| In the context of global warming,autumn cooling has occurred in central Eurasia since the beginning of the 21st century.Although some studies have shown that the cooling is closely related to the Pacific Decadal Oscillation and the Siberian high,the mechanism of autumn temperature change in Eurasia needs to be further explored.As an important part of climate change,snow cover can significantly change the underlying surface thermal conditions and thus affect the temperature.More and more attention has been paid to the influence of snow cover change on temperature.Most studies have qualitatively discussed the relationship between snow cover change and temperature in different regions,but the contribution of snow cover change to autumn temperature change in central Eurasia is still unclear.This study focused on the influence of snow cover change on autumn temperature change in central Eurasia(40-65°N,50-130°E),and carried out the following studies:Based on the Snow and Ice data of the Interactive Multisensor Snow and Ice Mapping System(IMS),the spatio-temporal variation characteristics of snow cover in central Eurasia during 2004-2020 were analyzed.By comparing and analyzing the applicability of different weather driving fields and parameterization schemes in the study area,a localized WRF(Weather Research and Forecasting)model is established.Based on the simulation results of control experiment and sensitivity experiment,the spatio-temporal pattern and contribution rate of snow cover change on autumn temperature during 2004-2020 were quantitatively evaluated.The mechanism of snow cover change on autumn temperature over central Eurasia was discussed from the perspective of radiation budget and energy flux.The main conclusions are as follows:(1)In recent 17 years,the annual variation of snow cover showed a unimodal curve(the peak was reached in January),and the interannual variation of snow cover percentage(SCP)in the study area showed a significant(p<0.05)increasing trend(SCP0.767%/10a).From different seasons,SCP showed a significant(p<0.05)increase trend in autumn,but showed an insignificant(p>0.05)decrease trend in other seasons,and the increase rate of SCP in autumn(5.38%/10a)was much higher than that in the whole year(0.767%/10a).(2)The snow cover frequency(SCF)in the study area showed an increasing trend(50.90%),and the distribution had obvious regional and seasonal differences.The high annual mean SCF value area(more than 70%)was mostly distributed in the high altitude area,and the SCF distribution in other areas gradually increased from low latitude to high latitude.Seasonally,SCFS decreased in spring and winter(70.2%and67.21%,respectively),and showed no change in most areas in summer.In autumn,SCFS showed an increasing trend,and the area of increasing trend accounted for89.32%.(3)Through the optimization of meteorological field driving data,parametric scheme combination and underlying surface data,a simulation test scheme suitable for central Eurasia was obtained,and a localized WRF model was constructed.The simulated air temperature showed a significant(p<0.001)positive correlation with the observed air temperature and CRU air temperature(R=0.948,0.978).The simulation verification results confirmed that the localized WRF model had a good simulation effect on autumn air temperature in the central Eurasia,which could meet the accuracy requirements of this study.(4)From 2004 to 2020,the autumn temperature in the study area showed an insignificant(p>0.05)decreasing trend(tendency rate was-0.867℃/10a),and from2004 to 2016,the cooling trend was more significant(p<0.05,change tendency rate was-2.15℃/10a).According to the spatial distribution of the change trend,the area proportion of the cooling area was 98.21%during the autumn of 2004-2020,and the area proportion of the significant(p<0.05)declining area was 16.03%.From 2004 to2016,the area of cooling area accounted for 98.73%,and the area of significant(p<0.05)declining area accounted for 37.88%.During 2004-2020 and 2004-2016,the fall temperature change caused by snow cover change showed a downward trend,and the fall cooling area caused by snow cover change during 2004-2016 was larger than that during 2004-2020,and the cooling area caused by snow cover change was mainly concentrated in the area with the most significant fall cooling trend.(5)The comparative analysis of the results of the control test and the sensitivity test showed that the contribution rate of snow cover to the change of autumn temperature trend during 2004-2020 and 2004-2016 was about 21.54%and 22.64%,respectively.The regions with high correlation between the autumn temperature change caused by snow cover and that of the same period from 2004 to 2020 were mainly distributed in the central part of the study area(48-60°N),including the West Siberian Plain,Kazak Hills,Sayan Mountains,Mongolia Plateau and the plateau along the Lena River(54%).The spatial correlation distribution from 2004 to 2016 was similar to that from 2004 to 2020,and the correlation in the central and western parts of the study area(50-55N°,70-80E°)was higher than that from 2004 to 2020,indicating that the variation of snow cover in these areas had a higher explanatory effect on the temperature change.(6)In terms of radiation components,the surface albedo(trend rate 0.014/10a)and upward short-wave radiation(annual trend rate 0.196 W/m~2)caused by snow cover change in central Eurasia showed a significant(p<0.05)increasing trend during the study period.The upward long-wave radiation and net radiation showed a significant(p<0.05)decreasing trend,and the annual trend rate was-0.119W/m~2 and-0.112W/m~2,respectively.From the perspective of energy flux,the sensible heat flux,latent heat flux and soil heat flux caused by snow cover change all showed a decreasing trend(the annual tendency rate was-0.057 W/m~2,-0.016 W/m~2 and-0.040 W/m~2,respectively).Relatively,snow cover had the best correlation with upward shortwave radiation(R=0.95,p<0.01),followed by surface albedo(R=0.94,p<0.01),and had the worst correlation with latent heat flux(R=-0.62,p<0.01).On the one hand,with the increase of snow cover,the surface albedo increases,which leads to the increase of short-wave radiation reflected by the ground and the decrease of short-wave radiation absorbed by the ground.As a result,the surface temperature decreases and the upward long-wave radiation decreases.On the other hand,the low thermal conductivity of snow cover obstructs the heat exchange between earth and air,leading to the decrease of sensible heat flux,latent heat flux and soil heat flux. |
