Published papers

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.

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.

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.].

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.

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.

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.

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.

© 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.

© 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.).