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  • 1
    Publication Date: 2013-01-03
    Description: [1]  CMIP5 multimodel ensemble projection of midlatitude storm track changes has been examined. Storm track activity is quantified by temporal variance of meridional wind and sea level pressure (psl), as well as cyclone track statistics. For the Southern Hemisphere (SH), CMIP5 models project clear poleward migration, upward expansion, and intensification of the storm track. For the Northern Hemisphere (NH), the models also project some poleward shift and upward expansion of the storm track in the upper troposphere/lower stratosphere, but mainly weakening of the storm track toward its equatorward flank in the troposphere. Consistent with these, CMIP5 models project significant increase in the frequency of extreme cyclones during the SH cool season, but significant decrease in such events in the NH. Comparisons with CMIP3 projections indicate high degrees of consistency for SH projections, but significant differences are found in the NH. Overall, CMIP5 models project larger decrease in storm track activity in the NH troposphere, especially over North America in winter, where psl variance as well as cyclone frequency and amplitude are all projected to decrease significantly. In terms of climatology, similar to CMIP3, most CMIP5 models simulate storm tracks that are too weak and display equatorward biases in their latitude. These biases have also been related to future projections. In the NH, the strength of a model's climatological storm track is negatively correlated with its projected amplitude change under global warming, while in the SH, models with large equatorward biases in storm track latitude tend to project larger poleward shifts.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
    Publication Date: 2016-02-28
    Description: Extratropical cyclones cause much of the high impact weather over the mid-latitudes. With increasing greenhouse gases, enhanced high-latitude warming will lead to weaker cyclone activity. Here we show that between 1979 and 2014, the number of strong cyclones in Northern Hemisphere in summer has decreased at a rate of 4% per decade, with even larger decrease found near northeastern North America. Climate models project a decrease in summer cyclone activity, but the observed decreasing rate is near the fastest projected. Decrease in summer cyclone activity will lead to decrease in cloud cover, giving rise to higher maximum temperature, potentially enhancing the increase in maximum temperature by 0.5 K or more over some regions. We also show that climate models may have biases in simulating the positive relationship between cyclone activity and cloud cover, potentially under-estimating the impacts of cyclone decrease on accentuating the future increase in maximum temperature.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
    Publication Date: 2013-12-11
    Description: [1]  Cyclones are responsible for much of the high impact weather in the extratropics, thus how they will change under global warming is of great concern. Several studies have used the multi-model climate simulations conducted under Phase 5 of the Coupled Model Intercomparison Project (CMIP5) to examine such changes. One study suggested that the frequency of strong cyclones is projected to decrease over the North Pacific, while another concluded that this frequency will increase. [2]  A single tracking algorithm has been used to derive cyclone statistics from 23 CMIP5 simulations using two different definitions of cyclones: cyclones as minima in total sea level pressure (SLP), or cyclones as minima in SLP perturbations about a large scale, low frequency background. When cyclones are defined by total SLP, the frequency of deep cyclones over the Pacific is projected to increase, while if cyclones are defined as perturbations, this frequency is projected to decrease. These differences are shown to be due to a projected deepening of the climatological mean Aleutian low. [3]  In view of these results, it is important to critically assess how cyclones should be defined. Preliminary results suggest that among CMIP5 simulations, over the Pacific, both the projected changes in the frequency of high wind events and mean available potential energy are better correlated with the projected changes in the frequency of cyclones defined as perturbations. It is concluded that more research should be done to quantify and understand the impacts of the different definitions of cyclones.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
    Publication Date: 2012-12-15
    Description: CMIP5 multimodel ensemble projection of midlatitude storm track changes has been examined. Storm track activity is quantified by temporal variance of meridional wind and sea level pressure (psl), as well as cyclone track statistics. For the Southern Hemisphere (SH), CMIP5 models project clear poleward migration, upward expansion, and intensification of the storm track. For the Northern Hemisphere (NH), the models also project some poleward shift and upward expansion of the storm track in the upper troposphere/lower stratosphere, but mainly weakening of the storm track toward its equatorward flank in the troposphere. Consistent with these, CMIP5 models project significant increase in the frequency of extreme cyclones during the SH cool season, but significant decrease in such events in the NH. Comparisons with CMIP3 projections indicate high degrees of consistency for SH projections, but significant differences are found in the NH. Overall, CMIP5 models project larger decrease in storm track activity in the NH troposphere, especially over North America in winter, where psl variance as well as cyclone frequency and amplitude are all projected to decrease significantly. In terms of climatology, similar to CMIP3, most CMIP5 models simulate storm tracks that are too weak and display equatorward biases in their latitude. These biases have also been related to future projections. In the NH, the strength of a model's climatological storm track is negatively correlated with its projected amplitude change under global warming, while in the SH, models with large equatorward biases in storm track latitude tend to project larger poleward shifts.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
    Publication Date: 2019
    Description: Abstract A planar array of dipoles with matched loads in air backed by a ground plane at a quarter wavelength would become an absorber with a −10‐dB bandwidth of about 10%. Obviously, if the air substrate behind the dipoles is replaced by a dielectric substrate, the physical thickness of the absorber, saying a quarter wavelength, can be reduced. Meanwhile, as the incident waves illuminate on the absorber, the reflections are mainly from three ways, which are the air‐dielectric interface, the back scattering of the loaded dipole as a receiving antenna, and the ground plane. This paper shows that with the combination of the three reflections and a suitable dielectric constant together with its dipole separations, the −10‐dB bandwidth of the absorber increases to 20% and beyond. To further decrease the thickness of the dipole‐based absorber, inductive ground plane is introduced to replace the normal ground; phase analyses of the dipole‐based absorber with inductive ground is also given to explain the feasibility of the proposed technique. Both simulations and measurement results show that, with the proposed technique above, not only the thickness of the dipole‐based absorber can be decreased from a quarter wavelength to less than fifth wavelength, but also the −10‐dB bandwidth of the reflection can be enhanced from 20% to 48% or so.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
    Publication Date: 2018-03-12
    Description: Storm tracks, defined as the preferred regions of extratropical synoptic-scale disturbances, have remarkable impacts on global weather and climate systems. Causes of interannual storm track variation have been investigated mostly from a troposphere perspective. As shown in this study, Northern Hemisphere winter storm tracks are significantly modulated by the tropical stratosphere through the Quasi-Biennial Oscillation (QBO). The North Pacific storm track (NPST) shifts poleward during the easterly QBO winters associated with a dipole change in the eddy refraction and baroclinicity. The North Atlantic storm track (NAST) varies vertically with a downward shrinking (upward expansion) in easterly (westerly) QBO winters associated with the change of the tropopause height. These results not only fill the knowledge gap of QBO-storm track relationship but also suggest a potential route to improve the seasonal prediction of extratropical storm activities owing to the high predictability of the QBO.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
    Publication Date: 1996-02-01
    Print ISSN: 0003-021X
    Electronic ISSN: 1558-9331
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Published by Wiley
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  • 8
    Publication Date: 2015-10-03
    Description: Open-pit mining activities in the oil sands region of Alberta, Canada, create disturbed lands that, by law, must be reclaimed to a land capability equivalent to that existed before the disturbance. Re-establishment of forest cover will be affected by the production and turnover rate of fine roots. However, the relationship between fine root dynamics and tree growth has not been studied in reclaimed oil sands sites. Fine root properties (root length density, mean surface area, total root biomass, and rates of root production, turnover, and decomposition) were assessed from May to October 2011 and 2012 using sequential coring and ingrowth core methods in lodgepole pine ( Pinus contorta Dougl.) and white spruce ( Picea glauca (Moench.) Voss) stands. The pine and spruce stands were planted on peat mineral soil mix placed over tailings sand and overburden substrates, respectively, in reclaimed oil sands sites in Alberta. We selected stands that form a productivity gradient (low, medium, and high productivities) of each tree species based on differences in tree height and diameter at breast height (DBH) increments. In lodgepole pine stands, fine root length density and fine root production, and turnover rates were in the order of high 〉 medium 〉 low productivity sites and were positively correlated with tree height and DBH and negatively correlated with soil salinity ( P  〈   0.05). In white spruce stands, fine root surface area was the only parameter that increased along the productivity gradient and was negatively correlated with soil compaction. In conclusion, fine root dynamics along the stand productivity gradients were closely linked to stand productivity and were affected by limiting soil properties related to the specific substrate used for reconstructing the reclaimed soil. Understanding the impact of soil properties on fine root dynamics and overall stand productivity will help improve land reclamation outcomes. Fine root properties were assessed for 2 years in lodgepole pine and white spruce stands planted on peat-mineral soil mix in reclaimed oil sand sites in Alberta, Canada. The study sites formed a stand productivity gradient of each tree species. Fine root dynamics of lodgepole pine was affected by salinity, while that of spruce by soil compaction along their respective stand productivity gradient. Understanding the impact of soil properties on fine root dynamics and overall stand productivity will help to improve the success of land reclamation.
    Electronic ISSN: 2045-7758
    Topics: Biology
    Published by Wiley
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  • 9
    Publication Date: 2014-07-13
    Description: A 12-m diameter radio telescope will be deployed to the Summit Station in Greenland to provide direct confirmation of a Super Massive Black Hole (SMBH) by observing its shadow image in the active galaxy M87. The telescope (Greenland Telescope: GLT) is to become one of the Very Long Baseline Interferometry (VLBI) stations at sub-millimeter (submm) regime, providing the longest baseline 〉 9,000 km to achieve an exceptional angular resolution of 20 µas at 350 GHz, which will enable us to resolve the shadow size of ~40 µas. The triangle with the longest baselines formed by the GLT, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, and the Submillimeter Array (SMA) in Hawaii will play a key role for the M87 observations. We have been working on the image simulations based on realistic conditions for a better understanding of the possible observed images. In parallel, retrofitting of the telescope and the site developments are in progress. Based on three years of opacity monitoring at 225 GHz, our measurements indicate that the site is excellent for submm observations, comparable to the ALMA site. The GLT is also expected to make single-dish observations up to 1.5 THz.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 10
    Publication Date: 2013-04-09
    Description: [1]  Many studies have attempted to estimate the equilibrium climate sensitivity (CS) to the doubling of CO 2 concentrations. One common methodology is to compare versions of Earth Models of Intermediate Complexity (EMICs) to spatially and/or temporally averaged historical observations. Despite the persistent efforts, CS remains uncertain. It is, thus far, unclear what is driving this uncertainty. Moreover, the effects of the internal climate variability on the CS estimates obtained using this method have not received thorough attention in the literature. [2]  Using a statistical approximator (“emulator”) of an EMIC, we show in an observation system simulation study, that unresolved internal climate variability appears to be a key driver of CS uncertainty (as measured by the 68% credible interval). We first simulate many realizations of pseudo-observations from an emulator at a “true” prescribed CS, and then re-estimate the CS using the pseudo-observations and an inverse parameter estimation method. [3]  We demonstrate that a single realization of the internal variability can result in a sizable discrepancy between the best CS estimate and the truth. Specifically, the average discrepancy is 0.84 °C, with the feasible range up to several °C. The results open the possibility that recent climate sensitivity estimates from global observations and EMICs are systematically considerably lower or higher than the truth, since they are typically based on the same realization of climate variability. This possibility should be investigated in future work. We also find that estimation uncertainties increase at higher climate sensitivities, suggesting that a high CS might be difficult to detect.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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