Effects of diabatic stability on turbulent biosphere-atmosphere exchange processes for momentum and scalars

Understanding and quantifying the exchange of momentum and scalars between vegetation and atmosphere is key to modelling the surface energy and mass balance and many biogeochemical cycles. It requires detailed knowledge of flow and transport processes. While many flow properties over vegetation canopies are relatively well known for neutral conditions less is known about the role played by diabatic stability and its effects on the efficiency of turbulent exchange of momentum and scalars. This may have important consequences given the relative infrequent occurrence of neutral compared with stable or unstable conditions, especially within forest canopies.

Momentum transport on the canopy scale is relatively well understood. The mixing layer analogy (MLA) and recent developments of this theory (Finnigan et al. 2009) can explain important features of the flow such as the occurrence of a flow instability due to an inflection point in the wind profile, the development of coherent structures and associated scale selection, an intense scalar micro-front in the convergence of sweeps and ejections, the dominance of sweeps in the canopy and of ejections further away from the rough surface. While there are some field data showing these processes during episodes of diabatic stability, most of our understanding is based on data from neutral or near-neutral conditions.

We present a detailed analysis how diabatic stability effects turbulent exchange and length scales of momentum and scalars in and above two open canopies, a Eucalyptus forest (LAI crown: 1.5, undergrtowth:1, height: 40 m) and a cork oak plantation (LAI: 1.4, undergrowth: 0.6, height: 10 m). Under neutral conditions coherent structures drive momentum and scalar exchange. Stable conditions will often lead to a decoupling of in and above canopy flows (van Gorsel et al. 2008); unstable conditions are associated with a less pronounced inflection of the wind profile at canopy top. As it is the inflection that leads to flow instabilities that trigger the development of coherent structures we can expect that weakening the inflection will result in an altered transport of momentum and scalars. To better understand the exchange processes in non-neutral conditions we use quadrant analysis to inform us on the relative contributions of sweeps and ejections to the total flux and to investigate under what diabatic conditions characteristics of momentum and scalar transport start to deviate from each other. Dissimilarity in the transport mechanisms is further reflected in differences between the mixing lengths for momentum and scalars. Their ratios (Prandtl and Schmidt numbers) are good markers of canopy flows that are dominated by coherent structures. The neutral values of the Prandtl number at canopy top is 0.5, much smaller than inertial sublayer values. One major outcome of this paper is an improved knowledge how these numbers vary with diabatic stability and what diabatic stabilities support coherent structures that are very efficient in exchanging momentum and scalar fluxes through the biosphere-atmosphere interface.

Finnigan JJ, Shaw RH, Patton EG (2009). Turbulence structure above a vegetation canopy. J. Fluid Mech. 637:387-424.

van Gorsel E., Finnigan JJ, Harman IN and Leuning R. (2008). Turbulence characteristics of canopy turbulence during the transition from convective to stable boundary layer. 18th Symposium on Boundary Layers and Turbulence, Stockholm, Sweden, June, 9-13, 2008, AMS.