Abstract
Climatic changes towards warmer temperatures require the need to improve the simplified vegetation scheme of the regional climate model COSMO-CLM, which is not capable of modelling complex processes which depend on temperature, water availability, and day length. Thus, we have implemented the physically based Ball-Berry approach coupled with photosynthesis processes based on Farquhar and Collatz models for C3 and C4 plants in the regional climate model COSMO-CLM (CCLM v 5.16). The implementation of the new algorithms includes the replacement of the “one-big leaf” approach by a “two-big leaf” one. We performed single column simulations with COSMO-CLM over three observational sites with C3 grass plants in Germany for the period from 2010 to 2015 (Parc, Linden and Lindenberg domain). Hereby, we tested three alternative formulations of the new algorithms against a reference simulation (CCLMref) with no changes. The first formulation (CCLM3.5) adapts the algorithms for stomatal resistance from the Community Land Model (CLM v3.5), which depend on leaf photosynthesis, CO2 partial and vapor pressure and maximum stomatal resistance. The second one (CCLM4.5) includes a soil water stress function as in CLM v4.5. The third one (CCLM4.5e) is similar to CCLM4.5, but with adapted equations for dry leaf calculations. The results revealed major differences in the annual cycle of stomatal resistance compared to the original algorithm (CCLMref) of the reference simulation. The largest changes in stomatal resistance are observed from October to April when stomata are closed while summer values are generally less than control values that come closer to measured values. The results indicate that changes in stomatal resistance and photosynthesis algorithms can improve the accuracy of other parameters of the COSMO-CLM model (e.g.: transpiration rate or total evapotranspiration). These results were received by comparing COSMO-CLM parameters with FLUXNET data, meteorological observations at the sites, and GLEAM and HYRAS datasets.
Highlights
30 The land surface processes significantly affect the conditions in the low-level atmosphere (Tölle and Churiulin 2021)
The results indicate that changes in stomatal resistance and photosynthesis algorithms can improve the 25 accuracy of other parameters of the Consortium for Small-Scale Modelling (COSMO)-CLM model (e.g.: transpiration rate or total evapotranspiration)
The results demonstrate that the main differences between the control simulation (CCLMref) and the experiment simulations (CCLMv3.5, CCLMv4.5 and CCLMv4.5e) are observed from September (October) to March (April), when stomata are closed or there are no leaves
Summary
30 The land surface processes significantly affect the conditions in the low-level atmosphere (Tölle and Churiulin 2021). The surface radiation budget and turbulent heat fluxes are controlled by near-surface atmospheric conditions. They determine the amount of energy and water available for heat and moistening the air over land. The main parameters, which determine the interactions between the land surface and atmosphere, are the soil water content (Koster et al, 2002) and the surface roughness (de Noblet-Ducoudre and Pitman, 2021). The impact of surface processes is evident in the low-level temperature, humidity, 35 the structure of the planetary boundary layer and precipitation (Arora, 2002). Tölle et al 2014 have shown in climate simulations at convection-permitting scale that vegetation type changes can have a significant impact on extreme temperatures. Atmospheric models have to represent the land surface processes in a realistic way
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