Effects of surface friction and turbulent mixing on long-term changes in the near-surface wind speed over the Eastern China Plain from 1981 to 2010

A significant slowdown in the near-surface wind speed (SWS) due to combined effects of the driving and drag forces of the atmosphere has been demonstrated in different regions in the globe. The drag force includes two sources: the friction force between the underlying surface and the bottom of the atmosphere, which is the external friction force (EFF), and the vertical exchange of the horizontal momentum induced by turbulent mixing, which is the turbulent friction force (TFF). In this paper, we propose a diagnostic method to separate the effects of the EFF and the TFF on long-term changes in the SWS over the Eastern China Plain (ECP) region from 1981 to 2010. The results show that the TFF could have caused an increase of 0.5 ± 0.2 m s− 1 in the SWS over the ECP region in the past 30 years and the TFF showed an increasing influence of 0.17 m s− 1 decade− 1. In contrast, the EFF distinctly decreased the SWS by an average of − 1.1 ± 0.4 m s− 1 and presented a significant decreasing trend of − 0.36 m s− 1 decade− 1. The effect of EFF is the main inducer of the observed regional long-term decrease of the SWS, which is in accordance with the distinct land use and cover change (LUCC) occurring in the ECP region in recent decades. Furthermore, the effects of the EFF and TFF on the changes in the SWS are more significant in large cities than those in small cities. The TFF effect can accelerate the SWS, with means of 0.5 ± 0.2 and 0.4 ± 0.2 m s− 1 in large and small cities, respectively. The EFF effect can decelerate the SWS, with means of − 1.2 ± 0.4 and − 0.7 ± 0.4 m s− 1 in large and small cities, respectively.