The diurnal cycle resonates with the large-scale circulation

Tropical deep convection is both driven by the large-scale circulation, such as the Hadley Cell, as well as by periodic heating, namely the diurnal cycle. In a suite of idealized cloud-resolving simulations we allow for an idealized Hadley Cell to evolve by imposing a spatial surface temperature gradient. In line with recent studies (O’Neill et al., 2017, Patrizio & Randall, 2019), we find that a large-scale oscillation of period T_L self-organizes, visible in many quantities, especially precipitation. The period T_L is found to depend approximately linearly on L, the meridional domain size. When we now impose a diurnal cycle at T_D=1day, the timeseries of precipitation is modified. For small L, the power spectrum shows that T_D dominates, whereas for large L the intrinsic mode T_L prevails. At intermediate T_L⋍T_D a resonance occurs, which causes the amplitude to nearly double, leading to more extreme rainfall. To gain insight, we propose a simple damped harmonic oscillator model with a spatial energy source, mimicking convective latent heat release. We associate the “spring constant” with the meridional domain size L. The coupling to the diurnal cycle is then incorporated by a periodic drive. Results show that the model can mimic the resonance at intermediate periods and the dominance of large-scale modes at large system sizes. We further find an envelope period which may give insight into the observed meandering of the ITCZ (Mapes et al., 2018). Our results could help understanding of tropical extreme events, e.g. MCS, and how they could be invigorated by the large scale circulation.