Key processes associated with the diurnal cycle of rainfall over East Amazon and their representation in global climate models
Abstract
Precipitation in the Amazon Basin is vital for the global hydrological and carbon cycles. However, current global climate models, which typically operate at horizontal resolutions of approximately 100 km, exhibit significant shortcomings in accurately representing precipitation patterns in the Amazon. Notably, these models suffer from a persistent dry bias over East Amazon during the local rainy season from March to May. This deficiency largely stems from the models’ inability to accurately capture the formation and southwestward inland propagation of diurnal rain belts that initiate along the northeast coast of Brazil. Observational diagnoses show that these diurnal rain events are closely linked to the westward propagation of lower-tropospheric buoyancy fluctuations, resulting from moisture advection by the easterly Amazon low-level jet. Model experiments utilizing regionally refined grids indicate that the representation of these diurnal rain belts, and consequently the mean precipitation pattern in East Amazon, is significantly improved in higher-resolution model simulations (from 25 km to 6 km). Further analysis reveals that this improved diurnal cycle of precipitation is primarily due to a substantial increase in resolved large-scale precipitation that responds to lower-tropospheric buoyancy forcing as the model resolution increases.