Bowen ratio reflects the partitioning between sensible and latent heat fluxes and plays a crucial role in land-atmosphere interaction. In this study, the spatiotemporal variations of Bowen ratio among 12 vegetation types were analyzed using observations from 203 FLUXNET sites worldwide and compared against Community Land Model. Results showed that the annual mean Bowen ratio across all sites was 1.48 ± 1.20 (mean ± SD). Sites with Bowen ratios less than 1 (39%, 80 sites) were found across all continents, and the ones with higher Bowen ratios (>3)(7%, 14 sites) appeared in dry and warm areas. Open shrublands showed the highest Bowen ratio (3.04 ± 0.58), whereas wetlands showed the lowest (0.74 ± 0.09). In terms of seasonality, Bowen ratio showed a U-curve with lower values in local summer and higher in spring and autumn in the northern hemisphere; the opposite occurred in the southern hemisphere. The spatiotemporal variations in Bowen ratio can be explained by climatic, geographical, and biological factors, with climate factors having the greatest impact. Bowen ratio increased under higher VPD (R = 0.45) and hotter (R=0.14) conditions with more shortwave radiation (R=0.39), and decreased with higher precipitation (R=-0.34), albedo (R=-0.16), and leaf area index (R=-0.25). CLM well reproduced the global annual mean Bowen ratio, but showed larger differences for certain vegetations types such as open shrublands (-1.51), woody savannas (+0.98). Our results could enhance our understanding of biotic and environmental controls on land surface energy fluxes and help improve land surface and climate models.
Forests play a pivotal role in regulating climate and sustaining the hydrological cycle. The biophysical impacts of forest on clouds, however, remain unclear due to the lack of direct observations. In this first global-scale observational study, we use long-term satellite-derived cloud cover data to show that forests can have opposite effects on summer cloud cover. We find enhanced cloud cover over most temperate and boreal forests, but inhibited cloud cover over Amazon, central Africa, and Southeast US. These cloud effects mainly arise from convection processes associated with forests. The spatial variation in the sign of cloud effects is driven by sensible heating where cloud enhancement (inhibition) is more likely to occur when sensible heat in forest is larger (smaller) than nearby nonforest. Ongoing forest cover loss has led to opposite cloud cover changes, with local cloud increase over forest loss hotspots in the Amazon (+0.78%), Indonesia (+1.19%), and Southeast US (+0.09%), but cloud reduction in East Siberia (-0.20%) from 2002-2018. Our data-driven assessment informs the climate effects of local-scale forest cover change and improves mechanistic understanding of forest-cloud interactions, the latter of which remains uncertain in Earth system models.