Abstract Previous studies demonstrated that the tropical cyclone (TC) boundary layer (TCBL) contains small‐scale coherent structures, such as roll vortices and tornado‐scale vortices (TSVs), and they play important roles in energy transport and intensity changes in TCs. However, little is known about how horizontal resolutions can affect storm‐scale and fine‐scale structures in the TCBL with the grid spacing decreasing from the turbulent gray zone (100 m–1 km) to the large‐eddy scale (<100 m). In this study, numerical experiments with the large eddy simulation technique are used to investigate the effect of the model horizontal resolution on the simulated TC‐scale and fine‐scale structures in the TCBL. The simulated TC tends to have a shallower BL, a lower BL jet associated with stronger near‐surface vertical wind shear in the TCBL when the grid spacing decreases from 333 to 111 m or 37 m. When the grid spacing decreases to 111 m or 37 m, the fine‐scale coherent structures and near‐surface wind streaks associated with roll vortices and TSVs can be simulated, and the characteristics of the simulated turbulent kinetic energy in the TCBL are in good agreement with observational studies. With the grid spacing of 111 and 37 m, no significant differences in TC intensity are found in terms of instantaneous maximum and azimuthal‐mean maximum wind speeds. This study suggests that simulations with the horizontal grid spacing of 100 m or less can simulate the small coherent structures in the TCBL and their effect on TC intensity.
Horns of Bovidae family represent extremely tough natural composites that resist flexural and impact damage during combats. However, the microstructure characteristics of the outer sheath of these horns and how this affects their mechanical properties and resistance to damage remain to be explored for the Bovinae subfamily. In this work we report that the Syncerus caffer horn sheath exhibits the highest toughness among the bovine horn sheaths. Its mechanical properties are closely related to its corrugated lamellae morphology. Finite element modeling verifies that the critical design characteristics of the curved corrugated lamellae with inter-lamellar connections has a profound effect on the flexural and impact mechanical behaviour of the sheath. Furthermore, the stress concentration at the ridge region for the highly curved-lamellar morphology is proved to possess rich disulphide crosslinking, suggesting a purposeful evolution of the heterogeneous lamellae structure. This work aims to provide insights into the toughening mechanisms of natural horn sheaths, and to encourage biomimetic design of superior structural materials.
Biochar materials are good reducers of nitrogen oxides. The composition and structure of biochar affect significantly its ability to reduce C–NO. In order to study the denitration of flue gases by biochar at high temperature, three kinds of biochar (bamboo charcoal (BC), rice husk ash (RHA), and straw charcoal (SC)) were mixed with cement raw meal in a fixed-bed quartz reactor at the temperature of 800–900 °C and O2 concentration of 0.5%–2%. The results showed that the initial denitration rate of BC was higher than that of RHA, and that of SC was the lowest. RHA had the largest specific surface area, and BC the smallest. The elements C, N, and O and the functional groups of the three types of biochar had a greater influence on the denitration rate than their structures. The denitration rate decreased faster as the O/C ratio increased, and the increase in the relative content of the N element induced the formation of nitrogen-containing functional groups catalyzing C–NO reduction. The content of the C–C bond affected directly the rate of denitration, and both (NCO)x and C–O bonds had a positive effect on the reduction capability of biochar. It can be concluded that the composition of biochar has an important effect on the reduction of C–NO.
It has been demonstrated that the tornado-scale vortex (TSV) is one of the fine-scale structures associated with extreme updrafts in the tropical cyclone boundary layer (TCBL), but the relationship between surface wind gusts and TSVs is still unclear. In this study, the one-second model output simulated in the Weather Research and Forecast (WRF) model with the large eddy simulation (WRF-LES) is used to investigate the relationships between TSVs and surface wind gusts. Results show that surface wind gust factors in the regions where TSVs are prevalent are significantly larger than those in other regions. 88% of the maximum gust factors associated with TSVs (vertical velocity larger than 20 m s −1 ) are larger than 1.25 (gust factors larger than 1.25 account for only 1% of the 1-min gust factors in the TC inner core), and the mean maximum 1-min gust factor associated with a TSV is larger than 1.3, while the mean 1-min gust factor in the TC inner core is only 1.1. The surface gust factors associated with TSVs in tropical cyclone eyewall can reach about 1.8, which can cause severe surface wind hazards. This study suggests that potential risk will increase in the regions where TSVs are prevalent because of the large wind gusts and gust factors. Finer real-time observations are needed to monitor the evolution of TSVs for improving the operational TC-related surface gust warnings.
Background: Accurate segmentation of tumor targets is critical for maximizing tumor control and minimizing normal tissue toxicity. We proposed a sequential and iterative U-Net (SI-Net) deep learning method to auto-segment the high-risk primary tumor clinical target volume (CTVp1) for treatment planning of nasopharyngeal carcinoma (NPC) radiotherapy. Methods: The SI-Net is a variant of the U-Net architecture. The input of SI-Net includes one CT image, the CTVp1 contour on this image, and the next CT image. The output is the predicted CTVp1 contour on the next CT image. We designed the SI-Net, using the left side to learn the volumetric features and the right to localize the contour on the next image. Two prediction directions, one from inferior to superior (forward direction) and the other from superior to inferior (backward direction), were tested. The performance was compared between the SI-Net and the U-Net using Dice similarity coefficient (DSC), Jaccard index (JI), average surface distance (ASD), and Hausdorff distance (HD) metrics. Results: The DSC and JI values from the forward direction SI-Net model were 5 and 6% higher than those from the U-Net model (0.84 ± 0.04 vs. 0.80 ± 0.05 and 0.74 ± 0.05 vs. 0.69 ± 0.05, p < 0.001). The smaller ASD and HD values also indicated a better performance (2.8 ± 1.0 vs. 3.3 ± 1.0 mm and 8.7 ± 2.5 vs. 9.7 ± 2.7 mm, p < 0.01) for the SI-Net model. For the backward direction SI-Net model, the DSC and JI values were still better than those from the U-Net model (p < 0.01), although there were no significant differences in ASD and HD. Conclusions: The SI-Net model preserved the continuity between adjacent images and thus improved the segmentation accuracy compared with the conventional U-Net model. This model has potential of improving the efficiency and consistence of CTVp1 contouring for NPC patients.
Tropical cyclone (TC) rapid intensification (RI) is usually accompanied by a rapid eyewall contraction, followed by a slow contraction, and then a nearly steady eyewall. However, this study shows that Hurricane Helene (2006) exhibited an eyewall expansion during its 30-h rapid intensification period. The possible environmental influence on the eyewall expansion during the RI of Helene is examined. It is found that the synoptic-scale circulations led to additional low-level inflows and upper-level outflows that may play an important role in the eyewall expansion during the RI of Helene. Examination of the divergence of the absolute angular momentum flux (AAMF) associated with the environmental circulation suggests that the synoptic-scale atmospheric circulation played an important role in the eyewall expansion during the RI of Helene. In the lower and middle troposphere, the synoptic-scale cross-equatorial flow, which was enhanced by the Helene-induced wave train, led to the horizontal convergence of absolute angular momentum flux, while the TC-trough interaction and the related outflow in the upper troposphere resulted in the divergence of AAMF. The environment-induced low-level convergence and upper-level divergence of AAMF were superimposed on the secondary circulation of Helene and may be important to the eyewall expansion during the RI by accelerating the tangential wind outside of the eyewall. This study suggests that RI can occur with an eyewall expansion.
Abstract Many studies have indicated that convective bursts (CBs) are closely related to tropical cyclone (TC) intensification, but few studies have been conducted on the mechanisms that control the formation and evolution of CBs. In this study, the 1‐min output data of a simulated TC are used to understand the convective extreme updrafts of CBs in the TC eyewall. Three different reference states including the local square‐area mean (LAM), the lower wavenumber‐components mean (WNM), and the arc‐area mean (ArcM) are used to calculate the driving forces of convective extreme updrafts. Contrary to the WNM and ArcM reference states, the LAM reference state struggles to capture realistic basic state structures. The LAM reference state can be modified through the inclusion of the mean hydrometeor mixing ratio in the basic state to produce physically consistent forcing in relation to the simulated convective extreme updrafts. The simulated convective extreme updrafts in the TC eyewall exhibit two peaks at middle and upper levels, respectively, since the effect of hydrometeor loading, decelerates the air parcels between the updraft maxima. While the positive buoyancy makes air parcels in the CBs accelerate at middle levels, in agreement with previous studies, it is found that, at the upper levels, both the positive buoyancy and the upward vertical perturbation pressure gradient force accelerate the air parcels. This study suggests that the vertical perturbation pressure gradient force also plays an important role in the formation of CBs in the TC eyewall.
It has been suggested that the inner eyewall structure may play an important role in the secondary eyewall formation (SEF) of tropical cyclones (TCs). This study is to further examine the role of the inner eyewall structure by comparing two numerical experiments, which were conducted with the same large-scale environment and initial and boundary conditions but different grid sizes. The SEF was simulated in the experiment with the finer grid spacing, but not in the other.Comparing the eyewall structure in the simulated TCs with and without the SEF indicates that the eyewall structure can play an important role in the SEF. For the simulated TC with the SEF, the eyewall is more upright with stronger updrafts, accompanied by a wide eyewall anvil at a higher altitude. Compared to the simulated TC without the SEF, diagnostic analysis reveals that the cooling outside the inner eyewall is induced by the sublimation, melting and evaporation of hydrometeors falling from the eyewall anvil. The cooling also induces upper-level dry, cool inflow below the anvil, prompting the subsidence and moat formation between the inner eyewall and the spiral rainband. In the simulated TC without the SEF, the cooling induced by the falling hydrometeors is significantly reduced and offset by the diabatic warming. There is no upper-level dry inflow below the anvil and no moat formation between the inner eyewall and the spiral rainband. This study suggests that a realistic simulation of the intense eyewall convection is important to the prediction of the SEF in the numerical forecasting model.
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