Satellite images reveal that, prior to the development of the MBVD, a chain of small vortices appeared on an east-west oriented cloud band along 39.8N line. This cloud band collocated a trough extending westward from a meso-α-scale low which moved eastward over the Japan Sea. The vortices eventually merged into the MBVD which had a horizontal scale of about 150km, and had a cloud-free eye and spiral-cloud bands.
A numerical simulation using Japan Meteorological Agency Non-hydrostatic Model (JMA-NHM) successfully reproduced the observed lifecycle of the MBVD. The results indicate that the development of the MBVD is divided into 3 stages: the early development stage in which a number of small vortices appear on a shear zone, the late development stage in which the merger of vortices proceeds and a few larger vortices are formed, and the mature stage in which only a single MBVD is present. We performed a detailed analysis on the structure and development mechanism at each stage.
At the early development stage, vortices have horizontal scales of 10km and intervals of 20-40km. To examine the development mechanism of these vortices, an eddy kinetic energy budget is analyzed. Horizontal shear production (HSP) is confined to low levels because the depth of shear zone is shallow at this stage. Buoyancy production (BP) and vertical shear production (VSP), both of which are related to cumulus convection, are dominant in upper levels. The net contribution of BP and VSP is larger than that of HSP, indicating that vortices at the early development stage are generated by strong updrafts of cumulus convection through stretching of vertical vorticity associated with horizontal shear and tilting of horizontal vorticity associated with vertical shear.
At the late development stage, the horizontal scale of the vortices increases to about 50km as a result of merger of vortices. The long axes of these vortices exhibit a clear tilt across the shear zone, which is consistent with a barotropic instability. The eddy kinetic energy budget confirms that HSP becomes relatively dominant since the depth of shear zone increases as the mixed layer develops.
The MBVD at the mature stage has a horizontal scale of about 100km in the vorticity field. A warm core structure is seen at its center, where a dry downdraft occupies. A backward trajectory analysis confirms that the warm core is formed due to adiabatic heating by the downdraft. Such a structure of the MBVD resembles to a tropical cyclone, which develops through a thermal instability such as CISK.
In order to examine the effects of physical processes on the development of the MBVD, performed are sensitivity experiments in which condensational heating and sensible and latent heat fluxes from the sea surface are switched on/off. 48 hour experiments in which the switch-on/off lasts for the whole integration time of 48 hours show that the condensational heating is essential to the development of the MBVD. The sensible heat flux makes the environment favorable for cumulus convection by destabilizing stratification in the lower layer and also for horizontal shear instability by increasing the depth of the shear zone. The latent heat flux moistens the environment and makes it favorable for cumulus convection. Furthermore, 9 hour experiments, in which the switch-on/off lasts for 9 hours, are performed in order to examine the effects of physical processes on the development of the MBVD at the mature stage. The condensational heating is found to be essential for the development at this stage. The effect of horizontal convergence of pre-existing water vapor also plays some important role in the development.