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Journal Articles

Each year NCAR/UCAR scientists and research staff disseminate the results of their research by publishing hundreds of articles. Our collection of Journal Articles contains peer-reviewed journal articles and published conference proceedings. We provide full text access to article where possible.


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A GLOBAL VIEW OF VELOCITY FLUCTUATIONS IN THE CORONA BELOW 1.3 WITH CoMP
The Coronal Multi-channel Polarimeter (CoMP) has previously demonstrated the presence of Doppler velocity fluctuations in the solar corona. The observed fluctuations are thought to be transverse waves, i.e., highly incompressible motions whose restoring force is dominated by the magnetic tension, some of which demonstrate clear periodicity. We aim to exploit CoMP's ability to provide high cadence observations of the off-limb corona to investigate the properties of velocity fluctuations in a range of coronal features, providing insight into how (whether) the properties of the waves are influenced by the varying magnetic topology in active regions, quiet Sun and open field regions. An analysis of Doppler velocity time-series of the solar corona from the 10747 angstrom Iron XIII line is performe, determining the velocity power spectrum and. using it as a tool to probe wave behavior. Further, the average phase speed and density for each region are estimated and used to compute the spectra for energy density and energy flux. In addition, we assess the noise levels associated with the CoMP data, deriving analytic formulae for the uncertainty on Doppler velocity measurements and providing a comparison by estimating the noise from the data. It is found that the entire corona is replete with transverse wave behavior. The corresponding power spectra indicate that the observed velocity fluctuations are predominately generated by stochastic processes, with the spectral slope of the power varying between the different magnetic regions. Most strikingly, all power spectra reveal the presence of enhanced power occurring at similar to 3 mHz, potentially implying that the excitation of coronal transverse waves by p-modes is a global phenomenon.
DETECTION OF SOLAR-LIKE OSCILLATIONS, OBSERVATIONAL CONSTRAINTS, AND STELLAR MODELS FOR CYG, THE BRIGHTEST STAR OBSERVED BY THE MISSION
theta Cygni is an F3 spectral type magnitude V = 4.48 main-sequence star that was the brightest star observed by the original Kepler spacecraft mission. Short-cadence (58.8 s) photometric data using a custom aperture were first obtained during Quarter 6 ( 2010 June-September). and subsequently in Quarters 8 and 12-17. We present analyses of solar-like oscillations based on Q6 and Q8 data, identifying angular degree l = 0, 1, and 2 modes with frequencies of 1000-2700 mu Hz, a large frequency separation of 83.9 +/- 0.4 mu Hz, and maximum oscillation amplitude at frequency nu(max) = 1829 +/- 54 mu Hz. We also present analyses of new ground-based spectroscopic observations, which, combined with interferometric angular diameter measurements, give T-eff = 6697 +/- 78 K, radius 1.49 +/- 0.03 Re-circle dot, [Fe/H] = -0.02 +/- 0.06 dex, and log g = 4.23 +/- 0.03. We calculate stellar models matching these constraints using the Yale Rotating Evolution Code and the Asteroseismic Modeling Portal. The best-fit models have masses of 1.35-1.39 M-circle dot and ages of 1.0-1.6 Gyr. theta Cyg's T-eff and log g place it cooler than the red edge of the gamma Doradus instability region established from pre-Kepler ground-based observations, but just at the red edge derived from pulsation modeling. The pulsation models show gamma Dor gravity modes driven by the convective blocking mechanism, with frequencies of 1-3 cycles per day (11 to 33 mu Hz). However, gravity modes were not seen in Kepler data; one signal at 1.776 cycles per day (20.56 mu Hz) may be attributable to a faint, possibly background, binary.
Effect of increasing CO on the terrestrial carbon cycle
Feedbacks from the terrestrial carbon cycle significantly affect future climate change. The CO2 concentration dependence of global terrestrial carbon storage is one of the largest and most uncertain feedbacks. Theory predicts the CO2 effect should have a tropical maximum, but a large terrestrial sink has been contradicted by analyses of atmospheric CO2 that do not show large tropical uptake. Our results, however, show significant tropical uptake and, combining tropical and extratropical fluxes, suggest that up to 60% of the present-day terrestrial sink is caused by increasing atmospheric CO2. This conclusion is consistent with a validated subset of atmospheric analyses, but uncertainty remains. Improved model diagnostics and new space-based observations can reduce the uncertainty of tropical and temperate zone carbon flux estimates. This analysis supports a significant feedback to future atmospheric CO2 concentrations from carbon uptake in terrestrial ecosystems caused by rising atmospheric CO2 concentrations. This feedback will have substantial tropical contributions, but the magnitude of future carbon uptake by tropical forests also depends on how they respond to climate change and requires their protection from deforestation.
Equatorial ionospheric plasma drifts and O concentration enhancements associated with disturbance dynamo during the 2015 St. Patrick's Day magnetic storm
Disturbance dynamo is an important dynamic process during magnetic storms. However, very few direct observations of dynamo-induced plasma drifts and ion composition changes in the equatorial ionosphere are available. In this study, we use measurements of the Defense Meteorological Satellite Program (DMSP) satellites to identify the characteristics of the disturbance dynamo process in the topside equatorial ionosphere near dawn during the magnetic storm with a minimum Dst of -223 nT on 17 March 2015. Data from four DMSP satellites with equatorial crossings at 0245, 0430, 0630, and 0730 LT are available for this case. The dynamo process was first observed in the postmidnight sector 3-4.7 h after the beginning of the storm main phase and lasted for 31 h, covering the second storm intensification and the initial 20 h of the recovery phase. The dynamo vertical ion drift was upward (up to 150-200 m s(-1)) in the postmidnight sector and downward (up to similar to 80 m s(-1)) in the early morning sector. The dynamo zonal ion drift was westward at these locations and reached similar to 100 m s(-1). The dynamo process caused large enhancements of the O+ concentration (the ratio of the oxygen ion density to the total ion density) at the altitude of 840 km near dawn. The O+ concentration increased from below 60% during the prestorm period to 80-90% during the storm time. More specifically, the O+ density was increased, and the H+ density was decreased. The variations of the O+ concentration were well correlated with the vertical ion drift.
Experimentally Determined Site-Specific Reactivity of the Gas-Phase OH and Cl + -Butanol Reactions Between 251 and 340 K
Product branching ratios for the gas-phase reactions of i-butanol, (CH3)2CHCH2OH, with OH radicals (251, 294, and 340 K) and Cl atoms (294 K) were quantified in an environmental chamber study and used to interpret i-butanol site-specific reactivity. i-Butyraldehyde, acetone, acetaldehyde, and formaldehyde were observed as major stable end products in both reaction systems with carbon mass balance indistinguishable from unity. Product branching ratios for OH oxidation were found to be temperature-dependent with the α, β, and γ channels changing from 34 ± 6 to 47 ± 1%, from 58 ± 6 to 37 ± 9%, and from 8 ± 1 to 16 ± 4%, respectively, between 251 and 340 K. Recommended temperature-dependent site-specific modified Arrhenius expressions for the OH reaction rate coefficient are (cm3 molecule-1 s-1): kα(T) = 8.64 × 10-18 × T1.91exp(666/T); kβ(T) = 5.15 × 10-19 × T2.04exp(1304/T); kγ(T) = 3.20 × 10-17 × T1.78exp(107/T); kOH(T) = 2.10 × 10-18 × T2exp(−23/T), where kTotal(T) = kα(T) + kβ(T) + kγ(T) + kOH(T). The expressions were constrained using the product branching ratios measured in this study and previous total phenomenological rate coefficient measurements. The site-specific expressions compare reasonably well with recent theoretical work. It is shown that use of i-butanol would result in acetone as the dominant degradation product under most atmospheric conditions.
Global burden of mortalities due to chronic exposure to ambient PM from open combustion of domestic waste
Uncontrolled combustion of domestic waste has been observed in many countries, creating concerns for air quality; however, the health implications have not yet been quantified. We incorporate the Wiedinmyer et al (2014 Environ. Sci. Technol. 48 9523-30) emissions inventory into the global chemical-transport model, GEOS-Chem, and provide a first estimate of premature adult mortalities from chronic exposure to ambient PM2.5 from uncontrolled combustion of domestic waste. Using the concentration-response functions (CRFs) of Burnett et al (2014 Environ. Health Perspect. 122 397-403), we estimate that waste-combustion emissions result in 270 000 (5th–95th: 213 000-328 000) premature adult mortalities per year. The confidence interval results only from uncertainty in the CRFs and assumes equal toxicity of waste-combustion PM2.5 to all other PM2.5 sources. We acknowledge that this result is likely sensitive to choice of chemical-transport model, CRFs, and emission inventories. Our central estimate equates to 9% of adult mortalities from exposure to ambient PM2.5 reported in the Global Burden of Disease Study 2010. Exposure to PM2.5 from waste combustion increases the risk of premature mortality by more than 0.5% for greater than 50% of the population. We consider sensitivity simulations to uncertainty in waste-combustion emission mass, the removal of waste-combustion emissions, and model resolution. A factor-of-2 uncertainty in waste-combustion PM2.5 leads to central estimates ranging from 138 000 to 518 000 mortalities per year for factors-of-2 reductions and increases, respectively. Complete removal of waste combustion would only avoid 191 000 (5th-95th: 151 000-224 000) mortalities per year (smaller than the total contributed premature mortalities due to nonlinear CRFs). Decreasing model resolution from 2° × 2.5° to 4° × 5° results in 16% fewer mortalities attributed to waste-combustion PM2.5, and over Asia, decreasing resolution from 0.5° × 0.666° to 2° × 2.5° results in 21% fewer mortalities attributed to waste-combustion PM2.5. Owing to coarse model resolution, our global estimates of premature mortality from waste-combustion PM2.5 are likely a lower bound.
Isotopic ordering in atmospheric O as a tracer of ozone photochemistry and the tropical atmosphere
The distribution of isotopes within O-2 molecules can be rapidly altered when they react with atomic oxygen. This mechanism is globally important: while other contributions to the global budget of O-2 impart isotopic signatures, the O(P-3)+O-2 reaction resets all such signatures in the atmosphere on subdecadal timescales. Consequently, the isotopic distribution within O-2 is determined by O-3 photochemistry and the circulation patterns that control where that photochemistry occurs. The variability of isotopic ordering in O-2 has not been established, however. We present new measurements of (OO)-O-18-O-18 in air (reported as (36) values) from the surface to 33km altitude. They confirm the basic features of the clumped-isotope budget of O-2: Stratospheric air has higher (36) values than tropospheric air (i.e., more (OO)-O-18-O-18), reflecting colder temperatures and fast photochemical cycling of O-3. Lower (36) values in the troposphere arise from photochemistry at warmer temperatures balanced by the influx of high-(36) air from the stratosphere. These observations agree with predictions derived from the GEOS-Chem chemical transport model, which provides additional insight. We find a link between tropical circulation patterns and regions where (36) values are reset in the troposphere. The dynamics of these regions influences lapse rates, vertical and horizontal patterns of O-2 reordering, and thus the isotopic distribution toward which O-2 is driven in the troposphere. Temporal variations in (36) values at the surface should therefore reflect changes in tropospheric temperatures, photochemistry, and circulation. Our results suggest that the tropospheric O-3 burden has remained within a 10% range since 1978.
Modeling the Geographic Distribution of and (Acari: Ixodidae) in the Contiguous United States
In addition to serving as vectors of several other human pathogens, the black-legged tick, Ixodes scapularis Say, and western black-legged tick, Ixodes pacificus Cooley and Kohls, are the primary vectors of the spirochete (Borrelia burgdorferi) that causes Lyme disease, the most common vector-borne disease in the United States. Over the past two decades, the geographic range of I. pacificus has changed modestly while, in contrast, the I. scapularis range has expanded substantially, which likely contributes to the concurrent expansion in the distribution of human Lyme disease cases in the Northeastern, North-Central and Mid-Atlantic states. Identifying counties that contain suitable habitat for these ticks that have not yet reported established vector populations can aid in targeting limited vector surveillance resources to areas where tick invasion and potential human risk are likely to occur. We used county-level vector distribution information and ensemble modeling to map the potential distribution of I. scapularis and I. pacificus in the contiguous United States as a function of climate, elevation, and forest cover. Results show that I. pacificus is currently present within much of the range classified by our model as suitable for establishment. In contrast, environmental conditions are suitable for I. scapularis to continue expanding its range into northwestern Minnesota, central and northern Michigan, within the Ohio River Valley, and inland from the southeastern and Gulf coasts. Overall, our ensemble models show suitable habitat for I. scapularis in 441 eastern counties and for I. pacificus in 11 western counties where surveillance records have not yet supported classification of the counties as established.
Modern precipitation δ O and trajectory analysis over the Himalaya-Tibet Orogen from ECHAM5-wiso simulations
Variations in oxygen isotope ratios (O-18) measured from modern precipitation and geologic archives provide a promising tool for understanding modern and past climate dynamics and tracking elevation changes over geologic time. In areas of extreme topography, such as the Tibetan Plateau, the interpretation of O-18 has proven challenging. This study investigates the climate controls on temporal (daily and 6h intervals) and spatial variations in present-day precipitation O-18 (O-18(p)) across the Tibetan Plateau using a 30year record produced from the European Centre/Hamburg ECHAM5-wiso global atmospheric general circulation model (GCM). Results indicate spatial and temporal agreement between model-predicted O-18(p) and observations. Large daily O-18(p) variations of -25 to +5 occur over the Tibetan Plateau throughout the 30 simulation years, along with interannual O-18(p) variations of similar to 2. Analysis of extreme daily O-18(p) indicates that extreme low values coincide with extreme highs in precipitation amount. During the summer, monsoon vapor transport from the north and southwest of the plateau generally corresponds with high O-18(p), whereas vapor transport from the Indian Ocean corresponds with average to low O-18(p). Thus, vapor source variations are one important cause of the spatial-temporal differences in O-18(p). Comparison of GCM and Rayleigh Distillation Model (RDM)-predicted O-18(p) indicates a modest agreement for the Himalaya region (averaged over 86 degrees-94 degrees E), confirming application of the simpler RDM approach for estimating O-18(p) lapse rates across Himalaya.
ON THE FINE STRUCTURE SPLITTING OF THE 3p 3d D AND 3p 3d D LEVELS
We study UV spectra obtained with the SO82-B slit spectrograph on board SKYLAB to estimate the fine structure (FS) splitting of the Cl-like 3p(4)3d(4)D(5/2) and 3p(4)3d(4)D(7/2) levels of Fe X. The splitting is of interest because the Zeeman effect mixes these levels, producing a "magnetically induced transition" (MIT) from 3p(4)3d(4)D(7/2) to 3p(5) (2)p(3/2)(o) for modest magnetic field strengths characteristic of the active solar corona. We estimate the splitting using the Ritz combination formula applied to two lines in the UV region of the spectrum close to 1603.2 angstrom, which decay from the level 3p(4)(D-1)3d (2)G(7/2) to these two lower levels. The MIT and accompanying spin-forbidden transition lie near 257 angstrom. By careful inspection of a deep exposure obtained with the S082B instrument, we derive a splitting of. less than or similar to 7 +/- 3 cm(-1). The upper limit arises because of a degeneracy between the effects of non-thermal line broadening and FS splitting for small values of the latter parameter. Although the data were recorded on photographic film, we solved for optimal values of line width and splitting of 8.3 +/- 0.9 and 3.6 +/- 2.7 cm(-1).
THE MOUNT WILSON OBSERVATORY -INDEX OF THE SUN
The most commonly used index of stellar magnetic activity is the instrumental flux scale of singly ionized calcium H & K line core emission, S, developed by the Mount Wilson Observatory (MWO) HK Project, or the derivative index ${R}_{\mathrm{HK}}^{\prime }$. Accurately placing the Sun on the S scale is important for comparing solar activity to that of the Sun-like stars. We present previously unpublished measurements of the reflected sunlight from the Moon using the second-generation MWO HK photometer during solar cycle 23 and determine cycle minimum ${S}_{23,\min }=0.1634\pm 0.0008$, amplitude ${\rm{\Delta }}{S}_{23}=0.0143\pm 0.0012$, and mean $\langle {S}_{23}\rangle =0.1701\pm 0.0005$. By establishing a proxy relationship with the closely related National Solar Observatory Sacramento Peak calcium K emission index, itself well correlated with the Kodaikanal Observatory plage index, we extend the MWO S time series to cover cycles 15–24 and find on average $\langle {S}_{\min }\rangle =0.1621\pm 0.0008$, $\langle {\rm{\Delta }}{S}_{\mathrm{cyc}}\rangle =0.0145\pm 0.0012$, $\langle {S}_{\mathrm{cyc}}\rangle =0.1694\pm 0.0005$. Our measurements represent an improvement over previous estimates that relied on stellar measurements or solar proxies with non-overlapping time series. We find good agreement from these results with measurements by the Solar-Stellar Spectrograph at Lowell Observatory, an independently calibrated instrument, which gives us additional confidence that we have accurately placed the Sun on the S-index flux scale.
The Resolution Dependence of Contiguous U.S. Precipitation Extremes in Response to CO Forcing
Precipitation extremes have a widespread impact on societies and ecosystems; it is therefore important to understand current and future patterns of extreme precipitation. Here, a set of new global coupled climate models with varying atmospheric resolution has been used to investigate the ability of these models to reproduce observed patterns of precipitation extremes and to investigate changes in these extremes in response to increased atmospheric CO2 concentrations. The atmospheric resolution was increased from 2 degrees x 2 degrees grid cells (typical resolution in the CMIP5 archive) to 0.25 degrees x 0.25 degrees (tropical cyclone permitting). Analysis has been confined to the contiguous United States (CONUS). It is shown that, for these models, integrating at higher atmospheric resolution improves all aspects of simulated extreme precipitation: spatial patterns, intensities, and seasonal timing. In response to 2 x CO2 concentrations, all models show a mean intensification of precipitation rates during extreme events of approximately 3%-4% K-1. However, projected regional patterns of changes in extremes are dependent on model resolution. For example, the highest-resolution models show increased precipitation rates during extreme events in the hurricane season in the U.S. Southeast; this increase is not found in the low-resolution model. These results emphasize that, for the study of extreme precipitation there is a minimum model resolution that is needed to capture the weather phenomena generating the extremes. Finally, the observed record and historical model experiments were used to investigate changes in the recent past. In part because of large intrinsic variability, no evidence was found for changes in extreme precipitation attributable to climate change in the available observed record.

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