Atmospheric transport in weak winds and very stable conditions is often characterized by phenomena collectively referred to as submeso motions since their time and spatial scales exceed those of turbulence, but are smaller than synoptic motions. Submeso motions invalidate models for turbulent dispersion and diffusion since their physics are not captured by current similarity theories. Typical phenomena in the weak-wind stable boundary layer include meandering motions, quasi two-dimensional vortices or wavelike motions. These motions may be subject to non-local forcing and sensitive to small topographic undulations. The invalidity of Taylor’s hypothesis for submeso motions requires the use of sensor networks to provide observations in both time and space domains simultaneously.
Material and Methods
We present the results from the series of Advanced Resolution Canopy Flow Observations (ARCFLO) experiments using a sensor network consisting of 12 sonic anemometers and 12 thermohygrometers. The objective of ARCFLO was to observe the flow and the turbulent and submeso transport at a high spatial and temporal resolution at 4 different sites in the Pacific Northwest, USA. The sites represented a variable degree of terrain complexity (flat to mountainous) and vegetation architecture (grass to forest)
In our study, a weak-wind regime was identified for each site using the threshold velocity at which the friction velocity becomes dependent upon the mean horizontal wind speed. We used the scalar average of the wind speed because the friction velocity showed a clearer dependence on the scalar average compared to the vector average of the wind velocity. It was found that the critical speed for the weak wind regime is higher in denser vegetation.