The Role of Cumulus Convection in Moistening the Larger-Scale Atmosphere over the Tropical Indian Ocean during CINDY/DYNAMO

The activity of cumulus convection in the tropical oceanic regions is strongly regulated by the larger-scale environmental atmosphere, while at the same time cumulus convection influences the larger-scale atmosphere. It is recognized that the spatial and temporal variability of moisture content play an important role in determining such multi-scale interaction processes: the down-scale regulation and up-scale influence. This study investigates the interaction between tropical cumulus convection and its environment in the tropical Indian Ocean during the active and in-active phases of Madden-Julian Oscillation (MJO) by conducting 100-m-mesh numerical simulations intended to sufficiently resolve mixing processes between cumulus clouds and their environments. The cases from the Cooperative Indian Ocean experiment on intraseasonal variability in the Year 2011 (CINDY2012)/Dynamics of MJO (DYNAMO) are examined. Numerical simulations are conducted with the use of the Weather Research and Forecasting (WRF) model with the nesting capability to resolve the area of interest at high resolutions. The nested computational domains cover the area of 2500 km by 2250 km, 1200 km by 1000 km, 300 km by 300 km, and 100 km by 60 km at 12.5-km, 2.5-km, 500-m, and 100-m horizontal grid spacing, respectively. The computational domains at the 500-m and 100-m grids are centered at the R/V Mirai observation site. The relationship among the tropospheric moisture variation, the diurnal variation of the atmospheric boundary layer, and the development and evolution of cumulus convection is specifically focused. Downscale regulation of environmental conditions on cumulus convection and upscale organization from boundary-layer disturbances to mesoscale convective systems are investigated. The vertical development of cumulus clouds are closely related to the relative humidity of the environment at levels lower than the cloud-top height. It is shown that the development and organization of cumulus clouds are significantly affected by moisture variability and the deep convective activity influences moisture fields at the larger-scales. The existence of undiluted updraft cores plays a key role in the interaction processes through undiluted updrafts' penetrating into the upper troposphere and moistening of the clouds' environment. The analysis with the use of thermodynamically conserved variables suggests that the mixing between clouds and their environments actively occurs. During the active phase of MJO in the later period of November 2011, the results from the 100-m-grid simulations clearly indicate that the environment is moistened by well-resolved cumulus clouds.