Xiaoxin Yang

A First-order Method to Identify Potentially Dangerous Glacial Lakes in a Region of the Southeastern Tibetan Plateau

Abstract Though glacial lake outburst floods have become an urgent issue on the Tibetan Plateau, no standardized methods have been proposed so far to identify and prioritize potentially dangerous glacial lakes (PDGLs). Here, we developed a first-order approach to identify PDGLs in the Boshula Mountain Range, southeastern Tibetan Plateau. Five variables—mother glacier area, distance between lake and glacier terminus, slope between lake and glacier, mean slope of moraine dam, and mother glacier snout steepness—were selected to identify PDGLs on the basis of four criteria we suggested. A fuzzy consistent matrix method was then applied to determine the weight of variables, and characteristic statistical values were used as thresholds to classify each variable. Out of 78 moraine-dammed lakes studied, we identified 8 glacial lakes as potentially very highly dangerous. We also validated our approach with 6 drained glacial lakes inside and outside our study area. Successfully identifying them as potentially very highly and/or highly dangerous lakes demonstrates the validity of the method.

Isotopic Signal of Earlier Summer Monsoon Onset in the Bay of Bengal

AbstractThe onset of the Asian summer monsoon is noticeably controversial, spatially and temporally. The stable oxygen isotope δ18O in precipitation has long been used to trace water vapor source, particularly to capture the summer monsoon precipitation signal. The abrupt decrease of precipitation δ18O in the Asian summer monsoon region closely corresponds to the summer monsoon onset. Two stations have therefore been set up at Guangzhou and Lulang in the East Asian summer monsoon domain to clarify the summer monsoon onset dates. Event-based precipitation δ18O during 2007/08 is much lower at Lulang than at Guangzhou and is attributable mainly to the altitude effect offset by different isotopic compositions in marine moisture sources. The earlier appearance of low δ18Owt at Lulang than at Guangzhou confirms the earlier summer monsoon onset in the Bay of Bengal. Isotopically identified summer monsoon evolutions from precipitation δ18O at both stations are verifiable with NCEP–NCAR reanalysis data, indicating...

Mean sediment residence time in barchan dunes

When a barchan dune migrates, the sediment trapped on its lee side is later mobilized when exposed on the stoss side. Then sand grains may undergo many dune turnover cycles before their ejection along the horns, but the amount of time a sand grain contributes to the dune morphodynamics remains unknown. To estimate such a residence time, we analyze sediment particle motions in steady state barchans by tracking individual cells of a 3-D cellular automaton dune model. The overall sediment flux may be decomposed into advective and dispersive fluxes to estimate the relative contribution of the underlying physical processes to the barchan shape. The net lateral sediment transport from the center to the horns indicates that dispersion on the stoss slope is more efficient than the convergent sediment fluxes associated with avalanches on the lee slope. The combined effect of these two antagonistic dispersive processes restricts the lateral mixing of sediment particles in the central region of barchans. Then, for different flow strengths and dune sizes, we find that the mean residence time of sediment particles in barchans is equal to the surface of the central longitudinal dune slices divided by the input sand flux. We infer that this central slice contains most of the relevant information about barchan morphodynamics. Finally, we initiate a discussion about sediment transport and memory in the presence of bed forms using the advantages of the particle tracking technique.

The Indian monsoonal influence on altitude effect of δ 18 O in surface water on southeast Tibetan Plateau

The altitude effect of δ18O is essential for the study of the paleo-elevation reconstruction and possible to be solved through modern process studies. This study presents new δ18O results from southeast Tibetan Plateau along two transects, the Zayu transect and the Lhasa-Nyang transect, with δ18O data from June to September representative of monsoon period and δ18O data during the rest of the year of non-monsoon period. Altitude effect outweighs the longitude and latitude effects in determining regional δ18O variation spatially. Relevant δ18O data from previous studies in the nearby region have also been combined to comprehensively understand the influence of different moisture sources on δ18O from local scale to regional scale. The δ18O in surface water in the southeast Tibetan Plateau and its nearby regions influenced by the Indian summer monsoon shows that single dominant moisture source or simple moisture sources lead to smaller altitudinal lapse rate, whilst growing contributions from local convection to precipitation enlarge δ18O-altitude rate. It thereupon reveals the significance of the Indian summer monsoon to the altitude effect of δ18O in surface water, and the complicated effect of local convection or westerlies evolution to the variation of altitudinal lapse rate. Paleo-monsoon evolution therefore should be considered when altitude effect is applied to paleo-elevation reconstruction for the Tibetan Plateau.