Search Results

You are looking at 1 - 4 of 4 items for

  • Author or Editor: Yasushi Fujiwara x
  • All content x
Clear All Modify Search
Yasushi Fujiwara and Yutaka Yoshikawa

Abstract

Wave-resolving simulations of monochromatic surface waves and Langmuir circulations (LCs) under an idealized condition are performed to investigate the dynamics of wave–current mutual interaction. When the Froude number (the ratio of the friction velocity of wind stress imposed at the surface and wave phase speed) is large, waves become refracted by the downwind jet associated with LCs and become amplitude modulated in the crosswind direction. In such cases, the simulations using the Craik–Leibovich (CL) equation with a prescribed horizontally uniform Stokes drift profile are found to underestimate the intensity of LCs. Vorticity budget analysis reveals that horizontal shear of Stokes drift induced by the wave modulation tilts the wind-driven vorticity to the downwind direction, intensifying the LCs that caused the waves to be modulated. Such an effect is not reproduced in the CL equation unless the Stokes drift of the waves modulated by LCs is prescribed. This intensification mechanism is similar to the CL1 mechanism in that the horizontal shear of the Stokes drift plays a key role, but it is more likely to occur because the shear in this interaction is automatically generated by the LCs whereas the shear in the CL1 mechanism is retained only when a particular phase relation between two crossing waves is kept locked for many periods.

Restricted access
Yasushi Fujiwara, Yutaka Yoshikawa, and Yoshimasa Matsumura

Abstract

Fujiwara et al. explicitly simulated Langmuir circulations using a wave-resolving simulation (WRS) technique and found that the residual wave effect on vorticity was well represented by the vortex force of the Craik–Leibovich (CL) equation, at least in the simulated situation. In response to the simulation results, Mellor has proposed a view that ubiquitous applicability of the CL formulation is still questionable and that the three-dimensional radiation stress (3DRS) formulation that he has derived encompasses both of the vortex force effect and an effect that is lower order in terms of wave steepness. Here, these opinions are discussed in terms of the approximations used in the wave-averaged formulations. The asymptotic expansion of the Eulerian-averaged momentum equation allows the separate discussion of two different wave effects: pressure correction and torque. It is argued that the approximation adopted in Mellor’s 3DRS formulation is presumably not accurate enough to properly parameterize the wave torque effect, and possible approaches to examine its performance are proposed. We agree with the view that the applicability of the CL formulation needs further investigation. WRS will be a helpful tool for this purpose.

Full access
Chusei Fujiwara, Kazuya Yamashita, Mikio Nakanishi, and Yasushi Fujiyoshi

Abstract

Atmospheric boundary layer (ABL) observations were conducted in an urban area (Sapporo, Japan) from April 2005 to July 2007 using a three-dimensional scanning coherent Doppler lidar. During this period, 50 dust devil–like vortices were detected in the area; they occurred during the daytime and were located at vertices or in the branches of convective cells (“fishnet” patterns of wind field). The diameters of the vortex cores ranged from 30 to 120 m, and maximum vorticity ranged from 0.15 to 0.26 s−1. More than 60% of the vortices were cyclonic; the rest were anticyclonic. The tangential velocity component of the strongest vortex varied from −5.4 to +1.4 m s−1, and the signal-to-noise ratio was weak in the core. Temporal changes were observed in the three-dimensional structures of two vortices from 1330 to 1354 (Japan standard time) 14 April 2005, and the temporal evolution of the stronger vortex was studied. The vortex initially formed along a low-level convergence line in a fishnet and developed vertically. Its vorticity increased with time in association with shrinkage in the core diameter.

Full access
Yasushi Fujiwara, Yutaka Yoshikawa, and Yoshimasa Matsumura

Abstract

The present study performs a wave-resolving simulation of wind-driven currents under monochromatic surface gravity waves using the latest nonhydrostatic free-surface numerical model. Here, phase speed of the waves is set much greater than the current speed. Roll structures very similar to observed Langmuir circulations (LCs) appear in the simulation only when both waves and down-wave surface currents are present, demonstrating that the rolls are driven by the wave–current interaction. A vorticity analysis of simulated mean flow reveals that the rolls are driven by the torque associated with wave motion, which arises from a correlation between wave-induced vorticity fluctuation and the wave motion itself. Furthermore, it is confirmed that the wave-induced torque is very well represented by the curl of the vortex force (VF), that is, the vector product of mean vorticity and Stokes drift velocity. Therefore, it is concluded that the simulated rolls are LCs and that the wave effects are well represented by the VF expression in the present simulation. The present study further revisits the scaling assumptions made by previous studies that derived VF formulation and shows that there is disagreement among the previous studies regarding the applicability of VF formulation when the wave orbital velocity (proportional to the amplitude times the frequency) is much smaller than the mean flow velocity. The result from the present simulation shows that the VF expression is still valid even with such small wave amplitudes, as long as phase speed of the waves is much greater than the current speed.

Full access