ALBERT

Description: Model simulations of future climates predict a poleward expansion of subtropical arid climates at the edges of earth's tropical belt, which would have significant environmental and societal impacts. This expansion may be related to the poleward shift of the Hadley cell edges, where subsidence stabilizes the atmosphere and suppresses precipitation. Understanding the primary drivers of tropical expansion is hampered by the myriad forcing agents in most model projections of future climate. While many previous studies have examined the response of idealized models to simplified climate forcings and the response of comprehensive climate models to more complex climate forcings, none have examined how comprehensive climate models respond to simplified climate forcings. To shed light on robust processes associated with tropical expansion, here we examine how the tropical belt width, as measured by the Hadley cell edges, responds to simplified forcings in the Geoengineering Model Intercomparison Project (GeoMIP). The tropical belt expands in response to a quadrupling of atmospheric carbon dioxide concentrations and contracts in response to a reduction in the solar constant, with a range of a factor of three in the response among nine models. Models with more surface warming and an overall stronger temperature response to quadrupled carbon dioxide exhibit greater tropical expansion, a robust result in spite of intermodel differences in the mean Hadley cell width, parameterizations, and numerical schemes. Under a scenario where the solar constant is reduced to offset an instantaneous quadrupling of carbon dioxide, the Hadley cells remain at their preindustrial width, despite the residual stratospheric cooling associated with elevated carbon dioxide levels. Quadrupled carbon dioxide produces greater tropical belt expansion in the Southern Hemisphere than in the Northern Hemisphere. This expansion is strongest in austral summer and autumn. Ozone depletion has been argued to cause this pattern of changes in observations and model experiments, but the results here indicate that seasonally- and hemispherically-asymmetric tropical expansion can be a basic response of the general circulation to climate forcings.

Description: Observational and modeling studies suggest that Earth's tropical belt has widened over the late 20th century and will continue to widen throughout the 21st century. Yet estimates of tropical width variations differ significantly across studies. This uncertainty, to an unknown degree, is partly due to the large variety of methods used in studies of the tropical width. Here, methods for eight commonly-used metrics of the tropical width are implemented in a Tropical-width Diagnostics code package (TropD) in the MATLAB programming language. To consolidate the various methods, the operations used in each of the implemented methods are reduced to two basic calculations: finding the latitude of a zero crossing, and finding the latitude of a maximum. A detailed description of the methods implemented in the code and of the code syntax is provided, followed by a method sensitivity analysis for each of the metrics. The analysis provides information on how to reduce the methodological component of the uncertainty associated with fundamental aspects of the calculations, such as monthly vs. seasonal averaging biases, grid dependence, sensitivity to noise, and sensitivity to threshold criteria.

Description: Observational and modeling studies suggest that Earth's tropical belt has widened over the late 20th century and will continue to widen throughout the 21st century. Yet, estimates of tropical-width variations differ significantly across studies. This uncertainty, to an unknown degree, is partly due to the large variety of methods used in studies of the tropical width. Here, methods for eight commonly used metrics of the tropical width are implemented in the Tropical-width Diagnostics (TropD) code package in the MATLAB programming language. To consolidate the various methods, the operations used in each of the implemented methods are reduced to two basic calculations: finding the latitude of a zero crossing and finding the latitude of a maximum. A detailed description of the methods implemented in the code and of the code syntax is provided, followed by a method sensitivity analysis for each of the metrics. The analysis provides information on how to reduce the methodological component of the uncertainty associated with fundamental aspects of the calculations, such as monthly vs. seasonal averaging biases, grid dependence, sensitivity to noise, and sensitivity to threshold criteria.

Description: Model simulations of future climates predict a poleward expansion of subtropical arid climates at the edges of Earth's tropical belt, which would have significant environmental and societal impacts. This expansion may be related to the poleward shift of the Hadley cell edges, where subsidence stabilizes the atmosphere and suppresses precipitation. Understanding the primary drivers of tropical expansion is hampered by the myriad forcing agents in most model projections of future climate. While many previous studies have examined the response of idealized models to simplified climate forcings and the response of comprehensive climate models to more complex climate forcings, few have examined how comprehensive climate models respond to simplified climate forcings. To shed light on robust processes associated with tropical expansion, here we examine how the tropical belt width, as measured by the Hadley cell edges, responds to simplified forcings in the Geoengineering Model Intercomparison Project (GeoMIP). The tropical belt expands in response to a quadrupling of atmospheric carbon dioxide concentrations and contracts in response to a reduction in the solar constant, with a range of a factor of 3 in the response among nine models. Models with more surface warming and an overall stronger temperature response to quadrupled carbon dioxide exhibit greater tropical expansion, a robust result in spite of inter-model differences in the mean Hadley cell width, parameterizations, and numerical schemes. Under a scenario where the solar constant is reduced to offset an instantaneous quadrupling of carbon dioxide, the Hadley cells remain at their preindustrial width, despite the residual stratospheric cooling associated with elevated carbon dioxide levels. Quadrupled carbon dioxide produces greater tropical belt expansion in the Southern Hemisphere than in the Northern Hemisphere. This expansion is strongest in austral summer and autumn. Ozone depletion has been argued to cause this pattern of changes in observations and model experiments, but the results here indicate that seasonally and hemispherically asymmetric tropical expansion can be a basic response of the general circulation to climate forcings.

Description: Jules Verne’s adventure novels Five Weeks in a Balloon and Around the World in 80 Days highlighted some of the great technological advances of the late 19th century that revolutionized travel and captured the imagination of the public [ Verne, 1863, 1873]. Among those inspired by the novels was Nellie Bly, an American journalist for the New York World , who set off in November 1889 to complete a journey by rail and steamship, following Verne’s imagined path around the world in a record 72 days [ Bly, 1890] (Figure 1). Fig. 1. In 1889–1890, real-life New York World reporter Nellie Bly completed Jules Verne’s imagined path (shown here) around the world in slightly less than Verne’s “80 days.” Neither Bly’s journey nor Verne’s Around the World in 80 Days actually involved balloon travel, but Verne’s book drew on his previous novel Five Weeks in a Balloon. The earlier novel inspired the idea of incorporating balloon travel for one leg of the trip in the 1956 movie Around the World in 80 Days that has become a beloved misconception about Verne’s later book. Credit: Roke/Wikimedia Commons CC BY-SA 3.0 Bly’s accounts demonstrated how new technology, such as the transcountry railroads in the United States and India and the Suez Canal, brought exotic destinations within reach. The revolutionary development of submarine cables and the electric telegraph allowed Bly to keep her editors, and the larger connected world, aware of her progress in near-real time. The France-U.S. collaborative Stratéole 2 project is planning its own series of balloon trips, which will circle the world near the equator for 80 days (more or less), as did these fictional and factual 19th century adventurers, demonstrating new technology and sending new observations from the voyage back via satellite. Drifting with the Winds Scientists with the Stratéole 2 project will release superpressure balloons, designed to drift in the lower stratosphere, from the Seychelles islands in the Indian Ocean (Figure 2). Superpressure balloons contain a fixed amount of helium sealed inside an envelope that does not stretch. This type of balloon is not fully inflated when it is launched, but it expands to its full volume as it rises to an altitude where the gas density inside the balloon matches the density of the surrounding air and where it drifts with the wind. Fig. 2. Early test flights of the French National Center for Space Studies superpressure balloon system during February–May 2010 followed a tropical route. The flight durations of the three balloons were 92, 78, and (yes!) 80 days. The traces of the balloon paths show some wave structure, and the balloon paths reversed direction when the quasi-biennial oscillation, a periodic east–west oscillating feature in tropical lower stratospheric winds, changed phase. Credit: A. Hertzog Each balloon will carry as many as four instruments. As they collect their high-accuracy measurements of meteorological variables, chemical tracers, clouds, and aerosols, their horizontal motions are nearly identical to those of the surrounding air mass. These measurements will advance our knowledge and understanding of cirrus clouds, aerosols, and equatorial waves in the tropical tropopause layer (TTL; the transition region between the troposphere and the stratosphere) and in the lower stratosphere. Shown here is a fully inflated superpressure balloon in the lab at the French National Center for Space Studies (CNES). Credit: Philippe Cocquerez, CNES The Stratéole 2 research program will begin with a five-balloon technology validation campaign in Northern Hemisphere (boreal) fall–winter 2018–2019, followed by 20 balloon flights in boreal fall–winter 2020–2021. In the second campaign, 10 balloons will fly at an altitude near 20 kilometers, just above the TTL, and another 10 will fly near 18 kilometers, within the TTL. From past experience, we expect each balloon to fly for more than 2 months. Typically, a balloon will fly for about 84 days before chaotic atmospheric motions or interactions with Rossby waves push it outside of the deep tropics. A final 20-balloon campaign in 2023–2024 will drift in the opposite direction because of the shifting phase of the quasi-biennial oscillation (QBO), a dominant, periodic east–west oscillating feature in tropical lower stratospheric winds. Challenges Aloft The Stratéole 2 campaign targets the TTL, the primary entry point for tropospheric air into the stratosphere. As air slowly ascends across the TTL, the coldest temperatures encountered at the cold point tropopause (CPT) freeze water vapor into ice crystals. The formation of ice crystals dehydrates the air and regulates the amount of humidity reaching the global stratosphere, giving the TTL an outsized importance considering its geographic extent. The ice crystals form thin cirrus clouds, which have a global impact on the balance between incoming solar radiation and radiation reflected back into space at tropical latitudes. Water vapor and cirrus feedbacks are extremely important in climate system models. The underlying processes that control the formation and sublimation (direct conversion of ice crystals to water vapor) of these clouds remain strongly debated. These processes involve the interplay of deep convection, microphysics, aerosols, wave-induced temperature variations with timescales ranging from minutes to weeks, and the balance of forces driving large-scale slow ascent of air in the tropics. The superposition of wave-induced fluctuations on the average upwelling motion forces the temperatures in the TTL to extreme values at the CPT—less than –94°C at times and well below those expected from radiative equilibrium. These same waves also drive the QBO, which has an important long-range indirect influence on high-latitude seasonal forecasts. The waves, generated by convection below, transport momentum vertically across the TTL and drive QBO wind variations as the momentum dissipates in the stratosphere. Satellite and in situ observations can track the wind reversals of the QBO, but most general circulation models cannot replicate the QBO using current methods. This shortcoming is due to a combination of inadequate spatial resolution and a lack of small-scale wave drag applied at the subgrid scale. Even when models do simulate the QBO, doubts remain on the contribution from various families of waves with different scales and frequencies. As a result, even models that internally generate a QBO were unable to forecast the anomalous disruption of the oscillation that occurred in February 2016 [ Osprey et al., 2016]. Science Objectives This superpressure balloon, shown here at launch, is not fully inflated. As it rises, the volume of helium sealed inside increases until the spherical balloon is fully inflated, giving the balloon a fixed density. Once the balloon has reached the atmospheric level where the air has the same density, it drifts with the wind, providing accurate wind measurements. Credit: Philippe Cocquerez, CNES The overarching objectives of Stratéole 2 are to explore processes that control the transfer of trace gases and momentum between the equatorial upper troposphere and lower stratosphere. The instruments will provide fine-scale measurements of water vapor, temperature, and aerosol/ice at the balloon gondola and also within several kilometers below flight level, documenting air composition and investigating the formation of cirrus in the upper TTL. The balloons also provide unique measurements of equatorial waves over the full spectrum from high-frequency buoyancy waves to planetary-scale equatorial waves, providing information needed to improve representation of these waves in climate models. Stratéole 2 balloons will sample the whole equatorial band from 20°S to 15°N, thus complementing the widespread (but limited-resolution) spaceborne observations and the high-resolution (but geographically restricted) airborne and ground-based measurements from previous field missions. Past balloon campaign measurements sampling the Antarctic stratospheric vortex [see Podglajen et al., 2016] have been used to make accurate estimates of wave momentum fluxes as well as to explain springtime stratospheric ozone loss rates; we expect similar successes with our current campaigns. Stratéole 2 balloon flights will collect measurements over oceanic areas that are otherwise devoid of any stratospheric wind measurements.Other Stratéole 2 science objectives include contributions to operational meteorology and satellite validation. Wind analyses and forecasts have notably large errors in the tropics because sparse tropical wind measurements cannot be modeled in a straightforward way through their dynamical relation to temperature, as they are at higher latitudes. Thus, reducing these errors requires a higher density of measurements. Stratéole 2 balloon flights will address this data shortage by providing unprecedented, accurate wind observations in the equatorial regions of the upper troposphere and lower stratosphere. In particular, the project will collect measurements over oceanic areas that are otherwise devoid of any stratospheric wind measurements. The data will also contribute to the validation of Atmospheric Dynamics Mission Aeolus (ADM-Aeolus) wind products. An innovative European Space Agency mission, ADM-Aeolus, due to be launched in September 2018, is designed to perform the first spaceborne wind lidar measurements, providing unprecedented global coverage. The ensemble of Stratéole 2 instrumentation includes in situ measurements of pressure, temperature, and winds every 30 seconds and less frequently sampled observations of ozone, aerosols, water vapor, and carbon dioxide, plus remotely sensed cloud structure from microlidar and directional radiative fluxes. Instruments providing profiles will include GPS radio occultation receivers that measure temperature profiles to the side of the balloons. Novel reel-down devices suspended as far as 2 kilometers directly below the balloons will also provide profiles to explore the fine-scale distribution of temperature, aerosol/ice, and humidity. Capturing temperature variations in high-resolution profiles, in particular, from the unique balloon platform, is an approach that will provide new insight into equatorial wave processes. Measuring ozone in combination with water vapor and carbon dioxide enables us to discover correlations among these tracers that describe transport processes at the top of the TTL, including convective overshoots that rapidly transport air from the surface into the TTL. Data Dissemination Within 12 months of the end of each balloon campaign, the Stratéole 2 data set will be freely available to the scientific community.The Stratéole 2 data policy is in compliance with World Meteorological Organization (WMO) Resolution 40 (WMO Cg-XII) on the policy and practice for the exchange of meteorological and related data and products. Within 12 months of the end of each balloon campaign, the Stratéole 2 data set will be freely available to the scientific community through the Stratéole 2 Data Archive Center (S2DAC), which is scheduled to launch its website in July 2018. S2DAC will collect and make available the balloon observations and associated ground-based and satellite data, reanalyses, and model outputs. The S2DAC includes a primary, full repository at the Dynamic Meteorology Laboratory (LMD) in France and a secondary mirror site at the Laboratory for Atmospheric and Space Physics (LASP) in Boulder, Colo., in the United States. In addition, during the balloon campaigns, a subset of the Stratéole 2 data set, specifically flight-level winds, will be disseminated on the Global Telecommunication System for their assimilation in numerical weather prediction systems. We invite and encourage the use of Stratéole 2 data by the broader scientific community, and potential users can watch for future campaign updates on the project website. Up, Up, and Away In the spirit of Verne’s imagined use of new technologies and Bly’s real-world application of those technologies to explore the world, the Stratéole 2 campaign will scientifically explore the tropical tropopause and lower stratosphere from a long-duration superpressure balloon platform. The use of multiple balloons will permit extensive exploration of the finely layered features and unique processes occurring in this remote part of the atmosphere. With the involvement of the broader scientific community, analyses of the Stratéole 2 measurements hold promise to provide a new and deeper understanding of these processes and the connections of this region to global chemistry, dynamics, and climate variability. Acknowledgments Major funding for the Stratéole 2 campaign is provided by France’s National Center for Space Studies (CNES) and National Center for Scientific Research (CNRS), as well as the U.S. National Science Foundation (NSF).

Description: Crude oil from oil sands will constitute a substantial share of future global oil demand. Oil sands deposits account for a third of globally proven oil reserves, underlie large natural forested areas, and have extraction methods requiring large volumes of freshwater. Yet little work has been done to quantify some of the main environmental impacts of oil sands operations. Here we examine forest loss and water use for the world's major oil sands deposits. We calculate actual and potential rates of water use and forest loss both in Canadian deposits, where oil sands extraction is already taking place, and in other major deposits worldwide. We estimated that their exploitation, given projected production trends, could result in 1.31 km 3  yr −1 of freshwater demand and 8700 km 2 of forest loss. The expected escalation in oil sands extraction thus portends extensive environmental impacts.

Description: Atmospheric transport model errors are one of the main contributors to the uncertainty affecting CO2 inverse flux estimates. In this study, we determine the leading causes of transport errors over the US upper Midwest with a large set of simulations generated with the Weather Research and Forecasting (WRF) mesoscale model. The various WRF simulations are performed using different meteorological driver datasets and physical parameterizations including planetary boundary layer (PBL) schemes, land surface models (LSMs), cumulus parameterizations and microphysics parameterizations. All the different model configurations were coupled to CO2 fluxes and lateral boundary conditions from the CarbonTracker inversion system to simulate atmospheric CO2 mole fractions. PBL height, wind speed, wind direction, and atmospheric CO2 mole fractions are compared to observations during a month in the summer of 2008, and statistical analyses were performed to evaluate the impact of both physics parameterizations and meteorological datasets on these variables. All of the physical parameterizations and the meteorological initial and boundary conditions contribute 3 to 4 ppm to the model-to-model variability in daytime PBL CO2 except for the microphysics parameterization which has a smaller contribution. PBL height varies across ensemble members by 300 to 400 m, and this variability is controlled by the same physics parameterizations. Daily PBL CO2 mole fraction errors are correlated with errors in the PBL height. We show that specific model configurations systematically overestimate or underestimate the PBL height averaged across the region with biases closely correlated with the choice of LSM, PBL scheme, and cumulus parameterization (CP). Domain average PBL wind speed is overestimated in nearly every model configuration. Both planetary boundary layer height (PBLH) and PBL wind speed biases show coherent spatial variations across the Midwest, with PBLH overestimated averaged across configurations by 300–400 m in the west, and PBL winds overestimated by about 1 m s−1 on average in the east. We find model configurations with lower biases averaged across the domain, but no single configuration is optimal across the entire region and for all meteorological variables. We conclude that model ensembles that include multiple physics parameterizations and meteorological initial conditions are likely to be necessary to encompass the atmospheric conditions most important to the transport of CO2 in the PBL, but that construction of such an ensemble will be challenging due to ensemble biases that vary across the region.

Description: Ozone forms in the Earth's atmosphere from the photodissociation of molecular oxygen, primarily in the tropical stratosphere. It is then transported to the extratropics by the Brewer–Dobson circulation (BDC), forming a protective ozone layer around the globe. Human emissions of halogen-containing ozone-depleting substances (hODSs) led to a decline in stratospheric ozone until they were banned by the Montreal Protocol, and since 1998 ozone in the upper stratosphere is rising again, likely the recovery from halogen-induced losses. Total column measurements of ozone between the Earth's surface and the top of the atmosphere indicate that the ozone layer has stopped declining across the globe, but no clear increase has been observed at latitudes between 60° S and 60° N outside the polar regions (60–90°). Here we report evidence from multiple satellite measurements that ozone in the lower stratosphere between 60° S and 60° N has indeed continued to decline since 1998. We find that, even though upper stratospheric ozone is recovering, the continuing downward trend in the lower stratosphere prevails, resulting in a downward trend in stratospheric column ozone between 60° S and 60° N. We find that total column ozone between 60° S and 60° N appears not to have decreased only because of increases in tropospheric column ozone that compensate for the stratospheric decreases. The reasons for the continued reduction of lower stratospheric ozone are not clear; models do not reproduce these trends, and thus the causes now urgently need to be established.

Description: A series of simulations using the NASA Goddard Earth Observing System Chemistry–Climate Model are analyzed in order to aid in the interpretation of observed interannual and sub-decadal variability in the tropical lower stratosphere over the past 35 years. The impact of El Niño–Southern Oscillation on temperature and water vapor in this region is nonlinear in boreal spring. While moderate El Niño events lead to cooling in this region, strong El Niño events lead to warming, even as the response of the large-scale Brewer–Dobson circulation appears to scale nearly linearly with El Niño. This nonlinearity is shown to arise from the response in the Indo-West Pacific to El Niño: strong El Niño events lead to tropospheric warming extending into the tropical tropopause layer and up to the cold point in this region, where it allows for more water vapor to enter the stratosphere. The net effect is that both strong La Niña and strong El Niño events lead to enhanced entry water vapor and stratospheric moistening in boreal spring and early summer. These results lead to the following interpretation of the contribution of sea surface temperatures to the decline in water vapor in the early 2000s: the very strong El Niño event in 1997/1998, followed by more than 2 consecutive years of La Niña, led to enhanced lower-stratospheric water vapor. As this period ended in early 2001, entry water vapor concentrations declined. This effect accounts for approximately one-quarter of the observed drop.

Description: Four in situ cavity ring-down spectrometers (G2132-i, Picarro, Inc.) measuring methane dry mole fraction (CH4), carbon dioxide dry mole fraction (CO2), and the isotopic ratio of methane (δ13CH4) were deployed at four towers in the Marcellus Shale natural gas extraction region of Pennsylvania. In this paper, we describe laboratory and field calibration of the analyzers for tower-based applications and characterize their performance in the field for the period January–December 2016. Prior to deployment, each analyzer was tested using bottles with various isotopic ratios, from biogenic to thermogenic source values, which were diluted to varying degrees in zero air, and an initial calibration was performed. Furthermore, at each tower location, three field tanks were employed, from ambient to high mole fractions, with various isotopic ratios. Two of these tanks were used to adjust the calibration of the analyzers on a daily basis. We also corrected for the cross-interference from ethane on the isotopic ratio of methane. Using an independent field tank for evaluation, the standard deviation of 4 h means of the isotopic ratio of methane difference from the known value was found to be 0.26 ‰ δ13CH4. Following improvements in the field tank testing scheme, the standard deviation of 4 h means was 0.11 ‰, well within the target compatibility of 0.2 ‰. Round-robin style testing using tanks with near-ambient isotopic ratios indicated mean errors of −0.14 to 0.03 ‰ for each of the analyzers. Flask to in situ comparisons showed mean differences over the year of 0.02 and 0.08 ‰, for the east and south towers, respectively. Regional sources in this region were difficult to differentiate from strong perturbations in the background. During the afternoon hours, the median differences of the isotopic ratio measured at three of the towers, compared to the background tower, were &minus0.15 to 0.12 ‰ with standard deviations of the 10 min isotopic ratio differences of 0.8 ‰. In terms of source attribution, analyzer compatibility of 0.2 ‰ δ13CH4 affords the ability to distinguish a 50 ppb CH4 peak from a biogenic source (at −60 ‰, for example) from one originating from a thermogenic source (−35 ‰), with the exact value dependent upon the source isotopic ratios. Using a Keeling plot approach for the non-afternoon data at a tower in the center of the study region, we determined the source isotopic signature to be −31.2 ± 1.9 ‰, within the wide range of values consistent with a deep-layer Marcellus natural gas source.