Midtropospheric Anticyclonic Vortex Street Associated with a Cloud Band near a Cold Front

Abstract

The characteristics and formation mechanism of an anticyclonic vortex street of meso-? scale, which appeared near a cold front around the Ryukyu Islands, Japan, on 11 April 1995, were examined by an analysis of observational data, a numerical simulation, and a linear stability theory. The vortex street was generated near the midtroposphere on the north edge of a cloud band along the cold front. The diameter of individual vortices was 70 km, and the interval between vortices was 100?200 km. The phase speed of the vortices was nearly equal to the wind speed at the 400-hPa level. The cloud band was accompanied by a weak wind region, in which the along-front wind was about 15 m s?1 weaker than that in the environment. A region of large anticyclonic horizontal shear (?10?4 s?1) existed on the north side of this weak wind region. To examine a detailed structure and formation mechanism of the weak wind region, a numerical simulation was performed. Regions of active convection and convective cloud bands, associated with a convergence line of the surface wind, were reproduced at the position where they were observed. The weak wind region accompanied by the cloud band was also reproduced. A budget analysis of the along-front momentum equation shows that the weak wind region was produced by vertical advection of horizontal momentum in the convective cloud band, which is resolved by the 15-km grid interval of the numerical model. The stability of the simulated along-front wind and temperature fields near the weak wind region was examined by a linear theory. It is found that there exist a number of growing modes, the disturbance kinetic energy of which is supplied by the kinetic energy of the basic flow (i.e., a barotropic instability). The fastest growing mode has a maximum amplitude at 450 hPa and is confined to the region of 450 hPa ± 100 hPa. The wavelength and phase velocity of the fastest growing mode agree well with those of the observed vortex street.