The Multiscale Impacts of Organized Convection in Global 2D Cloud Resolving Models

Two dimensional cloud resolving models showcase many of the multiscale structures present in the tropical atmosphere, but are computationally cheap enough to run in large horizontal domains. To investigate multiscale processes similar MJO and other tropical intraseasonal oscillations, we perform a set of idealized simulations using the System for Atmospheric Modeling (SAM) in a 30,000 km equatorial (f=0) domain with homogeneous sea surface temperature for a period of over 100 days. A constant -10 m/s background wind is used to stimulate convective activity, which shows a robust multiscale structure. Similar to previous studies, the variability in the first 40 days is dominated by eastward moving synoptic-scale convectively coupled waves (CCWs), which contain many westward propagating mesoscale squall-line systems. Later in the simulations, these CCWs subsequently give rise to a strong planetary scale circulation which propagates with respect to both the fixed reference frame and the frame following the background wind. This three scale hierarchy is present in simulations with fully interactive radiation as well as horizontally homogeneous prescribed cooling patterns, which indicates that this archetype of convective organization cannot be explained by radiative feedbacks alone.

Based on past work using multiscale asymptotic models for tropical dynamics, we hypothesize that the planetary scale oscillation is maintained by eddy flux divergences from the smaller scales. Simple low-pass filters in zonal and temporal space are used to decompose the model output into mesoscale, synoptic scale, and planetary scale components, which can then be used to compute eddy-flux terms. Unlike past studies focusing on self-aggregation feedbacks on the column budget of moist static energy, we highlight the importance of upscale momentum fluxes. In particular, a feedback analysis shows that the momentum fluxes due to synoptic scale waves consistently amplify the planetary scale kinetic energy, whereas the baroclinic generation term---although large in amplitude---oscillates from positive to negative. These results indicate that multiscale interactions can play an important role in the large-scale organization of realistic convection, even in the absence of radiative feedbacks.