Influence of Stability and Surface Roughness on Turbulence during the Stable Atmospheric Variability and Transport (SAVANT) Field Campaign

Harvesting of crops in a weakly sloping Midwestern field during the Stable Atmospheric Variability and Transport (SAVANT) observation campaign allowed for a systematic investigation of the influence of surface roughness and static stability magnitude on the applicability of the Monin–Obukhov similarity (MOST) and hockey-stick transition (HOST) theories during stable boundary layer periods. We analyze momentum flux and turbulent velocity scale V_TKE in three regimes, defined using the gradient Richardson number Ri and flux Richardson number Ri_f as regime 1 (0, Ri # 0.1 and 0, Ri_f # 0.1), regime 2 (0.1, Ri # 0.23 and 0.1, Ri_f # 0.23), and regime 3 (both Ri and Ri_f. 0.23). After harvest, in regime 1, stability varied from near-neutral to weakly stable and both MOST and HOST were applicable to estimate the momentum fluxes and V_TKE as a function of mean wind speed. In regime 2, the momentum flux deviated from the MOST linear relationship as stability increased. In regimes 1 and 2, a HOST-defined threshold wind speed V_s was identified beyond which V_TKE increased linearly with wind speed at a rate of 0.26 for all observation heights. Below this threshold wind speed, V_TKE behaved independent of mean wind and observation heights. Alternatively, for preharvest periods, MOST was applicable in regimes 1 and 2 for all heights and HOST was applicable with reduced V_s for heights above the crop layer. Regime 3 during pre-and postharvest consisted of strongly stable periods and very weak to weak winds, where MOST was found to be invalid and V_TKE remained low and independent of wind speed. The results suggest that roughness due to crops enhances the turbulence generation at lower wind speeds.