Spatio-temporal Change Dynamic Of Snowline On Tibetan Plateau

Snow cover and glacier is the important components of the earth surface of cryosphere as well as one of the important factors affecting climate change, of which the spatio-temporal distribution and change has been paid much attention from scientists of geoscience and climatology. As one of the most active natural factors, the research about changes in snow cover area has an important scientific meaning at the global, regional and local scales. As the zero-balanced surface of rainfall, the snowline can be defined as the lower limit of the perennially snow-covered area, and it is a geographic element as important as the tree line, the periglacial line and the soil line. The Tibet Plateau (TP), one of the low temperature zone as well as the major snow cover and glacier area in China, is located in the southwest and situated at4000m above sea level averagely. Because of the diversity and complexity in topography, there is a great climatic dissimilarity in the transmeridional direction. Accordingly, extracting the snowline accurately, studying the spatial distribution mechanics of snowline play an important role in geoscience, hydrology and climatology.Using remote sensing and geographic information system technologies, the method of extracting the snowline on Tibetan Plateau based on MODIS data was explored and the spatio-temporal information of snowline changes on TP from2001to2011was revealed. First, on the basis of topographic correction of MOD09GA based on CIVCO model, NDSI method was used to map snow cover. By using algorithms for cloud removal, MODIS daily cloudless snow classification products (CA) was produced; Second, using the CA and MCD12Q1data, combined with GIS spatial analysis method, the methods of snowline elevation extraction and snowline mapping were studied; Finally, by using the snowline extracting method based on MCD12Q1and the Universal Kringing method, the snowlines were extracted and the snowline fields were created on TP from2001to2011. The law of snowline distribution was analyzed and the spatio-temporal change of snowline on TP were dynamically monitored, including the maximum, minimum and average of snowline elevation during2001to2011. The results showed that:1) The MODIS/Terra daily snow cover product (i.e., MOD10A1) has a high omission error on mapping snow cover over the Tibetan Plateau. Compared to the "true value", the average Kappa coefficient, overall accuracy and snow classification accuracy of MOD10A1in verification zones (i.e., T1, T2, T3and T4) is0.57,85.69%and65.35%, respectively, while which of the COMOD is0.61,86.46%and73.66%, respectively. This suggests that COMOD has the capability to identify snow cover more effectively on TP.2) The snowline elevation, extracted by the approach based on MODIS daily cloudless snow classification products (CA) with topographic correction, has a different distribution information in each study region on TP. The accuracy of snowline elevation values were evaluated. The result shows that it underestimates the snowline elevation, and has a certain error compared the real snowline elevation with the extracted snowline elevation.3) Because the MCD12Q1has a comprehensive ice and snow cover information and the slope factor is taken into account in extracting snowline, the method based on MCD12Q1makes snowline extraction more scientific. The result of accuracy evaluation indicates that the average error of snowline elevation value is5.9%. The snow line field based on Kriging method underestimates the snowline elevation, and overestimates the sptail extent of the real snowline field. Compared with the snowline extrated by the CA approach, the method is more suitable to extract the snowline information on TP.4) From2001to2011, the spatial distribution of snowline on TP follows two basic laws: There is an overall distribution trend that the snowlines in south and west are higher than that in north and east, with a typical latitudinal zonality and longitudinal zonality; Snowline distribution has great regional differences, of which the high and low snowline elevation interlaced inside the plateau.5) From2001to2011, changing of the average, highest and lowest value of snowline elevation appears to be wavy. The average elevation for the eleven years maintain at about5000m and has a small fluctuation, of which the increasing range is less than100m; while the highest elevation and lowest elevation has a big fluctuation, of which the biggest change range are348m and319m respectively.