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It's studied how thermals, which cause guerrilla-heavy rainfall in future, are generated from urban area like Kobe City and break through atmospheric boundary layer (ABL). In particular, the phase that thermals and vortex tubes connect to genesis of cumulous and cumulonimbus. I think that aggregation may play an important role at the phase. But, by radar observation, this mechanism isn't clear yet. Hence it is better to compute by using urban meteorological large eddy simulation. In this study, I conducted idealized experiments in order to investigate on how the behavior of thermals and vortex tubes is around or above ABL.

Suddenly-generated and rapidly-developed isolated cumulonimbus often cause flash flood and inundation disasters. For these disasters prediction, it is significant to analyze flow structure in a cumulonimbus cloud before raindrop generation. To investigate the initiation process, we conducted observation in Kobe City, on August 2018 using a Ka-band Doppler radar, X-band radars and some sensors. Developing cumulonimbus clouds and non-precipitating cumulus were observed by Ka-band radar, and the former has high Radar reflectivity (Zh) in the upper part. This implies that strong updraft lifted up the cloud particles and particle size grew up in the developing cell. In addition, we evaluated the effect of spatial resolution and direction on the radar observation by analyzing the vortex structure simulated by a Large Eddy Simulation model. The simulation result show that the vorticity can be observed irrespective of the direction if a cell has a circulation in a spatial scale larger than observation.

It is aimed to analyze under what conditions thermals generated by the heat and shape effect of the city break through the atmospheric boundary layer. I used the urban meteorological Large Eddy Simulation model and the thermals over Kobe City are our targets. As a result, three factors were confirmed on the thermals which break through the atmosphere boundary layer. One factor is strong thermal buoyancy, and second factor is elimination of the stabilizing layer by another thermal and last factor is vapor amount. In addition, we could confirm the assembly of vortex. The assembly occur by vertical shear of horizontal wind. It is possible that the assembly strengthen each vortex. At last, I analyzed which factor is important for generating cumulus cloud.

It is aimed to analyze under what conditions the thermals generated by the heat and shape effect of the city breaks through the atmospheric boundary layer to generate cumulus clouds. Using the urban meteorological Large Eddy Simulation model, the thermals over Kobe City were targeted. As a result, the validity of the model was also assessed using the boundary layer radar. Two factors were confirmed on the thermals which break through the atmosphere boundary layer. One is strong thermal buoyancy, and the other is elimination of the stabilizing layer by another thermal. In addition, the relationship between the thermal and the vortex tube is also suggested, and further analysis will be carried out in the future.

In Japan, the frequency of Guerrilla-heavy rainfall is increasing, so it is quite important to predict heavy rainfall even within a few minutes. Nakakita et al. (2016) discovered the large vertical vorticity inside the baby cell aloft is useful to predict the Guerrilla-heavy rainfall, and also discovered the baby cells have vertical vortex tubes. By these results, we considered that the horizontal vortex tubes were caused by vertical shear of horizontal wind and these were tilted into vertical by an updraft. This hypothesis is popular in large scaled clouds, but it is not unclear in small scaled clouds. This paper shows two findings. One is that the strongest updraft exists between positive and negative vorticity using high ZDR column and multi Doppler technique. The other is that Guerrilla-heavy rainfall is caused by not so strong vertical shear as liner convective system. These unlock the developing process of small scaled cumulonimbus cloud.

In Japan, the frequency of Guerrilla-heavy rainfall is increasing, so it is quite important to predict the risk of heavy rainfall even within a few minutes. Nakakita et al. discovered the large vertical vorticity inside the baby cell aloft is useful to predict the Guerrilla-heavy rainfall, and also discovered the baby cells have vortex tubes which are similar to ones inside the initial cumulonimbus clouds of supercells. From this results, we considered that the vortex tubes inside the baby cells support fluid dynamics. This paper shows that the strongest updraft exists between positive vorticity and negative vorticity inside one sample cell using high ZDR column.

Today, frequency of Guerilla-heavy rainfall are increased in Japan. Nakakita et al. (2014) developed the risk prediction system with the vertical vorticity information in a convective cell at its first stage (hereafter, baby-cell). For more accurate system, we tried to understand how a cell develops to heavy rainfall by analyzing the structure of vortex tubes inside baby-cells. Vortex tubes are observed inside all sample cells with X-MP Doppler radar in Kansai area. For more detailed analysis, we are challenging to analyze vortex tubes with phased array radar in Osaka University.

26-27 Sep. 1996 (19th), 数値風洞を用いて、減衰一様等方性乱流の高分解能(空間格子点数512(exp 3))直接数値シミュレーション(DNS)を行い、流速、渦度および温度などの発達した種々の乱流場を得た。乱流場の流れ構造を調べるため、渦度および温度場を可視化した。その結果、まず、渦度の強い領域がシート状に渦層として集中し、次に渦層がケルビン・ヘルムホルツの不安定性により巻き上げられ、最後に微細なみみずと称する渦管になる。一方、強い温度勾配は、局所的なシート状層に集中する。, Direct Numerical Simulation (DNS) of decaying homogeneous isotropic turbulence with a high resolution (512(exp 3) spatial mesh points) is carried out using the numerical wind tunnel, and fully developed turbulent fields, such as the velocity, vorticity and temperature, are obtained. To search organized structures in the turbulent fields, the vorticity and temperature fields are visualized. As a result, the intense regions of vorticity concentrate into sheet-like layers at first. Then, the layers are rolled up by the Kelvin-Helmholtz instability, and ultimately, become to fine vortex tubes, which are called worms. On the other hand, the intense temperature gradient concentrates into local sheet-like layers., 資料番号: AA0001127002, レポート番号: NAL SP-35

航空宇宙技術研究所 6-7 JUN. 1996 東京 日本, National Aerospace Laboratory 6-7 JUN. 1996 Tokyo Japan, 減衰等方性乱流の直接数値シミュレーション(DNS)をNALの数値風洞(NWT)を用いて実施した。空間格子点を512(exp 3)点使いDNSによって、十分に発達した乱流とパッシブスカラー場を作った。流れの数値解析と可視化によって、乱流渦度の高い部分が初期段階ではシート状領域に集中すること、次いで、そのシートが巻き上り最終的にwormと呼ばれる管状渦を形成することが明らかになった。その渦管は低渦度領域でも観測できた。温度勾配の大きい部分もシート状領域に集中し、渦度とは対照的にシート状領域は不変のまま維持されることが分かった。, The Direct Numerical Simulation (DNS) of an isotropic turbulence decaying is performed using the NAL Numerical Wind Tunnel (NWT). The fully-developed turbulent flow and the passive scalar field are obtained through the DNS with 512(exp 3) mesh points. The numerical analysis and visualization of the flow reveal that the intense portions of the turbulent vorticity concentrate in the sheet-like regions at the early stage. Then, the sheets roll up and finally form vortex tubes that are called worm. The tubes are also observed in the lower-vorticity regions. Furthermore, the intense portions of temperature gradient concentrate in the sheet-like regions and remain unchanged contrary to those of the vorticity., 資料番号: AA0000685042, レポート番号: NAL SP-34

航空宇宙技術研究所 27-29 Sept.2000 東京 日本, National Aerospace Laboratory 27-29 Sept.2000 Tokyo Japan, 自然噴流あるいは音響励起平面噴流中で流れの可視化と熱線測定により、コヒーレント渦とエントレインメント間の関係を実験的に検討した。自然噴流については、非線形相互作用域で発生する対称渦配列から反対称渦配列に移行する時にエントレインメントが増加する。反対称的に励起すると、エントレインメントは非常に増進し、噴流幅が拡大する。対称的励起については、渦管は非線形相互作用域で波状配置を呈した。励起強度の増加に伴い、波状配置はかなり下流まで保たれるが、その規模は小さくなる。u(sub f)/U(sub 0)=±8%の時、渦のペアリングが起き、エネルギースペクトルに分数調波成分が現れる。, Relation between coherent vortices and entrainment was experimentally investigated in a natural or an acoustically excited plane jets through a flow visualization and hotwire measurement. For the natural jet, the entrainment increased when shift from the symmetric to the antisymmetric vortex arrangement occurred in the nonlinear interaction region. When antisymmetrically excited, the entrainment was greatly enhanced and jet width was enlarged. For the symmetric excitation, vortex tubes had wavy configuration in the nonlinear interaction region. As the intensity of excitation increased, the wavy configuration maintained until more downstream but its scale became smaller. At u(sub f)/U(sub 0) = +/-8 percent, the vortex pairing occurred and sub-harmonic components appeared in the energy spectra., 資料番号: AA0028637015, レポート番号: NAL SP-48T