Sho Yokota

Various phenomena on a water vortex in a cylindrical tank over a rotating bottom

Flows in a cylindrical tank over a rotating bottom are investigated by laboratory experiments. Despite the axisymmetry of the experimental setup, various anisotropic phenomena are observed. The slow rotation of the bottom disk induces a circular flow according to the axisymmetric environment, but polygonal vortices form under faster rotation. Between these two vortex flow states, the flow undergoes a transition with clear hysteresis during which the elliptical shape assumed under faster rotation is retained when the rotation is subsequently slowed to rates that previously supported axisymmetric flow. Sloshing is also observed; here, a calm circular flow state alternates with an oscillation of the water surface along the sidewall of the container. A phase diagram showing the phenomena observed under different combinations of the initial water depth at rest and the rotation rate of the bottom disk is developed following thorough experimental testing over a wide range of parameter values. The features of the dependences of the range for each phenomenon to occur on these parameters are also elucidated.

Ensemble experiments using a nested LETKF system to reproduce intense vortices associated with tornadoes of 6 May 2012 in Japan

Experiments simulating intense vortices associated with tornadoes that occurred on 6 May 2012 on the Kanto Plain, Japan, were performed with a nested local ensemble transform Kalman filter (LETKF) system. Intense vortices were reproduced by downscale experiments with a 12-member ensemble in which the initial conditions were obtained from the nested LETKF system analyses. The downscale experiments successfully generated intense vortices in three regions similar to the observed vortices, whereas only one tornado was reproduced by a deterministic forecast. The intense vorticity of the strongest tornado, which was observed in the southernmost region, was successfully reproduced by 10 of the 12 ensemble members. An examination of the results of the ensemble downscale experiments showed that the duration of intense vorticities tended to be longer when the vertical shear of the horizontal wind was larger and the lower airflow was more humid. Overall, the study results show that ensemble forecasts have the following merits: (1) probabilistic forecasts of the outbreak of intense vortices associated with tornadoes are possible; (2) the miss rate of outbreaks should decrease; and (3) environmental factors favoring outbreaks can be obtained by comparing the multiple possible scenarios of the ensemble forecasts.

The Tornadic Supercell on the Kanto Plain on 6 May 2012: Polarimetric Radar and Surface Data Assimilation with EnKF and Ensemble-Based Sensitivity Analysis

AbstractA tornadic supercell and associated low-level mesocyclone (LMC) observed on the Kanto Plain, Japan, on 6 May 2012 were predicted with a nonhydrostatic mesoscale model with a horizontal resolution of 350 m through assimilation of surface meteorological data (horizontal wind, temperature, and relative humidity) of high spatial density and C-band Doppler radar data (radial velocity and rainwater estimated from reflectivity and specific differential phase) with a local ensemble transform Kalman filter. With assimilation of both surface and radar data, a strong LMC was successfully predicted near the path of the actual tornado. When either surface or radar data were not assimilated, however, the LMC was not predicted. Therefore, both surface and radar data were essential for successful LMC forecasts. The factors controlling the strength of the predicted LMC, defined as a low-level maximum vertical vorticity, were clarified by an ensemble-based sensitivity analysis (ESA), which is a new approach for ana...

Tropical Cyclogenesis due to ITCZ Breakdown: Idealized Numerical Experiments and a Case Study of the Event in July 1988

AbstractThe mechanism of tropical cyclogenesis due to the breakdown of the intertropical convergence zone (ITCZ breakdown) and the structure of associated vortices are studied by numerical experiments using a nonhydrostatic mesoscale model. First, an idealized numerical experiment, in which a simple initial state without external disturbances is assumed, is performed without cumulus parameterization. A zonally uniform forcing of high sea surface temperature (SST) is imposed to generate an ITCZ-like structure. This “ITCZ” starts to undulate and eventually breaks down to form several tropical cyclones (TCs). These TCs merge and end up with a single TC. The energy budget analysis shows that barotropic instability of the low-level flow associated with the ITCZ is responsible for the genesis of vortices, and TC-scale buoyancy production soon takes over to contribute to the intensification of TCs. Conversion from the cumulus-scale kinetic energy into the TC-scale kinetic energy is found to be insignificant duri...

Important Factors for Tornadogenesis as Revealed by High-Resolution Ensemble Forecasts of the Tsukuba Supercell Tornado of 6 May 2012 in Japan

AbstractTo identify important factors for supercell tornadogenesis, 33-member ensemble forecasts of the supercell tornado that struck the city of Tsukuba, Japan, on 6 May 2012 were conducted using ...