Improvement of surface exchange coefficient parameterization and its application to regional numerical simulation

Land-atmosphere energy and moisture exchange can strongly influence local and regional climate changes. However, high uncertainties exist in the representation of land-atmosphere interactions in numerical models and the coupling strength between land and atmosphere is largely overestimated, in which the determination of surface exchange coefficient is one of the main problems. Here, we show the improvements from a dynamic vegetation-type-dependent exchange scheme in the offline Noah land surface model with multi-parameterization options and the Weather Research and Forecasting model when applied to China. Compared to the default schemes, the dynamic exchange scheme significantly reduces land-atmosphere coupling strength overestimations, and comparisons to flux tower observations reveal its capability to better match observed surface energy and water variations. In particular, the above remarkable improvements produced by the dynamic exchange scheme primarily occur in areas covered with short vegetation. The improved version benefits from the treatment of the roughness length for heat. Further, land-surface processes play significant roles in cloud formation and precipitation generation by affecting local planetary boundary layer profiles. The dynamical exchange scheme could narrow the positive discrepancies in the simulated precipitation. Using 3-km-resolution convection-permitting models for three heavy precipitation cases, the dynamic coupling simulations could achieve the closest agreement with the field observations, especially the intensity and location of the heaviest rainfall during the precipitation process. Overall, our findings highlight the applicability of the dynamic scheme as a better physical alternative to the current treatment of surface exchange processes in atmosphere coupling models and could help achieve more accurate simulations.