Hiroshi Niino

An improved Mellor-Yamada level 3 model with condensation physics : Its design and verification

A computational scheme for an improved Mellor–Yamada(M–Y) Level-3 model with condensation physics is proposedand its performance is examined against large-eddy-simulationdata on radiation fog. The improved M–Y model greatlycorrects several shortcomings of the original M–Y model:the underestimations of the mixed-layer depth and themagnitude of turbulent kinetic energy, and the discrepanciesin the formation and dissipation times of the fog. Inaddition the improved M–Y model can reproduce theoccurrence of Kelvin–Helmholtz instability and periodicoscillations due to its energy cycle. It is shown that theoptimization of both the closure constants and the masterlength scale is required for this improvement.The improved M–Y model has an improvement also in theLevel-2.5 version. Although the performance of theLevel-2.5 version is not so good as that of the Level-3version, the former has the advantage of relatively lowcomputational cost and is popularly used in operationalweather forecasts. Our computational scheme for theimproved M–Y model allows us to switch its hierarchylevels easily according to the purpose.

An Experimental and Theoretical Study of Barotropic Instability

Abstract The barotropic instability of horizontal shear flows is investigated by means of a laboratory experiment. Two kinds of basic flows with different velocity profiles am examined, one a free-shear layer and the other a jet. It is found that for both flows the stability is described by a single nondimensional parameter, a Reynolds number R=VL/v where V is the characteristic velocity of the basic flow, L=(E/4)¼H is the characteristic length of the basic flow, v the kinematic viscosity, H the depth of the fluid layer, and E=v(ΩH2)−1 the Ekman number, with Ω the angular velocity of the basic rotation. The experimentally-determined critical Reynolds number Rc and critical wavenumber kc show excellent agreement with those predicted by a linear stability theory in which both Ekman friction and internal viscosity are incorporated. It is found that the internal viscosity plays an important role in explaining the observed values of Rc and kc. When R is larger than Rc, several organized eddies develop along th...

Tornado-Producing Mini Supercells Associated with Typhoon 9019

On the night of 19 September 1990, nine mini-supercell storms were observed over the Kanto Plain of Japan in the northeast quadrant of Typhoon 9019. Three storms out of nine spawned a tornado. The characteristics of the storms and tornadoes were studied by means of a single Doppler radar, conventional radars, surface meteorological observations, and damage surveys. The mini-supercell storms exhibited characteristics similar to typical supercell storms over the Great Plains of the United States, such as hook echo, bounded weak echo region, slower movement relative to the mean wind, long lives, and rightward deviation of the storm motion relative to the mean wind shear was observed. They also displayed several differences from typical supercell storms with respect to the following points: 1) horizontal scale of the mesocyclone was smaller ; and 2) vertical vorticity was confined to lower levels (less than 5 km above ground level). These similarities and differences correspond with those observed for miniature supercells in Hurricane Danny’s environment. The storm environment for these mini supercells was characterized by modest convective available potential energy (about 1600 J kg21) and strong low-level wind shear with veering. The storm updrafts likely tilted the ambient horizontal vorticity associated with the strong low-level wind shear to generate mesocyclonic vertical vorticity. Two of the tornadoes were spawned by mini-supercell storms moving along a preexisting surface boundary that was accompanied by significant convergence, vertical vorticity, and horizontal gradients of temperature. An estimation of vertical vorticity near the ground in the boundary suggests a possibility that preexisting vertical vorticity contributed to supercell evolution resulting in tornadogenesis.

A Statistical Study of Tornadoes and Waterspouts in Japan from 1961 to 1993

Abstract A database on tornadoes and waterspouts in Japan from 1961 to 1993, which is believed to be most reliable at present, was compiled after an extensive survey of apparently almost all existing literature. Six hundred and seventy-seven tornadoes and 148 waterspouts were cataloged in the database, which is operated on a personal computer basis. Various statistical characteristics of tornadoes and waterspouts have been examined: 1) On average 20.5 tornadoes and 4.5 waterspouts occur per year in Japan. 2) Tornadoes occur most frequently in September and least frequently in March. Waterspouts occur most frequently in October. 3) Both tornadoes and waterspouts have high activities during daytime. The activity of tornadoes has two peaks between 1000 and 1100, and between 1500 and 1600 JST (Japan Standard Time). 4) About two-fifths of the tornadoes occur on multiple-tornado days. 5) The average lifetimes of tornadoes and waterspouts are 12 and 14 min, respectively. 6) About 15% of tornadoes are anticycloni...

Sediments of the Gulf of Thailand and Adjacent Continental Shelf

A total of 111 well-distributed sediment samples from the Gulf of Thailand and the adjacent continental shelf were analyzed. The material is dominantly detrital in origin, of modern distribution in the Gulf and on the inner third of the shelf, and relict from a time of lower sea level on the outer half of the shelf. The distribution pattern of organic matter caused by seasonal winds indicates upwelling probably along both sides of the Gulf. The patterns of grain size and of organic matter also suggest that a current flows into the Gulf at the north side of its mouth and that another flows out at the south side. Geological data supported by drill and magnetic measurements show that the long wide river valley at the head of the Gulf is a structural trough formed during the Late Tertiary. In the Gulf a closed depression represents an unfilled extension of this structural trough. Both areas together form an unfilled geosyncline which is still receiving sediments.

Numerical Simulation of Tornadogenesis in an Outer-Rainband Minisupercell of Typhoon Shanshan on 17 September 2006

Abstract On 17 September 2006, three tornadoes occurred along the east coast of Kyusyu Island in western Japan during the passage of an outer rainband in the right-front quadrant of Typhoon Shanshan. To clarify the structure of the tornado-producing storms and the mechanism of tornadogenesis, quadruply nested numerical simulations were performed using a nonhydrostatic model with an innermost horizontal grid spacing of 50 m. Several simulated convective storms in the outermost rainband exhibited characteristics of a minisupercell. One storm had a strong rotating updraft of more than 30 m s−1 and a large vertical vorticity exceeding 0.06 s−1. This storm spawned a tornado when the low-level mesocyclone intensified. The tornado was generated on the rear-flank gust front near the mesocyclone center when a secondary rear-flank downdraft (RFD) surge advanced cyclonically around the low-level mesocyclone and overtook the rear-flank gust front at its left-front edge. Backward trajectories and vorticity budget anal...

Dependence of Polar Low Development on Baroclinicity and Physical Processes: An Idealized High-Resolution Numerical Experiment

Abstract Polar low dynamics in an idealized atmosphere in which baroclinicity, stratification, and average temperature are varied in the typically observed range is investigated using a 5-km-resolution nonhydrostatic model. The baroclinicity is found to be the most important factor that strongly controls the polar low dynamics. When the baroclinicity is weak, a small, nearly axisymmetric vortex develops through a cooperative interaction between the vortex flow and cumulus convection. The surface friction promotes the vortex dynamics by transporting the sensible heat and moisture into the vortex center. The vortex development has a strong sensitivity to the initial perturbation. As the baroclinicity is increased, most of the characteristics of polar low dynamics change smoothly without showing any significant regime shift. The vortex for an intermediate baroclinicity, however, moves northward, which is a unique behavior. This is caused by vortex stretching on the northern side of the vortex where intense c...

A polar low over the Japan Sea on 21 January 1997. Part II: A numerical study

Abstract A remarkable isolated polar low observed over the Japan Sea on 21 January 1997 is studied by numerical simulations using a 20-km-mesh regional primitive equation model [Regional Spectral Model (RSM) of the Japan Meteorological Agency] and a 5-km-mesh nonhydrostatic mesoscale model [Meteorological Research Institute Nonhydrostatic Model (MRI-NHM)] and by sensitivity experiments using the MRI-NHM. The 24-h integration of RSM starting from 0000 UTC 21 January reproduces quite well the polar lows movement and development, including the initial wrapping of the west part of an east–west-oriented vorticity zone. The vorticity budget analysis indicates that the stretching term plays a dominant role in the polar low development. The simulation by MRI-NHM reproduces surprisingly well the wrapping of the western part of the east–west-oriented vortcity zone associated with a cloud band, the spiral-shaped cloud pattern, and the “eye” structure near the vortex center. The wind speed and resulting sea surface ...

A Polar Low over the Japan Sea on 21 January 1997. Part I: Observational Analysis

Abstract A polar low was observed over the Japan Sea on 21 January 1997. Part I of this paper focuses on the observational analyses by using all available observational data from surface meteorological stations, Geostationary Meteorological Satellite-5 (GMS-5), Sapporo and Niigata radars, 20-km resolution regional objective analysis (RANAL) data, Advanced Earth Observing System/NASA Scatterometer (ADEOS/NSCAT) wind data over oceans, and NOAA-14 Advanced Very High Resolution Radiometer (AVHRR) channel 2 and channel 4 images to document its spatial and temporal structures. The detailed evolutionary process of this polar low is described using GMS-5 data as well as Sapporo and Niigata radar data. This is the first paper to show the detailed evolution of a polar low feature by using radar data.

An Extension of the Mellor–Yamada Model to the Terra Incognita Zone for Dry Convective Mixed Layers in the Free Convection Regime

The terra incognita (TI) or grey zone arises in conventional planetary boundary-layer parametrizations when the grid resolution of a numerical model is comparable to the size of the energy-containing turbulent eddies