The importance of plant-water stress for predictions of ground-level ozone in a warm world

Abstract. Evapotranspiration is important for Earth’s water and energy cycles as it strongly affects air temperature, cloud
cover and precipitation. Leaf stomata are the conduit of transpiration and thus their opening is sensitive to weather and climate
conditions. This feedback can exacerbate heat waves and droughts and can play a role in their spatio-temporal propagation.
Therefore, the plant response to available water is a key element mediating vegetation-atmosphere interactions. Sustained high
temperatures strongly favor high ozone levels with significant negative effects on air quality and thus human health. Our study
assesses the process representation of evapotranspiration in the atmospheric chemistry model ECHAM/MESSy. Diverse water
stress parametrizations are implemented in a stomatal model based on CO₂ assimilation. The stress factors depend on either
soil moisture or leaf water potential and act directly on photosynthetic activity, mesophyll and stomatal conductance. Overall,
the new functionalities reduce the initial overestimation of evapotranspiration in the model globally by more than one order
of magnitude which is most important in the Southern Hemisphere. The intensity of simulated warm spells over continents
is significantly enhanced. With respect to ozone, we find that a realistic model representation of plant-water stress depresses
uptake by vegetation and enhances its photochemical production in the troposphere. These effects lead to a general increases
in simulated ground-level ozone which is most pronounced in the Southern Hemisphere over the continents. The uncertainties
for plant dynamics representation due to too shallow roots can be addressed by more sophisticated land surface models with
multi-layer soil schemes. In regions with low evaporative loss, however, the representation of precipitation remains the largest
uncertainty.