Separation of Zonally Elongated Large Cloud Disturbances over the Western Tropical Pacific

Abstract

 In this study, a data analysis was conducted to describe the common characteristics of “large-scale cloud separations,” in which zonally elongated cloud bands extending a few thousand kilometers over the western tropical Pacific are simultaneously separated into two or three zonally elongated bands. The separated cloud bands maintain their shapes for more than a day. In the present study, a case study was performed on four separation cases observed during the Intensive Obervation Period (IOP) of the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA-COARE).
 The northern and southern cloud bands consisted of clouds with fine line structures. Their orientations were in good agreement with the horizontal winds near the cloud top, indicating that the northern and southern cloud bands consisted of upper-tropospheric cirriform clouds. After the convective activity in the original cloud band weakened considerably, strong meridional winds remained in the upper troposphere and advected the separated cloud bands northward and southward.
 All the cloud separations occurred about half a day after the convective activity had peaked, and the onset time of the separations were fixed from evening to midnight. These facts indicate that the typical diurnal convective activity over the western tropical Pacific may play a role in the cloud separations. Westward-propagating cloud clusters were observed to the west of the original cloud band during the separations, which may be related to convectively coupled equatorial waves.