Published papers

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.

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.

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.

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.

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.

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 θe in the lowest layer suddenly increases inside the radius of maximum azimuthally averaged horizontal wind rma 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 rma, 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.

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 (Cd) and enthalpy (Ck), 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-Cd 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.

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.

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.