What causes faster and slower diurnal offshore rainfall propagation in New Guinea?

Twenty-year satellite observations of rainfall have shown offshore propagation of diurnal rainfall signals in northern coastal areas of New Guinea, with propagation speed varying from 8 m s^-1 to 12 m s^-1 even under similar weak offshore background wind conditions. This study investigates the mechanisms behind this variability in propagation speed using the Maritime Continent Austral summer climatology v1.0 (MCASClimate), a 10-year high resolution model simulations dataset. By calculating the rainfall propagation speed on days with pronounced propagation, we classify the top 30% and bottom 30% of propagation speeds as faster and slower groups, respectively.

 

The faster group exhibits a more widespread rainfall pattern, suggesting that inertial-gravity waves driven by land-sea thermal contrast is the dominant factor. Conversely, the slower group displays more concentrated rainfall, indicating the dominance of cold pool dynamics far offshore. The faster group is associated with clearer skies, allowing more shortwave radiation to be absorbed during the daytime, which enhances land-based convection and cold pools in the evening. This results in stronger land-sea temperature contrasts, driving more intense inertial-gravity waves that govern the rainfall propagation. In contrast, the slower group is influenced by stronger low level wind shear, which leads to convection initiation primarily at the cold pool leading edges, yielding slower propagation speeds. An interesting finding of this study is that, either cold pools or inertial-gravity waves can govern rainfall propagation over distances greater than 600 km in New Guinea, albeit with different propagation speed.