A Relationship Between Cloud Activity and the Low-Level Convergence as Observed in Reed-Recker’s Composite Easterly Waves

Abstract

The method proposed by Ogura and Cho is applied to the averaged easterly wave disturbances in the western Pacific based on Reed and Recker’s data set to determine the cloud mass flux distribution in various weather conditions, ranging from disturbed conditions in the trough region to a relatively undisturbed condition in the ridge region. The results show that shallow clouds are present practically everywhere in the wave disturbances, even in the ridge region where the large-scale vertical motion is downward. In contrast, deep clouds penetrating into the layer above the 500-mb level are present in a significant amount only in the region near the trough axis (most intense in the region east of the trough axis). More quantitatively, while the total mass flux associated with all cumulus clouds at the cloud base level is not correlated with the low-level, large-scale convergence, the mass flux due to deep clouds is found to increase linearly with the increase of low-level, large-scale convergence. This would suggest that the generation and maintenance of shallow clouds and deep clouds are controlled by separate mechanisms. The daily rainfall estimated on the basis of the computed convective mass flux is compared with observations with fairly good agreement.