Journal of the Meteorological Society of Japan. Ser. II
Online ISSN : 2186-9057
Print ISSN : 0026-1165
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  • Naoko SEINO, Ryoko ODA, Hirofumi SUGAWARA, Toshinori AOYAGI
    Type: Articles : Special issue on Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS)
    Article ID: 2018-029
    [Advance publication] Released: February 17, 2018
    JOURNALS FREE ACCESS ADVANCE PUBLICATION
     During the Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS) intensive observation period (IOP) in 2011-2013 summers, atmospheric environment of several heavy rainfalls was observed by means of radiosonde soundings in the Tokyo metropolitan area. We investigated formation and development processes of an extremely developed thunderstorm (Case 1 on 26 August 2011) and a moderately developed thunderstorm (Case 2 on 18 July 2013) observed in the TOMACS IOP, utilizing the radiosonde sounding data. Compared to Case 2, the mesoscale environment of the severe storm in Case 1 featured a lower level of free convection and a deeper layer of easterly flow. We carried out numerical simulations to investigate the formation processes of the convective systems in the two cases, using the Non-Hydrostatic Model (NHM) of the Japan Meteorological Agency (JMA) incorporating the Square Prism Urban Canopy (SPUC) scheme. Model results fairly represented the spatial distribution and amounts of the rainfall in both cases. In Case 1, the formation of a distinct convergence zone between easterly and southerly flows was the likely trigger of active convective systems around Tokyo. To further examine the urban impact on precipitation, we performed two comparative simulations, one using realistic current urban surface conditions (CRNT experiment) and the other using less urbanized surface conditions (LURB experiment). The CRNT experiment yielded more rainfall than the LURB experiment in the central urban area. It appears that higher temperatures caused by urbanization can lead to increased rainfall in Tokyo by intensifying convergence and ascending motion.
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  • Kazuo SAITO, Masaru KUNII, Kentaro ARAKI
    Type: Articles : Special issue on Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS)
    Article ID: 2018-027
    [Advance publication] Released: February 11, 2018
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      Local heavy rainfall of about 100 mm h-1 occurred in Tokyo and Kanagawa Prefecture on 26 August 2011. This rain was brought by a mesoscale convective system (MCS) that developed near a stationary front that slowly moved southward. In an analysis using geostationary multi-purpose satellite rapid scan images and dense automated weather station networks, development of the MCS occurred after the merging of sea breezes from the east (Kashima-nada) and the south (Tokyo Bay).

     Numerical experiments by the Japan Meteorological Agency (JMA) nonhydrostatic model (NHM) with horizontal resolutions of 10 km and 2 km using mesoscale 4D-VAR analysis of JMA for initial conditions tended to predict the position of intense rainfall areas west of observed positions. In the mesoscale ensemble forecast using perturbations from JMA’s one-week global ensemble prediction system (EPS) forecast, some ensemble members showed enhanced precipitation around Tokyo, but false precipitation areas appeared north of the Kanto and Hokuriku Districts.

     As an attempt to improve the model forecast, we modified the model, reducing the lower limit of subgrid deviation of water vapor condensation to diagnose the cloudiness for radiation. In the modified model simulation, surface temperatures around Tokyo increased by about 1°C and the position of the intense precipitation was improved, but the false precipitation areas in the Hokuriku District were also enhanced in the ensemble member which brought a better forecast than the control run.

     We also conducted ensemble prediction using a singular vector method based on NHM. One of the ensemble members unstabilized the lower atmosphere on the windward side of the Kanto District and suppressed the false precipitation in the Hokuriku District, and observed characteristics of the local heavy rainfall were well reproduced by NHM with a horizontal resolution of 2 km.

     A conceptual model of the initiation of deep convection by the formation of a low-level convergence zone succeeding merging of the two sea breezes from the east and south is proposed based on observations, previous studies, and numerical simulation results. In this event, the northerly ambient wind played an important role on the occurrence of the local heavy rainfall around Tokyo by suppressing the northward intrusion of the sea breeze from the south.

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  • Kozo OKAMOTO, Toshiyuki ISHIBASHI, Shoken ISHII, Philippe BARON, Kyoka ...
    Type: Articles
    Article ID: 2018-024
    [Advance publication] Released: February 05, 2018
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     This study evaluated the impact of a future space-borne Doppler wind lidar (DWL) on a super-low-altitude orbit using an observing system simulation experiment (OSSE) based on a sensitivity observing system experiment (SOSE) approach. Realistic atmospheric data, including wind and temperature, was provided as “pseudo-truth” (PT) to simulate DWL observations. Hourly aerosols and clouds that are consistent with PT winds were also created for the simulation. A full-scale lidar simulator, which is described in detail in the companion paper, simulated realistic line-of-sight wind measurements and observation quality information, such as signal-to-noise-ratio (SNR) and measurement error. Quality control (QC) procedures in the data assimilation system were developed to select high-quality DWL observations based on the averaged SNR from strong backscattering in the presence of aerosols or clouds. Also, DWL observation errors used in the assimilation were calculated using the measurement error estimated by the lidar simulator.

     The forecast impacts of DWL onboard polar- and tropical-orbiting satellites were assessed using the operational global data assimilation system. Data assimilation experiments were conducted in January and August in 2010 to assess overall impact and seasonal dependence. It is found that DWL on either polar- or tropical-orbiting satellites is overall beneficial for wind and temperature forecasts, with greater impacts for the January experiments. The relative forecast error reduction reaches almost 2 % in the tropics. An exception is a degradation in the southern hemisphere in August, suggesting a need to further refine observation error assignment and QC. A decisive conclusion cannot be drawn of the superiority of polar- or tropical-orbiting satellites due to their mixed impacts. This is probably related to the characteristics of error growth in the tropics. The limitations and possible underestimation of the DWL impacts, for example due to a simple observation error inflation setting, in the SOSE-OSSE are also discussed.

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  • Makoto KUJI, Atsumi MURASAKI, Masahiro HORI, Masataka SHIOBARA
    Type: Articles
    Article ID: 2018-025
    [Advance publication] Released: February 05, 2018
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     Cloud fractions were observed during research cruises onboard the research vessel (R/V) Shirase between Japan and Antarctica using a whole-sky camera and a ceilometer. The cruises, Japanese Antarctic Research Expeditions (JARE) 55 and 56, took place from November 2013 to April 2014 and November 2014 to April 2015, respectively. Cloud fractions were estimated from the whole-sky camera based on the sky brightness and spectral characteristics, while the ceilometer recorded the cloud occurrence frequency. According to the comparison of daily-averaged cloud fractions from the whole-sky camera with the ceilometer observations over the open ocean between Japan and Antarctica, the correlation coefficients were 0.87 and 0.93 for JARE 55 and 56, respectively. Overall, the results from both observation methods were consistent over the open ocean. Nevertheless, it was necessary to take surface conditions into consideration, particularly for the estimated cloud fractions from the whole-sky camera, because the contrast in brightness and spectral properties between cloudy and clear skies was lower over the sea ice region, owing to the higher surface albedo. Hence, the classification parameter was expressed as a function of sun elevation over the sea ice region in this study. This parameter was determined from part of the data over the sea ice region during JARE 55 and then applied to JARE 56 as well as to remaining data from JARE 55. As a result, the daily-averaged cloud fractions over the sea ice region were approximately 84% and 57% from JARE 55 and 56, respectively. The daily-averaged cloud fractions estimated from the whole-sky camera were also consistent with the ceilometer observations where the correlation coefficients with the sea ice region were 0.93 and 0.96 for JARE 55 and 56, respectively.
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  • Akihiro UCHIYAMA, Bin CHEN, Akihiro YAMAZAKI, Guangyu SHI, Rei KUDO, C ...
    Type: Articles
    Article ID: 2018-026
    [Advance publication] Released: February 05, 2018
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     The aerosol optical characteristics in the East Asian cities of Fukuoka and Beijing were measured from 2010 to 2014. These long-term season-crossing data were compared to understand the differences between the aerosol characteristics at a source and a downstream region. Previously, few long-term, season-crossing observations have been reported. Using a method developed by one of the present authors, the measurement data were analyzed so that the retrieved optical properties can be more accurate than those obtained in previous studies. Using these data, the aerosol characteristics and their frequency distributions were reliably obtained. In Fukuoka, the annual means of the extinction, scattering, and absorption coefficients Cext (525 nm), Csca (525 nm), and Cabs (520 nm) were 74.6, 66.1, and 8.1 M m−1, respectively, whereas those in Beijing were 412.1, 367.2, and 42.4 M m−1, respectively. The coefficients in Fukuoka were approximately one-fifth of those in Beijing. The single-scattering albedos ω 0 (525 nm) in Fukuoka and Beijing were 0.877 and 0.868, respectively. The asymmetry factors G (525 nm) in the two cities were 0.599 and 0.656, respectively. The extinction Ångström exponents αext in the two cities were 1.555 and 0.855, respectively. The absorption Ångström exponents αabs in the two cities were 1.106 and 0.977, respectively. The fine and coarse mode volume fractions in Fukuoka were approximately 80 % and 20 %, and those in Beijing were both approximately 50 % except in the summer.

     The Cext , Csca , and Cabs showed seasonal variation in both cities. Some other properties showed also seasonal variation. In particular, the seasonal variation in αabs was clear in both cities; it tended to be small in the summer and large in the winter. The frequency distributions of various parameters were also investigated. The frequency of Cext >500 M m−1 in Fukuoka was very low, and large Cext values were recorded more frequently in the spring than in other seasons. In Beijing, Cext > 1000 M m−1 values were recorded more frequently, and the frequency of 10 M m−1Cabs ≤ 60 M m−1 was high in the spring and summer. Furthermore, αabs < 1.0 values were recorded frequently, which cannot be explained by the simple external mixture of absorbing aerosols.

     To demonstrate the usefulness of the data obtained in this study, the relationships among αabs , αext , the volume size distribution, the imaginary part of the refractive index and ω 0 were investigated, and two characteristic cases in Beijing (winter) and Fukuoka (spring) were preliminarily analyzed.

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  • Hiroaki KAWASE, Takahiro SASAI, Takeshi YAMAZAKI, Rui ITO, Koji DAIRAK ...
    Type: Articles
    Article ID: 2018-022
    [Advance publication] Released: January 30, 2018
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     Geographical distributions of heavy snowfall, especially in the Pacific Ocean side of Japan, have not been elucidated due to low occurrence frequency of heavy snowfall and limited number of snow observation points. This study investigates the characteristics of synoptic conditions for heavy daily snowfall from western to northeastern Japan in the present climate, analyzing high-resolution regional climate ensemble experiments with 5-km grid spacing. The Japanese 55-year Reanalysis (JRA-55) and the 10-ensemble members of the database for Policy Decision making for Future climate change (d4PDF) historical experiments are applied to the lateral boundary conditions of the regional climate model. Dynamical downscaling using d4PDF (d4PDF-DS) enables us to evaluate much heavier snowfall events than those simulated by dynamical downscaling using JRA-55 (JRA55-DS).

     Over the Sea of Japan side, heavy snowfall occurs due to cold air outbreaks, while over the Pacific Ocean side, heavy snowfall is brought by extratropical cyclones passing along the Pacific Ocean coast. A comparison between JRA55-DS and d4PDF-DS indicates that heavier snowfall can occur due to more developed extratropical cyclones and enhanced cold air damming in the Tokyo metropolitan area. The geographical distributions of extremely heavy snowfall are different between two typical synoptic conditions, i.e., cold air outbreaks and extratropical cyclones. The difference is much clearer in the extremely heavy snowfall events than in all snowfall events. Heavy daily snowfall occurs in January and February on the Pacific Ocean side, in December and January on the Sea of Japan side, and in November and March in high mountainous areas. Saturated water vapor pressure is largest around 0 ℃ under the snowing conditions. Synoptic conditions from late fall to winter are closely related to preferable conditions for heavy snowfall over the mountainous areas where the surface air temperature is much less than 0 ℃ in the heavy snowfall events.

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  • Kei KAWAI, Kenji KAI, Yoshitaka JIN, Nobuo SUGIMOTO, Dashdondog BATDOR ...
    Type: Articles
    Article ID: 2018-023
    [Advance publication] Released: January 26, 2018
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     The Gobi Desert is one of the major sources of Asian dust, which influences the climate system both directly and indirectly through its long-range transport by the westerlies. In this desert, three ground-based lidars are operated in Dalanzadgad, Sainshand, and Zamyn-Uud, Mongolia. This study firstly combined these lidars into a lidar network and shows the spatial development of a dust layer over the desert and the long-range transport of the dust during 22–23 May 2013 via the lidar network. During this dust event, a cold front accompanying an extratropical cyclone moved southeastward across the desert and sequentially passed through Dalanzadgad, Sainshand, and Zamyn-Uud. In Dalanzadgad, in the central part of the desert, a dust storm occurred owing to the strong wind (6–10 m s -1) associated with the cold front and reached a top height of 1.6 km. Some of the dust floated at a height of 0.9–1.6 km along the cold frontal surface. In Sainshand and Zamyn-Uud, in the eastern part of the desert, the dust layer extended from the atmospheric boundary layer (ABL) to the free troposphere in the updraft region of warm air in the cold frontal system. Overall, while the dust layer was moving across the desert with the cold frontal system, it was developing up to the free troposphere. The mechanism of this development can be explained by the combination of two processes as follows: (1) continuous emission of dust from the desert surface to the ABL by the strong wind around the cold front and (2) continuous transport of the dust from the ABL to the free troposphere by the updraft of the warm air in the cold frontal system. This mechanism can contribute to the long-range transport of dust by the westerlies in the free troposphere.
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  • Keiichi ISHIOKA
    Type: Notes and Correspondence
    Article ID: 2018-019
    [Advance publication] Released: January 23, 2018
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      A new recurrence formula to calculate the associated Legendre functions is proposed for efficient computation of the spherical harmonic transform. This new recurrence formula makes the best use of the fused multiply-add (FMA) operations implemented in modern computers. The computational speeds in calculating the spherical harmonic transform are compared between a numerical code in which the new recurrence formula is implemented and other code using the traditional recurrence formula. This comparison shows that implementation of the new recurrence formula contributes to a faster transform. Furthermore, a scheme to maintain the accuracy of the transform, even when the truncation wavenumber is huge, is also explained.
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  • Nurfiena Sagita PUTRI, Hironobu IWABUCHI, Tadahiro HAYASAKA
    Type: Notes and Correspondence : Special issue on Meteorology and Climate Change Studies by Using the Geostationary Meteorological Satellite Himawari-8
    Article ID: 2018-020
    [Advance publication] Released: January 23, 2018
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     Two case studies of Mesoscale Convective System (MCS) in Indonesian region were conducted by applying an improved GTG tracking algorithm and ICAS algorithm to Himawari-8 AHI infrared data. The first case over Java Island showed a land-originating MCS in the boreal winter, which coincided with a wet phase of Madden-Julian Oscillation (MJO) over the Maritime Continent. The second case showed the evolution of MCS under the influence of a strong vertical wind shear during the boreal summer. The cloud top height (CTH) of deep convective part in the first case was larger than that in the second case, while the temporal evolution of CTH was similar between two cases. For the anvil part, the median CTH of the second case was relatively stable at around 13 km, while that of the first case showed a considerable temporal variation ranging from 14 to 16 km. The cloud-particle effective radius (CER) of anvil increased after the period of maximum deep convective CTH in both cases, although the CER was slightly larger in the second case than in the first case. These differences in cloud properties between two cases were attributable to the background wind profiles.
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  • Fumiaki FUJIBE
    Type: Articles
    Article ID: 2018-021
    [Advance publication] Released: January 23, 2018
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     Climatological features of surface air temperature variations on time scales of a few minutes to one hour were examined using one-minute data, spanning a four-year period, from 917 automated stations in Japan. The temperature time series was spectrally analyzed after the application of a Gaussian high-pass filter, and the variances with periods of 64 minutes or less were statistically analyzed as sub-hourly temperature variations. The result obtained shows that daytime temperature variation is observed throughout the country with relatively small regional differences. The amplitudes of daytime temperature variations were larger during spring and summer than those during autumn and winter, and under high temperature and sunny weather than under low temperature, no sunshine, and precipitation. A cross spectral analysis of temperature and wind speed reveals that temperature peaks tend to coincide with or lag behind wind speed minima. The variation is likely to correspond to the convective motion in the mixing layer. On the other hand, the intensity of nighttime temperature variation showed a large amount of scatter among stations, with exceptionally large variations during winter at some stations in northern and eastern Japan. Nighttime temperature variation tends to be in-phase with wind speed variation, with longer periods than daytime temperature variation, and is more intense under low temperature and low wind speed than under high temperature, high wind speed, and precipitation. Stations with large winter nighttime temperature variations tend to be located on a col or a slope, where the surface inversion layer is likely to be easily disturbed by any kind of atmospheric motion.
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  • Jingchao LONG, Yuqing WANG, Suping ZHANG
    Type: Articles
    Article ID: 2018-018
    [Advance publication] Released: January 15, 2018
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     The cloud variability and regime transition from-stratocumulus-to-cumulus across the sea surface temperature front in the Kuroshio region over the East China Sea are important regional climate features and may affect the earth’s energy balance. However, because of large uncertainties among available cloud products, it is unclear which cloud datasets are more reliable for use in studying the regional cloud features and to validate cloud simulations in the region by climate models. In this study, the monthly low cloud amount (LCA) and total cloud amount (TCA) datasets in the region from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), Moderate-resolution Imaging Spectroradiometer (MODIS) and International Comprehensive Ocean-Atmosphere Data Set (ICOADS) are validated against the combined product of CloudSat+CALIPSO (CC) in terms of the consistency and discrepancy in the climatologically mean, seasonal cycle, and interannual variation. The results show that LCA and TCA derived from MODIS and CALIPSO present relatively high consistency with CC data in the climatological annual mean and show similar behavior in seasonal cycle. The consistency in LCA between the three datasets and the CC is generally good in cold seasons (winter, spring and fall) but poor in summer. MODIS shows the best agreement with CC in fall with the correlation coefficient of 0.77 at the confidence level over 99%. CALIPSO and MODIS can provide competitive description of TCA in all seasons while ICOADS is good in terms of the climatological seasonal mean of TCA in winter only. Moreover, the interannual variation of LCA and TCA from all datasets is highly correlated with that from CC in both winter and spring with the Matching Score ranging between 2/3 and 1. Further analysis with long-term data suggests that both LCA and TCA from ICOADS and MODIS can be good references for the studies of cloud interannual variability in the region.
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  • V. CHANDRASEKAR, Haonan CHEN, Brenda PHILIPS
    Type: Articles : Special issue on Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS)
    Article ID: 2018-015
    [Advance publication] Released: January 12, 2018
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     The Center for Collaborative Adaptive Sensing of the Atmosphere (CASA) Dallas-Fort Worth (DFW) Urban Demonstration Network consists of a combination of high resolution X-band radar network and a National Weather Service S-band radar system (i.e., KFWS radar). Based primarily on these radars, CASA has developed end-to-end warning system that includes sensors, software architecture, products, data dissemination and visualization, and user decision making. This paper presents a technical summary of the DFW radar network for urban weather disaster detection and mitigation, from the perspective of tracking and warning of hails, tornadoes, and floods. Particularly, an overview of the X-band radar network design tradeoffs is presented. The architecture and associated algorithms for various product systems are described, including the real-time hail detection system, the multiple Doppler vector wind retrieval system, and the high-resolution quantitative precipitation estimation system. Sample products in the presence of high wind, tornado, hail, and flash flood are provided, and the systems’ performance is demonstrated through cross validation with ground observations and weather reports.
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  • Robert CIFELLI, V. CHANDRASEKAR, Haonan CHEN, Lynn E. JOHNSON
    Type: Articles : Special issue on Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS)
    Article ID: 2018-016
    [Advance publication] Released: January 12, 2018
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     An X-band radar system was deployed in Santa Clara, CA from February through May 2016 to support the National Weather Service in the event of potential flooding during one of the largest El Niños on record and to provide better understanding of rainfall processes occurring in the Bay Area. The system was also used to provide high quality precipitation estimation (quantitative precipitation estimation - QPE) for Santa Clara’s urban hydrologic modeling system. Although the Bay Area has coverage from the NEXRAD operational radar network, the combination of topographic influences and proximity to a maritime environment provide unique QPE challenges in this urban region. The X-band radar provided high quality rainfall estimates that performed better than NEXRAD, demonstrating the added value of the X-band system. High resolution rainfall monitoring systems in urban regions also provide a host of benefits across different sectors of the economy, including flood damage mitigation, water quality, water supply, and transportation.
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  • Takuya KAWABATA, Hans-Stefan BAUER, Thomas SCHWITALLA, Volker WULFMEYE ...
    Type: Articles : Special issue on Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS)
    Article ID: 2018-017
    [Advance publication] Released: December 27, 2017
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     In the preparation for polarimetric radar data assimilation, it is essential to examine the accuracy of forward operators based on different formulations. For this purpose, four forward operators that focus on warm rain condition are compared with both each other and actual observations with respect to their performance for C-band dual polarimetric radars. These operators mutually consider radar beam broadening and climatological beam bending. The first operator derives polarimetric parameters assuming an exponential raindrop size distribution obtained by the models and is based on fitting functions against scattering amplitudes. The other three converters estimate the mixing ratio of rainwater from the measured polarimetric parameters. The second converter uses both the horizontal reflectivity (ZH) and the differential reflectivity (ZDR), the third uses the specific differential phase (KDP), and the fourth uses both KDP and ZDP, respectively. Comparisons with modeled measurements show that the accuracy of the third converter is superior to the other two. Another evaluation with actual observations shows that the first converter has slightly higher fractions skill scores than the other three. Considering the attenuation effect, the fitting function and the operator only with KDP are found to be the most suitable for data assimilation at C-band.
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  • Jing XU, Yuqing WANG
    Type: Articles
    Article ID: 2018-014
    [Advance publication] Released: December 26, 2017
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     The dependence of intensification rate (IR) of a tropical cyclone (TC) on its initial structure, including the radius of maximum wind (RMW) and the radial decay rate of tangential wind outside the RMW, is examined based on ensemble of simulations using a nonhydrostatic axisymmetric cloud-resolving model. It is shown that the initial spinup period is shorter and the subsequent IR is larger for the storm with the initially smaller RMW or with the initially more rapid radial decay of tangential wind outside the RMW. The results show that the longevity of the initial spinup period is determined by how quickly the inner-core region becomes nearly saturated in the middle and lower troposphere and thus deep convection near the RMW is initiated and organized. Because of the larger volume and weaker Ekman pumping, the inner-core of the initially larger vortex takes longer time to become saturated and thus experiences a longer initial spinup period. The vortex initially with the larger RMW (with the slower radial decay of tangential wind outside the RMW) has lower inertial stability inside the RMW (higher inertial stability outside the RMW) develops more active convection in the outer-core region and weaker boundary-layer inflow in the inner-core region and thus experiences lower IR during the primary intensification stage.
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  • Hirofumi SUGAWARA, Ryoko ODA, Naoko SEINO
    Type: Articles : Special issue on Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS)
    Article ID: 2018-010
    [Advance publication] Released: December 21, 2017
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     Does the cities enhance precipitation? It is an unsettled question and the comprehensive answer has not been archived for it. This study focuses on the urban heat excess and evaluates its influence on atmospheric instability which is the background condition for the convective precipitation. A simple approach was developed that involved calculating the daytime evolution of the mixed layer over homogeneous ground surface. Calculations were based on the ensemble average of observations. The convective available potential energy (CAPE) was evaluated for both urban and rural land cover. Urban heat excess, which was 200 W m-2 higher in the urban than rural area, increased CAPE by 75 % comparing to the rural CAPE of 513 J kg-1. Results show that cities could cause favorable stratification of the atmosphere for convective precipitation.
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  • Stéphane BÉLAIR, Sylvie LEROYER, Naoko SEINO, Lubos SPACEK, Vanh SOUVA ...
    Type: Articles : Special issue on Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS)
    Article ID: 2018-011
    [Advance publication] Released: December 21, 2017
    JOURNALS FREE ACCESS ADVANCE PUBLICATION

     Heavy precipitation fell over Tokyo in the afternoon of 26 August 2011, leading to flooding and major disruptions for the population, businesses, and authorities. Over 150 mm of precipitation was observed over the city center on that day, with hourly accumulations reaching values as high as 90 mm in late afternoon. Numerical forecasts of this case were performed with a 250-m grid spacing version of the Global Environmental Multi-scale (GEM) model in the context of the Tokyo Metropolitan Area Convection Study (TOMACS). Although initialized only from a global 25-km upper-air analysis, results indicate that GEM is able to produce the intense precipitation over Tokyo at about the right location and time.

     A sensitivity test in which the urban surface scheme is switched off and replaced with tall grass suggests that the urban environment might have had considerable impact on precipitation intensity, but not on its occurrence or its timing. Based on diagnostics from the GEM integrations, the increased intensity of precipitation seems more related to an enhancement of lateral inflow of low-level moist static energy from Tokyo Bay than to augmented surface fluxes of heat and humidity from the city itself. The existence of low-level bands with locally high values of equivalent potential temperature indicates that the additional moist energy is distributed unevenly through the Tokyo area, an aspect of the simulation which is speculated to have directly contributed to the increase in precipitation intensity over the city.

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  • Tetsuya SANO, Satoru OISHI
    Type: Articles : Special issue on Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS)
    Article ID: 2018-012
    [Advance publication] Released: December 21, 2017
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     To elucidate the formation of a localized rainfall on a basin with heat and aridity under weak synoptic disturbance in summer, the characteristics of atmospheric conditions on the Kofu Basin preceding the appearance of primary precipitating cells were described from 23 localized rainfall events on the Kofu Basin on days of weak synoptic disturbance at the surface from 1 June to 30 September in 2012 to 2014. Furthermore, using the case study conducted on 25 July 2014, the formation of the atmospheric conditions was described from the standpoint of moisture behavior.

     Owing to the thermal contrast between the Kofu Basin with heat and aridity and the outside environment, the south-component wind blowing in the valley connecting it to the coastal region of Suruga Bay and the east-component wind blowing in the valley connecting it to the Kanto Plain entered the Kofu Basin as southwesterly wind and southeasterly wind, respectively, which caused an increase in the water vapor mixing ratio and a slight decrease in temperature at the surface. After that, the amount of precipitable water vapor derived by the global navigation satellite system observation (GNSS-PWV) at Nakamichi in the central region of the Kofu Basin increased abruptly after the moderate increase in GNSS-PWV at all the observation points on the Kofu Basin. Finally, a cloud appeared over the local region between the southwesterly wind and the southeasterly wind; the precipitating cells appeared here at 3.25 to 6.25 km above sea level.

     From the above results, the moisture transport to the Kofu Basin, the moisture concentration in the local region, and the appearance of precipitating cells were discussed as the formation of atmospheric conditions leading to a localized rainfall on a basin with heat and aridity.

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  • Guanghua CHEN, Ke WANG
    Type: Articles
    Article ID: 2018-013
    [Advance publication] Released: December 21, 2017
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     Although both the tropical cyclone (TC) peak seasons in 2016 and 1998 are in the decaying stage of a super El Niño, TC activities over the western North Pacific (WNP) exhibit vast differences. The TCs in 2016 feature more number, greater intensity and more distinct monthly variation of TC activity in contrast to those in 1998. The detailed comparison shows the warm sea surface temperature anomaly over the WNP in 2016 had a higher magnitude and a more eastward extension, compared with the case in 1998. Especially in August coincident with the enhanced Madden-Julian oscillation (MJO) westerly phase, more TCs clustered within the eastward-extending convective belt caused by the southwesterly surge. The mean longitude of TC genesis in 2016 shifted more eastward, favorable for the longer lifetime and greater intensity of the TCs. In terms of the extratropical influences, the cyclonic circulation anomaly associated with the Silk Road Pattern from the middle latitude penetrated southward and split the WNP subtropical high (WNPSH) into two components in August of 2016, causing the deep-tropospheric southerly steering flows in between and thus the TC northward-prone tracks. During the boreal autumn in 2016, the WNPSH was strengthened and stretched westward, producing the robust easterly steering flows that led to the successive TCs affecting the coastal areas of the East Asia.
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  • Chung-Chuan YANG, Chun-Chieh WU, Kevin K. W. CHEUNG
    Type: Articles
    Article ID: 2018-009
    [Advance publication] Released: December 15, 2017
    JOURNALS FREE ACCESS ADVANCE PUBLICATION

     The steering flow analysis based on potential vorticity (PV) diagnosis is used to examine the reasons why the National Centers for Environmental Prediction Global Forecast System (NCEP-GFS) model showed large track forecast errors with over-recurving movement in Typhoon Fengshen (2008). In particular, two forecasts initialized at 0000 UTC 19 and 20 June 2008 are demonstrated in this study. The deep-layer-mean (DLM) steering flow between 925 and 300 hPa with tropical cyclone components filtered out is directed to the west or northwest in the analysis field, which can account for the continuous westward and northwestward movement in the best track. However, the DLM steering flow is shown more toward the north in the forecast fields. Four distinct PV features associated with the corresponding subtropical high, monsoon trough, continental high, and midlatitude trough are identified to diagnose their balanced steering flows around the storm. The result based on PV analysis indicates that the reduced westward steering flow in the forecast field is mainly attributed to the subtropical high which is over-predicted to extend southwestward, as well as the continental high with underestimated coverage, as characterized by the geopotential height at 500 hPa. The steering flow associated with the monsoon trough plays an essential role while Typhoon Fengshen (2008) experiences northward recurvature in both analysis and forecast fields. Therefore the associated reduced westward steering flow in the NCEP-GFS model leads to the over-recurvature of Fengshen.

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  • Miho SEKIGUCHI, Hironobu IWABUCHI, Takashi M. NAGAO, Teruyuki NAKAJIMA
    Type: Articles : Special issue on Meteorology and Climate Change Studies by Using the Geostationary Meteorological Satellite Himawari-8
    Article ID: 2018-007
    [Advance publication] Released: December 08, 2017
    JOURNALS FREE ACCESS ADVANCE PUBLICATION

     We developed an atmospheric gas absorption table for the Advanced Himawari Imager (AHI) based on the correlated k-distribution (CKD) method with the optimization method, which was used to determine quadrature weights and abscissas. We incorporated the table and band information of the AHI into a multi-purpose atmospheric radiative transfer package, Rstar. We updated the package so that users could easily specify the satellite and band number. Use of this update made it possible for the optimized CKD method to carry out calculations rapidly and accurately. Rstar is easy for beginners to use and facilitates comparison of results. Cloud retrieval tests using different numbers of quadrature points showed that cloud retrievals could be significantly affected by the accuracy of the CKD model.

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  • Teruyuki KATO
    Type: Articles
    Article ID: 2018-008
    [Advance publication] Released: December 08, 2017
    JOURNALS FREE ACCESS ADVANCE PUBLICATION

     This study investigated the representative height of low-level water vapor field that can be used to examine the occurrence possibility of heavy rainfall in East Asia. First, cloud base heights (CBHs) of moist convection were statistically examined by performing simulations with a 1-km-resolution numerical model during April–August 2008, with a focus on Kyushu and Shikoku Islands, western Japan. CBHs of moist convection with strong updrafts were simulated mainly around 500 and 300 m heights above sea level over land and over the ocean, respectively. This result indicates that low-level humid air below a height of 500 m is very important for the initiation of strong moist convection. Moreover, the equivalent potential temperature θe at the CBHs was examined to clarify θe values of lifted air parcels initiating cumulonimbus development. This result showed that below the CBHs, θe was usually around 355 K.

     Next, given these results for the CBHs, θe at 500 m height from 10-km-resolution objective analysis data was statistically compared with θe at various heights and pressure levels over the ocean south of 35 °N in East Asia during June–September 2008. These comparisons showed that analyses at the 850-hPa level could not represent the low-level water vapor field, while the θe field at 850 hPa in the Baiu season was strongly influenced by convective activity over the Baiu frontal zone. The θ e field at 925 hPa also could not adequately represent the low-level water vapor field, but the difference in θ e between heights of 250 and 500 m was very small. Because high θ e layers must have some thickness, data at 500 m height can be considered representative of the low-level water vapor field in analyses examining the initiation of moist convection leading to heavy rainfall.

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  • Yuhei YAMAMOTO, Hirohiko ISHIKAWA
    Type: Articles : Special issue on Meteorology and Climate Change Studies by Using the Geostationary Meteorological Satellite Himawari-8
    Article ID: 2018-004
    [Advance publication] Released: November 24, 2017
    JOURNALS FREE ACCESS ADVANCE PUBLICATION
     The land surface emissivity (LSE) in the thermal infrared (TIR) is an essential parameter in the retrieving the land surface temperature (LST) from space. This paper describes the LSE maps in TIR three bands (centered at 10.4, 11.2 and 12.4 μm) used for retrieving the LST from Himawari-8. Himawari-8, a next-generation geostationary satellite has high spatial and temporal resolutions compared to previous geostationary satellites. Due to these improvements, it is expected that the Himawari-8 LST product contribute to observe small-scale environments in high-frequency. In this study, the LSE is estimated by a semi-empirical method, which is a combination of the classification based method and the normalized difference vegetation index (NDVI) thresholds method. The land cover classification information is taken from the Global Land Cover by National Mapping Organizations version3 (GLCNMO 2013). Material emissivities of soil, vegetation and others are taken from the MODIS UCSB emissivity library and the ASTER spectral library. This method is basically following to Peres and DaCamara (2005) but advanced considerations are added. They are phenology of vegetation, flooding of paddy field, snow/ice coverage and the internal reflections (cavity effect) in the urban area. The average cavity effect on LSE in urban canopy is approximately 0.01, but it reaches 0.02 in built-up area. The sensitivity analysis shows that the total LSE errors for three bands are less than 0.02. The LSE estimation is especially stable at the vegetation area, where the error is less than 0.01.
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  • Yuhei YAMAMOTO, Hirohiko ISHIKAWA, Yuichiro OKU, Zeyong HU
    Type: Articles : Special issue on Meteorology and Climate Change Studies by Using the Geostationary Meteorological Satellite Himawari-8
    Article ID: 2018-005
    [Advance publication] Released: November 24, 2017
    JOURNALS FREE ACCESS ADVANCE PUBLICATION
     This paper presents a method to estimate the land surface temperature (LST) from Himawari-8 data. The Advanced Himawari Imager (AHI) onboard Himawari-8 has three thermal infrared bands in the spectral range of 10–12.5 μm. We developed a nonlinear three-band algorithm (NTB) that makes the best use of these bands to estimate the LST. The formula of the algorithm includes ten coefficients. The optimum values of these coefficients were derived using a statistical regression method from the simulated data, as obtained by a radiative-transfer model. The simulated data sets corresponding to a variety of values of LST, surface emissivity, type and season of temperature and water vapor profiles. Viewing zenith angles (VZAs) from 0° to 60° were considered. For the coefficients obtained in this way, we checked the root-mean-square error (RMSE) in terms of the VZA, LST and precipitable water dependence. We showed that the NTB can accurately estimate the LST with the RMSE less than 0.9 K by comparison with the nonlinear split-window algorithm developed by Sobrino and Romaguera (2004). Moreover, we evaluated the sensitivities of the LST algorithms to the uncertainties in input data by using the dataset independent of dataset used to obtain coefficients. Consequently, we showed that the NTB has the highest robustness against the uncertainties in input data. Finally, the stepwise LST retrieval method were constructed. This method includes a simple cloud mask procedure and the LSE estimation. The LST product was evaluated using in-situ data over the Tibetan Plateau, and then the validity was confirmed.
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  • Hironobu IWABUCHI, Nurfiena Sagita PUTRI, Masanori SAITO, Yuka TOKORO, ...
    Type: Articles : Special issue on Meteorology and Climate Change Studies by Using the Geostationary Meteorological Satellite Himawari-8
    Article ID: 2018-001
    [Advance publication] Released: September 15, 2017
    JOURNALS FREE ACCESS ADVANCE PUBLICATION

      An algorithm for retrieving the macroscopic, physical and optical properties of clouds from thermal infrared measurements is applied to the Himawari-8 multiband observations. A sensitivity study demonstrates that the addition of the single CO2 band of Himawari-8 is effective for the estimation of cloud top height. For validation, retrieved cloud properties are compared systematically with collocated active remote sensing counterparts with small time lags. While retrievals agree reasonably for single-layer clouds, multilayer cloud systems with optically thin upper clouds overlying lower clouds are the major source of error in the present algorithm. Validation of cloud products is critical for identifying the characteristics, advantages and limitation of each product and should be continued in the future.

     As an application example, data are analyzed for eight days in the vicinity of the New Guinea to study the diurnal cycle of the cloud system. The present cloud property analysis investigates cloud evolution through separation of different cloud types and reveals typical features of diurnal cycles related to the topography. Over land, middle clouds increase from 0900 to 1200 local solar time (LST), deep convective clouds develop rapidly during 1200–1700 LST with a subsequent increase in cirrus and cirrostratus cloud amounts. Over the ocean near coastlines, a broad peak of convective cloud fraction is seen in the early morning. The present study demonstrates the utility of frequent observations by Himawari-8 for life cycle study of cloud systems, owing to the ability to capture their continuous temporal variations.

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  • Ryo OYAMA, Masahiro SAWADA, Kazuki SHIMOJI
    Type: Articles : Special issue on Weather and Environmental Studies Using the Geostationary Meteorological Satellite Himawari-8
    Article ID: 2017-024
    [Advance publication] Released: August 10, 2017
    JOURNALS FREE ACCESS ADVANCE PUBLICATION
     The high temporal and spatial resolutions of geostationary satellite observations achieved by recent technological advances have facilitated derivation of atmospheric motion vectors (AMVs), even in a tropical cyclone (TC) where the winds abruptly change. This study used TCs in the western North Pacific basin to investigate the ability of upper tropospheric AMVs to estimate TC intensity and structure. We first examined the relationships between the cloud-top wind fields captured by 6-hourly upper tropospheric AMVs derived from images of the Multi-functional Meteorological Satellite (MTSAT) and the surface maximum sustained wind (MSW) of the Japan Meteorological Agency best-track data for 44 TCs during 2011–2014. The correlation between the maximum tangential winds of the upper tropospheric AMVs (UMaxWinds) and MSWs was high, about 0.73, the suggestion being that the cyclonic circulation near the cloud top was intensified by the upward transport of absolute angular momentum within the TC inner core. The upper tropospheric AMVs also revealed that the mean radii of UMaxWinds and the maximum radial outflows shifted inward as the TC intensification rate became large, the implication being that low-level inflow was strong for TCs undergoing rapid intensification. We further examined the possibility of estimating the MSW by using 30-min-interval UMaxWinds derived from Himawari-8 target observations, which have been used to track TCs throughout their lifetimes. A case study using Typhoon Lionrock (1610) showed that the UMaxWinds captured changes of the cyclonic circulation near the cloud top within the inner core on a time scale shorter than one day. It was apparent that the increase of UMaxWind was associated with intensification of the TC warm core and shrinkage of UMaxWind radius. These results suggest that the Himawari-8 AMVs include useful information on TC intensification and related structural changes to support the TC intensity analysis and structure monitoring.
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