Temporal And Spatial Variations Of Extreme Temperature In Qinling Mountains During Recent 56 Years And Its Impact On Vegetation Coverage

Extreme temperature is an important part of the research on climate change.Under the background of global warming,exploring the changes in extreme temperature helps to understand regional climate change systematically,and it is of great significance to ecological and environmental protection and disaster warning in the context of climate change.In this paper,based on the daily temperature data observed from 32 standard meteorological stations from 1960 to 2015,and MODIS NDVI data during the period of 2000~2015,we studied the spatial-temporal variations and spatial difference of extreme temperature over Qinling Mountains during the last 56 years,as well as the effects of extreme temperature changes on NDVI by integrating the methods of Kriging interpolation,M-K mutation test,principal component analysis,correlation analysis and the RClim Dex model.The results and progress are as follows:(1)The frequency,intensity and duration of extreme temperature over Qinling Mountains showed an increasing trend.The impact of extreme temperature changes on regional warming was mainly due to the changes of warming indices.Among all the indices,only summer days,warm days,warm nights and cold days were the major contributors.The extreme warm indices over Qinling Mountains showed an upward trend during the last 56 years,while the extreme cold indices showed a downward trend.More importantly,the rate of warm indices were relatively larger.The tendency rate of monthly maximum(minimum)value of daily maximum temperature,monthly maximum(minimum)value of daily minimum temperature,summer days,tropical nights,warm days(nights),warm spell duration days,growing season length and diurnal temperature range showed significantly increasing trends at the rates of 0.17(0.35)?/10 a,0.07(0.18)?/10 a,3.7 d/10 a,1.57 d/10 a,1.4(1.37)d/10 a,0.98 d/10 a,3.55 d/10 a and 0.07 ?/10 a,respectively.While the occurrence of ice days,frost days,cold days(nights)and cold spell duration days had significantlydecreased by-0.8 d/10 a,-3.19 d/10 a,-1.08(-1.29)d/10 a and-0.94 d/10 a.The mutation years of extreme temperature indices were mainly concentrated in the 1990 s.The sensitive zones of extreme temperature change were Zhenan and Zhashui that located in the southern slope as well as Zhouzhi and Huxian in the northern slope,respectively.(2)The variation of extreme temperature in all seasons of Qinling Mountains showed a warming trend as a whole from 1960 to 2015,and the warming rate in summer was the lowest.Among them,the growth of high temperature dominated in spring and autumn,while the growth of low temperature dominated in summer and winter.Warm days(nights),monthly maximum value of daily maximum temperature,monthly maximum(minimum)value of daily minimum temperature in all seasons over Qinling Mountains showed an increasing trend,while cold days(nights)showed a decreasing trend during the last 56 years.Monthly minimum value of daily maximum temperature showed a decreasing trend in summer,but the trend increased in the other seasons.(3)Spatial differences existed in the distribution of the average temperature and extreme temperature over Qinling Mountains from 1960 to 2015,as well as the changes of extreme temperatures.The annual average temperature,average maximum temperature,and average minimum temperature over Qinling Mountains were 10.50 °C,16.47 °C,and 6.21 °C during the last 56 years,respectively.The temperature in southern slope of Qinling Mountains was higher than that in northern slope in the middle-altitude area and high-altitude area,and lower than that in northern slope in the low-altitude area.The difference in air temperature between the northern and southern slopes was smaller in the low-altitude area and greater in the middle-altitude area.High temperatures contributed to the temperature difference between the north and south slopes.The variation of the extreme temperature change was more obvious in frequency and durationin the northern slope while in intensity in the southern slope.Moreover,warming occurred mainly in the night in the northern slope but in the daytime in the southern slope.The warming rate of extreme temperature in Qinling Mountains increased with altitude.The change of frequency and intensity of extreme temperature was more obvious in the high-altitude area,and the duration of extreme temperature more obvious in the middle-altitude area.(4)The NDVI over Qinling Mountains showed an upward trend at the annual and seasonal scales from 2000 to 2015.The annual average NDVI over Qinling Mountains increased significantly at a rate of 0.02/10 a during the last 16 years,the increasing rate of average annual NDVI was greater in the southern slope.The high value areas of change rate were concentrated in most parts of the eastern section over Qinling Mountains,while the low value areas were located in Xi'an,the northern slope of Qinling Mountains.The NDVI in all seasons of Qinling Mountains increased significantly at the rates of 0.03/10 a,0.02 /10 a,0.04/10 a,and 0.02/10a(p<0.05),with the fastest increase in autumn.The seasonal average NDVI showed a downward trend in Xi'an and Weinan that located in the northern slope,as well as Shangluo in the southern slope.(5)The extreme temperature warming over Qinling Mountains had played an important role in promoting the annual average NDVI during the last 16 years,the increasing of diurnal temperature range had an inhibitory effect on the annual average NDVI.The extreme warm indices showed an upward trend,and the correlations with the annual average NDVI were mainly positive.The extreme cold indices showed a downward trend,but the correlations with the annual average NDVI were mainly negative.Summer days,tropical nights,warm days(nights),monthly maximum value of daily maximum(minimum)temperature,warm spell duration days and growing season length were non-significantlypositively correlated with the annual average NDVI,while non-significant negative correlations with the annual average NDVI existed in ice days,frost days,cold days(nights),monthly minimum value of daily maximum(minimum)temperature,diurnal temperature range and cold spell duration days.The proportions of pixels for significant spatial correlation between extreme temperature indices and annual average NDVI were different with the highest being 9.05%,for cold days,and the lowest being 0.62%,for monthly minimum value of daily maximum temperature.