| 1. |
- Bai, M. W., et al.
(författare)
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Different responses of the radial growth of the planted and natural forests to climate change in humid subtropical China
- 2020
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Ingår i: Geografiska Annaler Series a-Physical Geography. - : Informa UK Limited. - 0435-3676 .- 1468-0459. ; 102:3, s. 235-246
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Tidskriftsartikel (refereegranskat)abstract
- China holds the world's largest area of planted forests, which play a crucial role in mitigating climate change. Knowledge gaps remain on the responses of the growth of planted forests to climate changes in China, particularly for their long-term changes. This study compared the long-term tree-ring growth patterns and their responses to climate between planted and natural forests. We developed four chronologies from two typical conifer species ofPinus massonianaandCunninghamia lanceolatafrom planted and natural forests in humid subtropical China. One chronology is to our knowledge the longest chronology from planted forests in China spanning since 1944. Tree growths of both natural and planted forests are generally limited by summer drought due to the hot and dry summer in this area. In general, tree rings of the natural forests are more sensitive to climate than planted forests as indicated by higher inter-series correlations, which agree with previous studies. The natural forests are more drought sensitive wheareas the planted forests are relatively more limited by temperature. TheC. lanceolataforests showed more significant correlations with the accumulated drought thanP. massonianafor both the natural and planted forests.
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| 2. |
- Chen, D., et al.
(författare)
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A tree-ring delta O-18 based reconstruction of East Asia summer monsoon over the past two centuries
- 2020
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Ingår i: PLoS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 15:6
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Tidskriftsartikel (refereegranskat)abstract
- The East Asian summer monsoon (EASM) exhibits considerable decadal variations since the late 20th century. Efforts to examine long-term behaviors and dynamics of the EASM are impeded largely due to the shortness of instrumental meteorological records. So far, reconstructions of the EASM with annual resolution from its core regions remain limited. We conduct the first 200-year robust EASM reconstruction based on tree-ring cellulose delta O-18 records derived from Pinus massoniana trees growing in the middle Yangtze River basin, one of the core EASM areas. The delta O-18 chronology accounts for 46.2% of the actual variation in an index of the EASM from 1948 to 2014. The reconstructed EASM indicates that the monsoon intensity was below average before the 1950s, peaked in the 1950s-1970s, and then began to decline. The reconstructed EASM is negatively correlated with the El Nino-Southern Oscillation (ENSO), but this teleconnection is dynamic through time, i.e. enhanced (reduced) ENSO variability coheres with strong (weak) EASM-ENSO connections. In addition, despite high ENSO variability since the 1980s, the EASM-ENSO relationship weakened possibly due to anthropogenic impact, particularly aerosol emissions.
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| 3. |
- Chen, Deliang, 1961, et al.
(författare)
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Hydroclimate changes over Sweden in the twentieth and twenty-first centuries: a millennium perspective
- 2021
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Ingår i: Geografiska Annaler: Series A, Physical Geography. - : Informa UK Limited. - 0435-3676 .- 1468-0459. ; 103:2, s. 103-31
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Tidskriftsartikel (refereegranskat)abstract
- Climate change poses additional risks for natural and human systems including the hydrological cycle, leading to altered temporal and spatial variation of hydroclimatic conditions. This work assessed the current understanding of the dryness and wetness conditions in Sweden over the last two millenniums based on proxy and instrumental data, as well as climate model simulations and projections until the end of the twenty-first century. The assessment represents a summary of the existing literature concerning analysis of four selected indices for dry/wet conditions relating to precipitation, potential evapotranspiration (PET), and soil moisture (SPEI, PDSI [including scPDSI], SPI, and AI). SPEI considers both precipitation and PET and can show hydroclimatic conditions at different time scales. Therefore, it was chosen to summarize the past and future changes. A focus is put on dry conditions, as drought has strong influences on groundwater which is an important freshwater resource for Sweden. The millennium historical perspective reveals that the current climate is relatively wet and the future would become even wetter as a general wetting trend started some 120 years ago. However, there have been and will be large variations of both dry and wet conditions on short time scales, especially on decadal and interannual time scales. Further, the changes since the 1950s show a regional pattern with most significant wetting in the north, a slightly overall wetting in the south but a drying in central-eastern part including the island of Gotland since 1981. This pattern is broadly consistent with climate model projections for the future.
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| 4. |
- Chen, Deliang, 1961, et al.
(författare)
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Summary of a workshop on extreme weather events in a warming world organized by the Royal Swedish Academy of Sciences
- 2020
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Ingår i: Tellus Series B-Chemical and Physical Meteorology. - : Stockholm University Press. - 1600-0889 .- 0280-6509. ; 72:1
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Tidskriftsartikel (refereegranskat)abstract
- Climate change is not only about changes in means of climatic variables such as temperature, precipitation and wind, but also their extreme values which are of critical importance to human society and ecosystems. To inspire the Swedish climate research community and to promote assessments of international research on past and future changes in extreme weather events against the global climate change background, the Earth Science Class of the Royal Swedish Academy of Sciences organized a workshop entitled 'Extreme weather events in a warming world' in 2019. This article summarizes and synthesizes the key points from the presentations and discussions of the workshop on changes in floods, droughts, heat waves, as well as on tropical cyclones and extratropical storms. In addition to reviewing past achievements in these research fields and identifying research gaps with a focus on Sweden, future challenges and opportunities for the Swedish climate research community are highlighted.
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| 5. |
- Chen, Shiyin, et al.
(författare)
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Tree-ring recorded variations of 10 heavy metal elements over the past 168 years in southeastern China
- 2021
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Ingår i: Elementa: Science of the Anthropocene. - : University of California Press. - 2325-1026. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- Heavy metal pollution is a serious concern in the urban area of China. Understanding metal pollution history is crucial for setting up appropriate measures for pollution control. Herein, we report a record of concentrations of 10 heavy metals (Fe, Mn, Cu, Zn, Ni, Cr, Cd, Pb, Co, and Sr) in Pinus massoniana tree rings from Fuzhou City over the past 168 years, which represents the longest tree-ring chronology of heavy metals in China. The studied metals displayed contrasting distribution patterns. Among them, Mn and Sr showed the strongest migration trend with peak concentrations at the pith. Co, Cd, and Pb also showed distinctively high concentrations near the boundary between heartwood and sapwood. Ni, Cu, Cr, and Fe showed an increasing trend possibly due to migration toward bark caused by physiological activities and increasing tourism activities and traffic pollution. The other elements (Cr, Fe, and Zn) with low migration revealed the historical pollution possibly discharged by the Fuzhou Shipping Bureau and other anthropogenic activities. Strong correlations between Cu content and temperature were found, which provides an alternative tree-ring proxy for climate reconstruction. This study provides a long-term perspective of the joint impacts of physiological, environmental, and climatological factors on the concentrations of heavy metals in southeastern China.
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| 6. |
- Collier, Emily, et al.
(författare)
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The first ensemble of kilometer-scale simulations of a hydrological year over the third pole
- 2024
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Ingår i: Climate Dynamics. - 0930-7575 .- 1432-0894.
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Tidskriftsartikel (refereegranskat)abstract
- An accurate understanding of the current and future water cycle over the Third Pole is of great societal importance, given the role this region plays as a water tower for densely populated areas downstream. An emerging and promising approach for skillful climate assessments over regions of complex terrain is kilometer-scale climate modeling. As a foundational step towards such simulations over the Third Pole, we present a multi-model and multi-physics ensemble of kilometer-scale regional simulations for the hydrological year of October 2019 to September 2020. The ensemble consists of 13 simulations performed by an international consortium of 10 research groups, configured with a horizontal grid spacing ranging from 2.2 to 4km covering all of the Third Pole region. These simulations are driven by ERA5 and are part of a Coordinated Regional Climate Downscaling EXperiment Flagship Pilot Study on Convection-Permitting Third Pole. The simulations are compared against available gridded and in-situ observations and remote-sensing data, to assess the performance and spread of the model ensemble compared to the driving reanalysis during the cold and warm seasons. Although ensemble evaluation is hindered by large differences between the gridded precipitation datasets used as a reference over this region, we show that the ensemble improves on many warm-season precipitation metrics compared with ERA5, including most wet-day and hour statistics, and also adds value in the representation of wet spells in both seasons. As such, the ensemble will provide an invaluable resource for future improvements in the process understanding of the hydroclimate of this remote but important region.
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| 7. |
- Dahl, Martin, 1984-, et al.
(författare)
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A 2,000-Year Record of Eelgrass (Zostera marina L.) : Colonization Shows Substantial Gains in Blue Carbon Storage and Nutrient Retention
- 2024
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Ingår i: Global Biogeochemical Cycles. - : John Wiley & Sons. - 0886-6236 .- 1944-9224. ; 38:3
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Tidskriftsartikel (refereegranskat)abstract
- Assessing historical environmental conditions linked to habitat colonization is important for understanding long-term resilience and improving conservation and restoration efforts. Such information is lacking for the seagrass Zostera marina, an important foundation species across cold-temperate coastal areas of the Northern Hemisphere. Here, we reconstructed environmental conditions during the last 14,000 years from sediment cores in two eelgrass (Z. marina) meadows along the Swedish west coast, with the main aims to identify the time frame of seagrass colonization and describe subsequent biogeochemical changes following establishment. Based on vegetation proxies (lipid biomarkers), eelgrass colonization occurred about 2,000 years ago after geomorphological changes that resulted in a shallow, sheltered environment favoring seagrass growth. Seagrass establishment led to up to 20- and 24-fold increases in sedimentary carbon and nitrogen accumulation rates, respectively. This demonstrates the capacity of seagrasses as efficient ecosystem engineers and their role in global change mitigation and adaptation through CO2 removal, and nutrient and sediment retention. By combining regional climate projections and landscape models, we assessed potential climate change effects on seagrass growth, productivity and distribution until 2100. These predictions showed that seagrass meadows are mostly at risk from increased sedimentation and hydrodynamic changes, while the impact from sea level rise alone might be of less importance in the studied area. This study showcases the positive feedback between seagrass colonization and environmental conditions, which holds promise for successful conservation and restoration efforts aimed at supporting climate change mitigation and adaptation, and the provision of several other crucial ecosystem services. © 2024. The Authors.
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| 8. |
- Fang, Keyan, et al.
(författare)
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ENSO modulates wildfire activity in China
- 2021
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- China is a key region for understanding fire activity and the drivers of its variability under strict fire suppression policies. Here, we present a detailed fire occurrence dataset for China, the Wildfire Atlas of China (WFAC; 2005-2018), based on continuous monitoring from multiple satellites and calibrated against field observations. We find that wildfires across China mostly occur in the winter season from January to April and those fire occurrences generally show a decreasing trend after reaching a peak in 2007. Most wildfires (84%) occur in subtropical China, with two distinct clusters in its southwestern and southeastern parts. In southeastern China, wildfires are mainly promoted by low precipitation and high diurnal temperature ranges, the combination of which dries out plant tissue and fuel. In southwestern China, wildfires are mainly promoted by warm conditions that enhance evaporation from litter and dormant plant tissues. We further find a fire occurrence dipole between southwestern and southeastern China that is modulated by the El Nino-Southern Oscillation (ENSO). Fire activity in China and its associations with climate are not well quantified at a local scale. Here, the authors present a detailed fire occurrence dataset for China and find a dipole fire pattern between southwestern and southeastern China that is modulated by the El Nino-Southern Oscillation (ENSO).
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| 9. |
- He, M. S., et al.
(författare)
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A delayed modulation of solar ultraviolet radiation on the COVID-19 transmission reflects an incubation period
- 2022
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Ingår i: Meteorological Applications. - : Wiley. - 1350-4827 .- 1469-8080. ; 29:5
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Tidskriftsartikel (refereegranskat)abstract
- Laboratory experiments have revealed the meteorological sensitivity of the coronavirus disease 2019 (COVID-19) virus. However, no consensus has been reached about how outdoor meteorological conditions modulate the virus transmission as it is also constrained by non-meteorological conditions. Here, we identify the outbreak's evolution stage, constrained least by non-meteorological conditions, by searching the maximum correlation coefficient between the ultraviolet flux and the growth rate of cumulative confirmed cases at the country level. At this least-constrained stage, the cumulative cases count around 1300-3200, and the count's daily growth rate correlates with the ultraviolet flux and temperature significantly (correlation coefficients r = -0.54 +/- 0.09 and -0.39 +/- 0.10 at p<0.01$$ p, respectively), but not with precipitation, humidity, and wind. The ultraviolet correlation exhibits a delay of about 7 days, providing a meteorological measure of the incubation period. Our work reveals a seasonality of COVID-19 and a high risk of a pandemic resurgence in winter, implying a need for seasonal adaption in public policies.
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| 10. |
- Kukulies, Julia, et al.
(författare)
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Mesoscale convective systems in the third pole region: Characteristics, mechanisms and impact on precipitation
- 2023
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Ingår i: Frontiers in Earth Science. - 2296-6463. ; 11
-
Forskningsöversikt (refereegranskat)abstract
- The climate system of the Third Pole region, including the (TP) and its surroundings, is highly sensitive to global warming. Mesoscale convective systems (MCSs) are understood to be a vital component of this climate system. Driven by the monsoon circulation, surface heating, and large-scale and local moisture supply, they frequently occur during summer and mostly over the central and eastern TP as well as in the downstream regions. Further, MCSs have been highlighted as important contributors to total precipitation as they are efficient rain producers affecting water availability (seasonal precipitation) and potential flood risk (extreme precipitation) in the densely populated downstream regions. The availability of multi-decadal satellite observations and high-resolution climate model datasets has made it possible to study the role of MCSs in the under-observed TP water balance. However, the usage of different methods for MCS identification and the different focuses on specific subregions currently hamper a systematic and consistent assessment of the role played by MCSs and their impact on precipitation over the TP headwaters and its downstream regions. Here, we review observational and model studies of MCSs in the TP region within a common framework to elucidate their main characteristics, underlying mechanisms, and impact on seasonal and extreme precipitation. We also identify major knowledge gaps and provide suggestions on how these can be addressed using recently published high-resolution model datasets. Three important identified knowledge gaps are 1) the feedback of MCSs to other components of the TP climate system, 2) the impact of the changing climate on future MCS characteristics, and 3) the basin-scale assessment of flood and drought risks associated with changes in MCS frequency and intensity. A particularly promising tool to address these knowledge gaps are convection-permitting climate simulations. Therefore, the systematic evaluation of existing historical convection-permitting climate simulations over the TP is an urgent requirement for reliable future climate change assessments.
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| 11. |
- Lai, Hui-Wen, et al.
(författare)
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Regionalization of seasonal precipitation over the Tibetan plateau and associated large-scale atmospheric systems
- 2021
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 34:7, s. 2635-2651
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Tidskriftsartikel (refereegranskat)abstract
- Precipitation over the Tibetan Plateau (TP) has major societal impacts in South and East Asia, but its spatiotemporal variations are not well understood, mainly because of the sparsely distributed in situ observation sites. With the help of the Global Precipitation Measurement satellite product IMERG and the ERA5 dataset, distinct precipitation seasonality features over the TP were objectively classified using a self-organizing map algorithm fed with 10-day averaged precipitation from 2000 to 2019. The classification reveals three main precipitation regimes with distinct seasonality of precipitation: the winter peak, centered at the western plateau; the early summer peak, found on the eastern plateau; and the late summer peak, mainly located on the southwestern plateau. On a year-to-year basis, the winter peak regime is relatively robust, whereas the early summer and late summer peak regimes tend to shift mainly between the central and northern TP but are robust in the eastern and southwestern TP. A composite analysis shows that the winter peak regime experiences larger amounts of precipitation in winter and early spring when the westerly jet is anomalously strong to the north of the TP. Precipitation variations in the late summer peak regime are associated with intensity changes in the South Asian high and Indian summer monsoon. The precipitation in the early summer peak regime is correlated with the Indian summer monsoon together with anticyclonic circulation over the western North Pacific. The results provide a basic understanding of precipitation seasonality variations over the TP and associated large-scale conditions.
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| 12. |
- Lin, Changgui, 1985, et al.
(författare)
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Summer afternoon precipitation associated with wind convergence near the Himalayan glacier fronts
- 2021
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Ingår i: Atmospheric Research. - : Elsevier BV. - 0169-8095. ; 259
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Tidskriftsartikel (refereegranskat)abstract
- Little is known about the effects of glacier-air interactions on the Himalayan glacier mass balance. Until this knowledge gap is filled, a reliable projection of the future changes in the Himalayan glaciers is hardly possible. Here, we describe the drying effect of the katabatic winds on the up-valley summer monsoon flows by creating favorable conditions for local convergence-induced precipitation to occur near the glacier fronts. We postulate that this retarding effect on the up-valley monsoon flows results in a negative feedback mechanism mediated by glacier-air interactions, in which glacial retreat pushes precipitation upwards as the down-valley katabatic winds weaken, resulting in greater local precipitation and enhanced snow accumulation across the upper parts of the Himalayan glaciers. Our analyses are based on the exclusive data recorded in the Khumbu valley and the Langtang valley in the Nepalese Himalayas. These data revealed higher afternoon precipitation in summer associated with surface wind convergence near the glacier fronts and a sharp decrease in the temperature lapse rate over the glacier surfaces. The principle of the observed phenomena was proven by our high-resolution modeling sensitive experiment, which involved two simulations, one with the present glaciers and the other without. This numerical experiment also supports the proposed negative feedback. Furthermore, we report a low deuterium excess near the glacier fronts, indicating below-cloud re-evaporation facilitated by the local convergence induced by the dry katabatic winds. Our study suggests that current models may overestimate the retreat of Himalayan glaciers because they have completely ignored the glacier-air interactions. © 2021 The Author(s)
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| 13. |
- Lin, Q., et al.
(författare)
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Performance of the WRF Model at the Convection-Permitting Scale in Simulating Snowfall and Lake-Effect Snow Over the Tibetan Plateau
- 2023
-
Ingår i: Journal of Geophysical Research-Atmospheres. - 2169-897X. ; 128:16
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Tidskriftsartikel (refereegranskat)abstract
- This study investigated the performance of the Weather Research Forecasting (WRF) model at 4-km horizontal grid spacing in simulating precipitation, 2 m air temperature (T2), snowfall, and lake-effect snow (October 4-8, 2018) over the Tibetan Plateau (TP). Multiple simulations with different physical parameterization schemes (PPSs), including two planetary boundary layer schemes (Yonsei University and Mellor-Yamada-Janjic), no cumulus and multi-scale Kain-Fritsch, two land surface models (Noah and Noah-MP), and two microphysics schemes (Thompson and Milbrandt), were conducted and compared. Compared with gauge observations, all PPSs simulate mean daily precipitation with mean relative errors (MREs) of 27.7%-53.6%. Besides, spatial correlation coefficients (SCCs) between simulated and observed mean daily precipitation range from 0.56 to 0.71. For simulations of T2, all PPSs perform similarly well, even though the mean cold biases are up to about 3 degrees C. Meanwhile, all PPSs exhibit acceptable performance in simulating spatial distributions of snow depth, snow cover, and snowfall amount, with SCCs of 0.37-0.65 between simulations and observations. However, the WRF simulations significantly overestimate snow depth (similar to 0.4 cm mean error) and snowfall amount (MREs >372%). The Milbrandt scheme slightly outperforms the other PPSs in simulating snow-related variable magnitudes. Due to their inaccurate temperature and airflow modeling over the lake surface and its surroundings, none of the WRF simulations well reproduce the characteristics that more snow occurs over the lake and downwind area. Overall, this study provides a useful reference for future convection-permitting climate modeling of snow or other extreme events when using the WRF model in the TP and other alpine regions. Plain Language Summary Snow falls frequently in cold seasons, especially in alpine regions. When there is a lake, a very interesting snowfall phenomenon named lake-effect snow may happen, that is, more snow occurred over the lake and downwind areas. However, the lake-effect snow caused by large lakes over the Tibetan Plateau (TP) may induce snow disaster events. Thus, conducting reliable simulations of lake-effect snow events are essential for understanding the mechanism of these particular events over the TP. This study investigated the performance of the numerical model in simulating precipitation, 2 m air temperature (T2), snowfall, and lake-effect snow (October 4-8, 2018) over the TP. The results show that the simulated precipitation and T2 perform reasonably well, although wet and cold biases are observed. However, the investigated numerical model fails to reproduce the characteristics of this event well, which may be due to the inaccurate temperature and airflow modeling over the lake surface and its surroundings. Continued improvement is needed for future modeling. Hence, this study provides some suggestions for future numerical modeling of lake-effect snow or other snow events over the TP and other alpine regions.
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| 14. |
- Ma, Mengnan, et al.
(författare)
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Contribution of Recycled and External Advected Moisture to Precipitation and Its Inter-Annual Variation Over the Tibetan Plateau
- 2024
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-897X .- 2169-8996. ; 129
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we performed a high-resolution simulation using the Weather Research and Forecasting model, integrated with water vapor tracers, covering the years 2005–2019. Our objective was to obtain deeper insights into the spatiotemporal dynamics of external advected and local evaporative water vapor, and to elucidate their impact on precipitation patterns across the Tibetan Plateau (TP). Our findings underscore that a significant proportion of TP's precipitation originates from external advected water vapor, primarily entering through the western and southern boundaries. During summer, stronger zonal and meridional water vapor transport, driven by prevailing westerly winds and the Asian monsoon, significantly influences seasonal and spatial precipitation variations. Additionally, we observed that the inter-annual variation of precipitation is intricately linked to changes in the net water vapor influx, modulated by alterations in atmospheric circulation. We also analyze the Precipitation Recycling Ratio (PRR) which refers to the proportion of precipitation originated from local evaporative water vapor to the total precipitation, revealing distinctive elevation-dependent variations aligned with grassland distribution. Notably, PRR exhibits asynchronous shifts with precipitation at different timescales, potentially linked to soil moisture-precipitation feedback at intra-annual scales. Moreover, the investigation highlights that inter-annual variations in PRR are primarily linked to the inflow and outflow of water vapor as well as wind strength at 500hPa, particularly prominent during colder seasons, while thermal factors carry comparable weight to dynamical factors in warmer seasons.
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| 15. |
- Ma, Mengnan, et al.
(författare)
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High-resolution climate projection over the Tibetan Plateau using WRF forced by bias-corrected CESM
- 2023
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Ingår i: Atmospheric Research. - : Elsevier BV. - 0169-8095. ; 286
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Tidskriftsartikel (refereegranskat)abstract
- The Tibetan Plateau (TP) has undergone significant climate warming with a stronger amplitude than that experienced elsewhere in the Northern Hemisphere during the past years, but it is still challenging for most regional climate models to realistically simulate the present-day climate and promisingly project the future climate over the TP. In this study, high-resolution simulation using the Weather Research and Forecasting model (WRF) driven by bias-corrected CESM is conducted from 1979 to 2100, with the period from 2006 to 2100 under RCP4.5 and RCP8.5 (Representative Concentration Pathways) scenarios. The simulated present-day climate is evaluated firstly and then the future climate is studied secondly. The results show that compared with station observation, WRF successfully captures the spatial pattern of annual mean surface air temperature (T2m) and precipitation over the TP, with the spatial correlation coefficients larger than 0.95 for T2m and larger than 0.70 for precipitation. However, great underestimation of T2m over the southeastern TP is found in the cold season which is related to the underestimation of snow there, and the snow-temperature positive feedback develops. WRF shows limited ability in reducing the dry bias in summer, which is related to the simulated weaker water vapor transport over the southern and eastern TP. For the future changes, substantial warming, general increase in precipitation and decrease in snow are projected under RCP8.5. The warming magnitude is greater over the western TP where the more significant decrease of snow will occur by the end of 21st century. Projected precipitation tends to consistently decrease over the western TP and along the south flank of TP which is related to the low-level circulation change. The occurring frequency of light precipitation will decrease while that of non-precipitation and extreme precipitation will increase especially in the far future, with the more obvious change occurring under RCP8.5. Overall, WRF shows high ability in simulating the present-day climate and thus reliable performance in future climate projection.
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| 16. |
- Ma, Mengnan, et al.
(författare)
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Subdaily Extreme Precipitation and Its Linkage to Global Warming Over the Tibetan Plateau
- 2023
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-897X .- 2169-8996. ; 128:18
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Tidskriftsartikel (refereegranskat)abstract
- The spatiotemporal characteristics of subdaily extreme precipitation over the Tibetan Plateau (TP) have undergone significant changes due to global warming. In this study, we employed the high-resolution Weather Research and Forecasting regional climate model to conduct a series of historical and projection simulations under representative concentration pathways (RCPs), especially RCP4.5 and RCP8.5. The aim was to investigate the past and future climatologies and spatiotemporal evolution of subdaily precipitation extremes using newly proposed subdaily extreme precipitation indices (EPIs). The results show that projected changes in precipitation amount, particularly during wet hours, exhibit spatial disparaties. Notably, there are significant decreases along the southern border of the TP and over the western TP, while obvious increases are observed over the inner TP. The southeastern TP, western TP, and southern border of the TP are expected to experience less frequent, shorter duration, and more intense precipitation on an hourly basis. The TP, as a whole, has demonstrated significantly increasing trends in moderate-to-heavy precipitation frequencies, along with consistent decreasing trends in precipitation events with short, medium, and long durations. Furthermore, it is predicted that the relationship between extreme precipitation and temperature will deviate from the Clausius-Clapeyron (C-C) relationship toward the double C-C relationship in the far-future under RCP8.5, particularly over the southeastern TP. Additionally, there are robust correlations between the intensity-related EPIs and elevation. This indicates that, at the local scale, the complex topography of the region may play a crucial role in shaping the nonuniform distribution of precipitation extremes by modulating associated upward motion.
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| 17. |
- Ma, Mengnan, et al.
(författare)
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Summer regional climate simulations over Tibetan Plateau: from gray zone to convection permitting scale
- 2022
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894.
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Tidskriftsartikel (refereegranskat)abstract
- The Tibetan Plateau (TP) is often referred to as ‘the Third Pole’ and plays an essential role in the global climate. However, it remains challenging for most global and regional models to realistically simulate the characteristics of climate over the TP. In this study, two Weather Research and Forecasting model (WRF) experiments using spectral nudging with gray-zone (GZ9) and convection-permitting (CP3) resolution are conducted for summers from 2009 to 2018. The surface air temperature (T2m) and precipitation from the two simulations and the global reanalysis ERA5 are evaluated against in-situ observations. The results show that ERA5 has a general cold bias over southern TP, especially in maximum T2m (Tmax), and wet bias over whole TP. Both experiments can successfully capture the spatial pattern and daily variation of T2m and precipitation, though cold bias for temperature and dry bias for precipitation exist especially over the regions south of 35° N. Compared with ERA5, the added value of the two WRF experiments is mainly reflected in the reduced cold bias especially for Tmax with more improvement found in CP3 and the reduced wet bias. However, the ability of the convection-permitting WRF experiment in improving the simulation of precipitation seems limited when compared to the gray-zone WRF experiment, which may be related to the biases in physical parameterization and lack of representativeness of station observation. Further investigation into surface radiation budget reveals that the underestimation of net shortwave radiation contributes a lot to the cold bias of T2m over the southeastern TP in GZ9 which is improved in CP3. Compared with GZ9, CP3 shows that larger specific humidity at low-level (mid-high level) coexists with more precipitation (clouds) over the southern TP. This improvement is achieved by better depiction of topographic details, underlying surface and atmospheric processes, land–atmosphere interactions and so on, leading to stronger northward water vapor transport (WVT) in CP3, providing more water vapor for precipitation at surface and much wetter condition in the mid-high level.
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| 18. |
- Minola, Lorenzo, et al.
(författare)
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Climatology of near-surface wind speed from observational, reanalysis and high-resolution regional climate model data over the Tibetan Plateau
- 2023
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Ingår i: Climate Dynamics. - 0930-7575 .- 1432-0894.
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Tidskriftsartikel (refereegranskat)abstract
- As near-surface wind speed plays a role in regulating surface evaporation and thus the hydrological cycle, it is crucial to explore its spatio-temporal characteristics. However, in-situ measurements are scarce over the Tibetan Plateau, limiting the understanding of wind speed climate across this high-elevation region. This study explores the climatology of near-surface wind speed over the Tibetan Plateau by using for the first time homogenized observations together with reanalysis products and regional climate model simulations. Measuring stations across the center and the west of the plateau are at higher elevations and display higher mean and standard deviation, confirming that wind speed increases with increasing altitude. By exploring wind characteristics with a focus on seasonal cycle through cluster analysis, three regions of distinct wind regimes can be identified: (1) the central Tibetan Plateau, characterized by high elevation; (2) the eastern and the peripheral areas of the plateau; and (3) the Qaidam basin, a topographic depression strongly influenced by the blocking effect of the surrounding mountainous terrain. Notably, the ERA5 reanalysis, with its improvements in horizontal, vertical, and temporal spacing, model physics and data assimilation, demonstrates closer agreement to the measured wind conditions than its predecessor ERA-Interim. It successfully reproduces the three identified wind regimes. However, the newest ERA5-Land product does not show improvements compared to ERA5, most likely because they share most of the parametrizations. Furthermore, the two dynamical downscalings of ERA5 analyzed here fail to capture the observed wind statistics and exhibit notable biases and discrepancies also when investigating the diurnal variations. Consequently, these high-resolution downscaling products do not show add value in reproducing the observed climatology of wind speed compared to ERA5 over the Tibetan Plateau.
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| 19. |
- Niu, X. R., et al.
(författare)
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Elevation-Dependent Warming Over the Tibetan Plateau From an Ensemble of CORDEX-EA Regional Climate Simulations
- 2021
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Ingår i: Journal of Geophysical Research-Atmospheres. - : American Geophysical Union (AGU). - 2169-897X .- 2169-8996. ; 126:9
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Tidskriftsartikel (refereegranskat)abstract
- Under the Coordinated Regional Climate Downscaling Experiments-East Asia (CORDEX-EA-II), the outputs from two regional climate models (RCMs) driven by four global climate models (GCMs) are used to investigate the characteristics and possible mechanisms of the projected elevation-dependent warming (EDW) over the Tibetan Plateau (TP) under the Representative Concentration Pathway emission scenario 8.5 (RCP8.5). Results show that widespread warming over the TP is projected with considerable disagreements in warming intensity and the maximum warming center among RCMs. The largest spread in the surface air temperature (T-as) projections is found above 5,000 m, indicating that a large uncertainty exists over the higher elevations. A marked EDW signal over the TP is simulated under the RCP 8.5 by the multi-RCM ensemble mean for all seasons, particularly in autumn. Based on the analysis of the surface energy budget, it is found that the surface albedo feedback (SAF) is the primary contributor to EDW and acts as the main source of uncertainty in EDW projections among RCMs. The downward longwave radiation (DLW) is found to be the dominant factor in regulating T-as change over the TP, and its contribution to EDW is model-dependent. Furthermore, the structure and magnitude of projected EDW are sensitive to the RCM physics and driving GCM, as they can alter the projections of snow cover and albedo, which modulate the simulated SAF and its effect on EDW. Additionally, RegCM4 shows a higher sensitivity to the anthropogenic greenhouse forcing than WRF, evidenced by the larger temperature projections and stronger EDW signal in RegCM4.
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| 20. |
- Niu, Xiaorui, et al.
(författare)
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The performance of CORDEX-EA-II simulations in simulating seasonal temperature and elevation-dependent warming over the Tibetan Plateau
- 2021
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 57, s. 1135-1153
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Tidskriftsartikel (refereegranskat)abstract
- To explore the driving mechanisms of elevation-dependent warming (EDW) over the Tibetan Plateau (TP), the output from a suite of numerical experiments with different cumulus parameterization schemes (CPs) under the Coordinated Regional Climate Downscaling Experiments-East Asia (CORDEX-EA-II) project is examined. Results show that all experiments can broadly capture the observed temperature distributions over the TP with consistent cold biases, and the spread in temperature simulations commonly increases with elevation with the maximum located around 4000-5000 m. Such disagreements among the temperature simulations could to a large extent be explained by their spreads in the surface albedo feedback (SAF). All the experiments reproduce the observed EDW below 5000 m in winter but fail to capture the observed EDW above 4500 m in spring. Further analysis suggests that the simulated EDW during winter is mainly caused by the SAF, and the clear-sky downward longwave radiation (LWclr) plays a secondary role in shaping EDW. The models' inability in simulating EDW during spring is closely related to the SAF and the surface cloud radiative forcing (CRFs). Furthermore, the magnitude and structure of the simulated EDW are sensitive to the choice of CPs. Different CPs generate diverse snow cover fractions, which can modulate the simulated SAF and its effect on EDW. Also, the CPs show great influence on the LWclr via altering the low-level air temperature. Additionally, the mechanism for different temperature changes among the experiments varies with altitudes during summer and autumn, as the diverse temperature changes appear to be caused by the LWclr for the low altitudes while by the SAF for the middle-high altitudes.
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| 21. |
- Ou, Tinghai, et al.
(författare)
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Simulation of summer precipitation diurnal cycles over the Tibetan Plateau at the gray-zone grid spacing for cumulus parameterization
- 2020
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 54, s. 3525-3539
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Tidskriftsartikel (refereegranskat)abstract
- The Tibetan Plateau (TP) is often referred to as the “water tower of Asia” or the “Third Pole”. It remains a challenge for most global and regional models to realistically simulate precipitation, especially its diurnal cycles, over the TP. This study focuses on evaluating the summer (June–August) precipitation diurnal cycles over the TP simulated by the Weather Research and Forecasting (WRF) model. The horizontal resolution used in this study is 9 km, which is within the gray-zone grid spacing that a cumulus parameterization scheme (CU) may or may not be used. We conducted WRF simulations with different cumulus schemes (CU experiments) and a simulation without CU (No_CU experiment). The selected CUs include the Grell-3D Ensemble (Grell), New Simplified Arakawa-Schubert (NSAS), and Multiscale Kain-Fritsch (MSKF). These simulations are compared with both the in-situ observations and satellite products. Results show that the scale-aware MSKF outperforms the other CUs in simulating precipitation in terms of both the mean intensity and diurnal cycles. In addition, the peak time of precipitation intensity is better captured by all the CU experiments than by the No_CU experiment. However, all the CU experiments tend to overestimate the mean precipitation and simulate an earlier peak of precipitation frequency when compared to observations. The frequencies and initiation timings for short-duration (1–3 h) and long-duration (> 6 h) precipitation events are well captured by the No_CU experiment, while these features are poorly reproduced by the CU experiments. The results demonstrate simulation without a CU outperforms those with a CU at the gray-zone spatial resolution in regard to the precipitation diurnal cycles.
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| 22. |
- Ou, Tinghai, et al.
(författare)
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Wet bias of summer precipitation in the northwestern Tibetan Plateau in ERA5 is linked to overestimated lower-level southerly wind over the plateau
- 2023
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 61:5-6, s. 2139-53
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Tidskriftsartikel (refereegranskat)abstract
- The Tibetan Plateau (TP), also called the Third Pole, is considered to be “the world water tower”. The northwestern TP (NWTP), which has an average elevation higher than 4800m, is an arid region where the summer precipitation is largely overestimated by the ERA5 global reanalysis product. We hypothesize that this wet bias is mainly caused by unrealistic lower-level winds that trigger strong convection over the region; it can be reduced by using a high-resolution regional climate model with a large domain that allows realistically representing interactions between the Westerlies and Asian summer monsoons. Here, downscaling using the Weather Research and Forecasting (WRF) model driven by ERA5 was conducted with a large domain (8°‒50° N, 65°‒125° E) at 9km for the period 1979‒2019 (WRF9km). Precipitation values from WRF9km and ERA5 were evaluated against satellite observations; compared with ERA5, WRF9km captured the climatological summer precipitation over the NWTP with a much-reduced wet bias. The ERA5 overestimation is mainly caused by excessive convective precipitation, likely linked to strong vertical motions over the NWTP induced by an overestimated lower-level southerly wind.
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| 23. |
- Prein, Andreas F., et al.
(författare)
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Towards Ensemble-Based Kilometer-Scale Climate Simulations over the Third Pole Region
- 2022
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894.
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Tidskriftsartikel (refereegranskat)abstract
- The Tibetan Plateau and its surrounding mountains have an average elevation of 4,400m and a glaciated area of ∼ 100,000km 2 giving it the name “Third Pole (TP) region”. The TP is the headwater of many major rivers in Asia that provide fresh water to hundreds of millions of people. Climate change is altering the energy and water cycle of the TP at a record pace but the future of this region is highly uncertain due to major challenges in simulating weather and climate processes in this complex area. The Convection-Permitting Third Pole (CPTP) project is a Coordinated Regional Downscaling Experiment (CORDEX) Flagship Pilot Study (FPS) that aims to revolutionize our understanding of climate change impacts on the TP through ensemble-based, kilometer-scale climate modeling. Here we present the experimental design and first results from multi-model, multi-physics ensemble simulations of three case studies. The five participating modeling systems show high performance across a range of meteorological situations and are close to having ”observational quality” in simulating precipitation and near-surface temperature. This is partly due to the large differences between observational datasets in this region, which are the leading source of uncertainty in model evaluations. However, a systematic cold bias above 2000m exists in most modeling systems. Model physics sensitivity tests performed with the Weather Research and Forecasting (WRF) model show that planetary boundary layer (PBL) physics and microphysics contribute equally to model uncertainties. Additionally, larger domains result in better model performance. We conclude by describing high-priority research needs and the next steps in the CPTP project.
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| 24. |
- Shaw, T. E., et al.
(författare)
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Multi-decadal monsoon characteristics and glacier response in High Mountain Asia
- 2022
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Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 17:10
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Tidskriftsartikel (refereegranskat)abstract
- Glacier health across High Mountain Asia (HMA) is highly heterogeneous and strongly governed by regional climate, which is variably influenced by monsoon dynamics and the westerlies. We explore four decades of glacier energy and mass balance at three climatically distinct sites across HMA by utilising a detailed land surface model driven by bias-corrected Weather Research and Forecasting meteorological forcing. All three glaciers have experienced long-term mass losses (ranging from -0.04 +/- 0.09 to -0.59 +/- 0.20 m w.e. a(-1)) consistent with widespread warming across the region. However, complex and contrasting responses of glacier energy and mass balance to the patterns of the Indian Summer Monsoon were evident, largely driven by the role snowfall timing, amount and phase. A later monsoon onset generates less total snowfall to the glacier in the southeastern Tibetan Plateau during May-June, augmenting net shortwave radiation and affecting annual mass balance (-0.5 m w.e. on average compared to early onset years). Conversely, timing of the monsoon's arrival has limited impact for the Nepalese Himalaya which is more strongly governed by the temperature and snowfall amount during the core monsoon season. In the arid central Tibetan Plateau, a later monsoon arrival results in a 40 mm (58%) increase of May-June snowfall on average compared to early onset years, likely driven by the greater interaction of westerly storm events. Meanwhile, a late monsoon cessation at this site sees an average 200 mm (192%) increase in late summer precipitation due to monsoonal storms. A trend towards weaker intensity monsoon conditions in recent decades, combined with long-term warming patterns, has produced predominantly negative glacier mass balances for all sites (up to 1 m w.e. more mass loss in the Nepalese Himalaya compared to strong monsoon intensity years) but sub-regional variability in monsoon timing can additionally complicate this response.
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| 25. |
- Su, F., et al.
(författare)
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Contrasting Fate of Western Third Pole's Water Resources Under 21st Century Climate Change
- 2022
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Ingår i: Earth's Future. - 2328-4277. ; 10:9
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Tidskriftsartikel (refereegranskat)abstract
- Seasonal melting of glaciers and snow from the western Third Pole (TP) plays important role in sustaining water supplies downstream. However, the future water availability of the region, and even today's runoff regime, are both hotly debated and inadequately quantified. Here, we characterize the contemporary flow regimes and systematically assess the future evolution of total water availability, seasonal shifts, and dry and wet discharge extremes in four most meltwater-dominated basins in the western TP, by using a process-based, well-established glacier-hydrology model, well-constrained historical reference climate data, and the ensemble of 22 global climate models with an advanced statistical downscaling and bias correction technique. We show that these basins face sharply diverging water futures under 21st century climate change. In RCP scenarios 4.5 and 8.5, increased precipitation and glacier runoff in the Upper Indus and Yarkant basins more than compensate for decreased winter snow accumulation, boosting annual and summer water availability through the end of the century. In contrast, the Amu and Syr Darya basins will become more reliant on rainfall runoff as glacier ice and seasonal snow decline. Syr Darya summer river-flows, already low, will fall by 16%–30% by end-of-century, and striking increases in peak flood discharge (by >60%), drought duration (by >1 month) and drought intensity (by factor 4.6) will compound the considerable water-sharing challenges on this major transboundary river.
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| 26. |
- Sun, H., et al.
(författare)
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Corrected ERA5 Precipitation by Machine Learning Significantly Improved Flow Simulations for the Third Pole Basins
- 2022
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Ingår i: Journal of Hydrometeorology. - : American Meteorological Society. - 1525-755X .- 1525-7541. ; 23:10, s. 1663-1679
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Tidskriftsartikel (refereegranskat)abstract
- Precipitation is one of the most important atmospheric inputs to hydrological models. However, existing precipitation datasets for the Third Pole (TP) basins show large discrepancies in precipitation magnitudes and spatiotemporal patterns, which poses a great challenge to hydrological simulations in the TP basins. In this study, a gridded (10 km x 10 km) daily precipitation dataset is constructed through a random-forest-based machine learning algorithm (RF algorithm) correction of the ERA5 precipitation estimates based on 940 gauges in 11 upper basins of TP for 1951-2020. The dataset is evaluated by gauge observations at point scale and is inversely evaluated by the Variable Infiltration Capacity (VIC) hydrological model linked with a glacier melt algorithm (VIC-Glacier). The corrected ERA5 (ERA5_cor) agrees well with gauge observations after eliminating the severe overestimation in the original ERA5 precipitation. The corrections greatly reduce the original ERA5 precipitation estimates by 10%-50% in 11 basins of the TP and present more details on precipitation spatial variability. The inverse hydrological model evaluation demonstrates the accuracy and rationality, and we provide an updated estimate of runoff components contribution to total runoff in seven upper basins in the TP based on the VIC-Glacier model simulations with the ERA5_cor precipitation. This study provides good precipitation estimates with high spatiotemporal resolution for 11 upper basins in the TP, which are expected to facilitate the hydrological modeling and prediction studies in this high mountainous region. Significance StatementThe Third Pole (TP) is the source of water to the people living in the areas downstream. Precipitation is the key driver of the terrestrial hydrological cycle and the most important atmospheric input to land surface hydrological models. However, none of the current precipitation data are equally good for all the TP basins because of high variabilities in their magnitudes and spatiotemporal patterns, posing a great challenge to the hydrological simulation. Therefore, in this study, a gridded daily precipitation dataset (10 km x 10 km) is reconstructed through a random-forest-based machine learning algorithm correction of ERA5 precipitation estimates based on 940 gauges in 11 TP basins for 1951-2020. The data eliminate the severe overestimation of original ERA5 precipitation estimates and present more reasonable spatial variability, and also exhibit a high potential for hydrological application in the TP basins. This study provides long-term precipitation data for climate and hydrological studies and a reference for deriving precipitation in high mountainous regions with complex terrain and limited observations.
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| 27. |
- Sun, He, et al.
(författare)
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Hydrological Evaluation of High-Resolution Precipitation Estimates from the WRF Model in the Third Pole River Basins
- 2021
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Ingår i: Journal of Hydrometeorology. - 1525-755X. ; 22:8, s. 2055-71
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Tidskriftsartikel (refereegranskat)abstract
- In this study, two sets of precipitation estimates that are based on the regional Weather Research and Forecasting (WRF) Model—the high Asia refined analysis (HAR) and outputs with a 9-km resolution from WRF (WRF-9km)—are evaluated at both basin and point scales, and their potential hydrological utilities are investigated by driving the Variable Infiltration Capacity (VIC) large-scale land surface hydrological model in seven Third Pole (TP) basins. The regional climate model (RCM) tends to overestimate the gauge-based estimates by 20%–95% in annual means among the selected basins. Relative to the gauge observations, the RCM precipitation estimates can accurately detect daily precipitation events of varying intensities (with absolute bias < 3 mm). The WRF-9km exhibits a high potential for hydrological application in the monsoon-dominated basins in the southeastern TP (with NSE of 0.7–0.9 and bias from −11% to 3%), whereas the HAR performs well in the upper Indus and upper Brahmaputra basins (with NSE of 0.6 and bias from −15% to −9%). Both of the RCM precipitation estimates can accurately capture the magnitudes of low and moderate daily streamflow but show limited capabilities in flood prediction in most of the TP basins. This study provides a comprehensive evaluation of the strength and limitation of RCMs precipitation in hydrological modeling in the TP with complex terrains and sparse gauge observations.
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| 28. |
- Sun, He, et al.
(författare)
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Increased glacier melt enhances future extreme floods in the southern Tibetan Plateau
- 2024
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Ingår i: Advances in Climate Change Research. - 1674-9278 .- 2524-1761.
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Tidskriftsartikel (refereegranskat)abstract
- Mountainous areas are of special hydrological concern because topography and atmospheric conditions can result in large and sudden floods, posing serious risks to water-related safety in neighbouring countries. The Yarlung Zangbo (YZ) River basin is the largest river basin on the Tibetan Plateau (TP), but how floods will discharge in this basin and how the role of glacier melt in floods will change throughout the 21st-century under shared socioeconomic pathways scenarios (SSP2-4.5 and SSP5-8.5) remain unclear. Here, we comprehensively address this scientific question based on a well-validated large-scale glacier-hydrology model. The results indicate that extreme floods was projected to increase in the YZ basin, and was mainly reflected in increased duration (4–10 d per decade) and intensity (153–985 m3 s−1 per decade). Glacier runoff was projected to increase (2–30 mm per decade) throughout the 21st-century, but there was also a noticeable decrease or deceleration in glacier runoff growth in the late first half of the century under the SSP2-4.5, and in the latter half of the century under the SSP5-8.5. Glacier melt was projected to enhance the duration (12%–23%) and intensity (15%–21%) of extreme floods under both SSPs, which would aggravate the impact of future floods on the socioeconomics of the YZ basin. This effect was gradually overwhelmed by precipitation-induced floods from glacier areas to YZ outlet. This study takes the YZ basin as a projection framework example to help enrich the understanding of future flood hazards in basins affected by rainfall- or meltwater across the TP, and to help policy-makers and water managers develop future plans.
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| 29. |
- Wang, Xuejia, et al.
(författare)
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A climatology of surface–air temperature difference over the Tibetan Plateau: Results from multi-source reanalyses
- 2020
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Ingår i: International Journal of Climatology. - : Wiley. - 0899-8418 .- 1097-0088. ; 40:14, s. 6080-94
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Tidskriftsartikel (refereegranskat)abstract
- The Tibetan Plateau (TP), known as earth's “Third Pole,” influences regional and even global weather and climate systems through its mechanical and thermal-dynamical forcing. Near-surface (2 m) air temperature (Ta) and surface (skin) temperature (Ts) are two crucial parameters of land–atmosphere interactions and climate change. Their difference (ΔT = Ts − Ta) determines the heating source over the TP that drives the Asian summer monsoon. This study focuses on climatology, inter-annual variability, and long-term trend of ΔT over the TP in the last four decades (1979–2018), based on four latest reanalysis datasets including ERA-Interim, ERA5, MERRA2, and JRA55, along with observational data. We show that ΔT-based different datasets display fairly different climatology in terms of seasonality, spatial distribution, and long-term trend. ΔT exhibits a clear seasonality with negative value in winter and positive ones in summer despite different strengths and timings presented by the reanalyses. Along with global warming, all reanalyses except JRA55 exhibit obvious downwards trends of ΔT in a spatially non-uniform way. The median ΔT among the four reanalyses features uniform decreases in all seasons, with the most distinct area on the northern TP, as well as the largest and least decreases in autumn and spring, respectively. Further analysis shows that the differences in ΔT are most likely associated with discrepancies in radiation forcing, snow cover, wind speed, and boundary layer height within the reanalyses. The present findings highlight the difficulty for the state-of-the-art reanalyses to represent the climate change over the TP and point to possible factors behind the deficiencies identified.
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| 30. |
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| 31. |
- Wang, Xuejia, et al.
(författare)
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Effects of cumulus parameterization and land-surface hydrology schemes on Tibetan Plateau climate simulation during the wet season: insights from the RegCM4 model
- 2021
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 57:7-8, s. 1853-1879
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Tidskriftsartikel (refereegranskat)abstract
- Dynamical downscaling generally performs poorly on the Tibetan Plateau (TP), due to the region’s complex topography and several aspects of model physics, especially convection and land surface processes. This study investigated the effects of the cumulus parameterization scheme (CPS) and land-surface hydrology scheme (LSHS) on TP climate simulation during the wet season using the RegCM4 regional climate model. To address these issues and seek an optimal simulation, we conducted four experiments at a 20km resolution using various combinations of two CPSs (Grell and MIT-Emanuel), two LSHSs (the default TOPMODEL [TOP], and Variable Infiltration Capacity [VIC]). The simulations in terms of 2-m air temperature, precipitation (including large-scale precipitation [LSP] and convective precipitation [CP]), surface energy-water balance, as well as atmospheric moisture flux transport and vertical motion were compared with surface and satellite-based observations as well as the ERA5 reanalysis dataset for the period 2006–2016. The results revealed that the model using the Grell and TOP schemes better reproduced air temperature but with a warm bias, part of which could be significantly decreased by the MIT scheme. All schemes simulated a reasonable spatial distribution of precipitation, with the best performance in the experiment using the MIT and VIC schemes. Excessive precipitation was produced by the Grell scheme, mainly due to overestimated LSP, while the MIT scheme largely reduced the overestimation, and the simulated contribution of CP to total precipitation was in close agreement with the ERA5 data. The RegCM4 model satisfactorily captured diurnal cycles of precipitation amount and frequency, although there remained some differences in phase and magnitude, which were mainly caused by the CPSs. Relative to the Grell scheme, the MIT scheme yielded a weaker surface heating by reducing net radiation fluxes and the Bowen ratio. Consequently, anomalous moisture flux transport was substantially reduced over the southeastern TP, leading to a decrease in precipitation. The VIC scheme could also help decrease the wet bias by reducing surface heating. Further analysis indicated that the high CP in the MIT simulations could be attributed to destabilization in the low and mid-troposphere, while the VIC scheme tended to inhibit shallow convection, thereby decreasing CP. This study’s results also suggest that CPS interacts with LSHS to affect the simulated climate over the TP.
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| 32. |
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| 33. |
- Xing, Hang, et al.
(författare)
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Impacts of changes in climate extremes on wildfire occurrences in China
- 2023
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Ingår i: Ecological Indicators. - 1470-160X. ; 157
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Tidskriftsartikel (refereegranskat)abstract
- Climate change has caused more frequent instances of extreme climatic events around the world, being an influential factor on the occurrence of wildfires in China on large scale. However, the impact of changes in extreme climate on the occurrence of wildfires in different climate zones remains unclear. In the present study, 26 extreme climate indices were selected to analyze the thereof relationship with wildfire occurrences from 2005 to 2018 in different regions of China. Wildfires in China primarily occur in the south, with a measurable presence in the north. On an annual scale, the wildfire occurrences in southwestern China show stronger correlations with mean temperature than extreme temperature indices, but show stronger correlations with extreme precipitation indices than the total precipitation. On the contrary, the wildfire occurrences in southeastern China show stronger correlations with the total precipitation than extreme precipitation indices, but show stronger correlations with extreme temperature indices than the mean temperature. In Northeast China, wildfires show a more significant correlation with mean temperature than with any extreme climate indices, indicating a minimal impact from extreme climatic conditions. The fire-climate relationships in the main fire season (January-April) are similar to those in the annual scale. The wildfire occurrences in the southwestern, south-central, and southeastern China, which are located in the same latitudes, were affected by extreme climate indices of different types and on different time scales. Furthermore, we recommend that consecutive dry days (CDD) and diurnal temperature range (DTR) should be considered first when studying the relationship between wildfire occurrence and extreme climate in southwestern and southeastern China respectively.
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| 34. |
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| 35. |
- Zhou, Feifei, et al.
(författare)
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ENSO weakens the co-variability between the spring persistent rains and Asian summer monsoon: Evidences from tree-ring data in southeastern China
- 2024
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Ingår i: Journal of Hydrology. - 0022-1694. ; 634
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Tidskriftsartikel (refereegranskat)abstract
- The Spring Persistent Rains (SPR) and the Asian Summer Monsoon (ASM) are the two dominant rainfall systems in East Asia, providing together a majority of annual rainfall in southeastern China (SEC). Since observational data in SEC were mostly unavailable until the 1950s, proxy records that are capable of capturing the SPR and ASM variations are required to examine the long-term co-variability patterns between them. Tree-ring earlywood and latewood δ18O records in SEC were found to respond to relative humidity (RH) during the SPR and ASM seasons, respectively, allowing us, for the first time, to reconstruct the RH changes of SPR and ASM back to 1801. The two reconstructions can explain 44.9 % and 42.3 % of the instrumental variance. We observed a long-lasting wet epoch in the 1920s–60s for both the SPR and ASM, caused by a peak in the land–ocean thermal contrast. The El Niño-Southern Oscillation (ENSO) and the Intertropical Convergence Zone (ITCZ) were found to be the two leading tropical systems that modulated the SPR and ASM co-variability. During a period with weakened ENSO variance, the RH of SPR and ASM showed in-phase changes driven by the ITCZ. However, when the ENSO variance became strengthened, the co-variability collapsed since the ENSO can offset the influence of the ITCZ via teleconnections.
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| 36. |
- Zhou, Feifei, et al.
(författare)
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Summer heat induced the decline of Pinus taiwanensis forests at its southern limit in humid Subtropical China
- 2024
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Ingår i: Agricultural and Forest Meteorology. - 0168-1923. ; 350
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Tidskriftsartikel (refereegranskat)abstract
- Warming-induced aridity has caused forest decline and mortality for many sites with water-limiting conditions. However, equatorward rear-edge Pinus taiwanensis trees at the Daiyun Mountains in humid subtropical China are also suffering die-backs and decline, but the roles played by heat or drought stress still remain unclear. Here, we compared the tree-ring radial width, anatomical features, stable carbon isotope (δ13C) and intrinsic water use efficiency (iWUE) between die-back and healthy trees to elucidate potential causes driving the decline. Die-back trees showed sustained growth reductions and produced tracheids with thinner cell walls over the recent decade, indicative of reduced carbon assimilation. The climate response pattern and Vaganov-Shashkin (V-S) model indicated the critical role of summer (June-August) temperature in recent growth decline. Long-term higher wood δ13C and iWUE within die-back trees indicated that actual growth decline already started several decades earlier. This conservative growth strategy was at the cost of low efficiency of photosynthesis due to chronic stomatal closure. When the lethal heatwaves arrived, these weakened trees were not able to access sufficient carbonhydrates to maintain metabolism, causing a distinct decline and mortality. We concluded that recent decline in Pinus taiwanensis trees was mainly caused by long-term carbon starvation.
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| 37. |
- Zhou, Peifeng, et al.
(författare)
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WRF gray-zone dynamical downscaling over the Tibetan Plateau during 1999–2019: model performance and added value
- 2022
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894.
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Tidskriftsartikel (refereegranskat)abstract
- The Tibetan Plateau (TP) is an important component of the global climate system, while the characteristics of its climate are poorly represented in most regional climate models at coarse resolutions. In this study, a 20-year (2000–2019) dynamical downscaling simulation at the gray-zone resolution (9km) using the WRF model driven by the ERA5 reanalysis is conducted over the TP. Based on comparisons against in-situ observations and the Integrated Multi-satellite Retrievals for GPM (IMERG) version 6 satellite precipitation product, the assessment of basic climate variables, such as near-surface air temperature (T2m) and precipitation, is performed to evaluate the model’s performance and understand its added value better. Results show that both WRF and ERA5 can successfully reproduce the spatial patterns of annual mean and seasonal mean surface air temperature. However, significant cold and wet biases are found especially over the southeastern TP in ERA5, which are greatly improved in WRF with reduced RMSEs. Not only the climatological characteristics, but also the inter-annual variability and seasonal variation of T2m and precipitation are well captured by WRF which reduces the cold and wet biases especially in winter and summer compared to ERA5, respectively. Besides, at daily scale, the overestimation of precipitation in WRF and ERA5 is mainly caused by the overestimated precipitation frequency when precipitation intensity changed slightly. Furthermore, WRF outperforms ERA5 in capturing the diurnal variation of precipitation with more realistic peak time in all sub-regions over the TP. Further investigation into the mechanism of model bias reveals that less simulated snow cover fraction plays a crucial role in increasing the surface net energy by affecting surface albedo over the southeastern TP in WRF, leading to higher T2m. In addition, less water vapor transport from the southern boundary of TP leads to reduced wet bias in WRF, indicating that the added value in dynamical downscaling at gray-zone resolution is obtained by representing water vapor transport more realistically.
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