Synoptic Influences on Diurnal Rainfall Regimes over Puerto Rico during CPEX-AW

The objective of this study is to identify key processes that influence the diurnal cycle over Puerto Rico by understanding the effects of synoptic conditions. We use radiosonde and airborne data collected through the NASA CPEX-AW field campaign (August-September 2021) that revealed the detailed atmospheric structure around the island to achieve our objective, in addition to the network of station data over the island. Summer rainfall over Puerto Rico is dominated by its afternoon rainfall, but there is a large variability in its behavior that remains challenging to predict. We find that the background wind speed and humidity have strong influences on afternoon rainfall through different mechanisms. A stronger easterly background wind inhibits afternoon rainfall by reducing land-sea thermal contrast and weakening sea breeze convergence over the island. At the same time, a stronger inversion layer is often associated with a stronger background wind that further inhibits deep convection. The formation of strong sea breezes under a weak background wind condition enhances afternoon rainfall, exclusively over the island, while limited rainfall appears over the surrounding ocean. Large rainfall anomalies exclusively over the island tend to occur on days when humidity is not relatively high. However, when humidity is high and sea breezes are weak, enhanced rainfall occurs over both the island and the surrounding ocean due to the offshore movement of convective storms. Therefore, even when a similar amount of rainfall occurs over the island, its behavior and attributed processes can be different. The sources of variability in background wind and humidity are different (midlatitude vs. tropics, respectively) resulting in a wide range of synoptic conditions and associated effects on the island rainfall.