Statistics of Small-Scale Velocity Fluctuations and Internal Intermittency in Marine Stratocumulus Clouds

摘要

Clouds are known to be turbulent, but the details of their internal turbulent structure have been largely unexplored. Measurements of turbulent velocities in stratocumulus clouds presented here reveal an intermittent structure consistent with that observed in classic homogeneous isotropic turbulence. The measurements were taken close to cloud top in a 200-m-thick cloud layer over a path of approximately 6 km, using a hot-wire anemometer below a helicopter as part of the Airborne Cloud Turbulence Observation System (ACTOS) measurement system. Hot-wire signal artifacts resulting from droplet impacts are removed without significantly degrading the signal, such that high-order velocity structure functions can be evaluated. The structure function analysis for orders 2-8 show statistically significant departures from the Kolmogorov's 1941 scaling, yielding scaling exponents consistent with the Kolmogorov-Obukhov refined similarity hypothesis, with an intermittency exponent of 0.25. This is in agreement with the accepted value determined in single-phase flows under carefully controlled conditions, and no evidence is found of any departure from the large body of knowledge obtained from the laboratory on the finescale turbulence structure. This suggests that processes depending on the finescale structure of turbulence that cannot presently be measured in clouds can be explored in the laboratory setting. Since these findings pertain to clouds with relatively low liquid water content and weak turbulence, further work will be required to determine their applicability to other cloud types.