Yuichiro Oku

This paper presents a method for estimating the land surface temperature (LST) from Himawari-8 data. The Advanced Himawari Imager 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 10 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 correspond to a variety of values of LST, as well as 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 verified 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 an RMSE less than 0.9 K compared 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 the 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 was constructed. This method includes a simple cloud mask procedure and the land surface emissivity estimation. The LST product was evaluated using in-situ data over the Tibetan Plateau, and the validity was confirmed.

Recently heavy rains occur frequently and researchers consider a climate change may be one of the causes. Thus, this study estimates the influence of the sea surface temperature changes on rainfall intensity due to a Typhoon. First the Typhoon No.18, 2013 which caused heavy damage on Kyoto and Shiga prefectures due to the heavy rainfall is reproduced (control run). Then its SST global warming experiment is carried out using latest meso-scale meteorological model WRF. As the result of the SST global warming experiment, the catchment average rainfall of the Yodogawa river above Hirakata becomes 1.5 times larger than that of the control run. This result indicates a possibility that further heavier rainfall may occur in the future due to a climate change.

Abstract. Typhoon Talas (2011) caused extensive damage through landslides on the Kii Peninsula, Japan, in September 2011. The purpose of the present study is to quantitatively describe the potential for the occurrence of heavy rainfall-induced disasters if the typhoon track perturbs slightly or the typhoon intensifies. Regarding to the consideration of the track displacement of the typhoon, a procedure is proposed to generate different typhoon tracks perturbed from the original track of the typhoon. In this procedure, the position of a typhoon is artificially shifted at a certain time before landing in a physically consistent manner by applying potential vorticity inversion (PVI) methodology. After relocating the typhoon, the subsequent progress is simulated by a mesoscale meteorological model. Using the output, which consists of a set of realizations having different typhoon tracks, the worst-case scenario is discussed in terms of the soil water index (SWI) of the Kii Peninsula. The SWI is an indicator of the amount of water in soil that represents the hazard of landslide disasters. The maximum spatially averaged SWI is 1.10 times as large as that from the original typhoon track. Regarding the consideration of severer typhoon, the same method is used, but the intensity of the potential vorticity of a typhoon is artificially modified at the position instead of relocating potential vorticity to a different place. The maximum spatially averaged SWI is 1.28 times as large as that of the original typhoon intensity.

大阪市内60地点で行われた高細密な気温観測網の気温データを用いて，ヒートアイランド現象に着目した気温の日変化特性による大阪市域内の地域類型を，主成分分析とクラスター分析を用いて行った．その結果，大阪市域内において顕著なヒートアイランド現象の構造があることが改めて確認された．また，夏と冬で異なる気温の日変化特性を示す地点に着目し，市の中心部の地点との気温差の時間・季節変化から，その成因として従来から指摘されている夏の日中に顕著となる大阪湾からの海風による気温上昇の抑制と関連づけた．

This paper deals with a methodology to estimate the variations of flood hazard and risk due to a climate change. A pseudo global warming (pgw) experiment result of the Isewan Typhoon in 1959 is used as the rainfall input with a climate change effect to a distributed rainfall-runoff/flood inundation (DRR/FI) model. The simulated rainfalls of the control run (ctl) for the reproduction of the Isewan Typhoon as well as pgw experiment by JMA-NHM exhibit that, though the 48-hrs rainfall of ctl underestimates the observation, the maximum hourly rainfall of pgw becomes much higher than the observation. As the result, the simulated water level/discharge and inundation depth with pgw rainfall becomes all worse than those of the Isewan Typhoon.

Recently heavy rains occur frequently and researchers consider a climate change may be one of the causes. Thus, this study estimates the influence of the change of sea surface temperature on rainfall due to a Typhoon. First the Typhoon No.18, 2013 which caused heavy damage on Kyoto and Shiga prefectures due to the heavy rainfall is reproduced (control run). Then its SST global warming experiment is carried out using latest meso-scale meteorological Model WRF. As the result of the SST global warming experiment, the catchment average rainfall of Yodogawa river above Hirakata becomes 1.5 times larger than that of control run. This result indicates the possibility that further heavier rainfall may occur in the future due to a climate change.

The damage to society caused by tropical cyclones depends largely on the storm track relative to geography. A procedure is proposed to generate different typhoon tracks deviating from the original track of a given reference case. In this procedure, the position of a typhoon is artificially shifted at a certain time before landing in a physically consistent way by applying potential vorticity inversion methodology. After shifting the typhoon position, the subsequent progress is again simulated by a mesoscale weather model. The procedure is applied to a strong typhoon that emerged in a global warming experiment using an atmospheric general circulation model. Various realizations of typhoon landfall cases are generated. Using the output of a suite of realization of different tracks, the worst case scenario is discussed in terms of the river discharge in the Tone River basin, Japan. Copyright (c) 2013 John Wiley & Sons, Ltd.

Understanding and forecasting of summertime afternoon precipitation due to rapidly developing cumulonimbus clouds without any significant synoptic-scale influences are important to prevent and mitigate the induced disasters. Future changes in the behavior of such precipitation events have recently gained scientific and societal interests. This study investigates the environmental stability for afternoon precipitation that develops under synoptically undisturbed conditions in summer by using the outputs of 20-km-mesh, super-high-resolution atmospheric general circulation model (GCM) simulations for a present, a near-future, and a future climate under global warming with the Intergovernmental Panel on Climate Change A1B emission scenario. The Kanto Plain was chosen as the analysis area. After verifying the usefulness of the GCM present-climate outputs with observations and gridded mesoscale analyses, we examine the future changes in the environmental stability for the afternoon precipitation by conducting statistical analyses. In the future climates, temperature lapse rate decreased in the lower troposphere, while water vapor mixing ratio increased throughout the deep troposphere. The changes in the temperature and moisture profiles resulted in the increase in both precipitable water vapor and convective available potential energy. These projected changes will be enhanced with the future period. Furthermore, the statistical analyses for the differences of the stability parameters between no-rain and rain days under the synoptically undisturbed condition in each simulated climate period indicated that the representations of the stability parameters that distinguish no-rain and rain events are basically unchanged between the present and the future climates. This result suggests that the environmental characteristics favorable for afternoon precipitation in the synoptically undisturbed environments will not change under global warming.

It has been discussed that intensity of typhoon may increase in the future due to climate change. It is important to estimate the probable maximum magnitude of storm surges under the future climate for coastal disaster mitigation. In the present study, sensitivity of storm surge projection due to storm track in Tokyo Bay is discussed by employing the hundreds of different severe meteorological conditions generated by the potential vorticity inversion method. Ensemble numerical results show the most hazardous tropical cyclone track is different from most intense tropical cyclone track in Tokyo Bay. Estimated maximum storm surge in the Tokyo Bay is found to be 1.4 m which is the same level as the historical highest record.

It has been discussed that intensity of typhoon may increase in the future due to climate change, resulting in severe coastal disasters. It is important to estimate the probable maximum magnitude of storm surges under the future climate for coastal disaster mitigation. In the present study, uncertainty of storm surge projection was discussed by employing the hundreds of different severe meteorological conditions generated by the potential vorticity inversion method. Ensemble numerical simulation results show the most hazardous typhoon track is different from most intense typhoon's one. Estimated maximum storm surge in the Tokyo bay was 1.4 m which is the same level as the historical highest record.

This paper deals with the development of an integrated model for rainfall-runoff/flood inundation simulation and economic loss estimation, especially focusing on Sayo Town, Hyogo Prefecture, Japan where there was a severe flood disaster due to a heavy rainfall event on 9-10 August 2009. Firstly, the rainfall over the Sayogawa river catchment at the time is analyzed by using the composite radar and Radar-AMeDAS reanalysis data by the Japan Meteorological Agency in addition to the ground observation data. Second, the rainfall-runoff and flood inundation processes are simulated using the integrated model with the Radar-AMeDAS reanalysis data. The simulated inundation area shows the good agreement with the inundation record. Then, the economic loss estimation based on the simulated inundation area and depth is carried out and compared with the prospected value reported by the municipal government. The house/building economic loss estimated is 20.4 billion Japanese Yen, which agrees well with the reported value 18.5 billion Japanese Yen.

In this study, the changes of drought severity due to climate change were assessed using SPI. The maximum drought severity and frequency of effective and extreme drought were calculated. We also regionalized the drought severity using EOFs of variables to represent drought characteristics and K-means clustering method. As results, it was found that severity, frequency and seasonal variation might be changed significantly and that the regionalization scheme applied in this study can be an alternative to solve the problem of spatially high variation caused when we assess climate change impact using the outputs from climate model.

The algorithm used to retrieve the cloud effective particle radius from the 3.7 m band was adapted to the corresponding channel of the Japanese Advanced Meteorological Imager (JAMI) flown on board the Multi-functional Transport Satellite (MTSAT) geostationary platform. Snapshot comparisons with spatially well-resolved retrievals from the MODerate resolution Imaging Spectrometer (MODIS) instruments flown on the Terra polar platforms show qualitative agreement with MTSAT retrievals. The results of analysing daytime variation from eastern Asia to the northwest Pacific Ocean show that, not only is the effective particle radius smaller in continental clouds than in maritime clouds, but the daytime amplitude of the effective particle radius is also greater in continental clouds than in maritime clouds, where the effective particle radius value is approximately constant.

This study is an attempt to apply the surface energy flux computation algorithm to geostationary satellite data for the Tibetan Plateau to understand the surface energy budget distribution of sub-plateau scale. It is generally considered that the sensible heat flux accordingly decreases and the latent heat flux exceeds the sensible heat flux after the onset of the monsoons since surface moisture increased gradually. It is apparent from our result that this dramatic change cannot be seen all over the plateau, but only in the southeast part of the plateau. In the northwest part of the plateau, sensible heat flux is always greater than latent heat flux all through the year.

The mesoscale snow distribution over the Namco lake area of the Tibetan Plateau on October 2005 has been investigated in this paper. The base and revised experiments were conducted using the Weather Research Model (WRF) with three nested grids that included a 1 km finest grid centered on the Namco station. Our simulation ran from 6 October through to 10 October 2005, which was concurrent with long term meteorological observations. Evaluation against boundary layer meteorological tower measurements and flux observations showed that the model captured the observed 2 m temperature and 10 m winds reasonably well in the revised experiment. The results suggest that output snow depth maximum amounts from two simulated experiments were centered downwind of the Namco lakeshore. Modified surface state variable, for example, surface skin temperature on the lake help to increase simulated credibility.

ベンガル湾で発生したサイクロンにより，2007年11月のSidrでバングラデシュ，2008年5月のNargisでミャンマーにおいてそれぞれ大きな災害が発生した．本論文では，気象衛星写真，降雨レーダ画像をもとに気象特性を調べ，アラビア海やベンガル湾の北インド洋で発生するサイクロンの発生について，その発生や経路の特徴を明らかにした．

静止軌道衛星の赤外データから地表面温度を算出する手法をさらに発展させ，地表面熱収支量（地表面エネルギーフラックス）の算出する手法を開発した．静止軌道衛星ひまわりのデータから算出された地表面エネルギーフラックスは，地上観測で得られたそれの日変化や季節変化の傾向とよく一致していた．これまで，アジアモンスーン形成におけるチベット高原全体の熱力学効果は，高原上に設置された数少ない地上観測データのみで議論されてきたが，本手法の開発により高原全体の面的情報から評価することが可能となった．

Sensible and latent heat fluxes from the plateau surface are of great importance in the Asian monsoon system. Since the plateau occupies a wide area and the environmental conditions are severe to perform surface observation, the satellite remote sensing is inevitably a practical tool to estimate these fluxes from whole plateau surface. The in situ flux estimation is, however, necessary as a ground truth for the satellite remote sensing. It also gives scientific information in constructing land surface-atmosphere model, which shares an important part of data assimilation system using satellite data. There are several different approaches in estimating in situ heat fluxes. The simplest method uses operational observation and experimental parameters, and it gives steady continuous estimation. The more sophisticated Bowen ratio or profile observation gives the more precise information. The estimation with turbulence measurement together with the measurement of radiation and soil heat fluxes give detailed description of land surface atmosphere interaction suitable to model development. Since 1998, a combination of these methods has been applied to the Tibetan plateau. The efforts of these in situ flux observation and the current understandings are summarized in this presentation.

The diurnal, seasonal, and interannual variations in land surface temperature (LST) on the Tibetan Plateau from 1996 to 2002 are analyzed using the hourly LST dataset obtained by Japanese Geostationary Meteorological Satellite 5 (GMS-5) observations. Comparing LST retrieved from GMS-5 with independent precipitation amount data demonstrates the consistent and complementary relationship between them. The results indicate an increase in the LST over this period. The daily minimum has risen faster than the daily maximum, resulting in a narrowing of the diurnal range of LST. This is in agreement with the observed trends in both global and plateau near-surface air temperature. Since the near-surface air temperature is mainly controlled by LST, this result ensures a warming trend in near-surface air temperature.

極軌道衛星を用いた地表面温度算出手法は既に提案されているが，観測頻度や同時性の点で不十分であった．本論文では静止軌道衛星の赤外観測データを用いることにより，時空間的に均質な地表面温度算出を可能にした．静止軌道衛星ひまわりのデータを用いるにあたり衛星天頂角および大気中の水蒸気量に関わる補正が必要となるが，補正式を得るために衛星に搭載された放射計の感度特性まで考慮した緻密な放射伝達計算を行った．地表面温度算出の障害となる雲域の除去に関して，変動閾値を用いた新しい判別法を提案し，過不足無く雲領域を除去することに成功した．これらの工夫により，地上観測で得られる地表面温度との相関が0.8以上という極めて良好な算出結果を得た．