Buoyancy Flux and Its Environmental Modulation in the Vertical Development of Shallow Cumulus in the Central Amazon

This study investigates the vertical development of shallow cumulus (ShCu) clouds in the Central Amazon (CAMZ) by combining large-eddy simulations (LES) with observational data from the GoAmazon 2024 campaign. Analyses of turbulent kinetic energy (TKE) budgets, updraft mass flux, moisture flux, and surface fluxes across cloud sizes indicate that enhanced positive buoyancy flux in both the mixed and cloud layers is the primary driver of vertical cloud growth. Sensitivity LES experiments reveal that factors favoring buoyancy enhancement—and the transition to larger ShCu—include (1) increased morning relative humidity in the residual layer linked to cold, moist air advection; (2) warm-air advection in the subcloud layer; (3) large-scale morning ascent followed by weakened afternoon subsidence above the mixed layer; and (4) increased surface sensible heat flux. These processes collectively strengthen buoyancy flux and atmospheric instability, promoting deeper ShCu. Conversely, our results suggest that buoyancy reduction under unfavorable environmental conditions—which contrast with previously identified favorable factors for cloud growth—is driven by two physical mechanisms: (i) enhanced entrainment in smaller clouds with higher surface-area-to-volume ratios, increasing mixing and weakening updrafts; and (ii) dynamic suppression caused by vertical wind shear at cloud base, which induces downward pressure perturbations. Finally, variability in large-scale subsidence over the Central Amazon boundary layer is linked to synoptic upper-level circulation over southeastern Brazil, modulated by midlatitude wave disturbances propagating northeastward into subtropical South America.