Deforestation Impacts on Clouds and Precipitation Over Borneo Vary Across the Diurnal Cycle

The impact of tropical deforestation on clouds and precipitation remains uncertain due to complex interactions between land surface changes and atmospheric processes on convective and mesoscales. Here, we examine the impact of deforestation over Borneo in Southeast Asia using a pair of high-resolution large eddy simulations. Replacing tropical forest with oil palm plantations reduces surface roughness thereby making surface-atmosphere exchanges less efficient, leading to warmer land but a cooler near-surface atmosphere. In this moist environment, evapotranspiration can compensate for the added surface heating, thereby increasing local moisture availability. To examine how this impacts cloud formation, we identify and track tens of thousands of clouds to quantify shifts in the distribution of tropical convection across the diurnal cycle. Overall, deforestation suppresses shallow-to-deep convective development, though the shallow cumuli which do form start raining earlier in the day due to increased low-level moisture. However, this reduction in shallow cloud cover is not spatially uniform: in areas where the deforestation gradient is strong, midday shallow cumuli are enhanced by mesoscale vegetation breezes. Deforestation also weakens sea breeze-driven moisture convergence, leading to the relative enhancement of terminal congestus over deep convection and a shift in the onset of deep convection and precipitation towards later in the evening. Our findings emphasize that deforestation impacts can vary diurnally depending on cloud type and interactions with mesoscale phenomena, as well as spatially depending on the location relative to deforested regions. This variability should be incorporated when considering the overall impact of deforestation on clouds, rainfall, and climate.