Accelerating the spin-up of the coupled carbon and nitrogen cycle model in CLM4

Abstract

Abstract. The commonly adopted biogeochemistry spin-up process in an Earth system model (ESM) is to run the model for hundreds to thousands of years subject to periodic atmospheric forcing to reach dynamic steady state of the carbonnitrogen (CN) models. A variety of approaches have been proposed to reduce the computation time of the spin-up process. Significant improvement in computational efficiency has been made recently. However, a long simulation time is still required to reach the common convergence criteria of the coupled carbon-nitrogen model. A gradient projection method was proposed and used to further reduce the computation time after examining the trend of the dominant carbon pools. The Community Land Model version 4 (CLM4) with a carbon and nitrogen component was used in this study. From point-scale simulations, we found that the method can reduce the computation time by 20-69 % compared to one of the fastest approaches in the literature. We also found that the cyclic stability of total carbon for some cases differs from that of the periodic atmospheric forcing, and some cases even showed instability. Close examination showed that one case has a carbon periodicity much longer than that of the atmospheric forcing due to the annual fire disturbance that is longer than half a year. The rest was caused by the instability of water table calculation in the hydrology model of CLM4. The instability issue is resolved after we replaced the hydrology scheme in CLM4 with a flow model for variably saturated porous media.

The influence of two land‐surface hydrology schemes on the terrestrial carbon cycle of Africa: A regional climate model study

“…A portion of GPP is allocated to carbon stored in the leaf to form LAI and in return LAI contribute to form GPP. In fact, GPP and LAI are in a mutual feedback loop which means that decrease/increase of LAI leads to decrease/increase of GPP and vice versa (Fang et al ., 2015). These results are consistent with the results reported in Lei et al (2014).…”

“…A portion of GPP is allocated to the leaf and eventually used to form LAI. In return, LAI contributes to form GPP, that is, GPP and LAI is in a feedback loop (Fang et al ., 2015). This means that LAI and GPP are directly influenced by the soil moisture status (as represented by the runoff scheme) as will be discussed in Section 3.…”

“…Initializing the coupled RegCM4‐CLM45‐CN‐TOP/RegCM4‐CLM45‐CN‐VIC model requires a long‐term spinning up of the CN (which is provided by the NCAR) to ensure that the vegetation carbon and NEE are in equilibrium as recommended by Fang et al . (2015). The terrestrial carbon fluxes were analysed over the period 2001–2010 based on the availability of the CARDAMOM product; meanwhile the LAI is evaluated over the period 2000–2008 based on the availability of the moderate‐resolution imaging spectroradiometer (MODIS) satellite data.…”

Technical Note: An efficient method for accelerating the spin-up process for process-based biogeochemistry models

“…Qu et al: Technique for spin-up acceleration in TEM tively easy but will take a great amount of computing time to equilibrate. For models such as CLM, multiple methods have been tested to accelerate their spin-up process (e.g., Fang et al, 2015), the direct analytical solution method introduced in this study might be time consuming to achieve.…”

Technical Note: An efficient method for accelerating the spin-up process for process-based biogeochemistry models

“…To accelerate the spin-up for multiple soil carbon pool models with relatively simple and linear decomposition processes, implementing our method will still be rela-3972 Y. Qu et al: Technique for spin-up acceleration in TEM tively easy but will take a great amount of computing time to equilibrate. For models such as CLM, multiple methods have been tested to accelerate their spin-up process (e.g., Fang et al, 2015), the direct analytical solution method introduced in this study might be time consuming to achieve.…”