Using Cloud-Resolving Simulations to Evaluate the Sensitivity of the Shallow-to-Deep Convective Transition in the Amazon to the Vertical Distribution of Humidity

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 a challenge for a long time. Here, we assess the environmental controls in convective development through idealized high-resolution numerical simulations using the System of Atmospheric Model (SAM) version 6.11 at 250m and 500m horizontal resolution. Large-scale forcing based on the GoAmazon2014/5 experiment are used to drive the simulations for December 2014. The model consistently reproduces the GoAmazon observations for precipitation, moisture, and surface fluxes of radiation, latent and sensible heat. Next, we perform sensitivity experiments to evaluate the relative importance of moisture and vertical wind shear role in controlling convection. The experiments consist on perturbing the vertical profile of humidity or wind shear before the sun rises, and integrating the model for the rest of the day to observe the consequences on the STD transition. Our results show that deep convection over the Amazon is particularly sensitive to low-level environmental conditions. Notably, early morning low-level preconditioning is vital to daytime convection and precipitation. On the other hand, only unrealistically dry conditions in the free troposphere significantly inhibit deep cloud development. The low-level wind shear facilitates the late afternoon STD transition when the jet width is broadened, and with a moderate strength peaking from approximately 2 to 4 km. A more intense upper-level jet negatively affects the development of the deep clouds and substantially reduces the anvil. While low-level preconditioning is a necessary condition for deep convection, free troposphere humidity and vertical wind shear only modulate the late afternoon STD convection transition in the Amazon.