Detection and attribution (D&A) simulations were important components of CMIP5 and underpinned the climate change detection and attribution assessments of the Fifth Assessment Report of the ...Intergovernmental Panel on Climate Change. The primary goals of the Detection and Attribution Model Intercomparison Project (DAMIP) are to facilitate improved estimation of the contributions of anthropogenic and natural forcing changes to observed global warming as well as to observed global and regional changes in other climate variables; to contribute to the estimation of how historical emissions have altered and are altering contemporary climate risk; and to facilitate improved observationally constrained projections of future climate change. D&A studies typically require unforced control simulations and historical simulations including all major anthropogenic and natural forcings. Such simulations will be carried out as part of the DECK and the CMIP6 historical simulation. In addition D&A studies require simulations covering the historical period driven by individual forcings or subsets of forcings only: such simulations are proposed here. Key novel features of the experimental design presented here include firstly new historical simulations with aerosols-only, stratospheric-ozone-only, CO2-only, solar-only, and volcanic-only forcing, facilitating an improved estimation of the climate response to individual forcing, secondly future single forcing experiments, allowing observationally constrained projections of future climate change, and thirdly an experimental design which allows models with and without coupled atmospheric chemistry to be compared on an equal footing.
The lifetime of nitrous oxide, the third‐most‐important human‐emitted greenhouse gas, is based to date primarily on model studies or scaling to other gases. This work calculates a semiempirical ...lifetime based on Microwave Limb Sounder satellite measurements of stratospheric profiles of nitrous oxide, ozone, and temperature; laboratory cross‐section data for ozone and molecular oxygen plus kinetics for O(1D); the observed solar spectrum; and a simple radiative transfer model. The result is 116 ± 9 years. The observed monthly‐to‐biennial variations in lifetime and tropical abundance are well matched by four independent chemistry‐transport models driven by reanalysis meteorological fields for the period of observation (2005–2010), but all these models overestimate the lifetime due to lower abundances in the critical loss region near 32 km in the tropics. These models plus a chemistry‐climate model agree on the nitrous oxide feedback factor on its own lifetime of 0.94 ± 0.01, giving N2O perturbations an effective residence time of 109 years. Combining this new empirical lifetime with model estimates of residence time and preindustrial lifetime (123 years) adjusts our best estimates of the human‐natural balance of emissions today and improves the accuracy of projected nitrous oxide increases over this century.
Key Points
Nitrous oxide lifetime is computed empirically from MLS satellite data
Empirical N2O lifetimes compared with models including interannual variability
Results improve values for present anthropogenic and preindustrial emissions
Evaluating the representation of processes controlling tropical and subtropical tropospheric relative humidity (RH) in atmospheric general circulation models (GCMs) is crucial to assess the ...credibility of predicted climate changes. GCMs have long exhibited a moist bias in the tropical and subtropical mid and upper troposphere, which could be due to the mis‐representation of cloud processes or of the large‐scale circulation, or to excessive diffusion during water vapor transport. The goal of this study is to use observations of the water vapor isotopic ratio to understand the cause of this bias. We compare the three‐dimensional distribution of the water vapor isotopic ratio measured from space and ground to that simulated by several versions of the isotopic GCM LMDZ. We show that the combined evaluation of RH and of the water vapor isotopic composition makes it possible to discriminate the most likely cause of RH biases. Models characterized either by an excessive vertical diffusion, an excessive convective detrainment or an underestimated in situ cloud condensation will all produce a moist bias in the free troposphere. However, only an excessive vertical diffusion can lead to a reversed seasonality of the free tropospheric isotopic composition in the subtropics compared to observations. Comparing seven isotopic GCMs suggests that the moist bias found in many GCMs in the mid and upper troposphere most frequently results from an excessive diffusion during vertical water vapor transport. This study demonstrates the added value of water vapor isotopic measurements for interpreting shortcomings in the simulation of RH by climate models.
Key Points
Water vapor isotopes= observational diagnostics to evaluate RH in GCMs
Excessive vertical diffusion= widespread cause of moist bias in GCMs
Parameterized processes =larger uncertainty for RH than large‐scale circulation
Geomagnetic activity is thought to affect ozone and, possibly, climate in polar regions via energetic particle precipitation (EPP) but observational evidence of its importance in the seasonal ...stratospheric ozone variation on long time scales is still lacking. Here we fill this gap by showing that at high southern latitudes, late winter ozone series, covering the 1979–2014 period, exhibit an average stratospheric depletion of about 10–15% on a monthly basis caused by EPP. Daily observations indicate that every austral winter EPP‐induced low ozone concentrations appear at about 45 km in late June and descend later to 30 km, before disappearing by September. Such stratospheric variations are coupled with mesospheric ozone changes also driven by EPP. No significant correlation between these ozone variations and solar ultraviolet irradiance has been found. This suggests the need of including the EPP forcing in both ozone model simulations and trend analysis.
Key Points
Evaluation of the EPP‐induced O3 variability on long time scales
EPP causes an average upper stratospheric O3 depletion of about 10–15% on a monthly basis
Discrimination between EPP and solar irradiance effects on ozone
While knowledge of the energy inputs from the Sun (as it is the primary energy source) is important for understanding the solar-terrestrial system, of equal importance is the manner in which the ...terrestrial part of the system organizes itself in a quasi-equilibrium state to accommodate and re-emit this energy. The ROSMIC project (2014–2018 inclusive) was the component of SCOSTEP’s Variability of the Sun and Its Terrestrial Impact (VarSITI) program which supported research into the terrestrial component of this system. The four themes supported under ROSMIC are solar influence on climate, coupling by dynamics, trends in the mesosphere lower thermosphere, and trends and solar influence in the thermosphere. Over the course of the VarSITI program, scientific advances were made in all four themes. This included improvements in understanding (1) the transport of photochemically produced species from the thermosphere into the lower atmosphere; (2) the manner in which waves produced in the lower atmosphere propagate upward and influence the winds, dynamical variability, and transport of constituents in the mesosphere, ionosphere, and thermosphere; (3) the character of the long-term trends in the mesosphere and lower thermosphere; and (4) the trends and structural changes taking place in the thermosphere. This paper reviews the progress made in these four areas over the past 5 years and summarizes the anticipated research directions in these areas in the future. It also provides a physical context of the elements which maintain the structure of the terrestrial component of this system. The effects that changes to the atmosphere (such as those currently occurring as a result of anthropogenic influences) as well as plausible variations in solar activity may have on the solar terrestrial system need to be understood to support and guide future human activities on Earth.
Elevated stratopauses are typically associated with prolonged disturbed conditions in the Northern Hemisphere polar winter. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and ...the Microwave Limb Sounder (MLS) observed a short‐lived and highly zonally asymmetric stratopause at mesospheric altitudes in November 2009, the earliest in the season reported so far. The Arctic climatological winter stratopause vanished, and MIPAS and MLS measured temperatures of 260 K at 82 km and 250 K at 75 km, respectively, in a region smaller than in typical midwinter elevated stratopause events. Planetary wave activity was initially high. Zonal mean zonal winds and the poleward temperature gradient northward of 70°N stayed reversed during 7 days, but the mesosphere did not cool. Wave activity dropped until the eastward stratospheric winds resumed and a strong vortex restored in the mesosphere. The stratopause emerged at high altitudes, staying there for 2–5 days. It was accompanied by enhanced downward transport. It took the stratopause 9 days to move down to its typical winter altitudes.
Key Points
MIPAS and MLS observed a highly zonally asymmetric Arctic stratopause at high altitudes in November 2009 accompanied by a strong descent
The event has similarities with typical elevated stratopause events, but it is of smaller scale in terms of duration and extent
This is the first time a high‐altitude stratopause observed from space this early in the winter season is reported
Vertical profiles of nitric oxide (NO) concentration are derived between 120 and 250 km using updated NO emission rates measured by Sounding of the Atmosphere using Broadband Emission Radiometry ...(SABER) instrument on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. The Naval Research Laboratory Mass Spectrometer Incoherent Scatter Radar (MSIS) 2.1 model is used to provide the required parameters of temperature, atomic oxygen number density, and molecule oxygen number density needed to derive the NO concentrations using a non‐local thermodynamic equilibrium (non‐LTE) model. The SABER NO concentration shows a significant correlation with solar activity with larger peak NO concentrations and higher altitude extent during solar maximum years compared to those during the solar minimum years. The SABER NO agrees well with the MSIS 2.1 NO at altitudes above 120 km for all latitudes, while the pronounced SABER‐MSIS NO discrepancy below 120 km is likely due to the temperature underestimation by MSIS 2.1. A detailed error analysis is presented and considers systematic and random errors in all the terms in the non‐LTE model used to derive the NO concentration. Random error in MSIS 2.1 temperature and atomic oxygen dominates the uncertainty in single NO profiles above 120 km. We estimated a systematic error up to ∼36% between 120 and 250 km during solar maximum years.
Plain Language Summary
Nitric oxide (NO) is a minor constituent of the Earth's thermosphere and regulates the energy budget by emitting infrared radiation that cools the region. Understanding the distribution of the NO concentration is important in understanding the energy redistribution in the Earth's upper atmosphere. In this paper, a new NO concentration data set between 120 and 250 km from January 2002 to the present day is derived from the latest Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) NO emission measurements and Mass Spectrometer Incoherent Scatter Radar (MSIS) 2.1 empirical model of temperature and O, and O2 number density. The derived SABER NO concentration varies with solar activity. The SABER NO agrees well with the empirical MSIS 2.1 NO between 120 and 250 km. Below 120 km, the temperature may be warmer than predicted by MSIS 2.1, leading to a substantial difference between SABER NO and MSIS 2.1 NO. A comprehensive error analysis considering systematic and random errors in all the terms in the model used to derive the NO concentration from SABER data shows that the systematic uncertainty of the SABER NO density is up to 36% above 120 km.
Key Points
The nitric oxide (NO) 5.3 μm infrared emission rates derived from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) measurements have been updated
Vertical profiles of NO concentration, spanning 20+ years, are derived from the SABER NO emission rates, and Mass Spectrometer Incoherent Scatter Radar (MSIS) 2.1 T, n(O), and n(O2)
The SABER NO concentrations exhibit good agreement (±30%) with those in the MSIS 2.1 empirical model between 120 and 250 km
An analysis of recent observations (2004–2013) made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS) instrument indicate that total carbon (COx = CO + CO2) has been ...increasing rapidly in the lower thermosphere, above 10−3 hPa (90 km). The estimated trend (~9% per decade) is about a factor of 2 larger than the rate of increase that can be ascribed to anthropogenic emissions of CO2 (~5% per decade). Here we investigate whether the observed trends of CO2 and COx can be reproduced using the Whole Atmosphere Community Climate Model (WACCM), a comprehensive global model with interactive chemistry, wherein vertical eddy diffusion is estimated from a parameterization of gravity wave breaking that can respond to changes in the model climate. We find that the modeled trends of CO2 and COx do not differ significantly at any altitude from the value expected from anthropogenic increases of CO2 and that WACCM does not produce significant changes in eddy diffusivity. We show that the discrepancy between model and observations cannot be attributed to uncertainties associated with geophysical noise and instrumental effects, to difficulties separating a linear trend from the 11 year solar signal, or to sparse sampling by ACE‐FTS. Estimates of the impact of vertical diffusion on CO2 in the model indicate that a large increase in Kzz (~30% per decade) would be necessary to reconcile WACCM results with observations. It might be possible to ascertain whether such a large change in vertical mixing has in fact taken place by examining the trend of water vapor in the upper mesosphere.
Key Points
Observations show that CO2 in the lower thermosphere has increased rapidly since the early 2000s
The observed behavior cannot be simulated by a comprehensive climate‐chemistry model
Model and observations could be reconciled if vertical eddy mixing has increased by 30% per decade
The gravity wave drag parametrization of the Whole Atmosphere Community Climate Model (WACCM) has been modified to include the wave‐driven atmospheric vertical mixing caused by propagating, ...non‐breaking, gravity waves. The strength of this atmospheric mixing is represented in the model via the “effective wave diffusivity” coefficient (Kwave). Using Kwave, a new total dynamical diffusivity (KDyn) is defined. KDyn represents the vertical mixing of the atmosphere by both breaking (dissipating) and vertically propagating (non‐dissipating) gravity waves. Here we show that, when the new diffusivity is used, the downward fluxes of Fe and Na between 80 and 100 km largely increase. Larger meteoric ablation injection rates of these metals (within a factor 2 of measurements) can now be used in WACCM, which produce Na and Fe layers in good agreement with lidar observations. Mesospheric CO2 is also significantly impacted, with the largest CO2 concentration increase occurring between 80 and 90 km, where model‐observations agreement improves. However, in regions where the model overestimates CO2 concentration, the new parametrization exacerbates the model bias. The mesospheric cooling simulated by the new parametrization, while needed, is currently too strong almost everywhere. The summer mesopause in both hemispheres becomes too cold by about 30 K compared to observations, but it shifts upward, partially correcting the WACCM low summer mesopause. Our results highlight the far‐reaching implications and the necessity of representing vertically propagating non‐breaking gravity waves in climate models. This novel method of modeling gravity waves contributes to growing evidence that it is time to move away from dissipative‐only gravity wave parametrizations.
Plain Language Summary
Atmospheric gravity waves are generated in the lowest layers (∼bottom 10 km) of the atmosphere by processes such as weather systems and air masses interacting with the topography, and can propagate upward to ∼120 km. In this work, the representation of atmospheric gravity waves in a state‐of‐the‐art chemistry climate model, the Whole Atmosphere Community Climate Model (WACCM), has been updated. In the new model version, the mixing of the atmosphere caused by gravity waves that propagate upwards above 80 km and do not break is part of the total mixing of the atmosphere, which hitherto was considered to be caused by the turbulence created by gravity wave breaking. We show here that when this additional source of atmospheric mixing is taken into account, the WACCM model is better able to simulate the sodium and iron atom densities in the upper layers of the atmosphere (between ∼80 and 100 km), created by the ablation of cosmic dust. Additionally, gravity waves significantly affect the representation of CO2 mixing ratios and air temperature within the model. Our work is important because it shows that propagating gravity waves have non‐negligible impacts on basic properties of the Earth's atmosphere such as temperature, winds, and chemical species like CO2, and that the lack of their representation in current climate models needs to be addressed.
Key Points
A parametrization representing vertical mixing by non‐breaking gravity waves has been implemented in a chemistry‐climate model
Mesospheric CO2 and temperature are significantly impacted by the additional source of vertical mixing
Wave‐induced constituent transport largely reconciles the modeled mesospheric Na and Fe layers with the estimated meteoric injection rates
We use satellite observations and a numerical model to investigate polar nighttime ozone at the secondary maximum, around 90–95 km. Observations from the MIPAS and SABER satellite instruments ...indicate that the highest ozone mixing ratios are seen during the late fall to early winter period in both hemispheres and for all years examined. Simulations using the Whole Atmosphere Community Climate Model (WACCM) find qualitatively the same seasonal evolution. Analysis of WACCM results shows that the high ozone concentration is due in part to the relatively quiet dynamical conditions in early winter. The mean circulation, which brings warmer temperatures and higher concentrations of H, is weaker in early winter than during middle and late winter. H in the late fall to early winter period drops to the lowest levels seen during the year due to lack of a source from photochemistry, weak transport into the region by the mean circulation, and continual loss due to diffusive separation. The low concentration of H leads to higher ozone.
Key Points
Polar mesopause ozone has a seasonal maximum in late fallThe ozone maximum is observed by SABER and MIPAS and simulated by WACCMThe primary cause is low H due to weak production and transport
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