Research On The Characteristics Of The Temporal And Spatial Changes Of Snow Phenology In China And Its Response To Vegetation Phenology

As one of the most active factors in the cryosphere,snow cover plays an important role in the climate system,carbon sink level,ecological environment,energy exchange,and humanistic economy.As the temperature rises,it becomes especially important to explore the changes in snow cover,and snow phenology can quantify this change.At present,the main data sources are site observations and remote sensing methods.Due to their respective advantages and disadvantages,we have prepared China's long-term series of snow area products from 1981 to 2014 based on AVHRR.China is a major seasonal snow cover area.The study of seasonal snow cover changes has an important impact on climate and hydrology.Snow cover interacts with the ground surface,and the change of snow cover phenology profoundly affects the changes of vegetation ecology.The study of the correlation between snow cover and vegetation is of great significance.However,there are still relatively few related studies in China.Therefore,we explored the temporal and spatial variation of snow cover in China and the spatial distribution of correlation with vegetation.The specific content is as follows:(1)Preparation of snow cover area products.Take Landsat images as samples,use SNOWMAP algorithm to recognize snow as the true value,train and count the distribution of AVHRR feature parameters with snow and no snow,and establish a new snow recognition algorithm,and combining with the JASMESA cloud algorithm to prepare the primary products for the daily cloudless snow area in China.Since cloud is an important interference factor for snow recognition,two methods,Hidden Markov Random Field Method and Microwave Snow Water Equivalent Interpolation,are used to remove the cloud.Finally,the final product is prepared in combination with temperature and water data,and the accuracy of the product is verified with the measured data of the weather station.Compared with the JASMES algorithm,the snow recognition algorithm in this paper has more missing points and fewer points.The overall accuracy is 86.31% higher than that of the JASMES algorithm.(2)Phenological analysis of snow cover.Calculate the daily,monthly,and annual change trends of snow cover phenology parameters,as well as the temporal and spatial changes of snow area coverage,the start of snow cover days,the melt of snow cover days,and the length of snow period,in order to reveal the distribution and change characteristics of snow cover in China.China's main stable snow areas are mainly distributed in the three major snow areas of the Northeast(Northeast-Inner Mongolia Plateau),Northern Xinjiang,and Qinghai-Tibet Plateau.,of which the total snow area reached the largest in January.The overall snow cover in China shows a trend of decreasing snow area,delaying the first day of snow cover,advancing the end of snow cover,and shortening the length of the snow cover period.Different regions show different characteristics of change.In the Qinghai-Tibet Plateau,Altay,Tianshan,Greater Khingan Range and Lesser Khingan Mountains and other areas,the rate of change is large.In addition,China's snow cover presents the characteristics of lower latitude and lower altitude,the earlier the first day of snow cover,the later the last day,and the shorter the snow cover period.(3)Analysis of vegetation phenology.The greening period of natural vegetation in China is mainly two and a half months from late March to the end of May,the yellowing period is mainly from September to December,and the length of the growing period is the largest in the region of 150-180 days.China presents the characteristics of delayed replanting period,early yellowing period,and shortened growth period.However,the specific changes in different regions are different,and the rate of change is small,with a rate of 0-1 day/year being advanced or delayed.(4)The response relationship between snow phenology and vegetation phenology.Snow phenology parameters have different effects on vegetation phenology parameters,and the degree of influence is different in different regions.On the whole,the number of snow cover days has a greater impact on vegetation,while the first snow cover day,the last snow cover day,and the length of the snow cover period have relatively minor effects,except that the number of snow cover days is negatively correlated with the vegetation return period,and the end snow cover day has a negative correlation with the vegetation return period.,Both show positive correlation.Regionally,the snow cover days have a greater impact in Altay,Tianshan Mountains,central Inner Mongolia,the junction with Liaoning and Hebei,and parts of the Qinghai-Tibet Plateau.The first day of snow cover has less impact on the overall vegetation,and the snow cover lasts in Heilongjiang Province and Jilin Province.The southern part of the province has a greater impact,while the length of the snow cover period has a relatively greater impact in Altay,Inner Mongolia,northern Heilongjiang,and central Inner Mongolia.