Sensitivity of the Shallow-to-Deep Convective Transition to Moisture and Wind Shear in the Amazon

Abstract

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.