A new scheme that can define three rainy seasons, and the hydrological summer monsoon in Korea, has been proposed, examined, and verified to be effective. In the scheme, the Available Water Resources Index (AWRI) that is the accumulated precipitation value in which daily reduction of water, and the duration of accumulation are taken into account quantitatively.
With this scheme, the onset and ending dates of three rainy seasons are defined by such singularities as the smallest, the biggest, the minimum, and the maximum of the AWRI in a year. The intensities of these rainy seasons are defined by the value of the maximum AWRI, flood index, and drought index.
The first rainy season (Bom-Changma) starts in early April, when the minimum value of the AWRI in a year appears, and when the mean southerly wind at level become stronger then that at the 500 hPa level, and ends at May 15. The second rainy season (Changma) starts in late June and ends at 16-20 July. The third rainy season (Kaul-Changma) starts at mid August, and ends at 3-5 September that has the maximum value of the AWRI in a year, and that is the last date of mean southerly wind at the level.
Finally, the hydrological summer monsoon is proposed to be defined as the period of increasing water resources that is from the minimum to the maximum of the AWRI, from the onset of Bom-Changma till the end of Kaul-Changma. From a turning point of meridional wind system to the end of mean southerly wind in the low level atmosphere, and that is concurrent with the summer monsoon.

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Low stratiform clouds (LSCs) are of three different types: stratocumulus (Sc), stratus (St), and sky-obscuring fog (FOG). Ship-based cloud observations (September 1957-August 2002) and air-temperature and sea-level pressure data obtained from the ERA-40 reanalysis are used to investigate the seasonal relationships between the amounts of these LSC types and the estimated inversion strength (EIS) over the global ocean. Although it is known that a single linear relationship applies to the variations in the LSC amount as the sum of those of the LSC types and EIS, two relationships with different sensitivities are found between each LSC-type amount and EIS. The boundary lies at a sea surface temperature (SST) of approximately 16°C. The Sc amount is strongly correlated with EIS in the warm SST regime, whereas no correlation can be observed between them in the cold SST regime. In contrast, although FOG rarely occurs in the warm SST regime, its amount increases with EIS in the cold SST regime. The St amount increases with EIS in both regimes, with higher sensitivity in the cold SST regime. Examination of vertical layers contributing to EIS reveals that an increase in the inferred inversion strength between 850- and - levels corresponds to that in the Sc amount in the warm SST regime. In the cold SST regime, as EIS increases, relatively high values of inferred inversion strength between 700- and 850-hPa levels change to a rapid increase in that between - level and the surface, which coincides with the transition from Sc to FOG. Temperature advection implied by the air-sea temperature difference provides favorable conditions to the different variations in the two regimes: general occurrence of cold advection in the warm SST regime and cold-to-warm transition of advection in the cold SST regime.

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 Influence of a distantly located typhoon (TY), a weak westerly trough, and orography on the intense rainfall on September 14-15, 1965 over Gifu and Fukui Prefectures (~36°N, 137°E) in central Japan is studied by using European Center for Medium-Range Weather Forecasts 40-year reanalysis data and upper, surface, and local rain-gauge observation data.
 Prior to the intense rainfall, a long arc-shaped moisture front was intensified on September 13 along the northern periphery of the outer circulation of TY Tix (6524), which was distantly located from Japan. A rainfall of 50-100 mm d⁻¹ occurred in a long rainbelt along the Pacific coast of Japan, associated with the low-level convergence in the moisture front.
 A small depression formed over the Japan Sea on September 14 under the influence of a weak westerly shortwave trough in the middle troposphere. In the lower troposphere, a Λ-shaped trough extended from the outer circulation of the TY toward the depression and the moisture front protruded locally northward to the depression. Concurrently, with the protrusion of the moisture front, a meso-α-scale (~500 km) intense rainfall area protruded northward. In the meso-α-scale intense rain area, a south-north oriented narrow rainband formed along the east side of Suzuka-Ibuki Mountains on September 14. A maximum precipitation of ~800 mm d⁻¹ occurred in the rainband.
 This case may fall under “intense rainfall remotely affected by TY.” However, the present case is different from other cases studied in previous articles in that very intense precipitation occurred in a narrow south-north oriented rainband formed along mountains under the influence of the weak westerly shortwave trough.

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The synoptic-scale condition related to the intense rainfalls on August 17, 1968, over the Kiso?Hida and Nagara River Basin in the central part of Japan is studied by using European Center for Medium-Range Weather Forecasts 40-year reanalysis data and upper, synoptic-surface, local rain-gauge observation data and satellite cloud images.
The intense rainfalls occurred within a long cloud belt formed with a low-level moist belt (LMB), which had formed along the northwestern rim of the North Pacific subtropical anticyclone (NPSA). The LMB was sustained by large-scale moisture transport along the northern rim of the NPSA and mesoscale northward moisture transport along a small anticyclone embedded in the NPSA. In the lower and middle troposphere, dry air spread over the Japan Sea after the passage of a severe tropical storm (STS) over the Japan Sea owing to the prevailing westerlies to the southwest of the STS. The northern edge of the LMB was bounded by the dry air. The northern boundary of the LMB was signified as the dry front because of the strong moisture gradient and very weak thermal gradient.
Intense rainfalls occurred in the cloud belt, at a distance of ∼1200 km south of the STS, over the river basin where the large-scale and the mesoscale moist flows converged in association with the orographic convergence. Intense convective rainfalls were accompanied by the increase in the convective instability due to the southeastward intrusion of the middle tropospheric dry air over the LMB. Significant low-level jet stream and mesoscale depression were not found around the intense rainfall area.

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Global Water Cycle Experiment (GEWEX)-Asian Monsoon Experiment (GAME)/Huaihe River Basin Energy Water Cycle Experiments (GAME/HUBEX) were conducted during the Meiyu period in 1998 and 1999. Using infrared brightness temperature (T_BB) data of the Geostationary Meteorological Satellite (GMS)-5, we investigated the diurnal variation of 61 long-lasting cloud clusters that developed during GAME/HUBEX Intensive Observation Periods (IOPs). More than two-thirds of the cloud clusters, named nocturnal-type clusters, attained a convective peak between midnight and early morning, with most peaking between 00 and 02 LST. Almost all of these nocturnal-type cloud clusters developed in, or south of the Meiyu frontal zone. The other clusters, named evening-type cloud clusters, peaked from late afternoon to evening, and were less intense than the nocturnal clusters.
GAME-Reanalysis (Version 1.5) data showed that the ageostrophic wind component over a large domain to the south of the Meiyu front shifted from easterly to southerly, and increased the velocity of the southwesterly airflow largely at low levels at nighttime (02 LST). This southwesterly ageostrophic synoptic-scale low-level jet (S-LLJ) transported large amounts of water vapor to the Meiyu front, forcing large moisture convergence within, and immediately south of the Meiyu frontal zone. The low-level moisture convergence, and the S-LLJ itself are proposed to support the nocturnal evolution or redevelopment of cloud clusters.

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This report shows genesis of a polar low by an AGCM without specialized initial condition. A case of polar low genesis over the eastern coast of the Asian Continent, simulated in the seasonally varying climatological SST run by an AGCM (T42L52, primitive equation spectral model with 42 wave-number and 52 layers), is presented. A polar low simulated in January 22-23 of the fourth year (Y04) integration after the 10-year period of the spin-up is studied in comparison with polar lows described in several observational studies.
In January Y04, large-scale circulation systems, such as Asian winter monsoon, extratropical cyclones and upper cold lows are reasonably simulated. A typical polar low is formed in January 22 over the coastal sea area ∼1500 km west of the major extratropical cyclone that developed over the Northwestern Pacific Ocean, under the influence of a short wave trough, which propagates along the rim of an upper cold low. The polar low genesis takes place first in connection with a deepening of the surface trough, which extends westward from the major cyclone. The deepening of the surface trough in the zone of strong low-level thermal gradient over the coastal sea area suggests the important role of the low-level baroclinicity for the polar low genesis. The strong heating due to the energy supply from the sea surface contributes for the genesis of the polar low through the decreasing of the vertical stability, and the sustaining thermal gradient. Meanwhile, the heating around 700 hPa associated with the precipitation concentrated within the polar low indicates the influence of the condensation heating for the development of the polar low. Aforementioned various processes contribute together to generate and develop the polar low. The structure and the evolution process of the simulated polar low are consistent with those of the observed polar lows.
It is concluded that the realistic polar low genesis takes place in the model, when the large-scale phenomena such as the upper cold low, the short-wave trough, parent major cyclone and polar air outbreak are reasonably simulated. The present study is significant in presenting an AGCM simulation of a polar low for the first time.

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The impact of northerly surges of the East Asian winter monsoon on tropical cyclogenesis over the eastern Indian Ocean and maritime continent was examined for the 6-month period of October to March from 1979/1980 to 2006/2007 based on case studies and lag-composite analysis. We focused on long-lasting northerly surge events at 6- to 30-day (sub-monthly or intraseasonal) time scales over the South China Sea. In addition, we examined seasonal differences in the impact of northerly surges over the South China Sea on tropical atmospheric circulation.
The results show that northerly surges occur frequently in the period from October to March. Long-lasting northerly surges over the South China Sea intrude into tropical regions. Over the eastern Indian Ocean and maritime continent, the surges are associated with the appearance of tropical cyclones. However, the impact of these surges varies with the seasonal march. In October and November, tropical cyclones occur over the South China Sea during the northerly surge events, enhancing positive vorticity over the South China Sea. A cyclone pair symmetric with respect to the equator also appears over the eastern Indian Ocean in November and is responsible for the enhancement of the horizontal gradient of zonal wind by the northerly surge. In contrast, in December, January, and February, an asymmetric cyclone pair (the so-called Borneo vortex) develops around Borneo. The asymmetric cyclone pair around the maritime continent is associated with intensification of low-level wind along a channel between the islands of Borneo, Sumatra, and Java. In March, no clear tropical cyclone appears over the tropical regions in association with the northerly surge.

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Tropical cyclone Nargis was generated over the Bay of Bengal (BoB) in late April 2008, and it devastated the coastal area of Myanmar. This study reveals the environmental and external factors in its genesis.
It is found that large-scale environmental conditions around the genesis point were generally favorable for cyclogenesis. Positive lower-tropospheric relative vorticity and the weak vertical shear of horizontal wind were observed, which were distinctive features of April 2008 that are not seen in April climatology. Sea surface temperature and atmospheric thermodynamic profile were also favorable for cyclogenesis, as in climatology. Mid-tropospheric humidity was significantly low and unfavorable until six days before the genesis; however, it increased abruptly to become favorable just before the genesis.
The atmospheric disturbance that seemed to trigger the genesis was lower-tropospheric easterly surge blowing into the genesis point. Horizontal convergence at the head of the surge associated with active convection was considered as an External Forced Convergence that triggered cyclogenesis. In addition, the active convection seemed to contribute to the preconditioning of mid-tropospheric humidity. The easterly surge was associated with a cold surge in the eastern coastal area of the Eurasian continent, which is a typical feature of cold surges in April.

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 An investigation has been carried out using rainfall observation data, an analysis and forecast data by National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) to explain the environment and processes that lead to heavy rainfall in the early morning over the Korean peninsula during episodes of cloud clusters associated with mesoscale troughs (CCMTs). For this study, nine episodes with a maximum hourly rainfall amount in the early morning (i.e., 0300-0900 LST) are selected from seventeen heavy-rainfall episodes associated with CCMTs during 2001-2011. Case studies on two episodes have revealed that, for both episodes, 1) a low-level trough develops over eastern China and its coastal area during day time; 2) the strong southwesterly band (SWB; an area with wind speeds > 12.5 m s⁻¹) on the pressure level of

over the East China Sea, which is located southeast of the trough, strengthens and expands at night time toward the southwestern Korean peninsula; 3) the SWB supplies a large amount of moisture and increases convective instability over the southwestern Korean peninsula with a convection trigger mechanism (i.e., strong horizontal convergence); and 4) heavy rainfall occurs in the early morning over the southwestern Korean peninsula, where the exit region of the SWB is located. A mechanism for the SWB growth is presented. Furthermore, generality of the major results from the two case studies is verified using the results obtained for the composite fields of the nine CCMT episodes.

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A polar mesoscale cyclone (PMC) developed over the east coast of Asia in an atmospheric general circulation model (AGCM) that used seasonally varying climatological sea surface temperatures (SSTs). The AGCM had 52 layers and triangular spectral truncation at wavenumber 106 (T106L52). This study compares the model PMC, which formed in February, 8 years after model spin-up, with observational studies. The simulated PMC formed over the western Sea of Japan under the influence of a short-wave trough propagating around a cold upper low. PMC genesis occurred as a surface trough over the Sea of Japan. The surface trough extended northward from a major cyclone that developed over the western North Pacific south of Japan. Large sensible heat fluxes from the sea surface facilitated PMC genesis by decreasing the vertical stability and maintaining a thermal gradient in the lower troposphere. Signifi- cant weakening of the PMC as it passed over Japan suggests that energy supplied from the surface played an important role in the evolution of the PMC.
The PMC subsequently developed rapidly over the western North Pacific east of Japan, under the influence of the upper cold low and a short-wave trough. Precipitation, diabatic heating, and energy from the sea surface all increased as the PMC developed. The evolution of the simulated PMC matched the evolution of observed PMCs. Realistic formation of synoptic-scale circulation systems, such as the upper cold low, short-wave troughs, and a major cyclone in the model, are crucial if PMCs are to be simulated realistically in the AGCM.

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In this study, a stationary front is automatically detected from weather data using a U-Net deep convolutional neural network. The U-Net trained the transformation process from single/multiple physical quantities of weather data to detect stationary fronts using a 10-year data set. As a result of applying the trained U-Net to a 1-year untrained data set, the proposed approach succeeded in detecting the approximate shape of seasonal fronts with the exception of typhoons. In addition, the wind velocity (zonal and meridional components), wind direction, horizontal temperature gradient at 1000 hPa, relative humidity at

, and water vapor at 850 hPa yielded high detection performance. Because the shape of the front extracted from each physical quantity is occasionally different, it is important to comprehensively analyze the results to make a final determination.

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Structures of 266 tropical cyclones (TCs) that completed extratropical transition (ET) in the western North Pacific during 1979-2004 are examined using the JRA-25 dataset, in relation to the removal of TC wind profile retrievals (TCR) from each TC after its recurvature. The TCR is a kind of “TC bogusing” techniques, and it was expected that its removal may lead to an artificial change in the TC structure at the ET stage. Nearly half of the examined TCs have cold-core structures before the removal of the TCR. On average, the structural change associated with ET starts more than one day before the removal of the TCR. Direct impact of the warm-core structure of the TCR is limited within a radius of about 200 km from the TC center. Although discontinuity in the TC structure between the periods with and without TCR may be statistically significant, the differences in the indices are practically negligible.

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The ECMWF 2.5° gridded analysis was used to re-examine the evolution of the synoptic flow patterns and frontal structure of an early summer monsoon trough that occurred during 10-15 June 1975. A blocking pattern that began with an omega shape developed into a Rex pattern on 12 June. During 10-14 June, the blocking low pressure and associated trough axis were almost stagnant. As a result, the Mei-Yu front was quasi-stationary and affected the Taiwan area for more than four days.
Similar to other frontal systems during the early summer rainy season over southern China, this Mei-Yu front exhibited baroclinic characteristics in the subtropics. In the lower troposphere, appreciable temperature gradients and maximum frontogenesis due to horizontal deformation between the postfrontal northwesterlies and prefrontal southwesterlies were diagnosed. The western section (∼115°E) of this frontal system exhibited a marked northward vertical tilt. An upper-level jet near the tropopause was also present. A moist tongue was located south of the surface cold front within the low-level southwesterlies and extended vertically upward. A thermally direct circulation across the front with ascending motion within the prefrontal warm, moist air and descending motion within the postfrontal cold, dry air underneath the upper-level jet was diagnosed.

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 1982年7月23—24日の九州豪雨を伴った中規模（メソα規模）梅雨前線低気圧の構造と発達過程を観測データ・再解析データに基づいて調べた．この低気圧は21日中国大陸（〜33°N，105°E）の弱い傾圧場の中で降水を伴って発生した．発生期には低気圧は顕著な500hPaトラフを伴っていなかった．低気圧は発達しつつ東進し24日に対馬海峡に達し，低気圧暖域で持続した雲クラスターが豪雨をもたらした．23—24日にはオホーツク海高気圧から西方に伸びた高気圧域が低気圧の東側に停滞していた．低気圧の東縁辺に南風下層ジェット，水蒸気傾度，強い渦度，上昇流が発現した．25日以後，低気圧は高気圧域により東進を阻まれ衰弱し強い降水も終止した．本事例を先行研究で示された他の豪雨事例と比較した．豪雨を伴うトラフ・低気圧の状況は事例によりかなり異なる．

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 Polar mesoscale cyclones (PMCs) frequently develop over the Japan Sea. Genesis of PMCs over the East China Sea is rare, but can occur under certain synoptic-scale conditions. In this observational case study, the feature of a PMC generated over the eastern East China Sea on 20 February 1975 is studied using observation data including those obtained during Air-mass Transformation Experiment, satellite cloud images, and objective-reanalysis data.

 The PMC with a comma-cloud formed within cyclonic polar-air streams induced by an upper cold trough and a synoptic-scale parent cyclone that developed near Japan. Within 3-hour period after the generation of the PMC, its central pressure deepened from 1016 to 1012 hPa. Strong surface winds occurred in the trailing portion of the comma-cloud. The large-scale conditions for the PMC's genesis were characterized by the southward intruding of the cold core in the upper cold trough beyond 34°N to the East China Sea, positive vorticity advection at 500 hPa, and the moist-neutral layer formed over the warm Tsushima Current in the eastern East China Sea.

 The PMC, after passing over Kyushu, developed as it moved eastward along the Pacific coast of Japan. The PMC developed further into the secondary cyclone comparable to the parent cyclone, as it moved in the low-level baroclinic zone over the northwestern Pacific. The large-scale conditions for the development were characterized by the upper cold trough and the low-level baroclinic zone formed over the zone of maximum sea-surface temperature gradient along north of the Kuroshio extension.

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Dynamical downscaling simulations of MIROC6 (the sixth version of Model for Interdisciplinary Research on Climate) data using the Weather Research and Forecasting (WRF) model were performed to investigate future changes in the Japan Sea polar airmass convergence zones (JPCZs) related to heavy snowfall events in the Sea of Japan–side region of central and northern Japan. WRF dynamical downscaling simulations showed a northward shift of JPCZs and the associated changes in precipitation distribution. Rainfall increased significantly over the central part of the Sea of Japan and in the Tohoku region. On the other hand, snowfall decreased significantly over the entire Sea of Japan. However, there was a significant increase in snowfall in the inland mountainous areas of the Tohoku region. Jet streaks shift northward in the future climate, which was confirmed in the MIROC6 data. Reflecting such future environments, WRF dynamical downscaling simulations showed that a mesoscale cyclonic circulation anomaly passing over Hokkaido from the Sea of Japan, which is not resolved by MIROC6, contributes to the northward shift of JPCZs and the associated changes in rainfall and snowfall.

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　The brown planthopper, Nilaparvata lugens (Stål), a type of rice pest, immigrates from southern China to western Japan by drifting in the southwesterly environment during the Baiu/Meiyu rainy season. This study aims to investigate the preferable weather patterns for the immigration of brown planthoppers across the East China Sea. We conducted immigration runs using an advection diffusion model for brown planthoppers and projected the possibility of them landing in Kyushu in the model run onto 64 weather map patterns typically observed in June and July, identified by the self-organizing map analysis. The results showed that the immigration occurred under two specific weather map patterns: flow stagnation around western Japan and a low-level jet blowing over the East China Sea. Consistency between landing cases in model runs and the occurrence of the two weather patterns was found in the intraseasonal and interannual variability. We also estimated the change in the frequency of brown planthopper arrival in Kyushu based on the climate change dataset, the database for Policy Decision Making for Future climate change (d4PDF). It was found that the flow stagnation patterns increased in response to global change, at least in the d4PDF dataset. Finally, risk assessment for temperature change and a comparison with trap observations were discussed.

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Using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data and satellite-observed outgoing long-wave radiation (OLR) data, we examined the westward extension and eastward contraction of the North Pacific subtropical high in summer (NPSH). It was found that the NPSH shows a great variability in its western extent, both on the seasonal and interannual time scales. In order to examine the interannual variations of NPSH, we defined a NPSH index as the June-July-August (JJA) mean geopotential height anomalies at 850 hPa averaged over the west edge (110∼150°E, 10∼30°N) of NPSH. This index describes the year-to-year zonal displacement of NPSH. Composite analysis based on this NPSH index showed that there is a significant relation between zonal displacement of NPSH and intensity of atmospheric convection over the warm pool. A low-level cyclonic (anticyclonic) anomaly that is closely associated with the zonal shift of NPSH appears north of enhanced (weakened) atmospheric convection, i.e., the vorticity anomaly is found north of the divergence one.
Climatologically, the NPSH contracts eastward swiftly after pentad 40 (July 15 to 19). Such an eastward contraction is closely associated with the poleward shift of both NPSH and atmospheric convection over the tropical western Pacific warm pool. However, such seasonal variations of both NPSH and convection show distinct features between the summers with positive and negative NPSH indexes. During summers with positive NPSH index, NPSH and convection over the warm pool do not show an appreciable seasonal evolution. During summers with negative index, by contrast, they show a swift seasonal evolution after pentad 40.
Finally, we performed a vorticity analysis to explain the relation between the divergence and vorticity anomalies on the interannual time scale. The analysis shows that in the lower troposphere ( ), the advection of relative vorticity is comparable to the stretching and is responsible for the northward shift of the circulation anomaly relative to anomalous atmospheric convection. The difference from the theory of Gill (1980) is discussed. In the upper troposphere (200 hPa), the advection is slightly smaller than the stretching with opposite signs in East Asia and the western North Pacific, and thus the position of the vorticity anomaly is consistent with that of the stretching anomaly.

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Forest fires are a common and destructive natural disaster in Russia. Weather conditions during active forest fire periods in southern Sakha (Eastern Siberia) at high latitudes (58–65°N, 120–140°E) were evaluated. Periods of high fire activity during 2002 to 2016 were identified using MODIS (moderate resolution imaging spectroradiometer) hotspot data by considering the number of daily hotspots and their continuity. Weather conditions during the top seven periods of high fire activity were analyzed using atmospheric reanalysis data for upper (500 hPa) and lower levels (

). Our results showed that active fires occurred under varied weather conditions and it was difficult to find common weather patterns at both upper- and lower-levels during the seven most active fire periods. Furthermore, it was apparent that the northward movement of warm air masses (cTe: continental temperate) from lower latitudes (∼40°N) toward southern Sakha tended to exacerbate fires mainly due to strong wind conditions during the seven most active fire periods. In particular, on peak hotspot days, warm air masses from the south existed commonly near southern Sakha. This northward movement of warm air masses can be used to forecast fire and predict future fires in the region.

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