Xiaohua Gou

The soil moisture distribution, thawing–freezing processes and their effects on the seasonal transition on the Qinghai–Xizang (Tibetan) plateau

Abstract The soil energy–water distribution and freezing–thawing processes varied at different sites in the northern part of the Qinghai–Xizang (Tibetan) Plateau. The temporal and spatial variations of the soil moisture content were more complex than those of temperature. At the observation site, the soil moisture content increases with depth in certain layers but decreases in other layers. The freezing/thawing processes and the temperature distribution were strongly influenced by soil moisture content. During the summer monsoon period, the soil moisture contents at a depth of 10xa0cm for all sites were relatively high but differed spatially. In general, the shallow layers started to freeze in October and thaw in April at all sites, with a freezing period of about 6 months. However, the onset of freezing/thawing varied at different sites. The results suggest that the freezing process may prevent the soil moisture from evaporating. Although there is low precipitation in winter (freezing period), the soil moisture content is high when the soil begins to thaw. The abrupt increase/decrease of net atmospheric heating synchronized with the thawing/freezing of soil indicates that these processes may strongly influence the seasonal transition on the Qinghai–Xizang (Tibetan) Plateau.

Water Recycling between the Land Surface and Atmosphere on the Northern Tibetan Plateau—A Case Study at Flat Observation Sites

ABSTRACT High-resolution soil moisture, temperature, and precipitation data from the northern part of the Tibetan Plateau provide the basis for analyzing the cycling of water between the land surface and atmosphere. Data analyzed come from the Intensive Observation Period (IOP) of the GEWEX (Global Energy and Water Cycle Experiment) Asian Monsoon Experiment (GAME) on the Tibetan Plateau (GAME-Tibet). Observations from July to August 1998 show that evaporation from flat land surfaces was 177 mm on the south side of the Tanggula Mountains, and 73 mm on the north side. These represent about 73% and 58%, respectively, of the precipitation in the same period. Evaporation not only transports considerable water but also considerable energy from the land surface to the atmosphere, which can slow the rising of soil temperatures. Differences in the evaporation between the south and north sides of Tanggula Mountains is mainly caused by differences in precipitation.

Precipitation Distribution along the Qinghai-Xizang (Tibetan) Highway, Summer 1998

Geographic variations in summer (July–August) precipitation data for the northern Qinghai-Xizang (Tibetan) Plateau, collected during the IOP (Intense Observation Period) of GAME-Tibet, were examined. Results indicated that, basically, the daily precipitation amount consistently varies temporally at all sites. An increase in precipitation amount is evident from north to south. The distribution of summer monsoon precipitation is affected by latitude, altitude, and other factors (i.e. atmosphere circulation, water vapor transportation). The latitude effect of precipitation in July and August was significant. August precipitation varied with both latitude and altitude. The site with higher elevation received more precipitation. Topoclimatic controls operating over precipitation on slopes include elevation. The distribution of the precipitation during July–August mainly showed the effects of latitude, but this was modified by the altitude effect. Redistribution processes caused by topography induce increased precipitation with altitude at some sites. Owing to the complexity of surface conditions on the Qinghai-Xizang (Tibetan) Plateau, precipitation is controlled by many factors simultaneously. Without benefit of an expanded observation network, it is difficult to separate the effects of the many factors influencing the distribution of precipitation in the region.

The spatially heterogeneous distribution of precipitation of the Anduo area, Tibetan Plateau, in summer 1998

Abstract Examination of precipitation in the Tibetan Plateau is important to understanding the regional water cycle processes and the plateau-scale energy budget. Based on hourly precipitation data obtained during the GAME-Tibet intensive observation period at four sites, the spatial distribution of precipitation in summer 1998 within the Anduo area was examined. The results show that, between 1 July and 11 September 1998, the precipitation that occurred simultaneously (at the same hours) at the sites accounted for 6.9–15.3% of the total precipitation at each site during the study period. Even at the two observation sites that are only 20 km apart, the percentage of precipitation that occurred simultaneously was quite small. This indicates that precipitation occurred not only frequently but also very locally, except on several days with very strong monsoon precipitation. The limited observations highlight that the precipitation distribution is quite complex, and large-scale intensive precipitation observations are needed in the future to clarify the heterogeneity of precipitation on the Tibetan Plateau.

Correlation between precipitation and temperature variations in the past 300 years recorded in Guliya ice core, China

Abstract The Guliya ice cap, on the crest of the Kunlun Shan, central Asia, is an ideal site for acquiring ice cores for climate-change studies. Detailed analyses of the precipitation index (glacier accumulation) and the temperature proxy (δ18O) recorded in the Guliya ice core since 300 years BP show that precipitation correlates with temperature in this region. Climate conditions in the Guliya region since 300 years BP can be separated into three periods: warm and wet from AD 1690 to the end of the 18th century; cold and dry from the 19th century to the 1930s; and warm and wet again since the 1940s. During this period, the climate exhibits just two phases: warm/wet and cold/dry. Comparison of the temperatures and the precipitation recorded in the Guliya ice core shows that variations of temperature and precipitation in the region correlate quite well. However, changes in the precipitation regime appear to lag behind those of the temperature by 20–40 years. We believe this results from the larger heat capacity of the ocean relative to that of the land. Hence, ocean temperatures and corresponding evaporation rates change more slowly than do continental conditions. Additionally, however, positive feedback processes, such as increasing temperatures and precipitation improving vegetation, moisture retention and, hence, local convective precipitation probably play an important role. In this paper, we explain how the timescale of evolving vegetation and the feedback mechanism between precipitation and the temperature could help explain why the changes in precipitation lag those of temperature by 20–40 years over long periods. Taking this time lag into account, we should be able to predict future precipitation trends, based on observed temperature trends.

Effect of heavy snowfall on ground temperature, northern Tibetan Plateau

Abstract Very heavy snowfall occurred in the Amdo-Nagqu region during winter 1997/98, and enormous numbers of sheep and yaks died due to starvation and low temperatures. Some observation sites of the GEWEX (Global Energy and Water Cycle Experiment) Asian Monsoon Experiment (GAME)-Tibet are located in this area. In this paper, the variation of the ground temperature (GT) on the northern part of the Tibetan Plateau and its relationship with the heavy snow cover is analyzed based on the GAME-Tibet in situ observational data at several sites. The temporal and spatial differences of the variations of the daily maximum, daily minimum and range in GT are significant in 1997/98 in the northern part of the Tibetan Plateau. For example, at site D110, the daily range in GT fluctuated only 0.2˚C from the end of December 1997 to mid-April 1998, but in the north, at site D66, the daily range in GT fluctuated between 5˚C and ∼20˚C at the same depth and during the same period. At the southernmost site, MS3637, the daily range in GT fluctuated within 1.0˚C from mid-November to early February. From mid-February to mid-March, the daily range in GT increased and the peak was 8.1 ˚C. The temperature variation was related to the heavy snowfall that occurred on the northern Tibetan Plateau in winter 1997/98. The snow-cover conditions at different sites on the northern Tibetan Plateau were evaluated quantitatively from the variation of the GT at shallow depths.

Wavelet analysis reveals periodic oscillations in a 1700 year ice-core record from Guliya, China

Abstract Ice cores contribute important records of past climate changes. As one of the thickest ice caps in central Asia, the Guliya ice cap (35°17′ N, 81°29′ E) provides valuable information for this critical region about the past climate and environment changes. We used wavelet analysis to examine periodic temperature and precipitation oscillations over the past 1700 years recorded in the Guliya ice core. The results show non-linear oscillations in the ice-core records, with multiple timescales. Temperature records indicate persistent oscillations with periodicities of approximately 200, 150 and 70 years. Precipitation records show significant periodicities at 200, 100, 150 and 60 years. However, the amplitude modulation and frequency vary with time. Wavelet analysis can explore these time series in greater detail and furnish additional useful information.