A green leaf in the sun influences the formation of clouds in the sky. However, this fundamental biological, chemical and physical process at a tiny scale leads to a gap with the cloud scale. The accurate representations of all these scales is a challenge in weather and climate models. In his inaugural lecture on 13 April, Professor Jordi Vilà-Guerau de Arellano, appointed to a personal chair at Wageningen University & Research, will discuss this interaction between the atmospheric boundary layer and the Earth's surface.
When you breathe, you are breathing air from the lowest layer of the atmosphere, also known as the atmospheric boundary layer. Most human processes, such as traffic and industrial emissions, smog, solar and wind energy generation, but also oxygen production by plants, all take place in this the lower layer of the atmosphere in direct contact with the land. “In the Netherlands, this layer varies between approximately 100 metres thickness at night to as much as 1500 metres at the end of a sunny afternoon. To improve our understanding of the weather and climate system, we need to learn more about the processes that occur in this boundary layer,” Professor Vilà explains.
The current weather and climate models do not represent atmosphere-land interactions with sufficient accuracy: in the spatiotemporal scales occurring to less than a kilometre and less than a minute. The smallest scale of the current weather models is 5×5 kilometres, but this excludes phenomena that are smaller in scale in both space and time, such as evaporation by plants, turbulence, initial stages of cloud formation and aerosol formation. At a leaf level, plants respond immediately, within a minute or a few seconds. For example, if a tree's crown – or even a single leaf – is cast into shadow by cloud, stomata on the leaf, about half a hair’s thickness in size, open or close at the same time in their billions and so influence the amount of heat and moisture that rise hundreds of metres driven by ABL by turbulent motions and become clouds.
Professor Vilà offers the situation in the Amazon rainforest as an example during his lecture entitled Dialogues with the Atmospheric Boundary Layer – Integrating physics, chemistry and biology. Plants use the sun’s energy for photosynthesis, a process that costs energy, and leads to the release of water transpiration. The remaining solar available radiation heats the forest canopy . This less dense arises by turbulent motions that carry moisture previously evaporated through the leaf stomata. The turbulence of the rising air mixes the atmosphere (water, heat and atmospheric compounds) and leads to the condensation of water and the subsequent forming clouds, explains Professor Vilà. “But this interactive system it more complex. The cloud’s shadow falls over the canopy, leading to a decrease in photosynthesis. However, while the cloud acts as a barrier to direct solar radiation, indirect (diffuse) radiation increases. This diffuse light penetrates deeper into the forest and reaches the subcanopy, favouring the stomata in these leaves to open so that photosynthesis increases and more water is evaporated. In summary, heat and evaporation, regulated by the plant dynamics, are the keys to the occurrence and regulation of turbulence.”
Alongside moisture, various volatile organic compounds are also released by the stomata in the leaves. It is unclear why plants releases these compounds: as a defensive substance or to communicate among each others. “The relevant aspect is the following: these substances react with other chemical compounds and form tiny particles called aerosols, which serve as condensation nuclei to which moisture in the atmosphere attaches, a key and essential process in cloud formation. This is another phenomenon at a tiny (small) scale that should be included in weather and climate models,” explains the Professor in Meteorology and Air Quality.