Evaluating changes in the total evapotranspiration parameter due to the updates in stomatal resistance algorithms of COSMO-CLM model on the example of the exceptionally warm summer of 2013

Climatic changes will likely increase the frequency and intensity of extreme weather events (e.g. heat waves or droughts) in the future. Vegetation is one of the key factors, which has a significant impact on extreme temperatures, which is clearly evident in climate simulations at convection-permitting scale. The process of evapotranspiration is related to vegetation and controlled by stomatal resistance, which is playing a major role in regulating water loss and carbon uptake. However, the regional climate model of the Consortium for Small-scale Modelling (COSMO-CLM v5.16) uses a simplified stomatal resistance algorithm, which is not capable of modelling complex processes depending on temperature, water availability and day length. Here, we demonstrate the changes in the total evapotranspiration parameter caused by updates of the stomatal resistance algorithms based on the physically Bell-Berry approach coupled with the Farquhar photosynthesis model and “two-big leaf” approach. The latter is necessary for dividing the photosynthetic active radiation flux on two phases, which are sunlit and shaded. The algorithms from two different versions (v3.5 and v4.5) of the Community Land Model (CLM) were implemented. The stomatal resistance algorithm of CLM v3.5 depends on leaf photosynthesis, CO₂ partial and vapor pressure and minimum stomatal conductance. The algorithm of CLM v4.5 is additionally limited by the soil water stress function. In a third update, we also implemented in COSMO-CLM the water flux calculation algorithm based on CLM v4.5 for dry and wet leaf transpiration. Then single column simulations were conducted over three observational research domains with C₃ grass plants in Germany from 2001 to 2015 and analyzed for the exceptionally warm and dry summer 2013. Model results were compared with GLEAM data. Differences between simulations begin to appear with the leaf growth and reach the maximum values in summer months, especially in June 2013 when the standardized temperature anomaly was fixed. In June, the reference simulation reaches mean values of the total evapotranspiration equal to 2.7 mm month^-1, while the GLEAM datasets and experimental simulations show similar values in the range of 3.3 to 3.6 mm month ^-1. The simulations with the new algorithms have slightly greater correlation coefficient (0.879, 0.875 and 0.867) with GLEAM data than the reference simulation (0.856). Applied performance indices like Kling-Gupta Efficiency index (KGE > 0.77) and the distribution added value index (DAV > 0.007) confirm those results. Model results for the exceptional warm and dry summer 2013 showed that the new algorithms of stomatal resistance are much more sensitive to the changes in environmental conditions (e.q.: soil moisture deficit, warm temperatures), while the reference simulation demonstrates similar to usual summer values of stomatal resistance. We anticipate our results to be a starting point for more sophisticated developments in the COSMO-CLM model. The new stomatal resistance algorithms can be used for the modern algorithm for the leaf area index based on the biomass evolution.