不同环流背景下华北夏季午后降水特征及陆气耦合效应OA

North China,located at the northern margin of the East Asian monsoon and within a transitional zone between semi-humid and semi-arid climates,is characterized by complex underlying surface conditions and is widely recognized as a hotspot of land-atmosphere coupling.Summer precipitation is concentrated in this region and is frequently associated with afternoon convection,yet its formation is modulated by multiscale interactions between large-scale atmospheric circulation and local land surface processes.Although land-atmosphere coupling is recognized as an important factor influencing regional precipitation,how this coupling varies under different cir-culation regimes remains insufficiently understood.This study objectively classifies summer the dominant summer circulation regimes over North China and investigates their influences on summer afternoon precipitation and land-atmosphere coupling. Using ERA5 reanalysis data from 1981 to 2020,the K-Means clustering method is employed to objectively classify the summer 850 hPa wind fields over North China into three dominant circulation regimes.The analysis focuses on precipitation,vertical motion,moisture conditions,and key land surface variables,including soil mois-ture,sensible and latent heat fluxes,evaporation fraction,boundary layer height,and lifting condensation level def-icit.Conditional probability differences are calculated to quantify the effects of morning soil moisture,sensible heat flux,and evaporation fraction on the occurrence of afternoon precipitation.Furthermore,the land-atmosphere cou-pling pathway is decomposed into two components—land surface control and atmospheric response—to elucidate the underlying physical mechanisms. Three circulation regimes are identified:a southwesterly-dominated regime(P1,40.80%),an easterly-domi-nated regime(P2,35.58%),and a northwesterly-dominated regime(P3,23.62%).Under P1,precipitation inten-sity and frequency are the highest,with precipitation concentrated over central and northern North China.P2 is characterized by moderate precipitation,with the main rainband shifted southward,while P3 exhibits the weakest precipitation,which is primarily confined to coastal areas.Diurnal analysis reveals a bimodal precipitation cycle under all three regimes,with the afternoon peak strongest under P1 and weakest under P3.Analyses of vertical ve-locity and the moisture budget indicate that P1 is characterized by the strongest upward motion and most pro-nounced moisture convergence,providing the most favorable dynamical and moisture conditions for afternoon convection. Across all three regimes,afternoon precipitation occurs preferentially following mornings characterized by relatively dry soils,enhanced sensible heat flux,and a lower evaporation fraction.This preference is particularly pronounced over mountainous areas under P1.Analysis of the land-atmosphere coupling pathway reveals a signifi-cant positive correlation between morning soil moisture and evaporation fraction and a significant negative corre-lation between evaporation fraction and afternoon boundary layer height.These relationships indicate that drier soils enhance sensible heating,promote deeper boundary layer development,and facilitate the lifting of boundary layer air toward the lifting condensation level,thereby favoring convective initiation.Analysis of the lifting con-densation level deficit further confirms that the deficit is consistently smaller(i.e.,less negative)over drier soils than over wetter soils across all three regimes.The largest dry-wet contrast occurs under P1,further supporting the conclusion that land-atmosphere coupling is strongest under this regime. These findings demonstrate that large-scale circulation regimes not only directly modulate precipitation inten-sity and spatial distribution through their effect on atmospheric dynamics and moisture supply but also substantially influence the strength and spatial structure of land-atmosphere coupling.The southwesterly-dominated regime provides the most favorable combination of moisture availability,dynamical forcing,and land surface feed-back,resulting in the most frequent and intense afternoon precipitation.This study provides a physical framework for understanding the synergistic effects of large-scale circulation and land surface processes on afternoon convec-tion over North China.Future studies should further validate these findings and investigate the underlying mecha-nisms using multi-source observations and numerical model simulations.