We made case study on the meridional separation of zonally elongated large cloud band over the western tropical Pacific during TOGA-COARE with GMS-4 infrared T_BB data and ECMWF global objective analysis data. There were four cases of the separation of the cloud band. All separations during TOGA-COARE were observed over the ocean in the northern hemisphere.

In all cases, cloud bands, which developed with zonal extent up to several thousand kilometers, extend north-to-south simultaneously in zonal direction and separated into two or three sections. In terms that there was the strong divergence in the upper troposphere, and the northern and southern sections of separated cloud band had a fine line structure in agreement with the direction of the wind there, it was implied that the northern and southern sections of the separated cloud band was upper-tropospheric stratiform cloud extend from convective clouds in the cloud band. Although convective activity of the cloud band weakened when the separation started, there was strong meridional divergence in the upper troposphere and the northern and southern section of the separated cloud band continued moving northward and southward, respectively.

The clear-sky regions, extend between the northern and southern section of the separated cloud band, were formed by earlier dissipation of the stratiform clouds near the convective area than far from it. It was indeed partly observed that T_BB of line clouds away from convective cloud clusters decreased with time.

The separation of the cloud band had the zonal simultaneity in the development and separation. Indeed the local time when the cloud band developed and the separation started was fixed in a day. These simultaneities implied the relationship with the separation and the sun-synchronous diurnal convective activity over tropical ocean. Comparing with the mean diurnal convective activity over the western tropical Pacific during TOGA-COARE, convective activities of the separated cloud bands were in good agreement with that of mean diurnal convective activity.

During separation, westward-propagating cloud clusters were observed in the west where separation occured. Zonal scale of the separation and phase speed of propagation implies the relationship with the equatorially trapped n=1 westward-propagating inertio-gravity wave.