Deep convection is the primary influence on weather and climate in tropical regions. However, understanding and simulating the shallow-to-deep (STD) convective transition has long been challenging. Here, we conduct high-resolution numerical simulations to assess the environmental controls on the evolution of isolated convection in the Amazon during the wet season. Observations and large-scale forcing derived through the constrained variational analysis approach for the GoAmazon2014/5 experiments are used in the simulations and model validation. The model consistently reproduces the GOAmazon observations for precipitation, moisture, and surface fluxes of radiation, latent and sensible heat. Through sensitivity experiments, we examine the relative importance of moisture and vertical wind shear in controlling the STD convective transition. Reducing the pre-convective humidity within the lower 1.5 km significantly suppresses vertical development and lowers the ice water path. Additionally, the maximum precipitation rate decreases almost quadratically with column water vapor. Conversely, a reduction of column water vapor above 1.5 km by a factor of two or more is necessary to produce a comparable decrease in ice water path or precipitation. Moderate low-level wind shear facilitates the STD transition, leading to an earlier peak of ice water compared to stronger wind shear or its absence. Although upper-level wind shear negatively influences high cloud formation, its role in controlling the STD transition is relatively smaller than that of low-level wind shear. Our results help quantify the role of moisture and wind shear on the STD transition, but also suggest that dynamic factors may exert a more pronounced influence.

Tropical islands are simultaneously some of the most biodiverse and vulnerable places on Earth. Water resources help maintain the delicate balance on which the ecosystems and the population of tropical islands rely. Hydrogen and oxygen isotope analyses are a powerful tool in the study of the water cycle on tropical islands, although the scarcity of long-term and high-frequency data makes interpretation challenging. Here, a new dataset is presented based on weekly collection of rainfall H and O isotopic composition on the island of O‘ahu, Hawai‘i, beginning from July 2019 and still ongoing. Throughout this time, a variety of weather conditions have affected the island, each producing rainfall with different isotopic ratios: precipitation from Kona lows was found to have the lowest isotopic ratios, whereas trade-wind showers had the highest. These data also show some differences between the windward and the leeward side of the island, the latter being associated with higher rainfall isotope ratios due to increased rain evaporation. At all sites, the measured deuterium excess shows a marked seasonal cycle which is attributed to different origins of the air masses that are responsible for rainfall in the winter and summer months. The local meteoric water line is then determined and compared with similar lines for O‘ahu and other Hawaiian islands. Finally, a comparison is made with data collected on Hawai‘i Island for a longer period of time, and it is shown that the isotopic composition of rainfall exhibits significant interannual variability.