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ABSTRACT Drought is a recurrent natural disaster in northwestern China and it can have a substantial impact on the affected region; therefore, knowledge of drought variability is very important for drought adaptation and mitigation. However, the observed climate data are generally limited in length, which may hamper our understanding the full range of drought variability. Using tree-ring width measurements of 155 series from 81 Pinus tabulaeformis trees at three sites in the northern Min Mountains of northwestern China, the standardized precipitation evapotranspiration index ( SPEI) for May-August from 1804 to 2011 were reconstructed, which account for 64.8% of the variance in the instrumental record over 1954-2011. The new reconstruction is generally consistent with other moisture reconstructions nearby. Two notable features of the SPEI reconstruction is the severe drought starting in the late 1990s and the gradual drying trend since 1954. These features are also apparent in areas located in the north boundaries of Eastern Asian Summer Monsoon ( EASM), which indicates that the study area may be affected by the EASM. [ABSTRACT FROM AUTHOR]

Knowledge of natural long-term drought variability is essential for water resource management and planning, especially in arid and sub-arid regions of the world. In the eastern Qilian Mountains of China, long-term drought variability based on high-resolution proxy records such as tree-ring data are still scarce to date. Here we present a new tree-ring chronology from the eastern Qilian Mountains which provides a valuable 1,002-year record (1009-2010 CE) of drought variability. The new reconstruction of June-July 5-month scale standardized precipitation and evapotranspiration index is the first millennium tree-ring estimate of past climate developed in the eastern Qilian Mountains. The record shows that this region has experienced several persistent droughts and pluvials over the past millennium, with significantly drier climate during the fifteenth century and dramatic wetting since the nineteenth century. The low frequency generally agrees with other nearby studies based on both tree-ring data and other proxy data. [ABSTRACT FROM AUTHOR]

ABSTRACT Studies of regional drought history will be helpful for understanding current hydroclimate variability with global warming as well as predicting future hydroclimate shifts. Long-term tree-ring records are scarce in the western Qilian Mountains of northwestern China, which is also the western boundary of the natural distribution of Qilian Juniper ( Juniperus przewalskii Kom.) in this area. Here, we present an 850-year ( AD 1161-2010) reconstruction of May-July self-calibrating Palmer Drought Severity Index ( scPDSI) in the western Qilian Mountains that is based on two nearly millennia of ring-width chronologies derived from long-lived Qilian Juniper trees. The reconstruction suggests a relatively dry phase from the 15th century to the 18th century during the Little Ice Age ( LIA) and a relatively wet period over the past two centuries. This reconstruction is consistent with other tree-ring-based hydroclimatic reconstructions from the northeastern Tibetan Plateau, as well as the corresponding century-scale solar activity during the LIA. The three mega-drought periods ( AD 1260s-1340s, 1430s-1540s and 1640s-1740s) recorded by the tree-ring series also correspond to the Wolf, Spörer and Maunder solar activity minimum periods. Results of the multi-tape method analysis and wavelet analysis further confirmed the relationship between regional hydroclimate variability and solar activity forcing. [ABSTRACT FROM AUTHOR]

Accurate simulation of thermal and hydrological soil processes is highly important for studying cold region climates. In this paper, two thermal conductivity parameterization schemes for soil proposed by Johansen and Luo were combined with a groundwater parameterization scheme (JN and LN schemes) to modify the soil water and heat parameterization schemes in the Community Land Model (CLM3.5). Land surface processes in the Central Tibetan Plateau were simulated based on the original scheme (Farouki scheme), JN scheme, and LN scheme using the atmospheric forces from meteorological observations at Nameqie, and they were compared with observed data in the same period. We found that all three schemes underestimated the upward shortwave radiation and overestimated the upward longwave radiation and net radiation, but the predictions from the modified scheme better agreed with the observations. The three schemes simulated the average soil temperatures higher than the observation data. The modified schemes, particularly the LN scheme, resulted in better predictions of the soil temperature compared with the original CLM3.5. More thawing days were simulated by three schemes. The observed freezing and thawing proceeded from the upper layers to the lower layers; however, the frozen soil in the Farouki and LN schemes began to thaw in the upper layers and below ~160 cm. The differences in the simulated soil thermal conductivity among the three schemes increased with depth. The soil thermal conductivity simulated by the Farouki scheme was the highest, while the smallest values were generated by the JN scheme. The soil volumetric water content of the Farouki scheme below a 100 cm depth was obviously larger than the observed data. After considering water exchange between the soil column and its underlying aquifer, the JN scheme and, particularly, the LN scheme, simulated the soil volumetric water content slightly higher than but similarly to the observed data. [ABSTRACT FROM AUTHOR]

The effects of different RegCM4 land-surface schemes on Tibetan Plateau (TP) precipitation simulations were investigated. Two groups of ten-year (1992–2001) simulation experiments (hereafter referred to as BATS and CLM) were performed based on two land-surface schemes (BATS and CLM3.5, resp.) and were compared with observed data using the same domain, initial, and lateral boundary conditions, cumulus convective scheme, and spatial resolution. The results showed that the CLM monthly precipitation more closely matched the observed data compared with BATS. BATS and CLM both overestimated summer precipitation in the northern TP but underestimated summer precipitation in the southern TP. However, CLM, because of its detailed land-surface process descriptions, reduced the overestimated precipitation areas and magnitudes of BATS. Compared to CN05, the regional average summer precipitation in BATS and CLM was overestimated by 34.7% and underestimated by 24.7%, respectively. Higher soil moisture, evapotranspiration, and heating effects in the BATS experiment triggered changes in atmospheric circulation patterns over the TP. Moreover, BATS simulated the lower atmosphere as warmer and more humid and the upper atmosphere (~150 hPa) as colder than the CLM simulations; these characteristics likely increased the instability for moist convection and produced more summer precipitation. [ABSTRACT FROM AUTHOR]