著者
Shinya KOBAYASHI Yukinari OTA Yayoi HARADA Ayataka EBITA Masami MORIYA Hirokatsu ONODA Kazutoshi ONOGI Hirotaka KAMAHORI Chiaki KOBAYASHI Hirokazu ENDO Kengo MIYAOKA Kiyotoshi TAKAHASHI
出版者
(公社)日本気象学会
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
vol.93, no.1, pp.5-48, 2015 (Released:2015-03-18)
参考文献数
128
被引用文献数
213 783

The Japan Meteorological Agency (JMA) conducted the second Japanese global atmospheric reanalysis, called the Japanese 55-year Reanalysis or JRA-55. It covers the period from 1958, when regular radiosonde observations began on a global basis. JRA-55 is the first comprehensive reanalysis that has covered the last half-century since the European Centre for Medium-Range Weather Forecasts 45-year Reanalysis (ERA-40), and is the first one to apply four-dimensional variational analysis to this period. The main objectives of JRA-55 were to address issues found in previous reanalyses and to produce a comprehensive atmospheric dataset suitable for studying multidecadal variability and climate change. This paper describes the observations, data assimilation system, and forecast model used to produce JRA-55 as well as the basic characteristics of the JRA-55 product. JRA-55 has been produced with the TL319 version of JMA’s operational data assimilation system as of December 2009, which was extensively improved since the Japanese 25-year Reanalysis (JRA-25). It also uses several newly available and improved past observations. The resulting reanalysis products are considerably better than the JRA-25 product. Two major problems of JRA-25 were a cold bias in the lower stratosphere, which has been diminished, and a dry bias in the Amazon basin, which has been mitigated. The temporal consistency of temperature analysis has also been considerably improved compared to previous reanalysis products. Our initial quality evaluation revealed problems such as a warm bias in the upper troposphere, large upward imbalance in the global mean net energy fluxes at the top of the atmosphere and at the surface, excessive precipitation over the tropics, and unrealistic trends in analyzed tropical cyclone strength. This paper also assesses the impacts of model biases and changes in the observing system, and mentions efforts to further investigate the representation of low-frequency variability and trends in JRA-55.
著者
SAITO Kazuo KUNII Masaru ARAKI Kentaro
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2018-027, (Released:2018-02-11)
被引用文献数
2

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.
著者
Axel GABRIEL Dieter PETERS
出版者
(公社)日本気象学会
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
vol.86, no.5, pp.613-631, 2008 (Released:2008-11-13)
参考文献数
16
被引用文献数
19 26 26

The long-term behavior of Rossby wave breaking (RWB) events is investigated by a diagnosis separating in different asymmetric types of RWB, i.e., cyclonically sheared waves breaking predominantly pole-ward (P1) or equatorward (LC2) and anticyclonically sheared waves breaking predominantly poleward (P2) or equatorward (LC1). Generally, RWB can be identified by meridional overturning of potential vorticity (PV) on specific isentropes, but a separation in poleward or equatorward asymmetry is too difficult based on PV maps alone. For this paper, we use that northward or southward direction of the meridional wave flux component for quasi-stationary Rossby waves indicates cyclonically or anticyclonically sheared RWB. We demonstrate that the magnitude of the meridional wave fluxes gives a reliable measure of the asymmetric types of RWB when combining with PV diagnostics as well as with the geometry of large-scale diffluent/ confluent flow. Based on 45 winter periods of ECMWF Reanalysis (ERA-40), we found two pronounced regions for P1- and LC2-type events, over the northern North Pacific and the northern North Atlantic, and two extended belts of P2- and LC1-type RWB events, over the North Pacific/North America region and the North Atlantic/European-West Asian region. The results reveal that the long-term mean occurrence of poleward RWB is generally as large as that of equatorward RWB with local differences. Since poleward or equatorward RWB events influence different regions efficiently, e.g., by associated cut-off cyclones or anticyclones, the proposed diagnosis gives an important tool for interpreting long-term general circulation patterns and large-scale weather regimes.
著者
Youichi KAMAE Wei MEI Shang-Ping XIE
出版者
(公社)日本気象学会
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
vol.95, no.6, pp.411-431, 2017 (Released:2017-11-14)
参考文献数
76
被引用文献数
2

Eddy transport of atmospheric water vapor from the tropics is important for rainfall and related natural disasters in the middle latitudes. Atmospheric rivers (ARs), intense moisture plumes that are typically associated with extratropical cyclones, often produce heavy precipitation upon encountering topography on the west coasts of mid-latitude North America and Europe. ARs also occur over the northwestern Pacific and sometimes cause floods and landslides over East Asia, but the climatological relationship between ARs and heavy rainfall in this region remains unclear. Here we evaluate the contribution of ARs to the hydrological cycle over East Asia using high-resolution daily rainfall observations and an atmospheric reanalysis during 1958-2007. Despite their low occurrence, ARs account for 14-44 % of the total rainfall and 20-90 % of extreme heavy-rainfall events during spring, summer, and autumn. AR-related extreme rainfall is especially pronounced over western-to-southeastern slopes of terrains over the Korean Peninsula and Japan, owing to strong orographic effects and a stable direction of low-level moisture flows. A strong relationship between warm-season AR heavy rainfall and preceding-winter El Niño is identified since the 1970s, suggesting the potential of predicting heavy-rainfall risk over Korea and Japan at seasonal leads.
著者
Toshinori AOYAGI Nobuyuki KAYABA Naoko SEINO
出版者
(公社)日本気象学会
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
vol.90B, pp.11-31, 2012 (Released:2012-06-09)
参考文献数
32
被引用文献数
10 11 4

We investigated a warming trend in the Kanto-Koshin area during a 30-year period (1976-2006). The warming trends at AMeDAS stations were estimated to average a little less than 1.3°C/30 years in both summer and winter. These warming trends were considered to include the trends of large-scale and local-scale warming effects. Because a regional climate model with 20-km resolution without any urban parameterization could not well express the observed warming trends and their daily variations, we investigated whether a mesoscale atmospheric model with an urban canopy scheme could express them.To make the simulations realistic, we used 3 sets of real data: National Land Numerical Information datasets for the estimation of the land use area fractions, anthropogenic heat datasets varying in space and time, and GIS datasets of building shapes in the Tokyo Metropolis for the setting of building aspect ratios. The time integrations over 2 months were executed for both summer and winter. A certain level of correlation was found between the simulated temperature rises and the observed warming trends at the AMeDAS stations. The daily variation of the temperature rises in urban grids was higher at night than in the daytime, and its range was larger in winter than in summer. Such tendencies were consistent with the observational results.From factor analyses, we figured out the classic and some unexpected features of urban warming, as follows: (1) Land use distribution change (mainly caused by the decrease of vegetation cover) had the largest daytime warming effect, and the effect was larger in summer than in winter; (2) anthropogenic heat had a warming effect with 2 small peaks owing to the daily variation of the released heat and the timing of stable atmospheric layer formation; and (3) increased building height was the largest factor contributing to the temperature rises, with a single peak in early morning.
著者
Hironobu IWABUCHI Nurfiena Sagita PUTRI Masanori SAITO Yuka TOKORO Miho SEKIGUCHI Ping YANG Bryan A. BAUM
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
vol.96B, pp.27-42, 2018 (Released:2018-03-16)
参考文献数
35
被引用文献数
2

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.
著者
YOSHIDA Mayumi KIKUCHI Maki NAGAO Takashi M. MURAKAMI Hiroshi NOMAKI Tomoyuki HIGURASHI Akiko
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2018-039, (Released:2018-04-15)
被引用文献数
5

We develop a common retrieval algorithm of aerosol properties such as aerosol optical thickness, single-scattering albedo, and Ångström exponent for various satellite sensors over both land and ocean. The three main features of this algorithm are as follows: (1) automatic selection of the optimum channels for aerosol retrieval by introducing a weight for each channel to the object function, (2) setting common candidate aerosol models over land and ocean, and (3) preparation of lookup tables for every 1 nm in the range from 300 to 2500 nm of wavelength and weighting the radiance using the response function for each sensor. This method was applied to the Advanced Himawari Imager (AHI) on board the Japan Meteorological Agency’s geostationary satellite Himawari-8, and the results depicted an approximately continuous estimate of aerosol optical thickness over land and ocean. Further, the aerosol optical thickness estimated using our algorithm was generally consistent with the products from Moderate Resolution Imaging Spectroradiometer (MODIS) and Aerosol Robotic Network (AERONET). Additionally, we applied our algorithm to MODIS on board the Aqua satellite and then compared the retrieval results to those that were obtained using AHI. The comparisons of the aerosol optical thickness retrieved from different sensors with different viewing angles on board the geostationary and polar-orbiting satellites suggest an underestimation of aerosol optical thickness at the backscattering direction (or overestimated in other directions). The retrieval of aerosol properties using a common algorithm allows us to identify a weakness in the algorithm, which includes the assumptions in the aerosol model (e.g. sphericity or size distiribution).
著者
UEDA Hiroaki MIWA Kana KAMAE Youichi
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2018-044, (Released:2018-05-14)

The response of tropical cyclone (TC) activity to the El Niño-Southern Oscillation (ENSO) and coherent sea surface temperate (SST) anomaly in the Indian Ocean (IO) is investigated with a particular focus on the decaying phase of El Niño. The TC anomalies are obtained from the database for Policy Decision making for Future climate change (d4PDF). This dataset is based on 100-member ensemble simulations for the period of 1951-2010 by use of the state-of-the-art atmospheric general circulation model (AGCM) forced with observed SST as well as the historical radiative forcing. AGCM utilized in the d4PDF is the Meteorological Research Institute Atmospheric General Circulation Model with about 60km horizontal resolution. Our analysis reveals a prolonged decrease in TC frequency over the tropical western Pacific during the post El Niño years until the boreal fall. Dominance of anomalous anticyclone (AAC) over the western Pacific induced by the delayed warming in the tropical Indian Ocean is the main factor for the suppressed TC activity rather than the local SST change. In contrast, the TC number over the South China Sea tends to increase during the post-El Niño fall (September to November). The physical reason can be ascribed to the weakening of AAC associated with the termination of IO warming. Thus we demonstrate that the effect of the IO warming should be taken into account when the ENSO is considered as an environmental factor for predicting TC activity.
著者
JINNO Takuya MIYAKAWA Tomoki SATOH Masaki
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2019-017, (Released:2018-12-07)

In August 2016, a monsoon gyre persisted over the western North Pacific and was associated with the genesis of multiple devastating tropical cyclones. A series of hindcast simulations was performed using the nonhydrostatic icosahedral atmospheric model (NICAM) to reproduce the temporal evolution of this monsoon gyre. The simulations initiated at dates during the mature stage of the monsoon gyre successfully reproduced its termination and the subsequent intensification of the Bonin high, while the simulations initiated before the formation and during the developing stage of the gyre failed to reproduce subsequent gyre evolution even at a short lead time. These experiments further suggest a possibility that the development of the Bonin high is related to the termination of the monsoon gyre. High predictability of the termination is likely due to the predictable mid-latitudinal signals that intensify the Bonin high.
著者
ITO Rui AOYAGI Toshinori HORI Naoto OH'IZUMI Mitsuo KAWASE Hiroaki DAIRAKU Koji SEINO Naoko SASAKI Hidetaka
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2018-053, (Released:2018-08-24)

Accurate simulation of urban snow accumulation/melting processes is important to provide reliable information about climate change in snowy urban areas. The Japan Meteorological Agency operates a square prism urban canopy (SPUC) model within their regional model to simulate urban atmosphere. However, presently, this model takes no account of snow processes. Therefore, in this study, we enhanced the SPUC by introducing a snowpack scheme, and the simulated snow over Japanese urban areas was assessed by comparing the snow depths from the enhanced SPUC and from a simple biosphere (iSiB) model with the observations. Snowpack schemes based on two approaches were implemented. The diagnostic approach (sSPUCdgn) uses empirical factors for snow temperature and melting/freezing amounts and the Penman equation for heat fluxes, whereas the prognostic approach (sSPUCprg) calculates snow temperatures using heat fluxes estimated from bulk equations. Both snowpack schemes enabled the model to accurately reproduce the seasonal variations and peaks in snow depth, but it is necessary to use sSPUCprg if we wish to consider the physical processes in the snow layer. Compared with iSiB, sSPUCprg resulted in a good performance for the seasonal variations in snow depth, and the error fell to 20 %. While iSiB overestimated the snow depth, a cold bias of over 1°C appeared in the daily mean temperature, which can be attributed to excessive decreases in the snow surface temperature. sSPUCprg reduces the bias by a different calculation method for the snow surface temperature and by the inclusion of heated building walls without snow; consequently, the simulated snow depth is improved. sSPUCprg generated a relationship between the seasonal variations in snowfall and snow depth close to the observed relationship, with the correlation coefficient getting large. Therefore, the simulation accuracy of snowfall becomes more crucial for simulating the surface snow processes precisely by the enhanced SPUC.
著者
Akiyoshi WADA Ryo OYAMA
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
vol.96, no.6, pp.489-509, 2018 (Released:2018-11-22)
参考文献数
52

Typhoon Lionrock (2016) made landfall in the Pacific side of northern Japan. One of the intriguing events was consecutive deep convections (convective bursts, CBs) occurred before making landfall on 31 August. Lionrock paused the decay of the intensity of the storm, although sea surface cooling (SSC) was induced distinctly by Lionrock along the track. To examine the influence of CBs on changes in storm intensity during the decay phase, numerical simulations were conducted with a 3 km mesh coupled atmosphere-wave-ocean model. The coupled model successfully simulated the occurrence of CBs north of the near-surface-convergence area, which was formed by the confluent of the storm's tangential winds with near-surface frictional spiral inflow from the surrounding region where the significant wave height was high. Simultaneously, the relatively fast translation and asymmetric tropical cyclone (TC) structure were maintained. Lower tropospheric horizontal moisture fluxes have enhanced around the convergence area, although SSC resulted in reduction of the air-sea latent heat fluxes within the storm's inner core. Local occurrences of upward moisture fluxes associated with CBs increased the mid-to-upper tropospheric condensational heating on the upstream side. This caused local increase in lower-tropospheric pressure gradient on the upstream side. This was favorable for pausing the decay of the simulated storm intensity even during the decay phase. Sensitivity experiments regarding the execution time of the coupled model showed that the vertical moisture fluxes and number of CBs could increase around the surface frictional convergence area ahead of the storm when the coupled model was not used. This suggests that the storm in mid-latitude could locally increase the maximum surface wind speed under favorable oceanic conditions. The number and distribution of CBs are indeed sensitive to oceanic conditions and are considered to affect the storm-track simulation and maximum surface wind speeds.
著者
YUMIMOTO Keiya TANAKA Taichu Y. YOSHIDA Mayumi KIKUCHI Maki NAGAO Takashi M. MURAKAMI Hiroshi MAKI Takashi
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2018-035, (Released:2018-04-08)
被引用文献数
1

The Japan Meteorological Agency (JMA) launched a next-generation geostationary meteorological satellite (GMS), Himawari-8, on October 7, 2014 and began its operation on July 7, 2015. The Advanced Himawari Imager (AHI) onboard Himawari-8 has 16 observational bands that enable the retrieval of full-disk maps of aerosol optical properties (AOPs), including aerosol optical thickness (AOT) and the Ångström exponent (AE) with unprecedented spatial and temporal resolution. In this study, we combined an aerosol transport model with the Himawari-8 AOT using the data assimilation method, and performed aerosol assimilation and forecasting experiments on smoke from an intensive wildfire that occurred over Siberia between May 15 and 18, 2016. To effectively utilize the high observational frequency of Himawari-8, we assimilated 1-h merged AOTs generated through the combination of six AOT snapshots taken over 10-min intervals, three times per day. The heavy smoke originating from the wildfire was transported eastward behind a low-pressure trough, and covered northern Japan from May 19 to 20. The southern part of the smoke plume then traveled westward, in a clockwise flow associated with high pressure. The forecast without assimilation reproduced the transport of the smoke to northern Japan; however, it underestimated AOT and the extinction coefficient compared with observed values, mainly due to errors in the emission inventory. Data assimilation with the Himawari-8 AOT compensated for the underestimation and successfully forecasted the unique C-shaped distribution of the smoke. In particular, the assimilation of the Himawari-8 AOT during May 18 greatly improved the forecast of the southern part of the smoke flow. Our results indicate that the inheritance of assimilation cycles and the assimilation of more recent observations led to better forecasting in this case of a continental smoke outflow.
著者
Keiichi ISHIOKA
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
vol.96, no.2, pp.241-249, 2018 (Released:2018-03-27)
参考文献数
18

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 another 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.
著者
石原 正仁 藤吉 康志 田畑 明 榊原 均 赤枝 健冶 岡村 博文
出版者
社団法人日本気象学会
雑誌
Journal of the Meteorological Society of Japan. Ser. II (ISSN:00261165)
巻号頁・発行日
vol.73, no.2, pp.139-163, 1995-04-25
参考文献数
42
被引用文献数
19

「集中豪雨のメカニズムと予測に関する研究」の一環として1988年の梅雨期に九州北部を中心として実施された特別観測期間中に、梅雨前線に沿ってメソスケール降雨帯が発生し、最大総降水量178mmの大雨が発生した。2台のドップラーレーダーによる観測結果をもとに、この降雨帯のレーダーエコーと循環の3次元構造を解析し、その構造と維持過程を中心に議論する。降雨帯は1988年7月17日に発生し、7時間維持された。発生環境を見ると、大気下層の水平温度傾度が大きくはなく、熱力学的不安定度は熱帯と中緯度の中間であった。降雨帯の長さは170kmに達し、内部は対流性領域と層状性領域から構成されていた。降雨帯の走向は北西-南東であり、大気中層と下層の間の風の鉛直シヤーとほぼ平行であった。対流性領域にある既存の対流セルは降雨帯の走向に沿って移動し、周囲の南西風が入り込む降雨帯の南西端に新しい対流セルが次々と発生した。降雨帯の中には次のような特徴的な流れが確認された。:1)降雨帯の前部にある対流規模上昇流、2)降雨が最も強い領域にある対流性下降流、3)後部中層のエコーのノッチ(切れ目)からの乾燥空気の流入、4)この後部流入に接続するメソ下降流、5)対流規模下降流の下の大気最下層の前方と後方に進む発散流。これら最下層の発散流は周囲より4℃程度低温の寒気プールを作り、この寒気プールと降雨帯前方の暖湿な南西流との間にガストフロントが作られた。降雨帯後方にあった中層の総観規模の乾燥域は、最下層の暖湿気流とともに、降雨帯を維持するために重要な役割を果たした。高層データによると、雨滴の蒸発冷却によると思われる低温域が対流規模下降流とメソ下降流の中に存在した。後部流入にともなう乾燥空気は対流性領域の最下層まで達していた。こうした熱力学特徴は、ドップラーレーダー解析から得られた運動学的構造とよく適合した。降雨帯は中緯度の前線帯に発生したとはいえ、対流圏下層に限れば降雨帯の前後の熱力学的条件の差異は非常に小さかった。この降雨帯は、西ヨーロッパや北米太平洋岸の寒帯前線にともなって観測されるメソ対流システムよりも、熱帯や中緯度のスコールラインのような「自立型対流システム」に属するであろう。
著者
SHUSSE Yukari MAESAKA Takeshi KIEDA Kaori IWANAMI Koyuru
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2019-021, (Released:2018-12-13)

This study describes the spatial distribution of the melting layer (ML) in a winter stratiform precipitation system associated with a south-coast cyclone (SCC) on 30 January 2015 over the Kanto Plain, Japan, using an X-band polarimetric radar at Funabashi operated by the Ministry of Land, Infrastructure, Transport and Tourism. The detailed horizontal distribution of surface precipitation types based on Weather Reports from citizens provided by Weathernews Inc. (WNI reports) was also investigated in relation to the ML structure. Surface precipitation in the Kanto Region started with rain and then changed to snow around Tokyo. According to WNI reports, a large dry snow area had formed around Tokyo by 0900 Japan Standard Time (UTC + 9 hours), while surface rainfall continued in the southeast of the Kanto Plain (most part of Chiba and southern part of Kanagawa). A boundary line between the surface dry snow and rain areas became clear in the eastern part of Kanagawa and the northwestern part of Chiba. This boundary then gradually moved inland. Polarimetric ML signatures suggesting the presence of melting snow were continuously observed above the rainfall area in the southeast of the Kanto Plain. The polarimetric ML signatures, on the other hand, approached the ground near the surface dry snow-rain boundary while the surface snowfall was predominant around Tokyo. During the mature snowfall period around Tokyo, the ML vertically extended below 1 km above sea level near the surface dry snow-rain boundary, which indicates the presence of a local horizontal temperature gradient and a surrounding ~0°C near-isothermal layer. It is suggested that this vertically extending ML coincided with the edge of a cold air mass in the lower atmosphere, which often forms during snowfall associated with SCCs in the Kanto Region.
著者
ITO Takahiro NISHIMOTO Shusuke KANEHISA Hirotada
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2018-055, (Released:2018-09-07)

In this study, we show analytically that vortex Rossby waves (VRWs) with azimuthal wavenumber m =1 in a basic axisymmetric vortex can grow exponentially in a quasi-geostrophic system, although they cannot do so in a barotropic system. VRWs grow exponentially if Rayleigh’s condition and Fjørtoft’s condition are satisfied. Satisfying Rayleigh’s condition means that two horizontally aligned VRWs at two different radii propagate (here and hereafter “propagate” refers to propagation relative to the fluid) azimuthally counter to each other. Satisfying Fjørtoft’s condition means that the cyclonic advective angular velocity of the basic vortex is distributed radially so as to enable the VRWs to be phase-locked with each other. Under these conditions, a strong mutual interaction between the VRWs becomes possible, and thus they grow exponentially. In a barotropic system, even if Rayleigh’s condition is satisfied, the azimuthal counter propagation of VRWs with azimuthal wavenumber m =1 is so strong that phase-locking between them cannot occur, and thus they cannot grow exponentially. In a quasi-geostrophic system, however, the upper and lower VRWs of the first baroclinic vertical mode are equal in magnitude and have opposite signs. Because of this baroclinic structure, the azimuthal counter propagation of the horizontally aligned VRWs is suppressed by the vertical interactions between the upper and lower VRWs. Consequently, horizontally aligned VRWs with azimuthal wavenumber m =1 may become phase-locked, and hence they may grow exponentially. By analytically solving the linear problem of VRWs in a quasi-geostrophic system, we show that this is indeed the case.
著者
YAMAZAKI Akira HONDA Meiji KAWASE Hiroaki
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2019-012, (Released:2018-11-16)

This study found that regional snowfall distributions in a Japan-Sea side area of Japan are controlled by intraseasonal jet variability, particularly the 10-day-timescale quasi-stationary Rossby waves across the Eurasian continent and the atmospheric blocking over the East Asian region. This study mainly focused on the Niigata area, which is representative of heavy snowfall areas in Japan. Based on previous studies, three types of dominant snowfall distributions were defined: 1) the plain (P) type, which is characterized by heavy snowfall events predominant in coastal regions of the Niigata area, 2) the mountain (M) type, which occurs in the mountainous regions, and 3) the PM type, which occurs across the whole Niigata area. Our results revealed that all distribution types were related to the south-ward shift of the westerly jet over Japan associated with an intensified trough, i.e., cyclonic anomalies, originating from quasi-stationary Rossby waves along westerly jets over Eurasia (Eurasian jets). The cyclonic anomalies were found to be also related to blocking cyclones because the frequency of blocking events considerably increased in the East Siberian region. The mechanisms leading to the trough intensification were different among the events of the three snowfall types. The formation of Siberian blocking with relatively different positions and different paths of quasi-stationary Rossby wave packet propagation along Eurasian jets were evident in the distribution types. Therefore, local-scale snowfall distributions in the Japan-Sea side area are determined by anomalous large-scale circulations, which can be evidently distinguished in the global reanalysis data.
著者
VITANOVA Lidia Lazarova KUSAKA Hiroyuki DOAN Van Quang NISHI Akifumi
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2019-013, (Released:2018-11-16)

This study investigates the impact of urbanization on surface air temperature and the urban heat island (UHI) for Sendai City. We estimate the impacts of the urbaniza-tion during the 150-year period by comparing the 1850s to the 2000s case. We use the Weather Research and Forecasting (WRF) model with 1-km horizontal resolution and three land-use datasets, one for potential natural vegetation (PNV) data, the other two for realistic land-use data (the 1850s and 2000s). Results from the control simulation (2000s land-use case) are firstly verified against observations. The results show that the WRF model reasonably well reproduces the diurnal variation of the observed surface air temperatures in the 2000s land-use case at six stations in Miyagi prefecture. The model mean biases range from −0.29 to −1.18°C in August (10-year average) and from −0.44 to −1.50°C in February (10-year average). Secondly, the impacts of urbanization on the surface air temperature distribution in and around Sendai City are evaluated. In the 1850s land-use case, the very small urban area of Sendai City results in a negligible UHI. This case gives nearly the same surface air temperatures as experiments using the PNV. Comparing the simulated monthly mean surface air temperatures in the central part of Sendai City between the 1850s and 2000s land-use cases, we find that the monthly mean temperature for February in the 2000s is 1.40°C higher than that in the 1850s, whereas that for August is 1.30°C. Similarly, we find considerable nocturnal (1800–0500 JST) average surface air temperature increases of 2.20°C in February and 2.00°C in August.