The Freezing Level Height And Its Impact On Variation Of Water Resource In Qilian Mountains During 1979-2012

The freezing level height is a special layer in the troposphere and the lower layer. It reflects the lower troposphere atmosphere temperature condition and is rarely influenced by human activities such as urbanization and heat island effect compared with the measured surface temperature. Recently, the freezing level height widely attracted by global research field. The long-term changes of freezing level heights in Qilian Mountains, northeast Tibetan Plateau from 1979 to 2012 is analyzed using radiosonde data and two global analysis datasets including European Centre for Medium-Range Weather Forecasts reanalysis Interim(ERA) and National Centers for Environmental Prediction/Department of Energy Reanalysis 2(NCEP). The method of spatial difference, MK trend rate, correlation coefficient method and some other methods are applied. The temporal and spatial characteristics of freezing level heights in each season over Qilian Mountain during 1979-2012 are compared and analyzed. The relationship of freezing level height change with snow melt in spring, and the relationship of the freezing level height changes with small glaciers for different scales in summer are discussed. The significant of freezing level height in the cryosphere over mountains region is researched. In addition, Qiyi glacier which monitored records for many years is selected as a reference glacier in this paper. The relationship of the freezing level height with the equilibrium line altitude change and atmospheric glacier mass balance are quantitatively evaluated, and influence of water resources in arid areas is comprehensive assessed. The results showed that:(1)The freezing level heights based on three different datasets in the same season of Qilian Mountain region have the similar spatial distribution, and the reanalysis data based on ERA and NCEP in Qilian Mountain region have the same interval value. The freezing level height from three different datasets all show increasing trend in spring, summer and autumn, but the difference exists, in which significant increasing trend presents in summer for six stations. The trend in freezing level height of 95.2% and 52.3% of grids based on reanalysis including ERA and NCEP are statistically significant at 0.05 levels. The increasing spead of northern area in high altitude glaciers is much faster than south area. The tendency based on ERA is usually better than that in based on NCEP.(2)In resent 34 year, the freezing level height for eight stations in the Qilian mountain region shows increase trend, in which the significant trend presents in Dulan, Dunhuang, Mangya and Jiuquan being statistically significant at 0.05 level, while only decreases by-0.14m·a-1 in Golmud. The freezing level height obtained from different dataset all show increasing trend, and the trend of the freezing level height based on ERA is 5.4m·a-1(p<0.01).The trend of the freezing level height based on the ERA(5.1m per year, p<0.01)is slightly lower than the observation data, and the freezing level height based on NCEP shows the lower trend by 4m per year(p<0.05). Additionally, the annual average freezing level height is usually larger than reanalysis data.(3)The mutation in Dulan and Xining are in 1994, 1996 and 2004, and Mangya is in 1991, 1994 and 2001, while only 1994 and 1996 for Jiuquan and Dunhuang, respectively, and there is no mutation in Golmud, Yuzhong, Minqin and Zhangye. The freeing level height in summer is associated with latitude and altitude, which the decrease trend presents from low altitude to high altitude and the increasing trend appears with the rise of elevation.(4)The relationship between the freeing level height and the surface temperature has significant correlation with elevation of stations, indicating that the freezing level height is better at describing mountain air temperature of the surface in high altitude area and can be considered as meteorological parameters to estimate temperature in mountain regions. The predicated trend in future shows that the freezing level height change of the Qilian Mountains in future would be consistent with that in the past, and most of stations remain strong sustainability.(5)The date of melted snow and the freezing level height in spring has better correlation in the same area, and the freezing level height in the North River Basin in Hexi region increases the fastest and Qaidam river basin shows the slowest speed. For small glaciers, highly sensitive and the relatively rapid warming may lead to their huge melting and loss. In Hexi basin where small glaciers widely distribute, the effects of freezing level height change on the glacier shrinkage cannot be ignored.(6)The free-air freezing level height in summer shows good correlation with the material balance and equilibrium line altitude of glaciers, and most glaciers are statistically significant at the 0.01 level. After freezing level height increasing, the temperature in glacier regions and surroundings increases, but accumulation decreases, and zero equilibrium line increases and the material balance decreases. The free-air freezing level height of mountainous in the Qilian Mountain area is not significantly associated with runoff.