Evaluating the carbon and nitrogen cycles of the QUINCY terrestrial biosphere model using space-born optical remotely-sensed data

Accurate estimates of future land carbon sinks and thus the remaining carbon budget to achieve the Paris climate goals requires rigorous modelling of the carbon sequestration potential of the terrestrial biosphere. Estimating the terrestrial carbon budget requires an accurate understanding of the interlinkages between the land carbon and nitrogen cycles, yet coupled carbon-nitrogen cycle models exhibit large uncertainties. Leaf chlorophyll, chlleaf, is an indicator of the leaf nitrogen content stored within photosynthetic nitrogen pools and is central to the exchange of carbon, water and energy between the biosphere and the atmosphere. In this work, we harness an advanced remote sensing (RS) chlleaf product to evaluate a terrestrial biosphere model (TBM), QUantifying Interactions between terrestrial Nutrient CYcles and the climate system (QUINCY), which explicitly models chlleaf. We focus on comparing the spatial and seasonal patterns of modelled and observed chlleaf, and then further assessing if modelled leaf area and productivity agree with a RS leaf area index product and in-situ eddy covariance-based gross primary production, respectively. In addition, we conduct additional simulations to test two alternative formulations of leaf-internal nitrogen allocation within QUINCY. Our analysis over a globally representative set of locations reveals that QUINCY chlleaf magnitudes are mostly in line with the RS chlleaf values. However, QUINCY chlleaf tends to show a narrower numerical range compared to RS for specific ecosystem types, such as grasslands. While the seasonal cycle of QUINCY chlleaf mostly corresponds well to the observations, for many deciduous forests, the increase in QUINCY's chlleaf predictions in spring and the decrease in autumn were delayed compared to observations. Our results also show that compared to the original leaf nitrogen allocation scheme of QUINCY, the revised scheme produced a more reasonable sensitivity of gross primary production to increases in chlleaf. However, the revised scheme did not directly lead to improvement in simulating chlleaf and gross primary production. Our study shows the value of RS products linked to nitrogen cycle that will be useful in both carbon and nitrogen modelling, and paves way for closer linking of RS and TBMs.