Increasing trend of near-surface small wind in Northern China during 1984-2016

Previous studies have reported that the average near-surface wind speed and strong winds have shown a downward trend in China during the past few decades. However, few studies have investigated causes of increasing frequency of low wind speed days. Based on the data of 70 stations and reanalysis data, this paper studied the change of small wind (1-3 m s-1) in northern China from 1984 to 2016 in association with variations in surface heat fluxes. The frequency of small wind increased at the rate of 3.4 d-1 10 yr-1 and 8.9 d-1 10 yr-1 at 14:00 Beijing Time (BJT) in the afternoon and 2:00 BJT at night respectively. This increasing trend in the frequency of small wind in the daytime was related to a decrease of average wind speed in atmospheric boundary layer (ABL), which is partly caused by a decrease of surface upward sensible heat flux and therefore a decrease in both vertical turbulence activity and downward momentum transmit from high altitude to the surface. The increase of small wind frequency at night was mainly caused by a decrease of calm wind days. The increase in temperature inversion at the bottom of the boundary layer resulted in a more stable layer and a larger contribution of mechanical wind shear, which was helpful to increase surface wind by transmitting high wind speed at high altitude downward. This process took effect when wind speed was relatively lower at night. Finally, besides the changes in local surface heat flux, an increase of local pressure gradient among stations during past decades may also contribute to the decrease of calm wind frequency and the increase of small wind frequency.