Cloud Responses to Deforestation Vary Across the Diurnal Cycle

Among the most uncertain aspects of how land cover change impacts the Earth system is its effect on cloud formation. These cloud changes are important for both the hydrological cycle through shifts in precipitation, as well as the planetary energy balance through their radiative effects. However, quantifying land surface impacts on clouds remains a challenge, since the outcome depends on numerous convective and mesoscale processes that are not well-resolved in large-scale models. In this work, we focus on deforestation in Southeast Asia as a case study of extensive land cover changes in a complex convective environment. Using a combination of high-resolution atmospheric models and object-based analysis, we demonstrate that the cloud response to deforestation is not uniform and varies strongly across the diurnal cycle and with spatial scale.

We use the Regional Atmospheric Modeling System (RAMS) to conduct a pair of multi-day large eddy simulations (LES; Δx=150m) of shallow-to-deep convection over the island of Borneo. By using identical atmospheric boundary conditions but differing land surface properties, we explore the changes in convective initiation and development that occur due to realistic patterns of tropical deforestation. We use tobac (tracking and object-based analysis of clouds) to quantify shifts in the cloud population and find contrasting responses between various modes of tropical convection. Localized deforestation-driven changes to boundary layer processes result in widespread reductions in shallow cloud cover and suppression of the transition from shallow to deep convection. However, shallow cumuli which do form start raining earlier in the day, resulting in more widespread light rain throughout the afternoon. Furthermore, we also find localized increases in cloudiness along the boundary between forested and deforested areas due to surface heating-induced mesoscale circulations.

Our results demonstrate that land-atmosphere interactions and their implications for convection, hydrology, and radiation can vary greatly throughout the day, depending on prevailing cloud type and their interactions with mesoscale phenomena. We propose that this diurnal variability must be considered to more accurately capture the full impact of deforestation and other land cover changes on clouds, rainfall, and climate.