Midtropospheric anticyclonic vortex street associated with a cloud band near a cold front

The characteristics and formation mechanism of an anticyclonic vortex stree t of meso-beta scale, which appeared near a cold front around the Ryukyu Is lands, Japan, on 11 April 1995, were examined by an analysis of observation al data, a numerical simulation, and a linear stability theory. The vortex street was generated near the midtroposphere on the north edge o f a cloud band along the cold front. The diameter of individual vortices wa s 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-fr ont wind was about 15 m s(-1) weaker than that in the environment. A region of large anticyclonic horizontal sheer (similar to 10(-4) s(-1)) existed o n the north side of this weak wind region. To examine a detailed structure and formation mechanism of the weak a ind r egion. a numerical simulation was performed. Regions of active convection a nd convective cloud bands, associated with a convergence line of the surfac e wind, were reproduced at the position where they were observed. The weak wind region accompanied by the cloud band was also reproduced. A budget ana lysis 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 the re 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 in stability). 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.