УДК 551.578

Stable isotope variations in precipitation: simulations and comparison with observations

(Yunnan Plateau, High Asia)

© 2013 г. X.P. Zhang1, H.D. Guan12, Z.A. Sun3

College of Resources and Environmental Sciences, Hunan Normal University, Changsha 410081, China;

2National Centre for Groundwater Research and Training, Flinders University, Adelaide 5001, AU;

3Centre for Australian Weather and Climate Research, Melbourne 3001, Australia huade.guan@flinders.edu.au

Статья принята к печати 30 апреля 2013 г.

Amount effect, MWL (Meteoric Water Line), simulation, stable isotopes, Yunnan Plateau.

Количественный эффект, моделирование, плато Юннань, стабильные изотопы, MWL (линия метеорных вод).

Stable isotopes in precipitation, both liquid (rain) and solid (snow), can be suitable tracers for hydrological cycles because their concentrations reflect the cumulative record of physical phase changes. Distribution of stable isotopes in precipitation over High-Asian monsoon regions, including Yunnan Plateau is investigated. It has two noticeable features: firstly, the stable isotopes in precipitation distinctly decrease; and secondly, the stable isotopes in precipitation demonstrate smaller concentrations during rainy seasons and higher values during dry seasons. These features were found by the MUGCM simulation developed in the Melbourne Univertisty. Quantitative effect of the stable isotopes in precipitation takes place at different time scales, i.e. in diurnal, monthly or annual variations. Relative to observations, the simulated 518O in precipitation shows stronger dependence on precipitation. In the diurnal course, the simulated regression equations of 518O in precipitation versus precipitation amount are in good agreement with the observed values at Tengchong and Simao, except that the simulated 518O/P curve slope is slightly smaller than the observed one at Mingzi. In the monthly and annual courses both, the simulated and observed 518O/P slopes are smaller than it is in the diurnal course. For individual station, the local meteoric water line (LMWL) is simulated well at Mengzi and Tengchong. However, the simulated result does not reproduce truly the observed relationship between 5D and 518O in precipitation at Simao and Kunming where the LMWL inclination is larger 8.0, and a shift along the y-axis higher 10.0. In addition, all simulated LMWL slopes are higher the observed ones at four stations, suggesting that the GCM can overestimate the decreasing of Hydrogen Deuterium Oxide and, thus, underestimate the second-order parameter, i.e. the deuterium excess, in a particular region Yunnan.


Stable isotopes H218O and HDO (Hydrogen Deuterium Oxide) in precipitation are good tracers for hydro-logical cycles because their concentrations reflect cumulative record of physical phase changes [1-3, 6]. The best method to study isotope variations in precipitation is analysis of field samplings. However, the interpretation of the field data is frequently hampered by the incomplete records, limited number of simultaneous observations with climatic variables, and the short series of field data and thin network of points for samplings.

The only way to reconstruct space and time variations of stable isotopic compositions in water vapor and precipitation is to incorporate the stable isotope cycles into the atmosphere general circulation models (GCM) which simulate the global and regional features of atmospheric dynamics and thermodynamics in more details, with fully detailed hydrological cycles [8, 9]. With its better possibilities than a simple and idealized model, GCM can take into consideration the complexity of

dynamical and microphysical processes leading to formation of individual precipitation event, and also the fact that, on the average, the observed field data (e.g. in the monthly course) are the statistical result of successive precipitation events with various characteristics [10].

Yunnan is located in the Southwest Plateau including the Tibetan Plateau, and it is a part of High-Asian glacial zone where the sources of water vapor generating rainfall are very complicated. Evaporation of waters of the South China Sea, the Bay of the Bengal, the Arabian Sea and wet air flows across equator meet in this area, then they are transported into the middle-channel reaches of the Yangtze River and other East Asian region, and strongly affect the monsoon rainfall in these areas [7]. The Mount Yulong, the southernmost glacier-covered area in Eurasia, including China, is located in the investigated areas. There are 19 sub-tropical temperate glaciers on the mountain, controlled by the southwestern monsoon climate. During the past ten years, a few shallow holes have been drilled in the accumulation area of the largest glacier

1000 1500 2000 2500 3000 4000 5000 m

98° 100° 102° 104° 106°E

Fig. 1. Positions of sampling stations Mengzi, Tengchong, Simao and Kunming in Yunnan, southwest China

Рис. 1. Расположение станций, где велись наблюдения на плато Юннань, юго-восточный Китай: Менгзи, Тенгчонг, Симао и Кунминг

Baishui No. 1. So, studying features of stable isotopes in the water cycle in this region has important for restoration and interpretation of paleo-climatic and paleo-environ-mental records stored in the ice and snow.

The aims of this study are to analyze and compare features of stable isotope variations in precipitation in both simulated and observed data over Yunnan, China, using the MUGCM (The Melbourne University GCM), including amount effect and relationship between 5D and 518O at different time scales; to estimate ability of MUGCM to simulate stable isotopes in precipitation of monsoon regions; to investigate impact of atmospheric physical process on stable isotopic fractionation in the water cycle; to understand the isotope variation in the regional water cycle; and to provide a powerful tool to interpret the representation and climatic significance of stable isotope in precipitation of glaciated areas over High-Asian monsoon regions.

Principal data

Observational data. Data on stable isotope concentrations were obtained by the daily precipitation sampling at 20h BST (Beijing standard time) together with regular weather observations and measurements of surface air temperature and amount of precipitation. The sampling and the observations were carried out at three main national weather stations which are Mengzi (23.23°N, 103.23°E, 1301.7 m a.s.l.), Tengchong (25.10°N, 98.30°E, 1648.7 m a.s.l.) and Simao (22.40°N, 101.24°E, 1302.9 m a.s.l.), during the period from February to December, 2003. These three sampling stations and Kunming (25.10°N, 102.41°E, 1896.8 m a.s.l.) form the basis for this study (see Fig. 1). Totally 117, 139 and 104 water samples were collected during almost one year of samplings, corresponding to 117, 139 and 104 rainfall days, at Mengzi, Tengchong and Simao, respectively.

Kunming is one of Chinese sampling stations included into the global observational network established by International Atomic Energy Agency (IAEA) in cooperation with the World Meteorological Organization (WMO). There is a series of 15-year records of stable isotope obtained from 1986 to 2003 (absent from 1993 to 1995). Mean monthly stable isotope ratios in precipitation and relevant weather data are available from IAEA/WMO.

All precipitation samples collected in Mengzi, Tengchong and Simao were sealed in plastic bottles and kept in a freezing tank, and then measured for their oxygen-18 ratios using the Delta-Plus mass spectrometer at the Key Laboratory of Ice Core and Cold Regions Environments, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences. All these samples at three stations were sorted again in 2006. The water samples higher 2 mm in diurnal precipitation were selected. So, 85 samples at Mengzi, 105 samples at Tengchong, and 72 samples at Simao were obtained. These water samples were brought to the Institute for Hydrospheric-Atmospheric Sciences, Nagoya University for D and 18O measurements using the MAT-252 mass spectrometer. The measured ratio of oxygen-18 in samples 18O/16O (or D/H) is expressed in parts per thousand of their deviation relative to the Vienna standard mean ocean water (V-SMOW). The S18O (or SD) is defined by the following equation:

S18O (or SD) = (Rs/Rv-smow - 1)1000, (1)

where Rs and RV-SMOW represent the isotope ratio 18O/16O (or D/H) in water sample and in V-SMOW respectively. The measurement accuracy is ±0.1 %o (Delta-Plus mass spectrometer) and ±0.2 % (MAT-252 mass spectrometer) for 18O respectively and ±0.5 % (MAT-252 mass spectrometer) for D.

Simulation data. Simulated data were taken from the isotope runs of the MUGCM model developed by Melbourne University. This general circulation model is a spectral primitive equation model of the atmosphere based on the model of Bourke et al [4] and McAvaney et al [12]. For the study reported here MUGCM is configured to have a horizontal resolution denoted by rhomboidal truncation of harmonic series at wave number 21 (R21). To allow quadratic products to be calculated on a transformation grid with a good accuracy, the 3.25°* 5.625° grid points are required. In the vertical, there are 9 discrete levels in hybrid-sigma coordinates. The MUGCM isotope scheme is based on the earlier one implemented in GISS GCM [10], with a semi-Lagrangian moisture transport scheme.

The MUGCM includes also an interactive ocean surface with variable surface isotope ratios as well as formations of isotopes in snow and river runoff [5]. Detailed descriptions of MUGCM incorporating stable water isotope effect may be found in the paper of Noone and Simmonds [13].

Analysis of results

1. Stable isotopes in precipitation on the synoptic time scale

Variations of stable isotopes in precipitation.

Fig. 2, a, c and e show observed diurnal variations of S18O in precipitation and precipitation amounts at M

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