTL is diagnosed from radiosonde and ozonesonde profiles in the tropics and a climatology of this layer is presented. The radiative balance in the TTL is also characterized. The TTL is locally defined as extending from the level of the lapse rate minimum at 10-12 km to the cold point tropopause (CPT) at 16-17 km. The minimum in lapse rate represents the level of maximum convective impact on upper tropospheric temperatures, which is found to closely correspond to a minimum in ozone. Variations in this level are correlated with convective activity as measured by satellite brightness temperatures and Outgoing Longwave Radiation (OLR). At the cold point, the TTL height is nearly uniform throughout the tropics, and has a pronounced annual cycle. There are regional variations in the altitude of the lower boundary of the TTL. Interannual variations of the TTL result from changes in the large scale organization of convective activity, such as from the El-Niño Southern Oscillation (ENSO). Over the last 40 years, records indicate an increase (200-400 m) in the height of both the cold point tropopause and the level of minimum lapse rate. To better understand vertical transport in the TTL, the clear sky radiative heating rate is diagnosed using a sophisticated radiative transfer scheme. The level of zero radiative heating occurs roughly 1 km below the CPT, implying that convection needs to loft air 4-5 km above the base of the TTL if the air is to eventually enter the stratosphere.

著者

真鍋 淑郎

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.33, no.2, pp.76-88, 1955 (Released:2009-02-05)

参考文献数

7

被引用文献数

2 2

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Using 2-layer model which is the special case of the multiple layer model defined by Charney and Phillips, investigation is made, from the view point of numerical weather prediction, on the role of the tilt of trough relative to the vertical for the development of the pressure pattern. At the same time, the mechanism of the transition between the tilted and the non-tilted trough is discussed. Finally, corresponding to the above development, the energetics of westerly wave, i. e., the energy exchange between the potential and the kinetic energy is investigated. Here, the simple expression for the change of these energies is obtained.

著者

KODERA Kunihiko EGUCHI Nawo UEYAMA Rei FUNATSU Beatriz M. GAETANI Marco TAYLOR Christopher M.

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.2021-055, (Released:2021-05-14)

被引用文献数

1

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Previous studies have suggested that the recent increase in tropical extreme deep convection, in particular over Asia and Africa during the boreal summer, has occurred in association with a cooling in the tropical lower stratosphere. The present study is focused on the Sahel region of West Africa, where an increased occurrence of extreme precipitation events has been reported over recent decades. The results show that the changes over West Africa since the 1980s involve a cooling trend in the tropical lower stratosphere and tropopause layer, combined with a warming in the troposphere. This feature is similar to that which might result from increased greenhouse gas levels, but is distinct from the interannual variation of precipitation associated with the transport of water vapor from the Atlantic Ocean. It is suggested that the decrease in the vertical temperature gradient in the tropical tropopause region enhances extreme deep convection over the Sahel, where penetrating convection is frequent, whereas tropospheric warming suppresses the shallower convection over the Guinea Coast. The essential feature of the recent changes over West Africa is therefore the depth of convection, rather than the total amount of surface precipitation.

著者

SONG Jinjie KLOTZBACH Philip J. DUAN Yihong

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.2021-051, (Released:2021-04-27)

被引用文献数

3

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This study shows that the impact of El Niño-Southern Oscillation (ENSO) Modoki on boreal summer tropical cyclone (TC) formation over the western North Pacific (WNP) has experienced decadal changes during the past few decades. The correlation between the ENSO Modoki index (EMI) and TC frequency over the WNP is weak between 1975-1989, becomes strong and significant between 1990-2004, and becomes weak again between 2005-2019. Over the eastern part of the WNP, ENSO Modoki enhanced TC formation during 1990-2004 but did not significantly impact on the TC formation during 1975-1989 and 2005-2019. The difference in correlation strength primarily results from changes in large-scale features related to ENSO Modoki among the three sub-periods (1975-1989, 1990-2004 and 2005-2019). El Niño Modoki from 1990-2004 was characterized by a tripole SST pattern with maximum SST anomalies in the equatorial central Pacific, while from 1975-1989 and 2005-2019, the maximum SST anomalies were located over the subtropical northeastern Pacific. The two primary environmental variables likely leading to these observed relationships between ENSO Modoki and TCs were mid-level moisture (RH600) and low-level vorticity (VOR850). From 1990-2004, TC formation was enhanced both south of 20°N and north of 20°N. The increase in TC activity during El Niño Modoki south of 20°N was likely tied to greater RH600 and north of 20°N to larger cyclonic VOR850. In contrast, ENSO Modoki's impacts on both VOR850 and RH600 were weak from 1975-1989 and 2005-2019.

著者

SHIBUYA Ryosuke NAKANO Masuo KODAMA Chihiro NASUNO Tomoe KIKUCHI Kazuyoshi SATOH Masaki MIURA Hiroaki MIYAKAWA Tomoki

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.2021-046, (Released:2021-04-08)

被引用文献数

5

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In this study, we assessed the prediction skill of the Boreal Summer Intra-Seasonal Oscillation (BSISO) mode of one-month simulations using a global non-hydrostatic atmospheric model (NICAM) with explicit cloud microphysics and with a grid spacing of 14 km. The simulations were run as a series of hindcast experiments every day of August during 2000-2014; a total of 465 simulations were run with a 13950-day integration. On using forecast skill scores for statistical measurements, it was found that the model showed an overall BSISO prediction skill of approximately 24 days. The prediction skill tended to be slightly higher (∼ 2 days) when BSISO events began in the initial phases 7 to 1, which corresponded to the re-initiation phase of the BSISO, where a major convective center over the Philippine Sea decayed and a new convective envelope began aggregating over the western Indian Ocean. The phase speed and the evolution of the amplitude of the BSISO were well simulated by the model with a clear northwestward-southeastward tilted outgoing longwave radiation (OLR) structure over the Maritime continent and the western Pacific. However, the propagation speed was slower during phases 6-7, and the amplitude of the BSISO largely decayed during phases 8-1, which was likely to have been associated with the stagnant behavior of the convective cells over the Philippines. This stagnation of the propagation over the Philippines may be largely attributed to the small background southerlies bias in the model over the Philippines based on regression coefficient analysis using the moist static energy. The bias in the large-scale circulation was likely to have been associated with the bias in the moisture field and the associated background monsoonal circulation. We concluded that the model physics controlling the background fields are important factors for improving the BSISO prediction skill.

著者

KIKUCHI Kazuyoshi

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.2021-045, (Released:2021-03-30)

被引用文献数

44

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The boreal summer intraseasonal oscillation (BSISO) is among the most pronounced subseasonal variability in the tropics during boreal summer. Compared to its wintertime counterpart, the so-called Madden-Julian oscillation (MJO), the BSISO convection displays more complicated spatio-temporal evolution, characterized by northward propagation over the northern Indian Ocean and western North Pacific as well as eastward propagation along the equator. It exerts a strong influence on a broad range of tropical weather and climate phenomena such as tropical cyclogenesis, monsoon onset and active/break cycles, among others. Our fundamental understanding of the BSISO has steadily advanced: so far various aspects of the BSISO have been described and several theories aiming to explain its northward propagation have been proposed. Yet, our skill to simulate the BSISO by general circulation models remains unsatisfactory, though it has been improved. This paper reviews some fundamental aspects of the BSISO from the viewpoint of observation, theory, and modeling.

著者

Kenji Kai Kei Kawai Atsuya Ito Yuki Aizawa Yuki Minamoto Erdenebadrakh Munkhjargal Enkhbaatar Davaanyam

出版者

Meteorological Society of Japan

雑誌

SOLA (ISSN:13496476)

巻号頁・発行日

pp.2021-023, (Released:2021-06-18)

被引用文献数

5

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The Gobi Desert is a dominant source of dust on the Asian continent. In this study, we analysed the characteristics of a typical Mongolian dust storm and identified a prominent dust hotspot in the Gobi Desert. During a field survey from Ulaanbaatar (the capital of Mongolia) to Dalanzadgad in the Gobi Desert, we encountered a typical dust storm on 28 April 2019, exhibiting a distinct dust wall. The head of the dust storm crossed the road several kilometres ahead of our vehicle. The head of the storm had a height of 600 m, and its structure suggested that the dust storm was induced by a gravity current. We entered the front of the dust storm and measured a maximum wind speed of 18.2 m/s and a visibility of less than 20 m. The normalized dust number concentration at 7 μm was 59 cm−3. Moreover, Himawari-8 Dust RGB imagery showed that the dust storm occurred in an orographic convergence zone. This zone connects two valleys that are sandwiched between three mountains in the Gobi Desert: the Khangai, Altai, and Gurvan Saikhan Mountains. Our results suggest that this zone is a remarkable dust hotspot in the Gobi Desert.