Mikio Nakanishi

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.

Improvement Of The Mellor–Yamada Turbulence Closure Model Based On Large-Eddy Simulation Data

On the basis of data constructed with large-eddy simulation (LES), an attempt is made to improve the Mellor–Yamada (M–Y) turbulence closure model. Firstly, stably-stratified and convective planetary boundary layers without moisture are simulated by a LES model to obtain a database for the improvement. Secondly, based on the LES data, closure constants are re-evaluated and a new diagnostic equation for the master length scale L is proposed. The new equation is characterized by allowing L in the surface layer to vary with stability instead of constant kz, where k is the von Kármán constant, and z is height.The non-dimensional eddy-diffusivity coefficients calculated from the modifiedM–Y model are in satisfactory agreement with those from the LES data. It isfound that the modified M–Y model improves the original one largely, and thatthe improvement is achieved by considering buoyancy effects on the pressurecovariances andby using the newly proposed equation for L.

Large-eddy simulation of radiation fog

In order to study the three-dimensional structure of radiation fogand to obtain a basic understanding of its generation mechanism,a numerical experiment is performed with a large-eddysimulation model and compared with the observation at Cabauw in the Netherlands. After confirming that the results are insatisfactory agreement with the observations, the structure of thefog and its generation mechanism are examined in more detail.Before the fog forms, the atmosphere is stable and an inversionlayer exists almost adjacent to the ground surface. As the fog grows, however, the stratification is destabilized and a mixed layerdevelops gradually. The longwave radiative cooling near thefog top contributes to the destabilization more than thecondensational heating does.The evolution of the fog can be classified into three stagesaccording to the behaviour of turbulent kinetic energy (TKE):formation, development, and dissipation stages.The fog layer has different flow structures at each stage.During the formation stage, longitudinal rolls similar tostreaks in channel flows appear near the ground surface.The development stage is characterized by an initiation oftransverse bands due to Kelvin–Helmholtz instability anda sudden increase of TKE. During the dissipation stage, longitudinalrolls and polygonal cells due to convective instability are organized.

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

Diurnal Wind Cycles Forcing Inertial Oscillations: A Latitude-Dependent Resonance Phenomenon

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Interaction of an Asymmetric Double Vortex and Trochoidal Motion of a Tropical Cyclone with the Concentric Eyewall Structure

∼1 km or less. Here, we investigate a simple, plausible extension of the Mellor–Yamada (MY) level-3 scheme for TI-scale grid size using a large-eddy simulation (LES) as a benchmark. Horizontal filtering of the benchmark simulation data for the dry convective mixed layer in the free convection regime yields subfilter-scale components whose statistics are then retrieved for various filter sizes. This leads to a modified MY level-3 scheme for TI-scale grid sizes. The proposed TI scheme incorporates: (1) modification of various length scales in the conventional MY scheme by an empirical function that depends on the horizontal grid size normalized by the convective boundary-layer height; (2) a new length scale for horizontal turbulent fluxes; and (3) a linear relationship between the local dissipation length and subfilter-scale turbulent kinetic energy. A posteriori tests of the proposed TI scheme show a much improved performance compared with the conventional MY level-3 scheme. The ratio of the grid-scale to the subgrid-scale turbulent intensity is comparable to that obtained from the filtered LES solutions. Sensitivity tests show that the modification of the dissipation length scales has the largest impact, while the new length scale for horizontal fluxes also proves important. A simulation that includes all of the above modifications results in the optimum performance.

Destabilization of the Symmetric Vortex and Formation of the Elliptical Eye of Typhoon Herb

AbstractFor the last decade, horizontal roll vortices have been often observed in hurricane boundary layers (HBLs). In this study, a large-eddy simulation is performed to explore the formation mechanism of the horizontal roll vortices and their significance in a near-neutrally stratified HBL at 40 km (R40) and 100 km (R100) from the center of the hurricane. Results are examined through turbulence statistics and empirical orthogonal function (EOF) analysis. The EOF analysis and budgets of turbulent kinetic energy demonstrate that an inflection-point instability in the radial velocity profile is responsible for the roll vortices with horizontal wavelengths of 1.5–2.4 km in the HBL both for R40 and R100. The roll vortices for R40 are nearly aligned with the gradient wind, while those for R100 are oriented slightly to the left of that wind. Also the horizontal distributions of velocity fluctuations suggest the presence of streaklike structures at horizontal intervals of several hundred meters near the ground ...

Large-Eddy Simulation of a Residual Layer: Low-Level Jet, Convective Rolls, and Kelvin–Helmholtz Instability

AbstractThe latitudinal dependence of inertial oscillation (IO) in a diurnally evolving atmospheric boundary layer (ABL) is examined using a large-eddy simulation (LES). Previous studies that used LES were unable to simulate such an ABL on a time scale of several days because of high computational cost. By using an LES with a simple radiation scheme, the present study has succeeded in simulating the diurnal behavior of the ABL above the nocturnal stable layer as a function of the latitude. The reality of model simulations is confirmed by comparison with Wangara experiments.It is shown that a resonance-like amplification of the IO appears only at two latitudes where the respective inertial periods are 24 and 12 h. A horizontal wind oscillation with strong dependence on latitude is observed during an entire day. The oscillation amplitude is maximized slightly above the nocturnal stable layer. It seems that this maximum corresponds to the nocturnal low-level jet, whose mechanism is explained in terms of IO. ...