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Abstract: Based on the NCEP/NCAR reanalysis, the long-term changes in free-air freezing level height (FLH) in Northwest China during 1960–2012 were investigated. The results indicated that FLH of Northwest China has increased at a rate of approximate 1.2 gpm (geopotential meters) per year, which is statistically significant at the 0.01 level. Most grid boxes in Northwest China show consistent increasing trends, except the northern Tibetan Plateau. Periodic analyses demonstrate that primary period of FLH in Northwest China is approximately 15 years, and the different climate zones (eastern monsoon region, northwest arid region and Tibetan cold region) show similar periods. FLH is significantly related to surface air temperature during the study period (r = 0.90), especially in northwest arid region. The correlation coefficients are relatively higher in the colder months and lower in the hotter months. Altitude also correlates with mean FLH positively (r = 0.64) and trend magnitude in FLH negatively (r = −0.63). [Copyright &y& Elsevier]

Abstract: Climate extremes in the Tibetan Plateau may cause serious regional and global consequences, but observation-based research over the western Tibetan Plateau is scarce. In this paper, recent changes in extremes of temperature and precipitation over the western Tibetan Plateau from 1973 to 2011 are investigated. A total of 24 indices that represent extreme climate events are selected and calculated by using daily maximum and minimum temperature and precipitation data. Results demonstrate that most cold-related indices of temperature extremes (frost and ice days, cool nights and cool days, and cold spell duration indicators) show a significant decrease, and that both the coldest night and coldest day have increased during the study period. Warm-related indices of temperature extremes such as summer days, the warmest night and the warmest day, warm nights and warm days, and warm spell duration indicators all have increased. The diurnal temperature ranges show a decreasing trend with statistical significance, whereas growing season lengths have increased significantly. The change trends of precipitation extremes are nonsignificant. Nonsignificant increasing trends are detected for most indices including annual total wet-day precipitation, maximum 1-day and 5-day precipitation, very wet and extremely wet day precipitation, and consecutive wet days. However, nonsignificant decreasing trends are found for the number of heavy precipitation days, consecutive dry days and simple daily intensity index. [Copyright &y& Elsevier]

The ecohydrological separation hypothesis has generated considerable scientific interest and debate in ecohydrological studies, and it assumes there exist two water pools in subsurface water, one of which is soil water used by plants and the other is that supplied to groundwater. Although this hypothesis has been tested in several humid sites, a further test in arid and semiarid conditions is still needed. Based on the isotopic ratios in different water bodies collected during a 2‐year field work, the hypothesis was tested in a drier zone located in the Qilian Mountains of the northeastern Tibetan Plateau in order to investigate whether this separation phenomenon existed in a drier climate, and whether this may be a common characteristic or an exception. The results suggested that the hydrological separation does not necessarily exist and may not even be determined by stable isotopes; there is a clear need to more precise experimental methods. [ABSTRACT FROM AUTHOR]

The long-term changes of summer freezing level heights in the Qilian Mountains, northeast Tibetan Plateau from 1979 to 2012 is analyzed by using radiosonde data and two global analysis datasets including European Centre for Medium-Range Weather Forecasts re-analysis-Interim (ERA) and National Centers for Environmental Prediction/Department of Energy Reanalysis 2 (NCEP). The freezing level heights in the Qilian Mountains generally show an increasing trend from high latitude to low latitude as well as from low altitude to high altitude. The trend magnitude of ERA during 1979–2012 is approximately 54 m per decade ( p < 0.01) which is slightly larger than that derived from radiosonde-based series (51 m per decade, p < 0.01), and NCEP shows a relatively lower trend by 40 m per decade ( p < 0.05). The correlation between free-air freezing level height and surface air temperature is greatly related with the altitude of stations, and the correlation coefficients are generally larger in alpine station than those in plain stations. Glaciers in the Hexi drainage basin at the northern slope have experienced larger increasing trend magnitudes, which may significantly impact the regional hydrological process. [ABSTRACT FROM AUTHOR]

Based on the meteorological observations at three sites (4180 m, 4550 m and 5040 m a.s.l.) in Laohugou and the surrounding 7 national meteorological stations, the long-term surface air temperature of the glaciated region in the Qilian Mountains, northeastern Tibetan Plateau during 1957–2013 was reconstructed. The in-situ air temperature observed at 4180 m during 1959–1961 was used to verify the reconstructed temperature series. Several approaches of multiple regressions were applied to select the optimal method in estimating the daily air temperature, including the ordinary least square regression (OLSR), stepwise regression (SR), principal component regression (PCR) and partial least squares regression (PLSR), respectively. According to root mean square error, mean bias error, and correlation coefficient, OLSR is selected as the optimal approach in the study region. The reconstructed daily temperature correlates well with the synchronous observations, and correlation coefficients are higher than 0.96 ( p < 0.01) on an annual basis. All the three sites in the study region have experienced a warming trend, and annual mean temperature has increased by 0.31 C°, 0.35 C° and 0.37 C° per decade at 4180 m, 4550 m and 5040 m ( p < 0.01), respectively. The higher elevations usually have larger trend magnitudes, compared with the lower sites. The positive degree day (PDD, the sum of daily mean temperatures above ice point) has significantly increased by 27.8 C°, 12.8 C°, and 3.2 C° per decade at 4180 m, 4550 m and 5040 m, respectively, which also indicates the continuing shrinkage of glaciers for the study region. [ABSTRACT FROM AUTHOR]

Precipitation in different types has great influence not only on water resources and distribution of annual precipitation in cold regions, but also on the thermal regimes of frozen ground. The long-term variations of the ratios of snow, rain and sleet to precipitation were analyzed in this study. The 44 meteorological stations were selected, which are located on the Qinghai-Tibet Plateau (QTP) including permafrost and seasonal frozen ground regions. The results indicate that the monthly snow/precipitation ratio in permafrost regions is far higher than that in seasonal frozen ground regions, but the monthly rain/precipitation ratio in permafrost regions is lower than that in seasonal frozen ground regions, and the monthly variation of sleet/precipitation ratio is indistinctive. The annual ratios of snow and sleet to precipitation show decreasing trends in both regions, and annual rain/precipitation ratio shows an increasing tendency. The dropping magnitude of annual snow/precipitation ratio in seasonal frozen ground regions is larger than that in permafrost regions. The maximum and minimum annual snow/precipitation ratios in both regions occur in winter and summer, respectively, and the second is in spring, instead, the maximum and minimum annual rain/precipitation ratios appear in summer and winter, respectively. The spatial features of the ratios of snow and rain to precipitation are largely opposite, that is, the low snow/precipitation ratio stations are usually associated with the high rain/precipitation ratio, and prominent seasonal diversities of three ratios can be found from their spatial patterns. In addition, the decreasing (increasing) tendency of snow (rain)/precipitation ratio in spring and autumn is more significant than that in winter and summer, and these stations of snow/precipitation ratio with downward trends are mainly located at the edge of the QTP. The ratios of snow and sleet to precipitation will gradually decrease (increase), and the rain/precipitation ratio will increase (decrease) with the air temperature (altitude) rises, respectively, while there are significant seasonal discrepancies. Moreover, the altitude zone showing remarkable variations of the ratios of snow and rain to precipitation ranges from 3000 to 4000 m a.s.l, especially in spring and summer. [ABSTRACT FROM AUTHOR]