Impacts of Weak Sea Surface Temperature Warm Anomalies on Trade Wind Cloudiness in Large Eddy Simulations

This study investigates the impact of a weak sea surface temperature (SST) warm anomaly on trade cumulus cloudiness in an idealized and ensemble framework with cloud-resolving large-eddy simulations. Control and perturbation experiments are conducted respectively with spatially uniform and time invariant SST and a prescribed 25-km wide two-dimensional Gaussian warm SST anomaly (SSTA) of 0.5 K magnitude, using the mean large-scale environmental conditions and atmospheric forcings during the Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign. The ensemble-averaged differences between perturbation and control experiments show that the cloud fraction is enhanced over the downwind half of the prescribed warm SSTA, with the enhancement peaking slightly above the environmental lifting condensation level (LCL) and then decaying with height. Furthermore, the low-level cloud response (< 1 km) to warm SSTA is stronger and occurs more systematically across different ensemble members. This near-LCL cloud response is driven by enhanced surface buoyancy flux and turbulence over warm SSTA as opposed to SSTA-induced anomalous surface convergence and mesoscale upward motions. Process denial experiments indicate that the locally enhanced surface sensible and latent heat fluxes contribute almost equally to increase the near-LCL cloudiness, even though the locally enhanced surface sensible heat flux plays a dominant role in enhancing surface buoyancy flux. These results corroborate recent satellite composite results (Chen et al., 2023), suggesting that the observed increase of daily cloud fraction above warm SSTA is due to the more frequent turbulence-driven formation of very shallow cumuli within the trade-wind transition layer.