This study statistically investigates the characteristics of tropical cyclones (TCs) undergoing rapid intensification (RI) in the western North Pacific in the 37 years from 1979 to 2015 and the ...relevant atmospheric and oceanic environments. Among 900 TCs, 201 TCs undergoing RI (RI-TCs) are detected by our definition as a wind speed increase of 30 kt (15.4 m s−1) or more in a 24-h period. RI-TCs potentially occur throughout the year, with low variation in RI-TC occurrence rate among the seasons. Conversely, the annual occurrence of RI-TC varies widely. In El Niño years, TCs tend to undergo RI mainly as a result of average locations at the time of tropical storm formation (TSF) being farther east and south, whereas TCs experience RI less frequently in La Niña years. The occurrence rates of RI-TC increased from the 1990s to the late 2000s. The RI onset time is typically 0–66 h after the TSF and the duration that satisfies the criteria of RI is 1–2 days. RI frequently occurs over the zonally elongated area around the eastern Philippine Sea. The development stage and life-span are longer in RI-TCs than in TCs that do not undergo RI. RI-TCs are small at the time of TSF and tend to develop as intense TCs as a result of environmental conditions favorable for TC development, weak vertical wind shear, high convective available potential energy, and tropical cyclone heat potential. The occurrence rates of RI-TCs that make landfall in Japan and the Philippines are higher than in China and Vietnam.
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BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
The characteristics of tropical cyclones (TCs) in the summer and autumn seasons over the western North Pacific that are associated with different environmental factors that influence TC ...genesis (TCG) were studied. The authors objectively categorized factors into the five TCG factors classified by Ritchie and Holland: monsoon shear line (SL), monsoon confluence region (CR), monsoon gyre (GY), easterly wave (EW), and the Rossby wave energy dispersion from a preexisting TC (PTC). The GY-TCs tended to develop slowly, and the highest rates of occurrence of rapid intensification (RI) were found for the CR-TCs, whereas the GY-TCs rarely experienced RI. The average storm size of the GY-TCs at the time of formation was the largest of the averages among the TC types, while the EW- and PTC-TCs were smaller, although these differences disappeared at the mature time. There were no significant differences in the sea surface temperature (SST) beneath the TCs, but the tropical cyclone heat potential (TCHP) of the PTC-TCs was higher. The PTC-TCs tended to develop as intense TCs and exhibited favorable environmental characteristics, such as high TCHP, high convective available potential energy, and weak vertical shear. The occurrence rate of the PTC-TCs that made landfall in the Philippines was higher than the averages of the other TC types, whereas those of the EW-TCs (PTC-TCs) that made landfall in Japan (China) were lower. These results provide important information for use in disaster prevention.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
This study examined the statistical characteristics of tropical cyclones (TCs) for which the cyclogenesis (TCG) process was modulated by upper tropospheric cold lows (UCLs) over the western North ...Pacific during the 38 years from 1979 to 2016. Among the 965 TCs, 90 TCs (9 %, 2.4 per year) were defined as having TCG influenced by UCLs in the northwest quadrant of the TC region (UL-TCs). Most UL-TCs occurred in the summer, with large variability in the annual occurrence rate of UL-TCs during June to October, ranging from 0 to approximately 30 %. The annual variation was related to the activity of the Tibetan high and the summer temperature anomaly over Japan. The extremely hot summer of 2016 was partly enhanced by the intense Tibetan high, when 4 UL-TCs also occurred. The average location of UL-TCs at the time of TCG and tropical storm formation (TSF) was significantly farther to the north than the average location of TCs not formed under the influence of UCL (N-UL-TCs). Many UL-TCs occurred in lower tropospheric environments associated with the shear line or confluence regions. The UL-TCs tended to move northward, and the occurrence rate of UL-TCs that made landfall in Japan was approximately double that of other countries. The atmospheric environmental parameters around UL-TCs at the time of TCG were more favorable for the development of TCs than those around N-UL-TCs. In contrast, the atmospheric and oceanic environmental parameters around UL-TCs at the time of TSF were less favorable for the development of TCs, such that UL-TCs tended to remain at weak intensity.
Abstract
The balanced contribution to the intensification of a tropical cyclone simulated in the three-dimensional, nonhydrostatic, full-physics tropical cyclone model version 4 (TCM4), in particular ...the spinup of the outer-core circulation, is investigated by solving the Sawyer–Eliassen equation and by computing terms in the azimuthal-mean tangential wind tendency equation. Results demonstrate that the azimuthal-mean secondary circulation (radial and vertical circulation) and the spinup of the midtropospheric outer-core circulation in the simulated tropical cyclone are well captured by balance dynamics. The midtropospheric inflow develops in response to diabatic heating in mid–upper-tropospheric stratiform (anvil) clouds outside the eyewall in active spiral rainbands and transports absolute angular momentum inward to spin up the outer-core circulation. Although the azimuthal-mean diabatic heating rate in the eyewall is the largest, its contribution to radial winds and thus the spinup of outer-core circulation in the middle troposphere is rather weak. This is because the high inertial stability in the inner-core region resists the radial inflow in the middle troposphere, limiting the inward transport of absolute angular momentum. The result thus suggests that diabatic heating in spiral rainbands is the key to the continued growth of the storm-scale circulation.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Shallow coastal seawater response during the passage of near-landfall intensification (NLI) tropical cyclones (TCs) and non-NLI TCs was examined using oceanic and atmospheric reanalysis data and ...observations. The sea surface temperature ahead of the NLI-TC track is maintained or even increases when NLI-TC is approaching the land. The magnitude of the wind stress, which plays an important role in the NLI process, is related to the zonal surface wind on the right side of the tracks. Coastal mixed layer warming can be explained by Ekman transport under sustained wind stress due to surface wind forcing. The successive deepening of the coastal ocean boundary layer and the increase in warming in the subsurface seawater temperature by an average of 0.3°C, could maintain thermal capacity in a certain degree. This shallow coastal water response could partly explain the NLI progress in the northern South China Sea, indicating the importance of coastal ocean dynamics and air-sea interactions.
This study quantitatively evaluated the typhoon‐induced sea surface temperature (SST) cooling caused by typhoons Faxai (2019) and Hagibis (2019) using a high‐resolution ocean model and the cooling ...parameter (Co). Faxai and Hagibis both passed over the ocean south of the eastern part of the Japanese main island, but the associated average SST decreases differed. Faxai produced a decrease of less than 2°C, whereas Hagibis produced a decrease of more than 2°C. The average Co value was 1.6 for Faxai and 3.6 for Hagibis, indicating that SST was more easily cooled by Hagibis than by Faxai, consistent with the observations. The impact of ocean conditions on the typhoon‐induced SST cooling by Hagibis was 2.6 times larger than the impact by Faxai, indicating that the ocean before Hagibis passes is less hard to cool ocean than Faxai. In short, it is important for ocean cooling not only ocean conditions but also typhoon characteristics because in fact, Hagibis cooled the ocean more than Faxai. In addition, the impact of Hagibis's characteristics on the typhoon‐induced SST cooling was 4.8 times larger than the impact of Faxai's characteristics. Thus, SST was more likely to cool by typhoon characteristics in the case of Hagibis. In particular, among Hagibis's characteristics, typhoon size in the horizontal direction had the most efficient effect on SST cooling. Although Co does not consider the effects of the advection of ocean water, we suggest that Co is a practical indicator for estimating SST cooling caused by a typhoon and comparing factors of typhoon‐induced SST cooling in multiple cases.
This study quantitatively evaluated the typhoon‐induced sea surface temperature (SST) cooling caused by typhoons Faxai and Hagibis using a high‐resolution ocean model and the cooling parameter (Co). The average Co value was 1.6 for Faxai and 3.6 for Hagibis, indicating that SST was more easily cooled by Hagibis than Faxai, consistent with the observations. We suggest that Co is a practical indicator for estimating SST cooling caused by a typhoon.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
This study investigated the atmospheric and oceanic contributions to the genesis of Typhoon Faxai in 2019. Our statistical analysis using the tropical cyclone genesis score (TGS) attributed the ...tropical disturbance that developed into Faxai (Pre-Faxai) to easterly waves (EWs). The EW score evaluated by a grid version of the TGS (Grid-EW) averaged around the occurrence of Pre-Faxai was approximately twice as large as the climatological mean, and it was the second largest value in the past 38 years. The Pre-Faxai area with high Grid-EW scores could be traced back to the eastern North Pacific (ENP) around August 25, 2019. The lower-troposphere environment characterized by high Grid-EW scores was favorable for vortex formation because it provided a containment area for moisture entrained by the developing circulation or lofted by the deep convection therein. The Pre-Faxai area with high Grid-EW scores moved westward due to the background easterly flow over the ENP and then entered the western North Pacific (WNP). The Typhoon Intensity Forecast Scheme (TIFS) showed that the important environments for its genesis were ocean conditions and the vertical wind shear. The oceanic conditions contributed to the development of Pre-Faxai as it traveled over the WNP. The enhancement of vertical wind shear and subsequent suppression of the development of Pre-Faxai were caused by the lower-troposphere easterly winds associated with high EW scores. They were also caused by upper-troposphere westerly winds associated with an upper cold low northwest of Pre-Faxai. When the vertical shear decreased with weakening of the upper cold low, Pre-Faxai reached tropical storm intensity on September 4. Therefore, TGS and TIFS detected Pre-Faxai 10 days before the typhoon arose, which indicates that monitoring environmental factors such as EW and vertical wind shear are important for disaster prevention.
Intense tropical cyclones (TCs) sometimes cause huge disasters, so it is imperative to explore the impacts of climate change on such TCs. Therefore, the authors conducted numerical simulations of the ...most destructive historical TC in Japanese history, Typhoon Vera (1959), in the current climate and a global warming climate. The authors used four nonhydrostatic models with a horizontal resolution of 5 km: the cloud-resolving storm simulator, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, the Japan Meteorological Agency (JMA) operational nonhydrostatic mesoscale model, and the Weather Research and Forecasting Model. Initial and boundary conditions for the control simulation were provided by the Japanese 55-year Reanalysis dataset. Changes between the periods of 1979–2003 and 2075–99 were estimated from climate runs of a 20-km-mesh atmospheric general circulation model, and these changes were added to the initial and boundary conditions of the control simulation to produce the future climate conditions.
Although the representation of inner-core structures varies largely between the models, all models project an increase in the maximum intensity of future typhoons. It is found that structural changes only appeared around the storm center with sudden changes in precipitation and near-surface wind speeds as the radius of maximum wind speed (RMW) contracted. In the future climate, the water vapor mixing ratio in the lower troposphere increased by 3–4 g kg−1. The increased water vapor allowed the eyewall updrafts to form continuously inside the RMW and contributed to rapid condensation in the taller and more intense updrafts.
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BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
When Typhoon Songda (2004) was located southeast of Okinawa over the western North Pacific during 2-4 September 2004, a heavy rainfall event occurred over southern central Japan and its adjacent ...seas, more than 1200 km from the typhoon center. The Advanced Research version of the Weather Research and Forecast (WRF-ARW) model was used to investigate the possible remote effects of Typhoon Songda on this heavy precipitation event in Japan. The National Centers for Environmental Prediction (NCEP) global final (FNL) analysis was used to provide both the initial and lateral boundary conditions for the WRF model. The model was initialized at 1800 UTC 2 September and integrated until 1800 UTC 6 September 2004, during which Songda was a supertyphoon. Two primary numerical experiments were performed. In the control experiment, a bogus vortex was inserted into the FNL analysis to enhance the initial storm intensity such that the model typhoon had an intensity that was similar to that observed at the initial time. In the no-typhoon experiment, the vortex associated with Typhoon Songda in the FNL analysis was removed by a smoothing algorithm such that the typhoon signal did not appear at the initial time. As verified against various observations, the control experiment captured reasonably well the evolution of the storm and the spatial distribution and evolution of the precipitation, whereas the remote precipitation in Japan was largely suppressed in the no-typhoon experiment, indicting the significant far-reaching effects of Typhoon Songda. Songda enhanced the remote precipitation in Japan mainly through northward moisture transport into the preconditioned precipitation region by its outer circulation. The orographic forcing of the central mountains in Japan played a small role compared with Typhoon Songda in this extreme precipitation event.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
This study proposes a deep learning approach called SolaCam to accurately estimate solar radiation from the images captured by cameras. The proposed SolaCam performs deep learning by utilizing both ...image features and theoretical maximum solar radiation that vary with time and location. The trained model is capable of accurately estimating solar radiation on the ground surface from sky images captured by smartphones, fixed-point cameras, and other devices. The developed SolaCam can use a remote sensing function, which estimates solar radiation, on inexpensive camera-equipped devices.
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