2015 Volume 93A Pages 41-58
The cumulus convection activity in the tropical oceanic regions is strongly regulated by the large-scale environmental atmosphere, while at the same time, cumulus convection will influence the large-scale atmosphere. It is thus recognized that the spatiotemporal variability of moisture content plays an important role in determining such multiscale interaction processes relevant to tropical cumulus convection. This study investigates the relationship between cumulus convection and environmental moisture in the tropical Indian Ocean by conducting convection-resolving simulations through the nesting capability with which the innermost domain has the 100 m grid resolution. We examine the cases observed from October to November 2011 during the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011/Dynamics of the Madden-Julian oscillation (MJO) field experiment. Based on the favorable performance of the outermost domain simulations in reproducing eastward propagating signals over the Indian Ocean, the data obtained from the 100 m mesh simulations are examined. It is shown that the cloud cover whose tops exceed a middle level sharply increases with the increase in precipitable water vapor over about 55 mm. The increase in relative humidity in a lower layer results in the increase in cloud cover at a level above the humid layer. From the convection-resolving simulations, the existence of updraft cores that are less diluted with the environment is demonstrated. It is considered that cloud-core parcels are less susceptible to the negative effects of dilution with the environment and survive to penetrate to upper levels, which contributes to the moistening of the environmental atmosphere. The existence of updraft cores plays a key role in the inter-relationship between cumulus convection and its environment. The effects from cumulus clouds on their environment are regarded as a preconditioning influence for the convective initiation of MJO.