Effects of the surface coupling strength in the WRF/Noah-MP model on regional climate simulations over China

Land-atmosphere energy and moisture exchange can strongly influence local and regional climates. However, high uncertainty exists in the representation of land-atmosphere interactions in numerical models. Parameterization of surface exchange processes is greatly affected by parameter C-zil, which, however, is typically defined as a domain-wide constant value. In this study, we examine the sensitivity of regional climate simulations over China to different surface exchange strengths via three C-zil schemes (default (without C-zil), constant (C-zil = 0.1), and dynamic canopy-height-dependent C-zil-h schemes) within a 13-km-resolution Weather Research and Forecasting model coupled with the Noah land surface model with multiparameterization options (WRF/Noah-MP). Our results demonstrate that compared to the other two schemes, the C-zil-h scheme substantially reduces land-atmosphere coupling strength overestimation, and comparison to Chinese terrestrial ecosystem flux research network (ChinaFLUX) observations reveals the capability of the C-zil-h scheme to better match observed surface energy and water variations. The results of the application of the various C-zil schemes in four typical climate zones in China demonstrate that the C-zil-h simulations achieve the closest agreement with field observations. The C-zil-h scheme can narrow the positive discrepancies in the simulated precipitation and surface fluxes and the negative biases of the land surface temperature in Northeast China, North China, eastern Northwest China, and Southwest China. In particular, the above remarkable improvements produced by the C-zil-h scheme primarily occur in areas covered with short vegetation. Additionally, the precipitation simulated with the C-zil-h scheme exhibits more intricate and uncertain changes compared with surface flux simulations due to the nonlocal impacts of the surface exchange strength resulting from atmospheric fluidity. Overall, our findings highlight the applicability of the dynamic C-zil scheme as a better physical alternative to the current treatment of surface exchange processes in atmosphere coupling models.