Lan Cuo

Evaluation of the Global Climate Models in the CMIP5 over the Tibetan Plateau

AbstractThe performance of 24 GCMs available in the fifth phase of the Coupled Model Intercomparison Project (CMIP5) is evaluated over the eastern Tibetan Plateau (TP) by comparing the model outputs with ground observations for the period 1961–2005. The twenty-first century trends of precipitation and temperature based on the GCMs’ projections over the TP are also analyzed. The results suggest that for temperature most GCMs reasonably capture the climatological patterns and spatial variations of the observed climate. However, the majority of the models have cold biases, with a mean underestimation of 1.1°–2.5°C for the months December–May, and less than 1°C for June–October. For precipitation, the simulations of all models overestimate the observations in climatological annual means by 62.0%–183.0%, and only half of the 24 GCMs are able to reproduce the observed seasonal pattern, which demonstrates a critical need to improve precipitation-related processes in these models. All models produce a warming tre...

A Review of Quantitative Precipitation Forecasts and Their Use in Short- to Medium-Range Streamflow Forecasting

AbstractUnknown future precipitation is the dominant source of uncertainty for many streamflow forecasts. Numerical weather prediction (NWP) models can be used to generate quantitative precipitation forecasts (QPF) to reduce this uncertainty. The usability and usefulness of NWP model outputs depend on the application time and space scales as well as forecast lead time. For streamflow nowcasting (very short lead times; e.g., 12 h), many applications are based on measured in situ or radar-based real-time precipitation and/or the extrapolation of recent precipitation patterns. QPF based on NWP model output may be more useful in extending forecast lead time, particularly in the range of a few days to a week, although low NWP model skill remains a major obstacle. Ensemble outputs from NWP models are used to articulate QPF uncertainty, improve forecast skill, and extend forecast lead times. Hydrologic prediction driven by these ensembles has been an active research field, although operational adoption has lagge...

Assessing the Impacts of Global Warming on Snowpack in the Washington Cascades

Abstract The decrease in mountain snowpack associated with global warming is difficult to estimate in the presence of the large year-to-year natural variability in observations of snow-water equivalent (SWE). A more robust approach for inferring the impacts of global warming is to estimate the temperature sensitivity (λ) of spring snowpack and multiply it by putative past and future temperature rises observed across the Northern Hemisphere. Estimates of λ can be obtained from (i) simple geometric considerations based on the notion that as the seasonal-mean temperature rises by the amount δT, the freezing level and the entire snowpack should rise by the increment δT/Γ, where Γ is the mean lapse rate; (ii) the regression of 1 April SWE measurements upon mean winter temperatures; (iii) a hydrological model forced by daily temperature and precipitation observations; and (iv) the use of inferred accumulated snowfall derived from daily temperature and precipitation data as a proxy for SWE. All four methods yiel...

Changes in Moisture Flux over the Tibetan Plateau during 1979-2011 and Possible Mechanisms

AbstractChanges in moisture as represented by P − E (precipitation − evapotranspiration) and the possible causes over the Tibetan Plateau (TP) during 1979–2011 are examined based on the Global Land Data Assimilation Systems (GLDAS) ensemble mean runoff and reanalyses. It is found that the TP is getting wetter as a whole but with large spatial variations. The climatologically humid southeastern TP is getting drier while the vast arid and semiarid northwestern TP is getting wetter. The Clausius–Clapeyron relation cannot be used to explain the changes in P − E over the TP.Through decomposing the changes in P − E into three major components—dynamic, thermodynamic, and transient eddy components—it is noted that the dynamic component plays a key role in the changes of P − E over the TP. The thermodynamic component contributes positively over the southern and central TP whereas the transient eddy component tends to reinforce (offset) the dynamic component over the southern and parts of the northern TP (central T...

Climate Change on the Northern Tibetan Plateau during 1957–2009: Spatial Patterns and Possible Mechanisms

AbstractGridded daily precipitation, temperature minima and maxima, and wind speed are generated for the northern Tibetan Plateau (NTP) for 1957–2009 using observations from 81 surface stations. Evaluation reveals reasonable quality and suitability of the gridded data for climate and hydrology analysis. The Mann–Kendall trends of various climate elements of the gridded data show that NTP has in general experienced annually increasing temperature and decreasing wind speed but spatially varied precipitation changes. The northwest (northeast) NTP became dryer (wetter), while there were insignificant changes in precipitation in the south. Snowfall has decreased along high mountain ranges during the wet and warm season. Averaged over the entire NTP, snowfall, temperature minima and maxima, and wind speed experienced statistically significant linear trends at rates of −0.52 mm yr−1 (water equivalent), +0.04°C yr−1, +0.03°C yr−1, and −0.01 m s−1 yr−1, respectively. Correlation between precipitation/wind speed an...

Frozen soil degradation and its effects on surface hydrology in the northern Tibetan Plateau

Frozen soil was simulated at six seasonally frozen and seven permafrost stations over the northern Tibetan Plateau using the Variable Infiltration Capacity (VIC) model for the period of 1962–2009. The VIC model resolved the seasonal cycle and temporal evolution of the observed soil temperatures and liquid soil moisture well. The simulated long-term changes during 1962–2009 indicated mostly positive trends for both soil temperature and soil moisture, and negative trends for soil ice content at annual and monthly time scales, although differences existed among the stations, soil layers, and seasons. Increases in soil temperature were due mainly to increases in daily air temperature maxima and internal soil heat conduction, while decreases in soil ice content were related to the warming of frozen soil. For liquid soil moisture, increases in the cold months can be attributed to increases in soil temperature and enhanced soil ice melt while changes in the warm months were the results of competition between positive precipitation and negative soil temperature effects. Precipitation and liquid soil moisture were strongly correlated with evapotranspiration and runoff but had various degrees of correlations with base flow during May–September. Seasonally frozen stations displayed longer and more active hydrological processes than permafrost stations. Slight enhancement of the surface hydrological processes at the study stations was indicated, due to the combined effects of precipitation changes, which were dominant, and frozen soil degradation.

Changes in Moisture Flux over the Tibetan Plateau during 1979–2011: Insights from a High-Resolution Simulation

Net precipitation [precipitation minus evapotranspiration (P 2 E)] changes between 1979 and 2011 from a high-resolution regional climate simulation and its reanalysis forcing are analyzed over the Tibetan Plateau (TP) and compared to the Global Land Data Assimilation System (GLDAS) product. The high-resolution simulation better resolves precipitation changes than its coarse-resolution forcing, which contributes dominantly to the improvedP 2Echange in the regional simulation compared to the global reanalysis. Hence,the former may provide better insights about the drivers of P 2 E changes. The mechanism behind the P 2 E changes is explored by decomposing the column integrated moisture flux convergence into thermodynamic, dynamic, and transient eddy components. High-resolution climate simulation improves the spatial pattern of P 2 E changes over the best available global reanalysis. High-resolution climate simulation also facilitates new and substantial findings regarding the role of thermodynamics and transient eddies in P 2 E changes reflected in observed changes in major river basins fed by runoff from the TP. The analysis reveals the contrasting convergence/divergence changes between the northwestern and southeastern TP and feedback through latent heat release as an important mechanism leading to the mean P 2 E changes in the TP.

Topographic normalization for improving vegetation classification in a mountainous watershed in Northern Thailand

Land cover classifications are adversely affected by shading or topographic effects in mountainous areas in that the spectral properties of an entity in the shade appear to be different from those of the same entity in a sunlit area. Topographic effects can make it especially difficult to distinguish different successional stages of vegetation. The current work uses a simplified topographic normalization method to reduce the topographic effect and to improve land cover classification in a mountainous watershed in northern Thailand. Data used in the study were two Landsat 7 Enhanced Thematic Mapper Plus (ETM+) images acquired on 5 March 2000 and 7 February 2002, a digital elevation model, and Global Positioning Systems (GPS) ground truth data collected in July 2002 consisting of geographic location (latitude/longitude), feature information and ground reference photographs. A supervised land cover classification was conducted on original and normalized images. In general, the classification accuracy of the different successional stages of vegetation was improved in the normalized images.

A decreasing glacier mass balance gradient from the edge of the Upper Tarim Basin to the Karakoram during 2000-2014

In contrast to the glacier mass losses observed at other locations around the world, some glaciers in the High Mountains of Asia appear to have gained mass in recent decades. However, changes in digital elevation models indicate that glaciers in Karakoram and Pamir have gained mass, while recent laser altimetry data indicate mass gain centred on West Kunlun. Here, we obtain results that are essentially consistent with those from altimetry, but with two-dimensional observations and higher resolution. We produced elevation models using radar interferometry applied to bistatic data gathered between 2011 and 2014 and compared them to a model produced from bistatic data collected in 2000. The glaciers in West Kunlun, Eastern Pamir and the northern part of Karakoram experienced a clear mass gain of 0.043 ± 0.078~0.363 ± 0.065 m w.e. yr−1. The Karakoram showed a near-stable mass balance in its western part (−0.020 ± 0.064 m w.e. yr−1), while the Eastern Karakoram showed mass loss (−0.101 ± 0.058 m w.e. yr−1). Significant positive glacier mass balances are noted along the edge of the Upper Tarim Basin and indicate a decreasing gradient from northeast to southwest.

Effects of Urbanization and Climate Change on Peak Flows over the San Antonio River Basin, Texas

AbstractA thorough understanding of the peak flows under urbanization and climate change—with the associated uncertainties—is indispensable for mitigating the negative social, economic, and environmental impacts from flooding. In this paper, a case study was conducted by applying the Distributed Hydrology Soil Vegetation Model (DHSVM) to the San Antonio River basin (SARB), Texas. Historical and future land-cover maps were assembled to represent the urbanization process. Future climate and its uncertainties were represented by a series of designed scenarios using the Change Factor (CF) method. The factors were calculated by comparing the model ensemble from phase 5 of the Coupled Model Intercomparison Project (CMIP5) with baseline historical climatology during two future periods (2020–49, period 1; 2070–99, period 2). It was found that with urban impervious areas increasing alone, annual peak flows may increase from 601 (period 1) to 885 m3 s−1 (period 2). With regard to climate change, annual peak flows d...