On the Decrease of Bulk Drag Coefficient with Increasing Atmospheric Instability

Monin-Obukhov similarity theory (MOST) suggests a monotonic increase of bulk drag coefficient with increasing atmospheric instability at a fixed location, qualitatively supported by large-eddy simulation results. Observational studies over various underlying surfaces, however, reported decrease of bulk drag coefficient with increasing atmospheric instability even if the unstable conditions of interest are well within the range of applicability of MOST. The observed relationship between bulk drag coefficient and the stability parameter for unstable conditions exhibits huge scatter. Whether such scatter arises from physical processes or uncertainties in data analysis remains unclear. We apply a recently developed multi-sensor stationarity analysis technique to the Canopy Horizontal Array Turbulent Study (CHATS) data, obtaining stationary periods ranging from near neutral to free convection conditions. Preliminary results suggest that bulk drag coefficient can decrease by 40% when stratification becomes more unstable for situations with similar mean wind. For a given mean wind, the atmospheric boundary layer (ABL) may or may not involve organized convective structures. The presence of ABL-scale structures notably increases friction velocity, which increases both bulk drag coefficient and the magnitude of Obukhov length (i.e., decreases instability). Separating cases with or without the presence of organized ABL-scale structures remarkably reduces the scatter in observed relationship between bulk drag coefficient and the stability parameter for unstable conditions.