Joint modulation of coastal rainfall in Northeast Australia by local and large-scale forcings

This study investigates the impact of the interaction between large-scale and local-scale forcings in regulating rainfall patterns and their propagation over coastal areas of Northeast (NE) Australia using a convective-scale regional model run for 180 days. Over the coastal areas, spatially heterogeneous rainfall patterns are evident in both radar observations and model simulations. By classifying the characteristics of three distinct rainfall groups, we found that the rainfall propagation modulates the average rainfall patterns. Modelling results suggest that the large-scale background wind and local-scale land–sea breeze circulations are two important factors driving rainfall propagation. Offshore rainfall propagation, which is frequently observed in coastal regions in the tropics, occurs during the days with weak easterlies near the surface and strong upper-and mid-level westerlies. Rainfall is triggered during the afternoon by convergence driven by the sea breeze and then propagates offshore during the nighttime with the land breeze density current and large-scale background westerlies. In contrast, onshore rainfall propagation is observed during days with strong background easterlies from the surface to upper levels. For the No-Propagation group, rainfall occurs during the afternoon due to the convergence of sea breezes and low-level background westerlies, and it persists over land during the nighttime with low- and mid-level easterlies. Our results also suggest that the background wind regimes associated with different phases of intraseasonal variability modulate the direction and strength of rainfall propagation, leading to different coastal rainfall patterns.