Journal of the Meteorological Society of Japan. Ser. II
Online ISSN : 2186-9057
Print ISSN : 0026-1165
ISSN-L : 0026-1165
Advance online publication
Showing 1-20 articles out of 20 articles from Advance online publication
  • Izumi SAITO, Toshiyuki GOTOH, Takeshi WATANABE
    Type: Articles
    Article ID: 2019-049
    Published: 2019
    [Advance publication] Released: May 17, 2019
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     To consider the growth of cloud droplets by condensation in turbulence, the Fokker-Planck equation is derived for the droplet size distribution (droplet spectrum). This is an extension of the statistical theory proposed by Chandrakar and coauthors in 2016 for explaining the broadening of the droplet spectrum obtained from the ‘Π-chamber', a laboratory cloud chamber. In this Fokker-Planck equation, the diffusion term represents the broadening effect of the supersaturation fluctuation on the droplet spectrum. The aerosol (curvature and solute) effects are introduced into the Fokker-Planck equation as the zero flux boundary condition at R2=0, where R is the droplet radius, which is mathematically equivalent to the case of Brownian motion in the presence of a wall. The analytical expression for the droplet spectrum in the steady state is obtained and shown to be proportional to Rexp(-cR2), where c is a constant. We conduct direct numerical simulations of cloud droplets in turbulence and show that the results agree closely with the theoretical predictions and, when the computational domain is large enough to be comparable to the Π-chamber, agree with the results from the Π-chamber as well. We also show that the diffusion coefficient in the Fokker-Planck equation should be expressed in terms of the Lagrangian autocorrelation time of the supersaturation fluctuation in turbulent flow.

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  • Meng-Tze LEE, Pay-Liam LIN, Wei-Yu CHANG, Balaji Kumar SEELA, Jayalaks ...
    Type: Articles
    Article ID: 2019-048
    Published: 2019
    [Advance publication] Released: May 10, 2019
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     In the present work, long-term (10 years) raindrop size distribution (RSD) measurements from Joss-Waldvogel Disdrometer (JWD) installed at National Central University (NCU, 24°58′6″N 121°11′27″E), Taiwan and vertical profile of radar reflectivity were used to analyze the variations in gamma parameters of six seasons (winter, spring, mei-yu, summer, typhoon, and autumn) and types of precipitation. The normalized gamma distribution of RSD revealed that the highest mean Dm (Mass-weighted average diameter) values occurred in summer, whereas the highest mean log10Nw (normalized intercept parameter) values were found in winter. Furthermore, most of the rainfall rate falling at less than 20 mm h-1 occurs in Northern Taiwan. In this study, we used radar reflectivity to differentiate between convective and stratiform systems. It was revealed that the mean Dm values are higher in convective systems, whereas the mean log10Nw values are higher in stratiform systems. The structure of RSD in stratiform systems remains constant in all seasons; however, convection is similar to maritime type. The microphysical characteristics that are responsible for different RSD features in different seasons and types of precipitation are illustrated with the help of contoured frequency by altitude diagrams of radar reflectivity.
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  • Suranjith Bandara KORALEGEDARA, Chuan-Yao LIN, Yang-Fan SHENG
    Type: Articles
    Article ID: 2019-046
    Published: 2019
    [Advance publication] Released: April 22, 2019
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     This study analyzed the synoptic and mesoscale dynamics and underlying mechanism of an extreme rainfall and flood event that occurred in Sri Lanka from 14–17 May 2016 using the Weather Research and Forecasting Model simulations with a horizontal grid size of 3 km and observational data. This extreme rainfall event was associated with a low-pressure system (LPS) that originated over the Bay of Bengal in the Indian Ocean and passed over Sri Lanka. The observed maximum accumulation of rainfall during the event exceeded 300 mm at several weather stations during 15–16 May and resulted in severe flooding and landslides, particularly in the western part of the island. The model closely simulated the timing of the initiation of the LPS and its development along the east coast of Sri Lanka. The model could capture the overall rainfall tendency and pattern of this event. Synoptic and mesoscale analyses indicated that this extreme rainfall event occurred as the cumulative effect of a sustained low-level convergence zone generated by an enhanced westerly monsoon flow and the circulation of the LPS alongside a continuous supply of high-magnitude moisture, strong vertical motion, and orographic effects of the Central Mountains of Sri Lanka. Model sensitivity experiments indicated that rainfall over the western slope area of the mountains was enhanced by mountain lifting, whereas western coastal rainfall was reduced because the mountains blocked the northeasterly flow of the LPS.
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  • Akifumi NISHI, Hiroyuki KUSAKA
    Type: Articles
    Article ID: 2019-044
    Published: 2019
    [Advance publication] Released: April 19, 2019
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     This study numerically examined how the locally strong “Karakkaze” wind in the Kanto Plain of Japan is affected by terrain shape, particularly by a convex feature in the mountain range. Our method involved running idealized numerical simulations with the Weather Research and Forecast model with a horizontal grid spacing of 3 km. The results revealed that a strong-wind region formed in the lee area of the convex feature, hereafter the semi-basin, and leeward of the semi-basin. In contrast, weak-wind areas formed adjacent to the strong-wind region. These results were consistent with the basic features of the observed surface wind pattern of the Karakkaze during the winter monsoon. However, such a flow pattern did not appear in the numerical simulation with a mountain range that lacked a convex feature.

     Sensitivity experiments were also conducted to evaluate the detailed effects of a mountain range with convexity. Sensitivity experiments with different convex shapes revealed that strong winds appeared within and leeward of the semi-basin when the aspect ratio of convexity (ratio of the wave amplitude to the wavelength of the convexity) exceeded about 0.5. Sensitivity experiments on terrain shape suggested that saddles in the mountain range were not essential to the formation of the Karakkaze, but they could affect its strength. Sensitivity experiments on the mountain Froude number, Frm, showed that locally strong winds within and leeward of the semi-basin appeared only when the Frm was in the range 0.42–1.04. Sensitivity experiments with surface heat fluxes (SHFs) showed that the basic structure of the strong-wind region in the leeward plain of the convex feature did not depend strongly on SHFs. However, the addition of SHFs reduced the surface wind speed but increased the size of the strong-wind region.

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  • Boqi LIU, Congwen ZHU, Jingzhi SU, Shuangmei MA, Kang XU
    Type: Notes and Correspondence : Special Edition on Extreme Rainfall Events in 2017 and 2018
    Article ID: 2019-047
    Published: 2019
    [Advance publication] Released: April 19, 2019
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     The northward shift of the western North Pacific Subtropical High (WNPSH) in July 2018 broke the historical record since 1958 and resulted in extreme heat waves and casualties across Northeast Asia (NEA). The present work associated this extreme WNPSH anomaly with the anomalies of barotropic anticyclone above NEA originating from the strongest positive tri-pole pattern of sea surface temperature anomaly (SSTA) in the North Atlantic in July. Both data analysis and numerical experiments indicated that the positive tri-pole SSTA pattern could produce an upper-tropospheric wave source over the Europe, which stimulated an eastward propagating wave train along the subpolar westerly jet over the Eurasian continent. When its anticyclonic node reached NEA, the WNPSH started to shift northward. After the cyclonic node in the circulation anomaly encountered the Tibetan Plateau, atmospheric diabatic heating was enhanced over the eastern Tibetan Plateau, initiating another subtropical wave train, which furthered the northward shift of the WNPSH. Therefore, the wave source over Europe was critical for the northward shift of the WNPSH in July, connecting the tri-pole SSTA pattern in the North Atlantic with the WNPSH anomaly and maintaining the downstream effects of thermal forcing over the eastern Tibetan Plateau on the East Asian summer monsoon.
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  • Shun-ichi I. WATANABE, Akihiko MURATA, Hidetaka SASAKI, Hiroaki KAWASE ...
    Type: Articles
    Article ID: 2019-045
    Published: 2019
    [Advance publication] Released: April 15, 2019
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     This study evaluates possible changes in tropical cyclone (TC) precipitation over Japan under a future warmer climate using an ensemble projection generated by a non-hydrostatic regional climate model with a resolution of 5 km (NHRCM05) under the RCP8.5 scenario. NHRCM05 reproduces TC precipitation and TC intensity more accurately than does a general circulation model with a resolution of 20 km. The number of TCs approaching Japan is projected to decrease under the future climate, while the TC precipitation rate increases. As these two effects cancel each other out, total TC precipitation, and the frequency of the moderate TC precipitation that is usual under the present climate, show no significant change. On the other hand, the frequency of extreme TC precipitation increases significantly because the intensification in the TC precipitation rate outweighs the reduction in TC frequency. The increase in the TC precipitation rate is caused primarily by the increase in water vapor around the TCs, which in turn results from the change in environmental water vapor. The intensification and structural changes to TCs also contribute to the enhanced TC precipitation.
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  • Luteru TAUVALE, Kazuhisa TSUBOKI
    Type: Articles
    Article ID: 2019-042
    Published: 2019
    [Advance publication] Released: April 01, 2019
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     Geographic and meteorological characteristics of 479 tropical cyclones (TCs) in a study domain in the Southwest Pacific (defined by 135°E - 120°W and 5°S - 65°S) over the past 48 TC seasons from 1969–1970 to 2016–2017 were examined using the latest Southwest Pacific Enhanced Archive of Tropical Cyclones dataset. Examined metrics include the geographic distributions of TCs, numbers, intensity, length in days (TC days), accumulated cyclone energy (ACE), and power dissipation index (PDI). The results show increasing TC activities in the western, northwestern, northern and central subdomains of the nine subdomains in the study domain. The average latitudes of TC genesis and TC maximum intensity remained almost unchanged. Most of TCs took southward to southeastward paths, and the majority attained their maximum intensities in the western and central parts of the study domain. The annual number of TCs and TC days decreased over the study period, the numbers of stronger TCs slightly increased whereas stronger TC days increased. The highest annual lifetime-maximum intensity and average annual lifetime-maximum intensity also increased. The highest annual maximum intensification rates did not change much over the study period, nor did ACE and PDI. The results show correlations between highest annual lifetime-maximum intensity to the variations of average sea surface temperature (SST) as well as correlations between TC days to the variations of average SST in the region.
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  • Xiaoge XIN, Min WEI, Qingquan LI, Wei ZHOU, Yong LUO, Zongci ZHAO
    Type: Articles
    Article ID: 2019-043
    Published: 2019
    [Advance publication] Released: April 01, 2019
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     Two sets of decadal prediction experiments were performed with Beijing Climate Center climate system model version 1.1 (BCC-CSM1.1) with different initialization strategies. One experiment is relaxing modeled ocean temperature to the Simple Ocean Data Assimilation (SODA) reanalysis data (SODAInit). In the other (EnOI_HadInit) experiment, the modeled ocean temperature were relaxed toward the assimilated ocean data, which were generated by assimilating sea surface temperature (SST) of the Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) data to the ocean model of BCC-CSM1.1 using Ensemble Optimum Interpolation (EnOI) method. Comparisons between EnOI_HadInit and SODAInit hindcasts show that EnOI_HadInit is more skillful in predicting SST over the North Pacific, the southern Indian Ocean, and the North Atlantic. Improved prediction skill is also found for surface air temperature (SAT) over South Europe, North Africa, and Greenland, which is associated with the skillful prediction of the Atlantic multi-decadal oscillation in EnOI_HadInit. EnOI_HadInit and SODAInit are both skillful in predicting East Asian SAT, which is related to their skillful predictions of the tropical western Pacific SST. The result indicates that assimilated data generated by the ocean model of BCC-CSM1.1 with EnOI assimilation provide better initial conditions than SODA reanalysis data for the decadal predictions of BCC-CSM1.1.
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  • Jaechan LIM, Hyung-Min PARK
    Type: Notes and Correspondence
    Article ID: 2019-041
    Published: 2019
    [Advance publication] Released: March 19, 2019
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     In this paper, we propose an H-infinity (H) filtering approach for the prediction of bias in post-processing of model outputs and past measurements. This method adopts minimax strategy that is a solution for zero-sum games. The proposed H filtering approach minimizes maximum possible errors whereas a recently common approach that adopts the Kalman filtering (KF) minimizes the mean square errors. The proposed approach does not need the information of noise statistics unlike the method based on the KF, while training process is required. We show that the proposed approach outperforms the method based on the KF in experiments by applying real weather data in Korea.
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  • Hitoshi HIROSE, Shoichi SHIGE, Munehisa K. YAMAMOTO, Atsushi HIGUCHI
    Type: Articles
    Article ID: 2019-040
    Published: 2019
    [Advance publication] Released: March 15, 2019
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     We introduce a novel rainfall estimation algorithm with a random-forest machine-learning method only from Infrared (IR) observations. As training data, we use nine-band brightness temperature (BT) observations obtained from IR radiometers on the third-generation geostationary meteorological satellite (GEO) Himawari-8 and precipitation radar observations from the Global Precipitation Measurement core observatory. The Himawari-8 Rainfall estimation Algorithm (HRA) enables us to estimate rain rate with high spatial and temporal resolution (i.e., 0.04° every 10 min), covering the entire Himawari-8 observation area (i.e., 85°E–155°W, 60°S–60°N) based solely on satellite observations. We conducted a case analysis of the Kanto–Tohoku heavy rainfall event to compare rainfall estimation results of HRA and the near-real-time version of the Global Satellite Mapping of Precipitation (GSMaP_NRT), which combines global rainfall estimation products with microwave and IR BT observations obtained from satellites. In this case, HRA could estimate heavy rainfall from warm-type precipitating clouds, although GSMaP_NRT could not estimate heavy rainfall when the microwave satellites were unavailable. Further, a statistical analysis showed that the warm-type heavy rain seen in the Asian monsoon region occurred frequently when the BT differences between the 6.9-μm and 7.3-μm of water vapor (WV) bands (ΔT6.9–7.3) were small. Himawari-8 is the first GEO to include the 6.9-μm band which is sensitive to middle-to-upper tropospheric WV. An analysis for the weighting functions of the WV multibands revealed that ΔT6.9–7.3 became small as WV amount in the middle-to-upper troposphere was small and there were optically thick cloud with the cloud top near the middle troposphere. Statistical analyses during boreal summer (August and September 2015 and July 2016) and boreal winter (December 2015 and January and February 2016) indicate that HRA has higher estimation accuracy for heavy rain from warm-type precipitating clouds than a conventional rain estimation method based on only one IR band.
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  • Shingo WATANABE, Mikiko FUJITA, Sho KAWAZOE, Shiori SUGIMOTO, Yasuko O ...
    Type: Notes and Correspondence
    Article ID: 2019-038
    Published: 2019
    [Advance publication] Released: March 13, 2019
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     Future changes in the climatological distribution of clear air turbulence (CAT) and its seasonality over the North Pacific are estimated based on an ensemble of climate projections under warming for the globally averaged surface air temperature of 2 K relative to pre-industrial levels, which includes over 3000 years of ensembles using a 60-km atmospheric general circulation model (AGCM). The AGCM outputs are interpolated to a 1.25° horizontal resolution, and the climatological CAT frequency is computed. The CAT broadly decreases in the mid-latitude central to western North Pacific along with the anticyclonic (south) side of its present-day high-frequency band extending from Japan to the eastern North Pacific. Meanwhile, large relative increases are found outside the band, implying an increased risk of CAT encounters. Uncertainty in future CAT changes due to uncertainties in the spatial pattern of sea surface temperature change is addressed for the first time using six selected Climate Model Intercomparison Project Phase-5 (CMIP5) climate models. The uncertainty is greatest in the boreal winter and spring over the central North Pacific, and is associated with uncertainty in future changes in the jet stream and upper-level synoptic-scale disturbances.
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  • Haikun ZHAO, Liguang WU, Chao WANG, Philip J. KLOTZBACH
    Type: Articles
    Article ID: 2019-039
    Published: 2019
    [Advance publication] Released: March 11, 2019
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     Most studies have focused on variations of tropical cyclone (TC) frequency, intensity, and track over the western North Pacific (WNP), while variability of WNP TC season onset date (TCSO) has been less studied. Recent research has indicated a close association between WNP TCSO and sea surface temperature (SST) over the tropical Indian Ocean and the tropical central-eastern Pacific. This study finds the relationship between TCSO and SST underwent an interdecadal change in the late 1990s, likely due to a climate shift that occurred around that time. An observed significant correlation between TCSO and SST before the late 1990s and has been insignificant since that time. It was confrimed by the fact that ENSO positively correlates at 0.46 with TCSO from 1965-1999 (significant at the 95 % level), while the correlation becomes insignificant (0.16) during 1998-2016. Further analysis suggests that the close association between TCSO and SST is robust only for major El Niño events, with consistently extreme late TCSO following major El Niños during the satellite era. Accompanying the decay of major El Niños, tropical equatorial easterly anomalies in the WNP are driven by a Matsuno-Gill-type response to the specific SST anomaly pattern over the tropical Indo-Pacific sector. This in turn induces an anomalous anticyclone, anomalous westerly vertical wind shear, reduced mid-level moisture and suppressed convection over the WNP basin – all of which are unfavorable for WNP TCs, resulting in delayed TCSO following major El Niño events. These inter-decadal changes in the inter-annual correlation between TCSO and ENSO are largely due to the changing influence of moderate El Niño events on TCSO before and after the late 1990s. This study improves understanding of the ENSO-TC relationship, which should aid seasonal outlooks of WNP TC activity.
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  • Uju SHIN, Tae-Young LEE, Sang-Hun PARK
    Type: Articles
    Article ID: 2019-036
    Published: 2019
    [Advance publication] Released: February 19, 2019
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     An investigation has been carried out using rainfall observation data, National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) analysis and forecast data to explain the environment and processes that lead to heavy rainfall in the early morning over the Korean peninsula during episodes of cloud clusters (CCs) associated with mesoscale troughs (CCMTs). For this study, nine episodes with 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 the daytime; 2) the strong southwesterly band (SWB; an area with wind speeds > 12.5 m s-1) on the pressure level 925 hPa over the East China Sea, which is located southeast of the trough, strengthens and expands at nighttime toward the southwestern Korean peninsula; 3) the SWB supplies 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 9 CCMT episodes.
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  • Waheed IQBAL, Abdel HANNACHI, Toshihiko HIROOKA, Léon CHAFIK, Yayoi HA ...
    Type: Articles
    Article ID: 2019-037
    Published: 2019
    [Advance publication] Released: February 15, 2019
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     The interaction between the troposphere and the stratosphere has attracted the attention of climate scientists for several decades not least for the benefit it has on understanding dynamical processes and predictability. This interaction has been revived recently in regard to downward disturbance propagation effects on tropospheric circulations. The current study investigates such interactions over the North Atlantic region in relation to the eddy-driven jet stream. The atmospheric low-frequency variability in the winter over the North Atlantic sector is mainly associated with variations in the latitudinal positions of the North Atlantic eddy-driven jet stream. The Japanese Reanalysis JRA-55 data has been used to analyse the jet latitude statistics. The results reveal robust trimodality of the North Atlantic jet reflecting the latitudinal (i.e., northern, central and southern) positions in agreement with other reanalysis products. Thirty major sudden stratospheric warming events were analysed in relation to the three modes or regimes of the eddy-driven jet. The frequency of occurrence of the eddy-driven jet to be in a specific latitudinal position is largely related to the wave amplitude. The stratospheric polar vortex experiences significant changes via upward wave propagation associated with the jet positions. It is found that when the jet is close to its central mode the wave propagation of zonal wave number 2 from the troposphere to the stratosphere is significantly high. Eliassen-Palm fluxes from all waves and zonal wave number 1 depict deceleration of the stratospheric polar vortex for the eddy-driven jet with latitudinal position close to the northern mode. Plumb wave activity variations originate mainly in the Atlantic sector depending on the North Atlantic eddy-driven jet states. These significant associations between preferred latitudinal positions of the North Atlantic eddy-driven jet and the stratospheric dynamics may be a source of predictability.
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  • Tomoki MIYAKAWA, Hiroaki MIURA
    Type: Notes and Correspondence
    Article ID: 2019-034
    Published: 2019
    [Advance publication] Released: February 11, 2019
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     The properties of tropical convection are evaluated using one-month long simulation datasets produced by the non-hydrostatic icosahedral atmospheric model (NICAM) using 3.5-, 7-, and 14-km horizontal meshes with identical cloud-microphysics configurations. The simulations are targeted on the 2nd Madden-Julian oscillation (MJO) event observed in the CINDY2011/DYNAMO field campaign. An increase of high cloud fraction at 200 hPa level and a reduction of surface precipitation occur as the horizontal resolution increases, corresponding to the reduction of precipitation efficiency due to the shorter residence time inside stronger updrafts that occur at the higher resolution. The increase of high cloud fraction is followed by the warming of the troposphere, which results in an increase in the column water vapor and an elevation of the freezing level. The total water condensation is decreased at higher resolutions, which is likely due to a balance with the decreased outgoing longwave radiation (OLR). The reproduced MJOs, which accounted for a large portion of the tropical convections, were similar in the 3.5-km and 14-km simulations in terms of eastward propagation speeds and structures, including the characteristic westward tilt of the moisture anomaly with height. However, the amplitude of the anomalous MJO circulation was considerably smaller in the 3.5-km simulation. The robust resolution dependence and the interpretations presented in this study underline the necessity for a resolution-aware cloud-microphysics optimization method that will have value in the coming era of global cloud-resolving simulations.
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  • Takafumi SEGUCHI, Suginori IWASAKI, Masashi KAMOGAWA, Tomoki USHIYAMA, ...
    Type: Articles
    Article ID: 2019-033
    Published: 2019
    [Advance publication] Released: February 04, 2019
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     In the summer of 2016, 14 cases of jumping cirrus (JC) were observed around the Kanto region in Japan by ground-based, visible-light cameras. The cameras were set at the summit of Mt. Fuji and National Defense Academy (Kanagawa, Japan), and 15-second time-lapse photography was continually taken for the period. The location and spatial scale of the JC were calculated by measurements using the photometry of background stars in the nighttime and the geostationary meteorological satellite Himawari-8 infrared imagery. The environmental conditions of the JC were also investigated using radiosonde and Himawari-8 visible and infrared measurements. Comparing our cases to the JC in the United States of America (USA) reproduced by a three-dimensional, non-hydrostatic cloud model from previous studies, their motions, morphology, spatial and temporal scales showed similarities, although the horizontal scale of the JC and the magnitude of the underlying convection was relatively smaller in our cases. The sounding by the radiosonde in the vicinity of the storms showed that 3 of the 14 cases reached the stratosphere. However, the hydration of the lower stratosphere was not supported by an analysis of the brightness temperature difference (BTD) between 6.2 and 10.4 µm measured by Himawari-8. The averaged wind shear across the range of the jumping heights above the anvil was -1.1 ms-1 km-1. The maximum value of the convective available potential energy (CAPE) of the 14 cases was 1384 Jkg-1, which is several times smaller than those of the thunderstorm cases observed in the USA in previous numerical JC studies. This indicates that JC occurs from the cumulonimbus anvil top even if the convection is relatively weak. The motion of JC observed by visible-light cameras shows that it can transport moisture above the tops of the anvils of convective clouds regardless of its altitude as cloud ice appears to be sublimated.
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  • Tzu-Hsien KUO, Masataka MURAKAMI, Takuya TAJIRI, Narihiro ORIKASA
    Type: Articles
    Article ID: 2019-032
    Published: 2019
    [Advance publication] Released: February 02, 2019
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     Aluminum oxide (Al2O3) and iron oxide (Fe2O3) particles have been observed not only in industrial areas and their surroundings but also in natural atmospheric environments. These types of aerosols can influence aerosol–cloud interactions. In this study, physico-chemical properties, such as size distribution and ability to act as cloud condensation nuclei (CCN) as well as ice nucleating particles (INPs), of surrogates of ambient Al2O3 and Fe2O3 particles were investigated using a CCN counter, the Meteorological Research Institute’s (MRI’s) cloud simulation chamber, the MRI’s continuous flow diffusion chamber-type ice nucleus counter, and an array of aerosol instruments. The results indicated that their hygroscopicity parameter (κ-value) ranged from 0.01 to 0.03. This range is compatible with that of surrogates of mineral dust particles and is smaller than typical κ-values of atmospheric aerosols. On the other hand, based on their ice nucleation active surface site (INAS) densities, these materials may act as effective INPs via immersion freezing (i.e., ice nucleation triggered by particles immersed in water droplets). In the cloud chamber experiments, Al2O3 and Fe2O3 particles continuously nucleated ice crystals at temperatures < −14°C and < −20°C, respectively. This result indicates that the Al2O3 particles were better INPs than the Fe2O3 particles. Moreover, the INAS density of the Al2O3 particles was comparable to that of natural ambient dust.
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  • Type: Editorial
    Article ID: 2019-c
    Published: 2019
    [Advance publication] Released: February 02, 2019
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  • Peter SAUNDERS, Yafan YU, Zhaoxia PU
    Type: Articles
    Article ID: 2019-030
    Published: 2019
    [Advance publication] Released: January 25, 2019
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     Hurricane Joaquin, a notable hurricane over the Atlantic Ocean in 2015, is studied with emphasis on its unique hairpin turn that occurred between 2100 UTC 1 October and 0600 UTC 2 October 2015. A series of mesoscale high-resolution numerical simulations is performed with an advanced research version of the Weather Research and Forecasting (WRF) model. The sensitivity of numerical simulations to different cumulus, boundary layer, and microphysical parameterization schemes is examined to investigate the most relevant processes influencing the track evolution of Hurricane Joaquin. It is found that the numerical simulation of Hurricane Joaquin’s track is highly sensitive to the choice of cumulus scheme. Large-scale environmental conditions and hurricane inner-core structures are diagnosed. Results indicate that middle- to upper-level steering flows are crucial in influencing Joaquin’s track. Further investigation of the large-scale environment (e.g., middle- and upper-level trough, blocking high, thermal distribution, etc.) shows that middle-level blocking high plays an important role in Joaquin’s movement. The structure of the hurricane core region, including the vertical extent of diabatic heating, vertical velocity, and relative humidity, could also play an important role. Specifically, the asymmetry and local absolute vorticity tendency over the inner-core region and its vicinity has a strong implication for Joaquin’s hairpin turn.
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  • Akiyoshi WADA, Hiroshige TSUGUTI, Kozo OKAMOTO, Naoko SEINO
    Type: Articles : Special Edition on Tropical Cyclones in 2015–2016
    Article ID: 2019-029
    Published: 2019
    [Advance publication] Released: January 19, 2019
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     The September 2015 Kanto-Tohoku heavy rainfall occurred in a stationary linear convective system between Typhoons Kilo and Etau. We investigated the influence of sea surface temperature (SST) on the local heavy rainfall event using a regional air-sea strongly coupled data assimilation system based on the local ensemble transform Kalman filter (LETKF) and a nonhydrostatic atmosphere model (NHM) coupled with ocean-surface wave model and a multilayer ocean model together with the Advanced Microwave Scanning Radiometer 2 (AMSR2) level 2 (L2) SST product. From the validation of SST analyzed by the coupled data assimilation system with the Japanese geostationary satellite multi-functional transport satellite 2 hourly SST product and in situ observations at the moored buoy, we demonstrated that the coupled system with AMSR2 L2 SST led to improvement of the SST analysis. From the verification by using radiosonde observations and radar-raingauge rainfall analysis, the analysis of the lower-atmospheric components were improved by the air-sea coupled NHM-LETKF.

     The local torrential rain occurred around 37°N in the Tochigi prefecture was embedded in a stationary linear convective system. The location of the linear convective system corresponded to the synoptic-scale convergence area between the cyclonic circulation associated with Etau and easterly lower-tropospheric winds. Strong southerly winds associated with Etau caused enhancement of local convection periodically along the convergence area on the upwind side of the linear convective system and resulted in a wave-like train of the total water content around 4-8 km altitudes on the leeward side. The improvement of SST analysis could change not only the transition of Etau to the extratropical cyclone but also lower-tropospheric wind field and thereby the location of the stationary linear convective system with embedded local torrential rain.

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