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

We study the impact of the roughness sublayer (RSL), formed over a relatively sparse canopy, on the dynamics of convective-boundary layers, as well as on the exchange of reactants in and above the canopy. Our approach combines numerical experiments using an atmospheric mixed-layer model coupled to a land surface-vegetation representation and observations collected at different heights in and above the RSL during the Canopy Horizontal Array Turbulence Study (CHATS) field experiment near Dixon, California. Within the modelling system, the RSL is parameterized using a formulation that accounts for the deviation of the standard Monin-Obukhov Similarity Theory flux-profile relationships due to the presence of a canopy. We select two representative days with different wind conditions and canopy fetch to determine the sensitivity of the applied RSL formulation to the diurnal variability of thermodynamic and reactant profiles.

We present the diurnal evolution of the surface momentum, energy fluxes and boundary-layer height, as well as the evolution of the mean wind, potential temperature and specific humidity in and above the RSL. This study is extended to infer the fluxes of ozone, NO and NO2, using the observations of their mean gradients, applying different retrieval formulations including and omitting the RSL effects. Finally, these retrieved fluxes are used to determine the ozone budget in the convective boundary layer.

We find that the presence of an RSL has relatively small impact on surface fluxes (<3%). This is due to the compensating effect of the mean gradients of the state variables and the calculated drag coefficients for momentum and scalar, which are largely affected by the canopy presence. The friction velocity is found to be sensitive to variations in the canopy adjusted length scale and the u_*⁄U ratio. These variations of the friction velocity (up to 15% relative to the case-study value) resulted in an impact on the diurnal variation of the surface energy fluxes and consequently on the convective boundary-layer dynamics of up to 6%.

For the reactants, we find its maximal deposition near the canopy top. Furthermore, our results show significant differences in the retrieved deposition fluxes of the reactants depending on the method used for their calculation from the gradients (e.g. up to 30% error in retrieved ozone fluxes when RSL is omitted). Finally, although deposition plays an important contribution in the diurnal variability of the ozone budget, our calculations indicate the relevance of reactivity and non-local effect contributions such as entrainment from the free troposphere and advection.