| 21. |
- Bao, Fangling, et al.
(författare)
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Advancing Cloud Classification Over the Tibetan Plateau: A New Algorithm Reveals Seasonal and Diurnal Variations
- 2024
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Ingår i: GEOPHYSICAL RESEARCH LETTERS. - 0094-8276 .- 1944-8007. ; 51:13
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Tidskriftsartikel (refereegranskat)abstract
- The cloud classification algorithm widely used in the International Satellite Cloud Climatology Project (ISCCP) tends to underestimate low clouds over the Tibetan Plateau (TP), often mistaking water clouds for high-level clouds. To address this issue, we propose a new algorithm based on cloud-top temperature and optical thickness, which we apply to TP using Advanced Himawari Imager (AHI) geostationary satellite data. Compared with Clouds and the Earth's Radiant Energy System cloud-type products and ISCCP results obtained from AHI data, this new algorithm markedly improved low-cloud detection accuracy and better aligned with cloud phase results. Validation with lidar cloud-type products further confirmed the superiority of this new algorithm. Diurnal cloud variations over the TP show morning dominance shifting to afternoon high clouds and evening mid-level clouds. Winter is dominated by high clouds, summer by mid-level clouds, spring by daytime low clouds and nighttime high clouds, and autumn by low and mid-level clouds. The accurate identification of low clouds over the Tibetan Plateau (TP) is crucial for climate regulation, ecosystems, aviation safety, research, and modeling. However, satellite-based methods often miss these clouds, misclassifying them as high-level clouds. To remedy this, we developed a new algorithm using cloud-top temperature and optical thickness, applied to Advanced Himawari Imager data. This significantly improves low-cloud detection, better aligning with actual cloud phases. Simultaneously, we analyzed diurnal cloud variations over the TP with the new algorithm. Cloud types at different altitudes in the TP exhibit strong seasonality. The dominant cloud types in winter and summer are high and mid-level, respectively. In spring, low clouds dominate during the day (2:00-10:00 UTC), transitioning to high clouds at night (10:00-18:00 UTC), with mid-level clouds prevailing at other times. In autumn, low clouds dominate during the day, transitioning to mid-level clouds at other times, with fewer occurrences of high clouds. Employing cloud-top temperature instead of pressure resolves classification-phase inconsistencies for clouds in the Tibetan Plateau (TP) Lidar validation shows new algorithm's low cloud detection outperforms the conventional International Satellite Cloud Climatology Project algorithm for both TP and plains The study reveals significant diurnal and seasonal variations in low clouds over the TP
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| 22. |
- Barthel, Roland, 1967, et al.
(författare)
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Current understanding of groundwater recharge and groundwater drought in Sweden compared to countries with similar geology and climate
- 2021
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Ingår i: Geografiska Annaler: Series A, Physical Geography. - : Informa UK Limited. - 0435-3676 .- 1468-0459. ; 103:4, s. 323-345
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Tidskriftsartikel (refereegranskat)abstract
- Recently, groundwater in Sweden has attracted media attention due to supply shortages caused by dry periods and low groundwater levels. About half of Swedish drinking water stems from groundwater. The small Swedish aquifers are highly dependent on frequent and sufficient recharge. Groundwater recharge forms the link between meteorological and groundwater drought and thus the main link between climate change and sustainable water supply. This study evaluated whether the current knowledge on groundwater recharge and groundwater drought is sufficient to mitigate the impacts of climate change. A review of international literature on groundwater recharge in regions with conditions similar to Sweden was performed. National literature was compared and links to international studies evaluated. A survey among Swedish groundwater experts complemented the study. Findings are that research on groundwater recharge has been scarce in Sweden over the last decades and Swedish experts seem to not have taken much notice of international literature. It is concluded that Sweden is not well prepared to predict the impact of climate change on groundwater resources, as the most crucial process linking climate to groundwater is not well understood. The study has a strong focus on Sweden, however, the results are also relevant for countries with similar geology and climate. One main conclusion is that groundwater recharge studies often remain unique, and thus hardly transferable and comparable. Two or more independent studies are rarely applied in parallel, verification based on direct groundwater observations is not common. This raises concerns about the reliability of climate change impact predictions on groundwater.
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| 23. |
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| 24. |
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| 25. |
- Benestad, R.E, et al.
(författare)
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On using principal components to represent stations in empirical-statistical downscaling
- 2015
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 67:28326
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Tidskriftsartikel (refereegranskat)abstract
- We test a strategy for downscaling seasonal mean temperature for many locations within a region, based on principal component analysis (PCA), and assess potential benefits of this strategy which include an enhancement of the signal-to-noise ratio, more efficient computations, and reduced sensitivity to the choice of predictor domain. These conditions are tested in some case studies for parts of Europe (northern and central) and northern China. Results show that the downscaled results were not highly sensitive to whether a PCA-basis or a more traditional strategy was used. However, the results based on a PCA were associated with marginally and systematically higher correlation scores as well as lower root-mean-squared errors. The results were also consistent with the notion that PCA emphasises the large-scale dependency in the station data and an enhancement of the signal-to-noise ratio. Furthermore, the computations were more efficient when the predictands were represented in terms of principal components.
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| 26. |
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| 27. |
- Bibi, S., et al.
(författare)
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Climatic and associated cryospheric, biospheric, and hydrological changes on the Tibetan Plateau: a review
- 2018
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Ingår i: International Journal of Climatology. - : Wiley. - 0899-8418. ; 38
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Forskningsöversikt (refereegranskat)abstract
- We review recent climate changes over the Tibetan Plateau (TP) and associated responses of cryospheric, biospheric, and hydrological variables. We focused on surface air temperature, precipitation, seasonal snow cover, mountain glaciers, permafrost, freshwater ice cover, lakes, streamflow, and biological system changes. TP is getting warmer and wetter, and air temperature has increased significantly, particularly since the 1980s. Most significant warming trends have occurred in the northern TP. Slight increases in precipitation have occurred over the entire TP with clear spatial variability. Intensification of surface air temperature is associated with variation in precipitation and decreases in snow cover depth, spatial extent, and persistence. Rising surface temperatures have caused recession of glaciers, permafrost thawing, and thickening of the active layers over the permafrost. Changing temperatures, precipitation, and other climate system components have also affected the TP biological system. In addition, elevation-dependent changes in air temperature, wind speed, and summer precipitation have occurred in the TP and its surroundings in the past three decades. Before projecting multifaceted interactions and process responses to future climate change, further quantitative analysis and understanding of the change mechanisms is required.
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| 28. |
- Bibi, S., et al.
(författare)
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Response of Groundwater Storage and Recharge in the Qaidam Basin (Tibetan Plateau) to Climate Variations From 2002 to 2016
- 2019
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Ingår i: Journal of Geophysical Research-Atmospheres. - : American Geophysical Union (AGU). - 2169-897X .- 2169-8996. ; 124:17-17, s. 9918-9934
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Tidskriftsartikel (refereegranskat)abstract
- Groundwater (GW) and recharge as the main drivers of the water budget are challenging to quantify due to the complexity of hydrological processes and limited observations. Understanding these processes in relation to climate is crucial for evaluating future water availability of Tibetan Plateau. By computing storage changes in Gravity Recovery and Climate Experiment terrestrial water storage and Global Land Surface Data Assimilation System land surface state variables and water balance approach, we calculated GW storage changes and recharges. In the Qaidam Basin (northern Tibetan Plateau), terrestrial water storage from the Gravity Recovery and Climate Experiment revealed a significant increasing trend of 25.5 mm/year during 2002-2012. However, an obviously turning point was found around 2012 and terrestrial water storage revealed a significant decreasing rate of 37.9 mm/year during 2013-2016. Similarly, GW (recharge) had a significant increasing trend of 21.2 (4.5) mm/year before 2012 and a decreasing rate of 32.1 (10.9) mm/year after 2012. Domain-averaged difference (P-ET) between precipitation (P) and evapotranspiration (ET) also exhibited an increasing trend of 4.4 mm/year during 2002-2012 and a decreasing rate of 9.0 mm/year during 2013-2016. Precipitation followed dissimilar pattern with an increasing rate of 5.3 mm/year during 2002-2012 while no significant trend during 2013-2016. However, ET had a consistent increasing trend over the basin during the past 15 years (0.9 mm/year before 2012 and 9.0 mm/year thereafter). This study concluded that GW amount and distribution is mainly controlled by precipitation and ET. Decrease in precipitation at high elevations and increase in ET may impact future groundwater availability in this region.
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| 29. |
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| 30. |
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