Clouds, aerosols, and water and carbon cycles over the Southern Great Plains

Description

Clouds and aerosols can have significant impacts on carbon cycle dynamics by modifying both the quality and amount of solar radiation for photosynthesis. Changes in field-scale surface carbon dioxide fluxes were previously demonstrated in response to changes in diffuse light fraction associated with cloud cover and aerosol loading, yet the dynamics of these responses remain unexplored and are critical for predictive understanding of land climate. In particular, stomatal conductance has a dual role in controlling water and CO2 fluxes and can support feedbacks involving aerosols and clouds. We performed cross-spectral analyses of surface CO2 fluxes, latent heat fluxes, and the diffuse-direct fraction to identify potential coupling between solar radiation regimes and vegetation fluxes at time scales from daily to seasonal, using eddy covariance and collocated broadband solar radiometer data from the ARM Southern Great Plains Central Facility. We found moderate coherence (squared coherence of 0.5–0.7, statistically significant at the 95% confidence level) between diffuse fraction and both surface CO2 and latent heat fluxes at synoptic time scales of 2–4 days, with weaker evaporation and stronger respiratory CO2 fluxes (or weaker uptake) coinciding with enhanced diffuse fraction, during the growing season for winter wheat. These results suggest daily weather variability as the primary pathway for solar radiation to couple crop and climate systems and a potential role of vegetation in explaining the climatic drift seen in general circulation models. Simulations with a coupled Community Earth System Model (CESM) will be presented to elucidate the significance of land-atmosphere feedbacks in supporting the observed coherence between diffuse fraction and surface fluxes and its impact on longer-term carbon and water budgets.