| Snow cover is an important parameter in investigations of climate,surface radiation budget and hydrology cycle.It is crucial to monitor snow cover extent,especially in determining long-term changes in the climate system.The Tibetan Plateau is called "the Roof of the World",".This region is surrounded by mountain ranges such as the Himalayan range and north of Kunlun range.Due to the high elevation,the TP is one of the coldest places on the earth.On the global scale,the long term determination on snow on the TP can support the analysis of the climate of our country and the Asian summer monsoon,even the global changes.On the regional scale,the analysis of the long term data has a great significance for the warter supply of pastoral ecosystem,snow damage forecasting and flood forecasting.The study focused on snow extraction model and ecological application.We an alternative approach with dynamic thresholds to produce snow cover products(1982-2012).Based on these products we tried analyzed the spatial and temporal variability in snow cover in the TP.The results are as follows:(1)Based on LTDR data and digital elevation model we built a snow inversion model.On one hand,by digital elevation model the model eliminates the effect of shadows of mountains.On the other hand,the 8-day composition eliminates the interference of the cloud.The dynamic thresholds were used to solve effect of elevation on the snow surface brightness temperature values.The classification algorithm correctly identified the snow class at all stations in 93.9%of the cases.The classification quality reached a very good level(K=0.765).(2)On the basis of completed snow products,we analyzed the monthly snow variation on the TP.For the entire TP,the monthly mean SCA exhibited a bimodal distribution,with the maximum cover occurred in(29.4%)in March,and the low covers(6.7%and 6.3%)in September.The SCA(snow cover area)shows a steady decreasing trend from March to August and steady increasing trend from September to January(Figure 5).For the spatial variation,the four semiarid ecological zones exhibited the same variation trend,while the humid/semihumid ecological zone shows a clearly time leg.(3)According to YSCA statistic data,we found 1982,1984,1990,1997,1998,2003 and 2007 were abnormal snow cover year which can be associated to the former study(Wei,Huang,Chen et al.,2002).For the entire TP,the high snow cover occurred in 1982,1997 and 2007,vale values happened in 1984,1990,1998 and 2003.We divided the study period into two parts for the analysis over the all nine EGR.Comparing each EGR,we found HID 1,HIC1,HIC2,HIB1 have a good agreement in the variation trend.We calculated annual SCD anomalies and showed the result as two terms according to YSCA variation characteristics.As is depicted in Figure 7,the central areas of the TP were more changeable.This area was distributed around Tanggula mountains(Mts),also four Eco-geographic regions:HIC1,HIB1,the west both HID1 and HIC2.Nyainqtanglha Mts,Himalayas Mts and the west of Kunlun Mts had snow covers in most years,whereas the Qaidam basin and the southern Tibet valley(the deep valley between Himalayas and Gandise)exhibited were snow-free in most years.(4)Based on snow cover onset date snow cover melted date for the completion of the TP,we analyzed 1982-2012 temporal changes in phenology snow phenology,and found that snow phenology did not show zonal characteristics on latitude.Besides,snow cover begins or finishes melting from the hinterland of the Qinghai-Tibet Plateau to other areas.Some ecological zones shows a stronger fluctuation than other ecological zones.(5)Regressed YSCA with air temperature and precipitation,the relativity with temperature was negative in HIC1,HIB1,HIC2 and HIIA/Blin snow-fall season(autumn and winter)and the relativity with precipitation was positive in HIC1,HID3,HIC2,HIB1,HIIC2 and HIIA/B1 in winter while the relationship of both temperature and precipitation is insignificant in spring probably due to relative high temperature in the snow-melt season.However,these strong-relationship regions were mainly located the SRYYL area and the regions where upstreams come down on the TP.This conclusion may have reference meaning for flood prediction.(6)By the comparison of snow distribution characteristics of each ecological zone,the results show:Qinghai-Tibet Plateau Tanggula and its adjacent areas,the two semi-arid ecosystems partition,show the similar interannual fluctuation.These areas also show a significant negatively correlation with temperature and a significant positive correlation with precipitation.It should be pointed that these areas covered the source of the Yangtze River and the Yellow River-the Sanjiangyuan region,and also is the main grazing area,therefore,the research results exhibitd a great benefit for the flood and snow disaster prediction. |
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