This note describes a numerically stable version of the improved Mellor-Yamada (M-Y) Level-3 model proposed by Nakanishi and Niino [Nakanishi, M. and Niino, H.: 2004, Boundary-Layer Meteorol. 112, 1-31] and demonstrates its application to a regional prediction of advection fog. In order to ensure the realizability for the improved M-Y Level-3 model and its numerical stability, restrictions are imposed on computing stability functions, on L/q, the temperature and water-content variances, and their covariance, where L is the master length scale and q 2 /2 the turbulent kinetic energy per unit mass. The model with these restrictions predicts vertical profiles of mean quantities such as temperature that are in good agreement with those obtained from large-eddy simulation of a radiation fog. In a regional prediction, it also reasonably reproduces the satellite-observed horizontal distribution of an advection fog.

An improved Mellor-Yamada (MY) turbulence closure model (MYNN model: Mellor-YamadaNakanishi-Niino model) that we have developed is summarized and its performance is demonstrated against a large-eddy simulation (LES) of a convective boundary layer. Unlike the original MY model, the MYNN model considers e¤ects of buoyancy on pressure covariances and e¤ects of stability on the turbulent length scale, with model constants determined from a LES database. One-dimensional simulations of Day 33 of the Wangara field experiment, which was conducted in a flat area of southeastern Australia in 1967, are made by the MY and MYNN models and the results are compared with horizontal-average statistics obtained from a threedimensional LES. The MYNN model improves several weak points of the MY model such as an insu‰cient growth of the convective boundary layer, and underestimates of the turbulent kinetic energy and the turbulent length scale; it reproduces fairly well the results of the LES including the vertical distributions of the mean and turbulent quantities. The improved performance of the MYNN model relies mainly on the new formulation of the turbulent length scale that realistically increases with decreasing stability, and partly on the parameterization of the pressure covariances and the expression for stability functions for third-order turbulent fluxes.

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For 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 (RlOO) 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 inflectionpoint 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 RlOO. The roll vortices for R40 are nearly aligned with the gradient wind, while those for RlOO 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 surface. Internal gravity waves, Kelvin-Helmholtz waves, and entrainments occur above the HBL and are partly coupled with the roll vortices in the HBL. implying an enhancement of vertical transports of momentum and other quantities between the HBL and the free atmosphere.

Formation of dust devils in diurnally-evolving convective mixed layers is studied by means of a large eddy simulation. It is found that a weaker general wind and a stronger surface heat flux for which cellular convection rather than roll convection prevails are favorable for the formation of dust devils. The simulation results show that when the general wind is weak, the maximum vertical vorticity in the convective mixed layer is a monotonically increasing function of w à , where w à is the convective velocity scale for a convective mixed layer. Therefore, dust devils occur most frequently in the early afternoon when the heat flux is large and the convective mixed layer grows to a significant height.The simulated dust devils are found to have a horizontal length scale comparable with observed larger dust devils. They have either one-celled or two-celled structure. Some of them have a one-celled structure initially, but later evolve into a two-celled structure.

Dust devils are small-scale vertical vortices often observed over deserts or bare land during the daytime under fair weather conditions. Previous numerical studies have demonstrated that dust devil–like vertical vortices can be simulated in idealized convective mixed layers in the absence of background winds or environmental shear. Their formation mechanism, however, has not been completely clarified. In this paper, the authors attempt to clarify the vorticity source of a dust devil–like vortex by means of a large-eddy simulation, in which a material surface initially placed in the vortex is tracked backward and the circulation on the material surface is examined. The material surface is found to originate from downdrafts, which already have sufficient circulation. As the material surface converges toward the vortex, the vorticity is increased because of conservation of circulation. It is shown that a convective mixed layer is inherently accompanied by circulation, which is scaled by a product of the convective velocity scale and the depth of the convective mixed layer. This circulation is considered to be originally generated by tilting of baroclinically generated horizontal vorticity principally at middepths of the convective mixed layer.