研究者詳細 - 宮本　佳明

論文 - 宮本　佳明

分割表示 >> ／全件表示 30 件中 1 - 30 件目

Lightning frequency in an idealized hurricane-like vortex from initial to steady-state using a coupled meteorological and explicit bulk lightning model

Sato Y., Miyamoto Y., Tomita H.

Monthly Weather Review （Monthly Weather Review） 149 （ 3 ） 753 - 771 2021年02月

ISSN 00270644

The dependence of lightning frequency on the life cycle of an idealized tropical cyclone (TC) was investigated using a three-dimensional meteorological model coupled with an explicit lightning model. To investigate this dependence, an idealized numerical simulation covering the initial state to the steady state (SS) of an idealized TC was conducted. The simulation was consistent with the temporal evolution of lightning frequency reported by previous observational studies. Our analyses showed that the dependence originates from changes in the types of convective cloud with lightning over the life cycle of the TC. Before rapid intensification (RI) and in the early stage of RI, convective cloud cells that form under high-convective available potential energy (CAPE) conditions are the main contributors to lightning. As the TC reaches the late stage of RI and approaches SS, the secondary circulation becomes prominent and convective clouds in the eyewall region alongside the secondary circulation gradually become the main contributors to the lightning. In the convective cloud cells formed under high-CAPE conditions, upward velocity is strong and large charge density is provided through noninductive charge separation induced by graupel collisions. This large charge density frequently induces lightning in the clouds. On the other hand, the vertical velocity in the eyewall is weak, and it tends to contribute to lightning only when the TC reaches the mature stage. Our analyses imply that the maximum lightning frequency that occurs before the maximum intensity of a TC corresponds to the stage of a TC's life cycle in which convective cloud cells are generated most frequently and moisten the upper troposphere.

Access to Document (DOI)

Using satellite observations to evaluate the relationships between ice condensate, latent heat release, and tropical cyclone intensification in a mesoscale model

Wu S.N., Soden B.J., Miyamoto Y., Nolan D.S., Buehler S.A.

Monthly Weather Review （Monthly Weather Review） 149 （ 1 ） 113 - 129 2020年12月

ISSN 00270644

This study examines the relationship between frozen hydrometeors and latent heating in model simulations and evaluates the capability of the Weather Research and Forecasting (WRF) Model to reproduce the observed frozen hydrometeors and their relationship to tropical cyclone (TC) intensification. Previous modeling studies have emphasized the importance of both the amount and location of latent heating inmodulating the evolution of TCintensity.However, the lack of observations limits a full understanding of its importance in the real atmosphere. Idealized simulations usingWRF indicate that latent heating is strongly correlated to the amount of ice water content, suggesting that ice water content can serve as an observable proxy for latent heat release in themid- to upper troposphere.Based on this result, satellite observations are used to create storm-centered composites of ice water path as a function of TC intensity. The model reasonably captures the vertical and horizontal distribution of icewater content and its dependence upon TC intensity, with differences typically less than 20%. The model also captures the signature of increased ice water content for intensifying TCs, suggesting that observations of ice water content provide a useful diagnostic for understanding and evaluating model simulations of TC intensification.

Access to Document (DOI)

An energy balance model for low--level clouds based on a simulation resolving mesoscale motions

Miyamoto Y., Sato Y., Nishizawa S., Yashiro H., Seiki T., Noda A.T.

Journal of the Meteorological Society of Japan （Journal of the Meteorological Society of Japan） 98 （ 5 ） 987 - 1004 2020年10月

ISSN 00261165

This study proposes a new energy balance model to determine the cloud fraction of low-level clouds. It is assumed that the horizontal cloud field consists of several individual cloud cells with a similar structure. Using a high–resolution simulation dataset with a wide numerical domain, we conducted an energy budget analysis. Here we show that the energy injected into the domain by surface flux is approximately balanced with that loss due to radiation and advection due to large–scale motion. The analysis of cloud cells within the simulated cloud field showed that the cloud field consists of a number of cloud cells with similar structures. We developed a simple model for the cloud fraction from the energy conservation equation. The cloud fraction determined using the model developed in this study was able to quantitatively captured the simulated cloud fraction.

Access to Document (DOI)

Impacts of number of cloud condensation nuclei on two-dimensional moist rayleigh convection

Miyamoto Y., Nishizawa S., Tomita H.

Journal of the Meteorological Society of Japan （Journal of the Meteorological Society of Japan） 98 （ 2 ） 437 - 453 2020年

ISSN 00261165

The impacts of the number density of cloud condensation nuclei (CCN) and other thermodynamic quantities on moist Rayleigh convection were examined. A numerical model, consisting of a simple two–dimensional equation for Boussinesq air and a sophisticated double moment microphysics scheme, was developed. The impact of the number of CCN is most prominent in the initially formed convection, whereas the convection in the quasi–steady state does not significantly depend on the number of CCN. It is suggested that the former convection is driven by a mechanism without a background circulation, such as parcel theory. In contrast, the latter convection appears to be driven by the statically unstable background layer. Incorporating the cloud microphysics reduces the integrated kinetic energy and number of convective cells (increases the distance between the cells), with some exceptions, which are consistent with previous studies. These features are not largely sensitive to the number of CCN. It is shown in this study that the reduction in kinetic energy is mainly due to condensation (evaporation) in the upper (lower) layer, which tends to stabilize the fluid. The ensemble simulation shows that the sensitivity of the moist processes to changes in the temperature at the bottom boundary, temperature lapse rate, water vapor mixing ratio, and CCN is qualitatively similar to that in the control simulation. The impact becomes strong with increasing temperature lapse rate. The number of convective cells in a domain decreases with the degree of supersaturation or an increase in the domain-integrated condensate.

Access to Document (DOI)

Large dependency of charge distribution in a tropical cyclone inner core upon aerosol number concentration

Sato Y., Miyamoto Y., Tomita H.

Progress in Earth and Planetary Science （Progress in Earth and Planetary Science） 6 （ 1 ） 2019年12月

The impacts of aerosols on the charge distribution of hydrometeors and lightning flash density in a tropical cyclone (TC) were investigated using a meteorological model coupled with an explicit lightning model. The meteorological model successfully simulated the tripole structure of charge density distribution in a TC, as reported by previous studies. The impacts of aerosols were investigated through a sensitivity experiment with changing the aerosol number concentration. The tripole structure became unclear with increasing aerosol number concentrations. The positive charge distribution located in the lower layer was not seen, and raindrops with negative charge distribution reached the surface. As a result, the vertical structure of the charge density was dipolar in the polluted case. As the tripole structure shifted to dipole, the magnitude of the electric field tended to be large, and the flash number was large. By contrast, in the pristine case, the tripole structure was dominant, and the flash number was much smaller than in the polluted case. [Figure not available: see fulltext.].

Access to Document (DOI)

On the correlation between total condensate and moist heating in tropical cyclones and applications for diagnosing intensity

Nolan D.S., Miyamoto Y., Wu S.N., Soden B.J.

Monthly Weather Review （Monthly Weather Review） 147 （ 10 ） 3759 - 3784 2019年

ISSN 00270644

While the transfer of moist enthalpy from the ocean to the atmosphere is the fundamental energy source for tropical cyclones, the release of latent heat in moist convection is the mechanism by which this energy is converted into the kinetic and potential energy of these storms. Most observational estimates of this heat release rely on satellite estimates of rain rate. Here, examination of five high-resolution numerical simulations of tropical cyclones reveals that there is a close correlation between the total condensate and the total heating, even though the former quantity is an amount and the latter is a rate. This relationship is due to the fact that for condensate to be sustained at large values, it must be rapidly replaced by new condensate and associated latent heating. Total condensate and total rain rate within fixed radial distances such as 111 km also show good correlations with the current intensity of the storm, but surprisingly, high values of condensate at high altitudes and close to the storm center are not good predictors of imminent intensification. These relationships are confirmed with an additional ensemble of 270 idealized simulations of tropical cyclones with varying sizes and intensities. Finally, simulated measurements of total condensate are computed from narrow swaths modeled after the cloud profiling radar on the CloudSat satellite. Despite their narrow footprint and the fact that they rarely cut through the exact center of the cyclone, these estimates of total condensate also show a useful correlation with current intensity.

Access to Document (DOI)

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年11月

ISSN 00224928

© 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.

Access to Document (DOI)

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年11月

ISSN 08948755

© 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.

Access to Document (DOI)

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

Miyamoto Y., Nolan D.

Journal of the Atmospheric Sciences （Journal of the Atmospheric Sciences） 75 （ 2 ） 555 - 569 2018年02月

ISSN 0022-4928

© 2018 American Meteorological Society. 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.

Access to Document (DOI)

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年06月

ISSN 0094-8276

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.

Access to Document (DOI)

Tropical cyclone intensity change and axisymmetricity deduced from GSMaP

Shimada Udai, Aonashi Kazumasa, Miyamoto Yoshiaki

Monthly Weather Review 145 （ 3 ） 1003 - 1017 2017年01月

ISSN 0027-0644

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.

Access to Document (DOI)

The effect of water vapor on tropical cyclone genesis

Yoshida Ryuji, Miyamoto Yoshiaki, Tomita Hirofumi, Kajikawa Yoshiyuki

Journal of the Meteorological Society of Japan 95 （ 1 ） 35 - 47 2017年01月

ISSN 0026-1165

The environmental conditions for tropical cyclone genesis are examined by numerical experiment. We focus on the case of a non-developing disturbance showed features for tropical cyclone genesis in the Pacific Area Long-term Atmospheric observation for Understanding climate change in 2010 (PALAU2010) observation campaign over the western North Pacific. We clarify the importance of the presence of abundant moisture around the disturbance for continuous convection and demonstrate that the collocation of a mid-level vortex and a low-level vortex, i.e., the persistence of an upright structure of vortices, is important in tropical cyclone genesis. We conduct two numerical experiments using the Weather Research and Forecasting Model Advanced Research WRF model in double nested domains with a horizontal grid space of 27 km and 9 km for the outer domain and the inner domain, respectively. The first experiment is based on reanalysis data (a control experiment) and the second includes increased water vapor content over the northwestern dry area of the disturbance. In the control experiment, the disturbance did not develop into a tropical cyclone in spite of the existence of the mid-level and low-level vortices. In contrast, the sensitivity experiment shows that a tropical cyclone was formed from the disturbance with increased water vapor content. The presence of persistent upright vortices was supported by continuous convection until the genesis of the tropical cyclone.

Access to Document (DOI)

Resolution dependence of deep convections in a global simulation from over 10-kilometer to sub-kilometer grid spacing

Kajikawa Yoshiyuki, Miyamoto Yoshiaki, Yoshida Ryuji, Yamaura Tsuyoshi, Yashiro Hisashi, Tomita Hirofumi

Progress in Earth and Planetary Science 3 （ 1 ） 2016年12月

The success of sub-kilometer global atmospheric simulation opens the door for resolving deep convections, which are fundamental elements of cloudy disturbances that drive global circulation. A previous study found that the essential change in the simulated convection properties occurred at a grid spacing of about 2 km as a global mean. In grid-refinement experiments, we conducted further comprehensive analysis of the global-mean state and the characteristics of deep convection, to clarify the difference of the essential change by location and environment. We found that the essential change in convection properties was different in the location and environment for each cloudy disturbance. The convections over the tropics show larger resolution dependence than convections over mid-latitudes, whereas no significant difference was found in convections over land or ocean. Furthermore, convections over cloudy disturbances [(i.e., Madden-Julian oscillation (MJO), tropical cyclones (TCs)] show essential change of convection properties at about 1 km grid spacing, suggesting resolution dependence. As a result, convections not categorized as cloudy disturbances make a large contribution to the global-mean convection properties. This implies that convections in disturbances are largely affected organization processes and hence have more horizontal resolution dependence. In contrast, other categorized convections that are not involved in major cloudy disturbances show the essential change at about 2 km grid spacing. This affects the latitude difference of the resolution dependence of convection properties and hence the zonal-mean outgoing longwave radiation (OLR). Despite the diversity of convection properties, most convections are resolved at less than 1 km grid spacing. In the future, longer integration of global atmosphere, to 0.87 km grid spacing, will stimulate significant discussion about the interaction between the convections and cloudy disturbances.

Access to Document (DOI)

Precursors of deep moist convection in a subkilometer global simulation

Miyamoto Yoshiaki, Yamaura Tsuyoshi, Yoshida Ryuji, Yashiro Hisashi, Tomita Hirofumi, Kajikawa Yoshiyuki

Journal of Geophysical Research 121 （ 20 ） 12,080 - 12,088 2016年10月

ISSN 0148-0227

Deepmoist convection in the atmosphere plays an important role in cloudy weather disturbances, such as hurricanes, and even in the global climate. The convection often causes disastrous heavy rainfall, and predicting such convection is therefore critical for both disaster prevention and climate projection. Although the key parameters for convection have been pointed out, understanding the preprocesses of convection is a challenging issue. Here we identified the precursors of convection by analyzing a global simulated data set with very high resolution in time and space. We found that the mass convergence near the Earth’s surface changed significantly several minutes before the initiation of early convection (the formation of cumulus clouds), which occurred with the increase in the convective available potential energy (CAPE). Decomposition of the statistical data revealed that a higher-CAPE environment resulted in stronger convection than in the stronger-convergence case. Furthermore, for the stronger-convergence case, the precursor was detected earlier than the total average (10-15 min before the initiation), whereas the amplitude of maximum velocity was not so strong as the higher-CAPE case. This suggests that the strength of convection is connected with CAPE, and the predictability is sensitive to the convergence.

Access to Document (DOI)

Numerical experiments on the transition of tornado-like vortices by a nonhydrostatic axisymmetric model

Hayashi Yuhei, Takemi Tetsuya, Miyamoto Yoshiaki

Journal of Wind Engineering 41 （ 4 ） 125 - 135 2016年10月

ISSN 1349-3507

The transition of tornado-like vortices is numerically investigated with the use of a nonhydrostatic axisymmetric model. Tornado-like vortices are generated by imposing a specified external force along the central axis of the computational domain that is rotating at a constant angular velocity. When the vortex transition from one to two-celled structure occurs, there is an overshoot of the radial inflow near the surface. The overshooting flow turns upward, directing outward to a larger radius. At the same time, downdraft penetrates to the surface at the rotating axis. The overshoot is generated by coupled effects of inward advection and inward pressure gradient force. The downdraft is caused by downward pressure gradient.

Access to Document (DOI)

Impacts of cloud microphysics on trade wind cumulus

Sato Yousuke, Nishizawa Seiya, Yashiro Hisashi, Miyamoto Yoshiaki, Kajikawa Yoshiyuki, Tomita Hirofumi

Progress in Earth and Planetary Science 2 （ 1 ） 2015年12月

This study investigated the impact of several cloud microphysical schemes on the trade wind cumulus in the large eddy simulation model. To highlight the differences due to the cloud microphysical component, we developed a fully compressible large eddy simulation model, which excluded the implicit scheme and approximations as much as possible. The three microphysical schemes, the one-moment bulk, two-moment bulk, and spectral bin schemes were used for sensitivity experiments in which the other components were fixed. Our new large eddy simulation model using a spectral bin scheme successfully reproduced trade wind cumuli, and reliable model performance was confirmed. Results of the sensitivity experiments indicated that precipitation simulated by the one-moment bulk scheme started earlier, and its total amount was larger than that of the other models. By contrast, precipitation simulated by the two-moment scheme started late, and its total amount was small. These results support those of a previous study. The analyses revealed that the expression of two processes, (1) the generation of cloud particles and (2) the conversion from small droplets to raindrops, were crucial to the results. The fast conversion from cloud to rain and the large amount of newly generated cloud particles at the cloud base led to evaporative cooling and subsequent stabilization in the sub-cloud layer. The latent heat released at higher layers by the condensation of cloud particles resulted in the development of the boundary layer top height.

Access to Document (DOI)

Influence of grid aspect ratio on planetary boundary layer turbulence in large-eddy simulations

Nishizawa S., Yashiro H., Sato Y., Miyamoto Y., Tomita H.

Geoscientific Model Development 8 （ 10 ） 3393 - 3419 2015年10月

ISSN 1991-959X

We examine the influence of the grid aspect ratio of horizontal to vertical grid spacing on turbulence in the planetary boundary layer (PBL) in a large-eddy simulation (LES). In order to clarify and distinguish them from other artificial effects caused by numerical schemes, we used a fully compressible meteorological LES model with a fully explicit scheme of temporal integration. The influences are investigated with a series of sensitivity tests with parameter sweeps of spatial resolution and grid aspect ratio. We confirmed that the mixing length of the eddy viscosity and diffusion due to sub-grid-scale turbulence plays an essential role in reproducing the theoretical -5/3 slope of the energy spectrum. If we define the filter length in LES modeling based on consideration of the numerical scheme, and introduce a corrective factor for the grid aspect ratio into the mixing length, the theoretical slope of the energy spectrum can be obtained; otherwise, spurious energy piling appears at high wave numbers. We also found that the grid aspect ratio has influence on the turbulent statistics, especially the skewness of the vertical velocity near the top of the PBL, which becomes spuriously large with large aspect ratio, even if a reasonable spectrum is obtained.

Access to Document (DOI)

Performance assessment of a triple-frequency spaceborne cloud-precipitation radar concept using a global cloud-resolving model

Leinonen J., Lebsock M. D., Tanelli S., Suzuki K., Yashiro H., Miyamoto Y.

Atmospheric Measurement Techniques 8 （ 8 ） 3493 - 3517 2015年08月

ISSN 1867-1381

Multi-frequency radars offer enhanced detection of clouds and precipitation compared to single-frequency systems, and are able to make more accurate retrievals when several frequencies are available simultaneously. An evaluation of a spaceborne three-frequency Ku-/Ka-/W-band radar system is presented in this study, based on modeling radar reflectivities from the results of a global cloud-resolving model with a 875 m grid spacing. To produce the reflectivities, a scattering model has been developed for each of the hydrometeor types produced by the model, as well as for melting snow. The effects of attenuation and multiple scattering on the radar signal are modeled using a radiative transfer model, while nonuniform beam filling is reproduced with spatial averaging. The combined effects of these are then quantified both globally and in six localized case studies. Two different orbital scenarios using the same radar are compared. Overall, based on the results, it is expected that the proposed radar would detect a high-quality signal in most clouds and precipitation. The main exceptions are the thinnest clouds that are below the detection threshold of the W-band channel, and at the opposite end of the scale, heavy convective rainfall where a combination of attenuation, multiple scattering and nonuniform beam filling commonly cause significant deterioration of the signal; thus, while the latter can be generally detected, the quality of the retrievals is likely to be degraded.

Access to Document (DOI)

A linear thermal stability analysis of discretized fluid equations

Miyamoto Yoshiaki, Ito Junshi, Nishizawa Seiya, Tomita Hirofumi

Theoretical and Computational Fluid Dynamics 29 （ 3 ） 155 - 169 2015年06月

ISSN 0935-4964

The effects of discretization on the equations, and their solutions, describing Rayleigh–Bénard convection are studied through linear stability analysis and numerical integration of the discretized equations. Linear stability analyses of the discretized equations were conducted in the usual manner except that the assumed solution contained discretized components (e.g., spatial grid interval in the x direction, (Formula presented.). As the resolution became infinitely high (Formula presented.), the solutions approached those obtained from the continuous equations. The wavenumber of the maximum growth rate increased with increasing $${Delta x}$$Δx until the wavenumber reached a minimum resolvable resolution, (Formula presented.). Therefore, the discretization of equations tends to reproduce higher-wavenumber structures than those predicted by the continuous equations. This behavior is counter intuitive and opposed to the expectation of (Formula presented.) leading to blurred simulated convection structures. However, when the analysis is conducted for discretized equations that are not combined into a single equation, as is the case for practically solved numerical models, the maximum growing wavenumber rather tends to decrease with increasing $${Delta x}$$Δx as intuitively expected. The degree of the decrease depends on the discretization accuracy of the first-order differentials. When the accuracy of the discretization scheme is of low order, the wavenumber monotonically decreases with increasing (Formula presented.). On the other hand, when higher-order schemes are used for the discretization, the wavenumber does increase with increasing (Formula presented.), a similar trend to that in the case of the single-discretized equation for smaller (Formula presented.).

Access to Document (DOI)

Horizontal distance of each cumulus and cloud broadening distance determine cloud cover

Sato Yousuke, Miyamoto Yoshiaki, Nishizawa Seiya, Yashiro Hisashi, Kajikawa Yoshiyuki, Yoshida Ryuji, Yamaura Tsuyoshi, Tomita Hirofumi

Scientific Online Letters on the Atmosphere 11 （ 2015 ） 75 - 79 2015年01月

ISSN 1349-6476

We examine how cloud cover is determined in shallow-cloud areas by using large-eddy simulation with an extremely wide domain, which covers the transition phase from cumulus-understratocumulus to shallow-cumulus regimes. The relationship between two distances is critical to cloud cover. One characteristic distance is the horizontal distance between cumulus clouds, and the other is the broadening distance of anvil-like stratiform cloud at the top of the boundary layer. High cloud cover occurs with a long distance of broadening and short distances between cumuli. In contrast, low cloud cover appears with a short distance of broadening and a long distance between cumuli. The contrast of the two distances is rooted in aerosol amount and the strength of the surface heat flux. The relationship between these two distances can be applied to estimating the cloud cover below sharp inversions.

Access to Document (DOI)

A triggering mechanism for rapid intensification of tropical cyclones

Miyamoto Yoshiaki, Takemi Tetsuya

Journal of the Atmospheric Sciences 72 （ 7 ） 2666 - 2681 2015年01月

ISSN 0022-4928

Triggering processes for the rapidly intensifying phase of a tropical cyclone (TC) were investigated on the basis of numerical experiments using a three-dimensional nonhydrostatic model. The results revealed that the rapid intensification of the simulated TC commenced following the formation of a circular cloud, which occurred about 12 h after the TC became essentially axisymmetric. The circular cloud (eyewall) evolved from a cloudy convective cell that was originally generated near the radius of maximum wind speed (RMW). The development of the convective cell in the eyewall was closely related to the radial location of the strong boundary layer convergence of axisymmetric flow. The radius of maximum convergence (RMC) was small relative to the RMW when the TC vortex was weak, which is consistent with the boundary layer theory for a rotating fluid system on a frictional surface. As the TC intensified, the RMC approached the RMW. An eyewall was very likely to form in the simulated TC when the RMC approached the RMW. Because the RMC is theoretically determined by a Rossby number defined by the maximum tangential velocity, RMW, and Coriolis parameter, a series of numerical experiments was conducted by changing the three parameters. The results were consistent with the hypothesis that intensification occurs earlier for larger Rossby numbers. This finding indicates that initial TC vortices with larger Rossby numbers are more likely to experience rapid intensification and, hence, to evolve into strong hurricanes.

Access to Document (DOI)

Corrigendum to

Sato Yousuke, Nishizawa Seiya, Yashiro Hisashi, Miyamoto Yoshiaki, Tomita Hirofumi

Scientific Online Letters on the Atmosphere 11 （ 2015 ） c1 2015年01月

ISSN 1349-6476

Access to Document (DOI)

Does convection vary in different cloud disturbances?

Miyamoto Yoshiaki, Yoshida Ryuji, Yamaura Tsuyoshi, Yashiro Hisashi, Tomita Hirofumi, Kajikawa Yoshiyuki

Atmospheric Science Letters 2015年01月

ISSN 1530-261X

Deep moist convection is an element of cloud disturbances such as Madden-Julian oscillation (MJO), tropical cyclones (TCs), mid-latitudinal low depressions (MDL), and fronts (FRT). However, differences in convection characteristics in disturbances remain unclear. We first clarified the statistical features of the convection (structure, intensity, and environmental parameters) in various disturbances simulated by a global simulation with a sub-kilometre grid spacing. The convection in MJO (TC) was tall with a strong (weak) upward motion, and was driven by large convective available potential energy (strong low-level convergence). The convection in MDL and FRT was shorter and characterized by a strong vertical wind shear.

Access to Document (DOI)

Potential of retrieving shallow-cloud life cycle from future generation satellite observations through cloud evolution diagrams

Sato Yousuke, Nishizawa Seiya, Yashiro Hisashi, Miyamoto Yoshiaki, Tomita Hirofumi

Scientific Online Letters on the Atmosphere 10 （ 1 ） 10 - 14 2014年02月

ISSN 1349-6476

We investigated the temporal evolution of two types of diagram produced by a three-dimensional large-eddy simulation: the Contoured Frequency Optical Depth Diagram (CFODD) and the correlation pattern between the cloud droplet effective radius and the cloud optical thickness (RE-COT pattern). The CFODD obtained by statistically method, in the past study, suggested that the transition of cloud stages were included in the diagram, but this paper is the first paper that confirms similar structure in the CFODD obtained by time-series analysis. Our results suggest that a part of the cloud lifecycle; transition from the non-drizzle to drizzle stage can be well expressed by the CFODD of individual clouds. While, the cloud lifecycle on the RE-COT pattern was obscure. This advantage of CFODD over the RE-COT pattern arises because the CFODD uses vertical information of the inner cloud, whereas the effective radius of the RE-COT diagram presents property near the cloud top. Our results emphasize that future satellite observations should retrieve vertical information such as the effective radius at each height with high frequency to capture the lifecycle of individual clouds. This paper presents useful suggestion that will be useful to the satellite user community for retrieving the shallow-cloud lifecycle.

Access to Document (DOI)

Gradient wind balance in tropical cyclones in high-resolution global experiments

Miyamoto Yoshiaki, Satoh Masaki, Tomita Masaki, Oouchi Kazuyoshi, Yamada Yohei, Kodama Chihiro, Kinter James

Monthly Weather Review 142 （ 5 ） 1908 - 1926 2014年01月

ISSN 0027-0644

The degree of gradient wind balance was investigated in a number of tropical cyclones (TCs) simulated under realistic environments. The results of global-scale numerical simulations without cumulus parameterization were used, with a horizontal mesh size of 7 km. On average, azimuthally averaged maximum tangential velocities at 850 (925) hPa in the simulated TCs were 0.72% (1.95%) faster than gradient wind- balanced tangential velocity (GWV) during quasi-steady periods. Of the simulated TCs, 75% satisfied the gradient wind balance at the radius of maximum wind speed (RMW) at 850 and at 925 hPa to within about 4.0%. These results were qualitatively similar to those obtained during the intensification phase. In contrast, averages of the maximum and minimum deviations from the GWV, in all the azimuths at the RMW, achieved up to 40% of the maximum tangential velocity. Azimuthally averaged tangential velocities exceeded the GWV (i.e., supergradient) inside the RMW in the lower troposphere, whereas the velocities were close to or slightly slower than GWV (i.e., subgradient) in the other regions. The tangential velocities at 925 hPa were faster (slower) in the right-hand (left hand) side of the TC motion. When the tangential velocities at theRMW were supergradient, the primary circulation tended to decay rapidly in the vertical direction and slowly in the radial direction, and the eyewall updraft and the RMWwere at larger radii. Statistical analyses revealed that the TC with supergradient wind at theRMWat 850 hPa was characterized by stronger intensity, larger RMW, more axisymmetric structure, and an intensity stronger than potential intensity.

Access to Document (DOI)

Deep moist atmospheric convection in a subkilometer global simulation

Miyamoto Yoshiaki, Kajikawa Yoshiyuki, Yoshida Ryuji, Yamaura Tsuyoshi, Yashiro Hisashi, Tomita Hirofumi

Geophysical Research Letters 40 （ 18 ） 4922 - 4926 2013年09月

ISSN 0094-8276

Deep moist atmospheric convection is a key element of the weather and climate system for transporting mass, momentum, and thermal energy. It has been challenging to simulate convection realistically in global atmospheric models because of the large gap in spatial scales between convection (10⁰ km) and global motions (10⁴ km). We conducted the first ever subkilometer global simulation and described the features of convection. Through a series of grid-refinement resolution testing, we found that an essential change for convection statistics occurred around 2 km grid spacing. The convection structure, number of convective cells, and distance to the nearest convective cell dramatically changed at this resolution. The convection core was resolved using multiple grids in simulations with grid spacings less than 2.0 km.

Access to Document (DOI)

A transition mechanism for the spontaneous axisymmetric intensification of tropical cyclones

Miyamoto Yoshiaki, Takemi Tetsuya

Journal of the Atmospheric Sciences 70 （ 1 ） 112 - 129 2013年01月

ISSN 0022-4928

A mechanism for the transition of tropical cyclones (TCs) to the spontaneous rapid intensification (RI) phase is proposed based on numerical results of a three-dimensional full-physics model. The intensification phase of the simulated TC is divided into three subphases according to the rate of intensification: 1) a slowly intensifying phase, 2) an RI phase, and 3) an adjustment phase toward the quasi-steady state. The evolution of a TC vortex is diagnosed by the energy budget analysis and the degree of axisymmetric structure of the TC vortex, and the simulated TC is determined to be axisymmetrized 12 h before the onset of RI. It is found that equivalent potential temperature θ_e in the lowest layer suddenly increases inside the radius of maximum azimuthally averaged horizontal wind r_ma after the TC becomes nearly axisymmetric. Forward trajectory analyses revealed that the enhanced convective instability in the TC core region where the eyewall subsequently forms results from the increased inertial stability of the TC core after the axisymmetrization. Since fluid parcels remain longer inside r_ma, owing to the increased inertial stability, the parcels obtain more enthalpy from the underlying ocean. As a result, low-level θe and hence convective available potential energy (CAPE) increase. Under the condition with increased CAPE, the eyewall is intensified and the secondary circulation is enhanced, leading to the increased convergence of low-level inflow; this process is considered to be the trigger of RI. Once the eyewall forms, the simulated TC starts its RI.

Access to Document (DOI)

Effects of surface exchange coefficients for high wind speeds on intensity and structure of tropical cyclones

Miyamoto Yoshiaki, Takemi Tetsuya

Theoretical and Applied Mechanics Japan 59 275 - 283 2011年10月

ISSN 1348-0693

Numerical simulations for Typhoon Ioke (2006) are carried out by using the WRF model and that with the revised formulation of exchange coefficients for momentum (C_d) and enthalpy (C_k), in order to examine the influences of surface exchange coefficients in strong winds upon the structure and intensity of tropical cyclones. The simulated results show that the reduced exchange coefficients in strong winds do not significantly affect the minimum surface pressure of the tropical cyclone while the maximum wind speed at the surface increases significantly. Furthermore, it is found that the convective intensity is significantly decreased in the reduced-C_d case specifically in the eyewall region. It is also shown that in spite of the reduced drag coefficient at high wind speed, the surface convergence does not change.

Short-time-scale processes in a mature hurricane as a response to sea surface fluctuations

Ito Kosuke, Ishikawa Yoichi, Miyamoto Yoshiaki, Awaji Toshiyuki

Journal of the Atmospheric Sciences 68 （ 10 ） 2250 - 2272 2011年10月

ISSN 0022-4928

To clarify the effect of fluctuations in surface stress and heat fluxes on the intensity of a mature-state hurricane, a sensitivity analysis is performed by using a cloud-permitting nonhydrostatic axisymmetric adjoint model. The response function of our experiment is tangential velocity at the top of the boundary layer in the eyewall. As a result of an integration backward to 4 min prior to the specified time, a dipole pattern appears in the sensitivity fields with respect to the vertical velocity, the potential temperature, and the mixing ratio of water vapor. A positive (negative) sensitivity is found in the hurricane interior (exterior) relative to the verification region. It exhibits an increase of tangential velocity 4 min after the introduction of positive (negative) perturbations in potential temperature or in the mixing ratio of water vapor in the interior (exterior). These sensitivities are not related to the changes in the central pressure field. With further backward integration, the sensitivity signals reach down to the surface and are located in the exterior region of the hurricane. While the sensitivity with respect to surface friction (heat flux) is strongly negative (positive) within a certain radius, the sensitivity can be positive (negative) beyond that radius. This means that both stronger friction and a reduction in moist air supply in the exterior region of the hurricane can serve to strengthen the maximum tangential velocity. To the authors' knowledge, this effect has not been explained in previous studies.

Access to Document (DOI)

An effective radius of the sea surface enthalpy flux for the maintenance of a tropical cyclone

Miyamoto Y., Takemi T.

Atmospheric Science Letters 11 （ 4 ） 278 - 282 2010年10月

ISSN 1530-261X

This study found that there is a radius within which the sea surface flux for moist enthalpy plays a vital role in determining the intensity of a tropical cyclone. From the results of the numerical experiments using an axisymmetric nonhydrostatic model, it was shown that when the sea surface fluxes are modified within the radius, the intensity suddenly changes on a short timescale. As long as the surface enthalpy flux diminishes outside this radius, the tropical cyclone intensity does not decrease. In the simulated tropical cyclone, this radius locates 7-8 times the radius of the maximum wind speed.

Access to Document (DOI)

　前のページ - 次のページ　

宮本 佳明

論文 - 宮本 佳明

宮本　佳明

論文 - 宮本　佳明