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By characterizing the dynamics of a convective boundary layer above a relativelysparse and uniform orchard canopy, we investigated the impact of theroughness-sublayer (RSL) representation on the predicted diurnal variabilityof surface fluxes and state variables. Our approach combined numericalexperiments, using an atmospheric mixed-layer model including a land-surface-vegetation representation, and measurements from the CanopyHorizontal Array Turbulence Study (CHATS) field experiment near Dixon,California. The RSL is parameterized using an additional factor in thestandard Monin–Obukhov similarity theory flux-profile relationships thattakes into account the canopy influence on the atmospheric flow. We selecteda representative case characterized by southerly wind conditions to ensurewell-developed RSL over the orchard canopy. We then investigated thesensitivity of the diurnal variability of the boundary-layer dynamics to thechanges in the RSL key scales, the canopy adjustment length scale, Lc, andthe β = u*/|U| ratio at the top of the canopy due to their stabilityand dependence on canopy structure. We found that the inclusion of the RSLparameterization resulted in improved prediction of the diurnal evolution ofthe near-surface mean quantities (e.g. up to 50 % for the wind velocity)and transfer (drag) coefficients. We found relatively insignificant effectson the modelled surface fluxes (e.g. up to 5 % for the friction velocity,while 3 % for the sensible and latent heat), which is due to thecompensating effect between the mean gradients and the drag coefficients,both of which are largely affected by the RSL parameterization. When varyingLc (from 10 to 20 m) and β (from 0.25 to 0.4 m), based on observationalevidence, the predicted friction velocity is found to vary by up to 25 %and the modelled surface-energy fluxes (sensible heat, SH, and latent heat of evaporation, LE) vary up to 2 and 9 %. Consequently, the boundary-layer height varies up to 6 %.Furthermore, our analysis indicated that to interpret the CHATS measurementsabove the canopy, the contributions of non-local effects such as entrainment,subsidence and the advection of heat and moisture over the CHATS site need tobe taken into account

Integrating canopy and large-scale effects in the convective boundary-layer dynamics during the CHATS experiment