| 1. |
- 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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| 2. |
- Chen, Hans, 1988, et al.
(författare)
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A robust mode of climate variability in the Arctic: The Barents Oscillation
- 2013
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 40:11, s. 2856-2861
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Tidskriftsartikel (refereegranskat)abstract
- The Barents Oscillation (BO) is an anomalous wintertime atmospheric circulation pattern in the Northern Hemisphere that has been linked to the meridional flow over the Nordic Seas. There are speculations that the BO has important implications for the Arctic climate; however, it has also been suggested that the pattern is an artifact of Empirical Orthogonal Function (EOF) analysis due to an eastward shift of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO). In this study, EOF analyses are performed to show that a robust pattern resembling the BO can be found during different time periods, even when the AO/NAO is relatively stationary. This BO has a high and stable temporal correlation with the geostrophic zonal wind over the Barents Sea, while the contribution from the AO/NAO is small. The surface air temperature anomalies over the Barents Sea are closely associated with this mode of climate variability.
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| 3. |
- Guan, Yanlong, et al.
(författare)
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Elevation Regulates the Response of Climate Heterogeneity to Climate Change
- 2024
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Ingår i: GEOPHYSICAL RESEARCH LETTERS. - 0094-8276 .- 1944-8007. ; 51:12
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Tidskriftsartikel (refereegranskat)abstract
- Climate change represents a profound threat to the diversity and stability of global climate zones. However, the complex interplay between climate change and elevation in shaping climate heterogeneity is not yet fully understood. Here, we combine Shannon's diversity index (SHDI) with the K & ouml;ppen-Geiger climate classification to explore the altitudinal distributions of global climate heterogeneity; and their responses to climate change. The study reveals a distinctive pattern: SHDI, a proxy for climate heterogeneity tends to slow down or decline at lower elevations with increasing temperatures, while at higher elevations, it continues to rise due to continuing cold conditions. Examination of climate simulations, both with and without anthropogenic forcing, confirms that observed changes in climate heterogeneity are primarily attributable to anthropogenic climate change within these high-elevation regions. This study underscores the importance of high-elevation regions as not only custodians of diverse climate types but also potential refuges for species fleeing warmer climates. Climate change is threatening the diversity and stability of global climate patterns. But we're still not completely sure how climate change interacts with elevation to affect climate heterogeneity. In this study, we looked at how climate heterogeneity changes with altitude and responds to climate change. We found that as temperatures rise, the climate diversity tends to decrease at lower elevations, but it increases at higher elevations. We used climate simulations to show that these changes can be attributed to anthropogenic climate change. This study shows that high-elevation regions are important because they can sustain diverse climates and are likely to be a safe haven for plants and animals when climate diversity continues to decline at lower elevations. We employed a high-resolution climate data set to analyze changes in global climate heterogeneity With increasing temperatures, global climate heterogeneity amplifies at higher elevations, while diminishing at lower altitudes Anthropogenic climate change primarily drives alterations in climate heterogeneity at higher elevations
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| 4. |
- He, B., et al.
(författare)
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Lengthening Dry Spells Intensify Summer Heatwaves
- 2022
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 49:19
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Tidskriftsartikel (refereegranskat)abstract
- A lengthening of dry spells (DSLs) has been reported by some regional studies, but its linkage with heatwaves via the feedback between soil moisture and air temperature is still not clear on the global and continental scales. Here we examine increases in the length of DSLs during summer over the global continents using in situ precipitation records. Globally, the average DSL has increased by 0.46 day/decade since the 1970s along with increased high-pressure anomalies which are found to be an important reason for the intensification of heatwaves as suggested by the robust and widespread relationships between the DSL and heatwave duration and severity in the northern extratropics. The average DSL associated with a heatwave declined over lands, implying a strengthening coupling between precipitation anomalies and heatwaves. The findings of this study suggest that the precipitation variations associated with changes in DSLs should be considered in attributions of temperature extremes.
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| 5. |
- Hu, Zengyun, et al.
(författare)
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CCHZ-DISO: A Timely New Assessment System for Data Quality or Model Performance From Da Dao Zhi Jian
- 2022
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 49:23
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Tidskriftsartikel (refereegranskat)abstract
- With the rapid development of big data, assessment of data quality or model performance has become a hot scientific question. However, most existing lots of metrics focus on specific aspects of the assessment, and comprehensive assessment is rare. Therefore, it is very necessary to develop new assessment system. To address this problem, a new assessment system is constructed which is named after Chen, Chen, Hu, and Zhou (CCHZ)-distance between indices of simulation and observation (DISO) according to the contributions of Xi Chen, Deliang Chen, Zengyun Hu, and Qiming Zhou. CCHZ-DISO system builds on the Euclidean Distance and flexible determination of statistical metrics and their numbers. Due to its simplicity and flexibility, CCHZ-DISO can be readily and widely applied to any subject of science. Therefore, it follows the principle of the Chinese philosopher Lao Zi's Da Dao Zhi Jian which means that the most basic truth is very simple.
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| 6. |
- Huang, Jianbin, et al.
(författare)
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The Amplified Arctic Warming in the Recent Decades may Have Been Overestimated by CMIP5 Models
- 2019
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:22, s. 13338-13345
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Tidskriftsartikel (refereegranskat)abstract
- ©2019. The Authors. Realistically representing the Arctic amplification in global climate models (GCMs) represents a key to accurately predict the climate system's response to increasing anthropogenic forcings. We examined the amplified Arctic warming over the past century simulated by 36 state-of-the-art GCMs against observation. We found a clear difference between the simulations and the observation in terms of the evolution of the secular warming rates. The observed rates of the secular Arctic warming increase from 0.14 °C/10a in the early 1890s to 0.21 °C/10a in the mid-2010s, while the GCMs show a negligible trend to 0.35 °C/10a at the corresponding times. The overestimation of the secular warming rate in the GCMs starts from the mid-twentieth century and aggravates with time. Further analysis indicates that the overestimation mainly comes from the exaggerated heating contribution from the Arctic sea ice melting. This result implies that the future secular Arctic warming may have been over-projected.
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| 7. |
- Li, Tiewei, et al.
(författare)
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Increasing Sensitivity of Tree Radial Growth to Precipitation
- 2024
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Ingår i: Geophysical Research Letters. - 1944-8007 .- 0094-8276. ; 51:16
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Tidskriftsartikel (refereegranskat)abstract
- The sensitivity of tree growth to precipitation regulates their responses to drought, and is a crucial metric for predicting ecosystem dynamics and vulnerability. Sensitivity may be changing with continuing climate change, yet a comprehensive assessment of its change is still lacking. We utilized tree ring measurements from 3,044 sites, climate data and CO2 concentrations obtained from monitoring stations, combined with dynamic global vegetation models to investigate spatiotemporal changes in the sensitivity over the past century. We observed an increasing sensitivity since around 1950. This increased sensitivity was particularly pronounced in arid biomes due to the combined effect of increased precipitation and elevated CO2. While elevated CO2 reduced the sensitivity of the humid regions, the intensified water pressure caused by decreased precipitation still increased the sensitivity. Our findings suggest an escalating vulnerability of tree growth to precipitation change, which may increase the risk of tree mortality under future intensified drought.
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| 8. |
- Liu, B., et al.
(författare)
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Global and Polar Region Temperature Change Induced by Single Mega Volcanic Eruption Based on Community Earth System Model Simulation
- 2020
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 47:18
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Tidskriftsartikel (refereegranskat)abstract
- In order to understand the pure long-term influence of single mega volcanic eruption (SMVE) of universal significance on global and polar region temperature changes, the AD 1258 Samalas mega volcanic eruption in Indonesia which is the largest eruption over the past millennium is selected as an ideal eruption for simulation study based on Community Earth System Model. Both reconstructions and simulations show that the Northern Hemisphere experienced nearly two decades of strong cooling after the Samalas mega eruption. The significant cooling in the Arctic lasts for 16 years, while the cooling in the Antarctic lasts only 2 years. As the volcanic aerosol gradually disappears, stronger cooling occurs in Arctic winter, and warming occurs in Antarctic winter. This asymmetric temperature changes over Arctic and Antarctic after SMVE (such as Samalas) are caused by the combined effects of albedo feedback and ocean-atmosphere heat exchange related to sea ice.
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| 9. |
- Liu, Fei, et al.
(författare)
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How Do Tropical, Northern Hemispheric, and Southern Hemispheric Volcanic Eruptions Affect ENSO Under Different Initial Ocean Conditions?
- 2018
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 45:23, s. 13041-13049
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Tidskriftsartikel (refereegranskat)abstract
- ©2018. American Geophysical Union. All Rights Reserved. Current understanding of volcanic effects on El Niño–Southern Oscillation in terms of eruption type and initial ocean condition (IOC) remains elusive. We use last-millennium proxy reconstructions to show how volcanic impacts depend on eruption type and IOC. When the IOC is not a strong El Niño, it is likely that an El Niño will mature in the second winter following 79% (p<0.01) of Northern Hemispheric eruptions and in the first winter following 81% (p<0.01) of tropical and 69% of Southern Hemispheric eruptions. For a strong El Niño-IOC, no significant El Niño will occur in the first winter after any type of eruption. The eruptions need to be large enough to cause these diverse effects. Our last-millennium simulation confirms the IOC effect, except that a La Niña occurs in the first winter following most tropical eruptions due to overestimated volcanic cooling in the model.
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| 10. |
- Liu, W. B., et al.
(författare)
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Global Freshwater Availability Below Normal Conditions and Population Impact Under 1.5 and 2 degrees C Stabilization Scenarios
- 2018
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 45:18, s. 9803-9813
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Tidskriftsartikel (refereegranskat)abstract
- Based on the large ensembles of the half a degree additional warming, prognosis, and projected impacts historical, +1.5 and +2 degrees C experiments, we quantify changes in the magnitude of water availability (i.e., precipitation minus actual evapotranspiration; a function of monthly precipitation flux, latent heat flux, and surface air temperature) below normal conditions (less than median, e.g., 20th percentile water availability). We found that, relative to the historical experiment, water availability below normal conditions of the +1.5 and +2 degrees C experiments would decrease in the midlatitudes and the tropics, indicating that hydrological drought is likely to increase in warmer worlds. These cause more (less) people in East Asia, Central Europe, South Asia, and Southeast Asia (West Africa and Alaska/Northwest Canada) to be exposed to water shortage. Stabilizing warming at 1.5 degrees C instead of 2 degrees C would limit population impact in most of the regions, less effective in Alaska/Northwest Canada, Southeast Asia, and Amazon. Globally, this reduced population impact is similar to 117 million people. Plain Language Summary This study emerges from the lack of scientific investigations to inform climate policy about differences between two global warming targets (i.e., 1.5 and 2 degrees C) for the "Intergovernmental Panel on Climate Change Special Report on Global Warming of 1.5 degrees C." We seek to understand the following: How would water availability below normal conditions (the drier end of hydrological extremes) change at these targets? How would they affect the water shortage of human society? Could we limit the impact by stabilizing the global warming at 1.5 degrees C instead of 2 degrees C? To address these questions, we employ the HAPPI (half a degree additional warming, prognosis, and projected impacts) experiments, explicitly designed to differentiate impacts between these targets. Relative to the historical period, future water availability below normal conditions (less than median, e.g., 20th percentile or lower) would decrease in the midlatitudes and the tropics; the globe and most of the regions would endure water shortages. Relative to the 2 degrees C warming target, stabilizing temperature increase at 1.5 degrees C would constrain adverse impact on people suffering water shortages in most of the regions (particularly Central Europe, East Africa, East Asia, South Asia, and West Africa) but ineffective in Alaska/Northwest Canada, Southeast Asia, and Amazon. A global sum of this reduced risk is similar to 117 million people.
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