How Vegetation Controls Boundary-Layer and Cloud Development: the Memory Effect

The feedbacks between vegetation and clouds play a key role in the development and structure of the atmospheric boundary layer (ABL). As the vegetation reacts to local cloud shading by closing its stomata, it affects the partitioning of latent and sensible heat at the surface. Ultimately, this affects the size and strength of thermals, thereby affecting cloud growth driven by turbulent transport. This has large consequences on cloud development and furthermore impacts the atmospheric structure, leading to a further decrease in ABL moisture content and a diminished decrease in atmospheric turbulence and boundary-layer growth compared to conditions with fixed Bowen ratios. With the use of a large-eddy simulation (LES) coupled to an interactive land-surface model, we investigated this vegetation-cloud coupling systematically by differing in wind conditions and biological factors. Considering a situation in which the sun is directly overhead, the cloud is directly located over the shaded area, which idealises the interactions, as the decrease in radiation exerts a lowering in local turbulence towards the cloud. However, when wind increases (e.g. > 7.5 m/s) roll vortices form that organise the clouds in street patterns. A direct impact of this wind shear situation is that boundary-layer thermals tilt and consequently the shaded area is not positioned at the cloud's root, thus preventing a direct coupling between the reduced surface fluxes by shading and cloud development.

By analysing the characteristic length scales of the boundary-layer thermals and clouds, we show that the adaptation time of the plants' stomata to changes in radiation is key in the vegetation-cloud interaction in wind situations, as this response causes the plants to ‘memorise' the clouds, which ultimately couples the system. By systematically breaking down the complexity of the system, we find that this memorising effect results in heterogeneous structures at the surface, initialised by cloud street shading. These patterns in vegetation response have an impact on the characteristic length scales of clouds and thermals, indicating that vegetation plays a role in the transition from shallow to deep convection. Bearing in mind that this memory effect is not yet introduced in global and regional atmospheric models, we advocate that this localised interaction plays an essential role in modifying boundary-layer dynamics and the subsequent cloud development and that its effects should be taken into account.