| The investigation of near-surface wind speed(SWS)is helpful to strengthen the understanding of the air stagnation and drought event,is helpful to intensify the understanding of the feedback mechanism between SWS,aerosol and boundary layer stability,is beneficial to development and evaluation of wind energy.Therefore,the studies of the long-term changes in SWS and their causes are very important for recognizing the effects of anthropogenic activities and natural climate changes.According to the limitations and drawbacks in the previous studies over China,in this paper,the observation,sounding,and reanalysis datasets,meanwhile,the statistical diagnostic analysis,dynamical diagnostic analysis,statistical downscaling,and numerical simulation,are used to investigate the spatio-temporal characteristics of wind speed in Eastern China,reveal the physical mechanisms of SWS changes,and estimate the possible climatic trend of SWS in the future under the different emission scenarios.The results are shown as followsThe long-term reduction in SWS was found in China during the study period.The long-term trends of probabilities of 0.3?1.5 and 1.6?3.3 m/s increased,while the probabilities of 3.4?5.4 m/s,5.5?7.9 m/s and greater than 8.0 m/s declined.Due to the increase in surface roughness attributed to LUCC or urbanization,the probabilities for 3.4?5.4,5.5?7.9 and greater than 8.0 m/s were lower in large city than that observed in small city.The probability of wind speed beyond 3.0 m/s was 6.5%,1.2%and 11.0%for all stations,large city and small city,respectively.Additionally,the reduction in wind speed occurred from the near-surface to 300 hPa,while the PGF also exhibited a synchronous reduction.The influence of PGF on the decrease of wind speed was relative smaller at near surface than that at higher levels(850-300 hPa).At the near-surface,71.8%of changes in annual mean SWS could be attributed to the changes of PGF.At the higher levels,most of the changes in wind speed(>90%)was induced by PGF.The SWS showed a significant decrease in the last 30 years,but a short-term strengthening in SWS was observed during the winter since 2000 in Eastern China.The short-term strengthening of SWS during winter was mainly induced by the changes of the pressure-gradient force(PGF).The effects of Northern Hemisphere Annular mode(NAM)and Pacific Decadal Oscillation(PDO)on the changes in SWS were pronounced.NAM mainly induced the inter-annual fluctuations of SWS over study region.At the positive NAM phases,the zonal westerly decreased over the sub-tropical and mid-latitudes,both the Ferrel cell over the high-latitudes and direct cell over the mid-latitudes increased,which caused the anomaly of meridional circulation,and thereby caused reduction of the surface air temperature(SAT)difference between mid-and high-latitudes,and therefore SWS declined in the study region.The inverse circulation patterns were generated accompanied by the negative NAM phases.PDO mainly affected the decadal changes of SWS in Eastern China.During the cool PDO phases,the North Pacific was controlled by the anti-cyclone,the positive anomaly of SST motivated a positive anomaly of SAT,southerly anomaly in the mid-and high-latitudes and northerly anomaly in low-latitudes over the East Asia.The northerly anomaly transported the cool air from the higher latitudes to the lower latitudes,and caused the north-south SAT difference declined.These changes mainly influenced the decadal changes in first mode of SWS;the explained variance reached 33.5%.Moreover,the mid-latitudes prevailed easterly anomaly accompanied by the anti-cyclone.These changes mainly induced the decadal changes in second mode of SWS;the explained variance was 19.7%.The inverse circulation patterns were generated accompanied by the warm PDO phases.The SWS based on land use cover data for the 2010s(LUC 10)was lower than that based on land use cover data from the 1980s(LUC80)by a difference of 0.17 m/s;meanwhile the LUCC effects also resulted in a decrease of 9.0%of the probability in strong wind.The LUCC effects induced significant alteration of the roughness length,caused changes in the drag coefficient and friction velocity,and thereby decrease SWS.A 0.1 m increase in roughness length could cause a 0.003 increase in drag coefficient and a 0.015 m/s increase in friction velocity.The contributions of LUCC to the SWS changes vary among different regions.The significant decrease of SWS over the middle reaches of the Yangtze River was induced by the changes from closed shrubland and cropland/natural vegetation mosaic to evergreen broadleaf and deciduous broadleaf forest.The slowdown in SWS over the Shandong Peninsula,the Beijing-Tianjin-Hebei region,the Yangtze River Delta,and the Pearl River Delta can be attributed to the extension of urban and built-up areas and the decrease of croplands and the cropland/natural vegetation mosaic.Reduction in SWS caused by LUCC was also revealed by the friction wind model(FWM),and which presented more significant effects of LUCC on decrease in SWS than WRF.Furthermore,the effects of LUCC on SWS could be attribute to the external friction force(EFF),turbulence mixing friction force(TFF)could induce the increase of SWS.Consequently,if the TFF was not be included,the SWS could show more significant slowdown with the only EFF effects.Some regional characteristics of SWS cannot be well captured by CMIP5 models,and all models underestimated the long-term slowdown in SWSs during 1979-2005.Compared to the traditional arithmetic mean ensemble method(AMEM),the results based on weighted-mean ensemble method(WMEM)were more credible than AMEM.Compared with WMEM,the smaller error was obtained in the statistical downscaling model(SDM).The results of SDM showed that the increase of greenhouse gas emission can induce significant decrease of SWS in future.The probability of annual mean wind speed exceeding 2.37 m s-1 decreased by 12.1%under RCP8.5 relative to RCP4.5 scenario.Consequently,the increase in greenhouse gas emission maybe cause the decrease of strong SWS. |
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