A 22-Year Evaluation of Convection Reaching the Stratosphere Over the United States

Stratosphere-reaching moist convection can significantly alter the dynamics, chemistry, and climate of the Earth system. This study seeks to add to the emerging understanding of the frequency, depth, and stratospheric impact of such events using 22 years (1996-2017) of ground-based radar observations in the contiguous United States. While most prior studies identify such storms using the temperature lapse-rate tropopause (LRT) as a troposphere-stratosphere boundary, this study is the first to identify convection that reaches into stratospheric air below the LRT (tropopause depressions, excluding folds) as well. It is found that tropopause depression (TD) overshooting and LRT overshooting occur at similar frequency over the United States, with TD overshooting being more episodic in nature than LRT overshooting. TD overshooting is also found more often throughout the cooler months of the year, while LRT overshooting dominates all overshooting in the summer months. Stratospheric residence of overshoot material, as estimated using trajectory calculations driven by large-scale winds, suggests that the vast majority of TD overshoot material does not remain in the stratosphere within 5 days downstream and rarely impacts altitudes more than 1 km above the LRT. Conversely, the majority of LRT overshoot material remains in the stratosphere downstream and routinely impacts altitudes >1 and >2 km above the tropopause.

Identifying Outflow Regions of North American Monsoon Anticyclone-Mediated Meridional Transport of Convectively Influenced Air Masses in the Lower Stratosphere

We analyzed the effect of the North American monsoon anticyclone (NAMA) on the meridional transport of summertime cross-tropopause convective outflow by applying a trajectory analysis to a climatology of convective overshooting tops (OTs) identified in GOES satellite images, which covers the domain from 29°S to 68°N and from 205°W to 1.25°W for the time period of May to September, 2013. From this analysis, we identify seasonal development of geographically distinct outflow regions of convectively influenced air masses (CIAMs) from the NAMA circulation to the global stratosphere and quantify the associated meridional displacement of CIAMs. We find that prior to the development of the NAMA, the majority of CIAMs exit the study area in a southeastern region between 5°N and 35°N at 45°W (75.5% in May). During July and August, when the NAMA is strongest, two additional outflow regions develop that constitute the majority of outflow: 68.1% in a northeastern region between 35°N and 60°N at 45°W and 13.4% in a southwestern region between 5°N and 35°N at 145°W. The shift in the location of most CIAM outflow from the pre-NAMA southeastern region to NAMA-dependent northeastern and southwestern regions corresponds to a change in average meridional displacement of CIAMs from 3.3° northward in May to 24.5° northward in July and August. Meridional transport of CIAMs through persistent outflow regions from the NAMA circulation to the global stratosphere has the potential to impact global stratospheric composition beyond convective source regions.

A 13-year Trajectory-Based Analysis of Convection-Driven Changes in Upper Troposphere Lower Stratosphere Composition Over the United States

Moist convection frequently reaches the tropopause and alters the distribution and concentration of radiatively important trace gases in the upper troposphere and lower stratosphere (UTLS), but the overall impact of convection on regional and global UTLS composition remains largely unknown. To improve understanding of convection-driven changes in water vapor (H2O), ozone (O3), and carbon monoxide (CO) in the UTLS, this study utilizes 13 years of observations of satellite-based trace gas profiles from the Microwave Limb Sounder (MLS) aboard the Aura satellite and convection from the operational network of ground-based weather radars in the United States. Locations with and without convection identified via radar are matched with downstream MLS observations through three-dimensional, kinematic forward trajectories, providing two populations of trace gas observations for analysis. These populations are further classified as belonging to extratropical or tropical environments based on the tropopause pressure at the MLS profile location. Extratropical regions are further separated by tropopause type (single or double), revealing differing impacts. Results show that convection typically moistens the UT by up to 300% and the LS by up to 100%, largely reduces O3 by up to 40%, and increases CO by up to 50%. Changes in H2O and O3 are robust, with LS O3 reduced more by convection within tropical environments, where the median concentration decrease is 34% at ~2 km above tropopause, compared to 24% in extratropical environments. Quantification of CO changes from convection is less reliable due to differences being near the MLS measurement precision and accuracy.

Water heavily fractionated as it ascends on Mars as revealed by ExoMars/NOMAD

Isotopic ratios and, in particular, the water D/H ratio are powerful tracers of the evolution and transport of water on Mars. From measurements performed with ExoMars/NOMAD, we observe marked and rapid variability of the D/H along altitude on Mars and across the whole planet. The observations (from April 2018 to April 2019) sample a broad range of events on Mars, including a global dust storm, the evolution of water released from the southern polar cap during southern summer, the equinox phases, and a short but intense regional dust storm. In three instances, we observe water at very high altitudes (>80 km), the prime region where water is photodissociated and starts its escape to space. Rayleigh distillation appears the be the driving force affecting the D/H in many cases, yet in some instances, the exchange of water reservoirs with distinctive D/H could be responsible.

The Chicago Water Isotope Spectrometer (ChiWIS-lab): A tunable diode laser spectrometer for chamber-based measurements of water vapor isotopic evolution during cirrus formation

We describe a new tunable diode laser (TDL) absorption instrument, the Chicago Water Isotope Spectrometer, designed for measurements of vapor-phase water isotopologues in conditions characteristic of the upper troposphere [190-235 K temperature and 2-500 parts per million volume (ppmv) water vapor]. The instrument is primarily targeted for measuring the evolving ratio of HDO/H₂O during experiments in the "Aerosol Interaction and Dynamics in the Atmosphere" (AIDA) cloud chamber. The spectrometer scans absorption lines of both H₂O and HDO near the 2.64 µm wavelength in a single current sweep, increasing the accuracy of isotopic ratio measurements. At AIDA, the instrument is configured with a 256-m path length White cell for in situ measurements, and effective sensitivity can be augmented by enhancing the HDO content of chamber water vapor by an order of magnitude. The instrument has participated to date in the 2012-2013 IsoCloud campaigns studying isotopic partitioning during the formation of cirrus clouds and in the AquaVIT-II instrument intercomparison campaign. Realized precisions for 1-s measurements during these campaigns were 22 ppbv for H₂O and 16 ppbv for HDO, equivalent to relative precisions of less than 0.5% for each species at 8 ppmv water vapor. The 1-s precision of the [HDO]/[H₂O] ratio measurement ranged from 1.6‰ to 5.6‰ over the range of experimental conditions. H₂O measurements showed agreement with calculated saturation vapor pressure to within 1% in conditions of sublimating ice and agreement with other AIDA instruments (the AIDA SP-APicT reference TDL instrument and an MBW 373LX chilled mirror hygrometer) to within 2.5% and 3.8%, respectively, over conditions suitable for all instruments (temperatures from 204 K to 234 K and H₂O content equivalent to 15-700 ppmv at 200 hPa).

Gas-Phase Isotopic Fractionation Study of Singly and Doubly Deuterated Isotopologues of Water in the H-D Exchange Reaction by Cavity Ring-Down Spectroscopy

The underlying mechanisms of the triple-oxygen (¹⁶O, ¹⁷O, and ¹⁸O) isotopic content of deuterated (D) isotopologues of water in H-D exchange reactions in the gas phase remain elusive. Herein, we have demonstrated a high-resolution gas-phase spectral analysis of doubly (D₂O) and singly (HDO) deuterated isotopologues of water in the region around 7.8 μm using quantum cascade laser-based cavity ring-down spectroscopy. Isotopic fractionations among doubly and singly deuterated species of water, D₂¹⁶O, HD¹⁶O, HD¹⁷O, and HD¹⁸O, in the gas phase were carried out by probing the fundamental and hot band transitions in the ν₂ (bending) mode of D₂O and the fundamental ν₂ transitions for the other water isotopes. We subsequently investigated the fractionations of different D-enriched water isotopologues for the H-D exchange reaction using various mixtures of D₂O in H₂O. We explored the potential role of triple-oxygen isotopic contents through enrichments and depletions of HD¹⁶O, HD¹⁷O, and HD¹⁸O, involved in the H-D reaction. Our first clear, direct, and quantitative experimental evidence reveals a new picture of gas-phase isotopic fractionation chemistry in a mixture of light and heavy water (H₂O-D₂O).

Identifying Source Regions and the Distribution of Cross-Tropopause Convective Outflow Over North America During the Warm Season

We analyzed the interaction between the North American monsoon anticyclone (NAMA) and summertime cross-tropopause convective outflow by applying a trajectory analysis to a climatology of convective overshooting tops (OTs) identified in GOES satellite images, which covers the domain from 29°S to 68°N and from 205 to 1.25°W for the time period of May through September 2013. With this analysis we identified seasonally, geographically, and altitude-dependent variability in NAMA strength and in cross-tropopause convection that control their interaction. We find that the NAMA has the strongest impact on the circulation of convectively influenced air masses in August. Over the entire time period examined the intertropical convergence zone contributes the majority of OTs with a larger fraction of total OTs at 370 K (on average 70%) than at 400 K (on average 52%). During August at 370 K, the convectively influenced air masses within the NAMA circulation, as determined by the trajectory analysis, are primarily sourced from the intertropical convergence zone (monthly average of 66.1%), while at 400 K the Sierra Madres and the Central United States combined constitute the dominant source region (monthly average of 44.1%, compared to 36.6% of the combined Intertropical Convergence Zone regions). When evaluating the impact of cross-tropopause convection on the composition and chemistry of the upper troposphere and lower stratosphere, the effects of the NAMA on both the distribution of convective outflow and the residence time of convectively influenced air masses within the NAMA region must be considered.

Modeling the Effect of Potential Nitric Acid Removal During Convective Injection of Water Vapor Over the Central United States on the Chemical Composition of the Lower Stratosphere

Tropopause-penetrating convection is a frequent seasonal feature of the Central United States climate. This convection presents the potential for consistent transport of water vapor into the upper troposphere and lower stratosphere (UTLS) through the lofting of ice, which then sublimates. Water vapor enhancements associated with convective ice lofting have been observed in both in situ and satellite measurements. These water vapor enhancements can increase the probability of sulfate aerosol-catalyzed heterogeneous reactions that convert reservoir chlorine (HCl and ClONO2) to free radical chlorine (Cl and ClO) that leads to catalytic ozone loss. In addition to water vapor transport, lofted ice may also scavenge nitric acid and further impact the chlorine activation chemistry of the UTLS. We present a photochemical model that resolves the vertical chemical structure of the UTLS to explore the effect of water vapor enhancements and potential additional nitric acid removal. The model is used to define the response of stratospheric column ozone to the range of convective water vapor transported and the temperature variability of the lower stratosphere currently observed over the Central United States in conjunction with potential nitric acid removal and to scenarios of elevated sulfate aerosol surface area density representative of possible future volcanic eruptions or solar radiation management. We find that the effect of HNO3 removal is dependent on the magnitude of nitric acid removal and has the greatest potential to increase chlorine activation and ozone loss under UTLS conditions that weakly favor the chlorine activation heterogeneous reactions by reducing NOx sources.

Coupling free radical catalysis, climate change, and human health

We present the chain of mechanisms linking free radical catalytic loss of stratospheric ozone, specifically over the central United States in summer, to increased climate forcing by CO2 and CH4 from fossil fuel use. This case directly engages detailed knowledge, emerging from in situ aircraft observations over the polar regions in winter, defining the temperature and water vapor dependence of the kinetics of heterogeneous catalytic conversion of inorganic chlorine (HCl and ClONO2) to free radical form (ClO). Analysis is placed in the context of irreversible changes to specific subsystems of the climate, most notably coupled feedbacks that link rapid changes in the Arctic with the discovery that convective storms over the central US in summer both suppress temperatures and inject water vapor deep into the stratosphere. This places the lower stratosphere over the US in summer within the same photochemical catalytic domain as the lower stratosphere of the Arctic in winter engaging the risk of amplifying the rate limiting step in the ClO dimer catalytic mechanism by some six orders of magnitude. This transitions the catalytic loss rate of ozone in lower stratosphere over the United States in summer from HOx radical control to ClOx radical control, increasing the overall ozone loss rate by some two orders of magnitude over that of the unperturbed state. Thus we address, through a combination of observations and modeling, the mechanistic foundation defining why stratospheric ozone, vulnerable to increased climate forcing, is one of the most delicate aspects of habitability on the planet.

Laboratory measurements of HDO/H2O isotopic fractionation during ice deposition in simulated cirrus clouds

The stable isotopologues of water have been used in atmospheric and climate studies for over 50 years, because their strong temperature-dependent preferential condensation makes them useful diagnostics of the hydrological cycle. However, the degree of preferential condensation between vapor and ice has never been directly measured at temperatures below 233 K (-40 °C), conditions necessary to form cirrus clouds in the Earth's atmosphere, routinely observed in polar regions, and typical for the near-surface atmospheric layers of Mars. Models generally assume an extrapolation from the warmer experiments of Merlivat and Nief [Merlivat L, Nief G (1967) Tellus 19:122-127]. Nonequilibrium kinetic effects that should alter preferential partitioning have also not been well characterized experimentally. We present here direct measurements of HDO/H₂O equilibrium fractionation between vapor and ice ([Formula: see text]) at cirrus-relevant temperatures, using in situ spectroscopic measurements of the evolving isotopic composition of water vapor during cirrus formation experiments in a cloud chamber. We rule out the recent proposed upward modification of [Formula: see text], and find values slightly lower than Merlivat and Nief. These experiments also allow us to make a quantitative validation of the kinetic modification expected to occur in supersaturated conditions in the ice-vapor system. In a subset of diffusion-limited experiments, we show that kinetic isotope effects are indeed consistent with published models, including allowing for small surface effects. These results are fundamental for inferring processes on Earth and other planets from water isotopic measurements. They also demonstrate the utility of dynamic in situ experiments for studying fractionation in geochemical systems.

Stable isotopes in atmospheric water vapor and applications to the hydrologic cycle

The measurement and simulation of water vapor isotopic composition has matured rapidly over the last decade, with long-term datasets and comprehensive modeling capabilities now available. Theories for water vapor isotopic composition have been developed by extending the theories that have been used for the isotopic composition of precipitation to include a more nuanced understanding of evaporation, large-scale mixing, deep convection, and kinetic fractionation. The technologies for in-situ and remote sensing measurements of water vapor isotopic composition have developed especially rapidly over the last decade, with discrete water vapor sampling methods, based on mass spectroscopy, giving way to laser spectroscopic methods and satellite- and ground-based infrared absorption techniques. The simulation of water vapor isotopic composition has evolved from General Circulation Model (GCM) methods for simulating precipitation isotopic composition to sophisticated isotope-enabled microphysics schemes using higher-order moments for water- and ice-size distributions. The incorporation of isotopes into GCMs has enabled more detailed diagnostics of the water cycle and has led to improvements in its simulation. The combination of improved measurement and modeling of water vapor isotopic composition opens the door to new advances in our understanding of the atmospheric water cycle, in processes ranging from the marine boundary layer, through deep convection and tropospheric mixing, and into the water cycle of the stratosphere. Finally, studies of the processes governing modern water vapor isotopic composition provide an improved framework for the interpretation of paleoclimate proxy records of the hydrological cycle.

Composition and physical properties of the Asian Tropopause Aerosol Layer and the North American Tropospheric Aerosol Layer

Recent studies revealed layers of enhanced aerosol scattering in the upper troposphere and lower stratosphere over Asia (Asian Tropopause Aerosol Layer (ATAL)) and North America (North American Tropospheric Aerosol Layer (NATAL)). We use a sectional aerosol model (Community Aerosol and Radiation Model for Atmospheres (CARMA)) coupled with the Community Earth System Model version 1 (CESM1) to explore the composition and optical properties of these aerosol layers. The observed aerosol extinction enhancement is reproduced by CESM1/CARMA. Both model and observations indicate a strong gradient of the sulfur-to-carbon ratio from Europe to the Asia on constant pressure surfaces. We found that the ATAL is mostly composed of sulfates, surface-emitted organics, and secondary organics; the NATAL is mostly composed of sulfates and secondary organics. The model also suggests that emission increases in Asia between 2000 and 2010 led to an increase of aerosol optical depth of the ATAL by 0.002 on average which is consistent with observations.

KEY POINTS

The Asian Tropopause Aerosol Layer is composed of sulfate, primary organics, and secondary organics The North American Tropospheric Aerosol Layer is mostly composed of sulfate and secondary organics Aerosol Optical Depth of Asian Tropopause Aerosol Layer increases by 0.002 from 2000 to 2010.