Miyamoto, Yoshiaki



Faculty of Environment and Information Studies (Shonan Fujisawa)


Associate Professor

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  • A dynamical mechanism for secondary eyewall formation in tropical cyclones

    Miyamoto Y., Nolan D., Sugimoto N.

    Journal of the Atmospheric Sciences (Journal of the Atmospheric Sciences)  75 ( 11 ) 3965 - 3986 2018.11

    ISSN  00224928

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    © 2018 American Meteorological Society. This study proposes that secondary eyewall formation (SEF) of tropical cyclones (TCs) can be attributed to an instability of flow in the free atmosphere coupled with Ekman pumping. Unstable solutions of a 1.5-layer shallow-water system are obtained under fast-wind speed conditions in the free atmosphere. The instability condition derived in the linear model indicates the importance of the ratio of angular velocity to vorticity, and the condition is more likely to be satisfied when the ratio is large and its radial gradient is positive. Thus, fast angular velocity, low absolute vertical vorticity, small negative radial gradient of angular velocity, and large negative gradient of vertical vorticity are favorable. Eigenvalue analyses are performed over a wide range of parameters using a vorticity profile with an infinitesimal secondary maximum. The growth rate increases with vorticity outside the radius of maximum wind (RMW), the radius of the secondary vorticity maximum, its magnitude, and the Rossby number defined by maximum tangential velocity, the RMW, and the Coriolis parameter. Furthermore, the growth rate is positive only between 2 and 7 times the RMW, and it is negative close to or far outside the RMW. These features are consistent with previous studies on SEF. A dimensionless quantity γ obtained from the unstable condition in the linear theory is applied to SEF events simulated by two different full-physics numerical models;γ increases several hours before a secondary peak of tangential velocity forms, suggesting that the initial process of SEF can be attributed to the proposed mechanism.

  • Characteristics of tropical cyclone rapid intensification over the Western North Pacific

    Fudeyasu H., Ito K., Miyamoto Y.

    Journal of Climate (Journal of Climate)  31 ( 21 ) 8917 - 8930 2018.11

    ISSN  08948755

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    © 2018 American Meteorological Society. 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 (15.4ms-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 ofRI-TCs thatmake landfall in Japan and the Philippines are higher than in China and Vietnam.

  • Structural changes preceding rapid intensification in tropical cyclones as shown in a large ensemble of idealized simulations

    Miyamoto Yoshiaki, Nolan David S.

    Journal of the Atmospheric Sciences (Journal of the Atmospheric Sciences)  75 ( 2 ) 555 - 569 2018.02

    ISSN  0022-4928

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    <p>Structural changes that precede rapid intensification (RI) of tropical cyclones (TCs) are examined in a full-physics model by conducting a large ensemble (270) of idealized TC simulations. The processes leading to RI in a representative case with moderate shear are consistent with previous studies for weakly sheared cases. The most distinct changes are that the vortex tilt and the vortex size begin to decrease more rapidly 6 h before the onset of RI. A vorticity budget analysis for the upper layer around the low-level center reveals that the vertical vorticity is increased by vertical advection, stretching, and tilting terms before RI, whereas the horizontal advection is small. Thus, the upright vortex structure is not achieved through a vortex alignment process but rather is built upward by deep convection. The ensemble simulations are generated by changing the intensity and size of the initial vortex, the magnitude of vertical wind shear, and the translation speed. The ensemble members that show RI are consistent with the control case and many previous studies: before the onset of RI, the intensity gradually increases, the radius of maximum tangential wind (RMW) decreases, the flow structure becomes more symmetric, the vortex tilt decreases, and the radius of maximum convergence approaches the radius of maximum winds. A dimensionless parameter representing a tendency for the formation of the vertically upright structure is considered. The product of this parameter and the local Rossby number is significantly larger for TCs that exhibit RI in the next 24 h.</p>

  • An analytical model of maximum potential intensity for tropical cyclones incorporating the effect of ocean mixing

    Miyamoto Yoshiaki, Bryan George H., Rotunno Richard

    Geophysical Research Letters 44 ( 11 ) 5826 - 5835 2017.06

    ISSN  0094-8276

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    <p>An analytical model of maximum potential intensity (PI) for tropical cyclones (TCs) incorporating wind-induced ocean cooling is developed on the basis of Emanuel's PI theory. The model consists of a one-dimensional ocean and an axisymmetric TC vortex that is translating at a constant speed. The model advances upon previous approaches by accounting for TC size and shape. The PI is determined at the radius of maximum winds by radially integrating the effect of ocean mixing using a specified wind profile. The resulting analytic ocean cooling agrees well with numerical solutions, and the new PI model better captures maximum intensities of observed TCs compared with Emanuel's original PI. It is also shown that the degree of cooling (i.e., ocean's feedback to atmosphere) can be quantified by a dimensionless parameter, representing the ratio of atmospheric forcing to ocean stability.</p>

  • Tropical cyclone intensity change and axisymmetricity deduced from GSMaP

    Shimada Udai, Aonashi Kazumasa, Miyamoto Yoshiaki

    Monthly Weather Review 145 ( 3 ) 1003 - 1017 2017.01

    ISSN  0027-0644

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    <p>The relationship of tropical cyclone (TC) future intensity change to current intensity and current axisymmetricity deduced from hourly Global Satellite Mapping of Precipitation (GSMaP) data was investigated. Axisymmetricity is a metric that correlates positively with the magnitude of the axisymmetric component of the rainfall rate and negatively with the magnitude of the asymmetric component. The samples used were all of the TCs that existed in the western North Pacific basin during the years 2000-15. The results showed that, during the development stage, the intensification rate at the current time, and 6 and 12 h after the current time was strongly related to both the current intensity and axisymmetricity. On average, the higher the axisymmetricity, the larger the intensity change in the next 24 h for TCs with a current central pressure (maximum sustained wind) between 945 and 995 hPa (85 and 40 kt). The mean value of the axisymmetricity for TCs experiencing rapid intensification (RI) was much higher than that for non-RI TCs for current intensities of 960-990 hPa. The new observational evidence for the intensification process presented here is consistent with the findings of previous theoretical studies emphasizing the role of the axisymmetric component of diabatic heating.</p>

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  • 台風の強度・構造に対するエアロゾルの影響


    MEXT,JSPS, Grant-in-Aid for Scientific Research, 宮本 佳明, Grant-in-Aid for Research Activity Start-up , Principal Investigator


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