High frequency isotopic composition measurements to classify cloud induced turbulent patterns above the amazon rain forest

During the 2022 CloudRoots-Amazonia field campaign high-frequency measurements of state variables and atmospheric composition were taken at the Amazon Tall Tower Observatory (ATTO) in Brazil. Specifically, we measured wind fields, radiation, H₂O and CO₂ mole fractions, and H₂O and CO₂ isotopic compositions at 4Hz or faster at 57m height. A main objective was to use these high-frequency measurements to investigate how the coherent canopy-atmosphere turbulent structures are influenced by the non-stationary passage of clouds. Novel in our investigation is the use of quadrant analysis in combination with high frequency isotopic composition measurements, as well as our approach to finding lag between cloud passages and turbulent variables.

Using quadrant analysis to distinguish sweeping and ejection motions, we find that the passage of clouds influences the transport of scalars and energy. Preceding the passage of well-developed cumulus clouds, we see that the gust front forcefully ejects this subcanopy air into the atmospheric mixed layer. It seems that this is an effective upward transport mechanism for CO₂ and other scalars emitted by the soil, plant roots, and understory. The understory separation was based on the q_CO2’ > 0, q_H2O’ > 0 quadrant. Keeling plots of this quadrant, made using the dD isotope of H₂O, indicate strong midday depletion of understory water vapour (-20 ‰). This effect can only be explained by a major downwards moisture flux from the atmosphere into the soils through the process of condensation, even - or especially – when the air temperatures are highest. Finally, we show what the responses of the major fluxes (H, LE, F_CO2) and their respective isotopologues are to the passage of clouds, including their lag times. Our study contributes to an improved quantification and understanding of the canopy-atmosphere fluxes influenced by the perpetual presence of clouds.