Search

Solar driven energetic particle precipitation (EPP) is an important factor in polar atmospheric ozone balance and has been linked to ground-level regional climate variability. However, the linking mechanism has remained ambiguous. The observed and simulated ground-level changes start well before the processes from the main candidate, the so-called EPP-indirect effect, would start. Here we show that initial reduction of polar mesospheric ozone and the resulting change in atmospheric heating rapidly couples to dynamics, transferring the signal downwards, shifting the tropospheric jet polewards. This pathway is not constrained to the polar vortex. Rather, a subtropical route initiated by a changing wind shear plays a key role. Our results show that the signal propagates downwards in timescales consistent with observed tropospheric level climatic changes linked to EPP. This pathway, from mesospheric ozone to regional climate, is independent of the EPP-indirect effect, and solves the long-standing mechanism problem for EPP effects on climate. Energetically charged particles affect polar ozone levels and have been linked to surface climate variability. This study discusses the pathway that explains the rapid connection from high altitude mesospheric ozone to surface regional climate. [ABSTRACT FROM AUTHOR]

Estimates of aviation effective radiative forcing (ERF) indicate that contrail cirrus is currently its largest contributor, although with a substantial associated uncertainty of ∼ 70 %. Here, we implement the contrail parameterisation developed for the Community Atmosphere Model (CAM) in the UK Met Office Unified Model (UM), allowing us to compare, for the first time, the impact of key features of the host climate model on contrail cirrus ERF. We find that differences in background humidity between the models result in the UM-simulated contrail fractions being 2 to 3 times larger than in CAM. Additionally, the models show contrasting responses in overall global cloud fraction, with contrails increasing the total cloud fraction in the UM and decreasing it in CAM. Differences in the complexity of the cloud microphysics schemes lead to significant differences in simulated changes to cloud ice water content due to aviation. After compensating for the unrealistically low contrail optical depth in the UM, we estimate the 2018 contrail cirrus ERF to be 40.8 mW m−2 in the UM, compared to 60.1 mW m−2 in CAM. These values highlight the substantial uncertainty in contrail cirrus ERF due to differences in microphysics and radiation schemes between the two models. We also find a factor-of-8 uncertainty in contrail cirrus ERF due to existing uncertainty in contrail cirrus optical depth. Future research should focus on better representing microphysical and radiative contrail characteristics in climate models and on improved observational constraints. [ABSTRACT FROM AUTHOR]

There is considerable academic interest in the potential for air quality improvement as a co-benefit of climate change mitigation. Few studies use regional air quality models for simulating future co-benefits, but many use global chemistry–climate model output. Using regional atmospheric chemistry could provide a better representation of air quality changes than global chemistry–climate models, especially by improving the representation of elevated urban concentrations. We use a detailed regional atmospheric-chemistry model (WRF-Chem v4.2) to model European air quality in 2050 compared to 2014 following three climate change mitigation scenarios. We represent different climate futures by using air pollutant emissions and chemical boundary conditions (from CESM2-WACCM output) for three shared socioeconomic pathways (SSP1-2.6, SSP2-4.5 and SSP3-7.0: high-, medium- and low-mitigation pathways respectively). We find that in 2050, following SSP1-2.6, mean population-weighted PM 2.5 concentrations across European countries are reduced by 52 % compared to 2014. Under SSP2-4.5, this average reduction is 34%. The smallest average reduction is 18 %, achieved following SSP3-7.0. Maximum 6-monthly-mean daily-maximum 8 h (6mDM8h) ozone (O 3) is reduced across Europe by 15 % following SSP1-2.6 and by 3 % following SSP2-4.5, but it increases by 13 % following SSP3-7.0. This demonstrates clear co-benefits of climate mitigation. The additional resolution allows us to analyse regional differences and identify key sectors. We find that the mitigation of agricultural emissions will be key for attaining meaningful co-benefits of mitigation policies, as evidenced by the importance of changes in NO 3 aerosol mass to future PM 2.5 air quality and changes in CH 4 emissions to future O 3 air quality. [ABSTRACT FROM AUTHOR]

The detection and characterization of Earth-like planets around Sun-like stars is an important goal of exoplanetary research, given their promise for hosting potentially habitable conditions. Key orbital parameters, such as eccentricity, can influence a planet's climate response and, as a consequence, affect its potential habitability. Utilizing the Earth System Model – the Whole Atmosphere Community Climate Model (WACCM6), we simulated Earth-like exoplanets with two different orbital parameters: one circular (⁠|$e = 0$|⁠) and another highly eccentric (⁠|$e = 0.4$|⁠), both with zero obliquity but fixing the annual mean insolation. The highly eccentric case exhibits a 1.9 K warmer surface temperature due to lower surface and cloud albedo and a weaker longwave cloud forcing. Exploring the annual global mean climate difference, we analysed latitudinal and seasonal variations in hydrological cycle variables, such as sea ice, land snow, and clouds. Land habitability metrics based on temperature and precipitation reveal that the |$e=0.4$| case has over 25 per cent more habitable land area for more than 80 per cent of its orbit, compared with the |$e=0$| case. Additionally, the global circulation pattern shifts from a three-cell to a two-cell system in the |$e=0.4$| case, expanding the Hadley cell to higher latitudes, enhancing meridional latent heat transport, and improving land habitability at higher latitudes. Our study suggests that Earth-like exoplanets with high eccentricity orbiting Sun-like stars may have greater land habitability than their circular counterparts, due to seasonally warmer surface temperatures and more evenly distributed precipitation over land. [ABSTRACT FROM AUTHOR]

The radiative infrared cooling of CO 2 in the middle atmosphere, where it emits under non-local thermodynamic equilibrium (non-LTE) conditions, is a crucial contribution to the energy balance of this region and hence to establishing its thermal structure. The non-LTE computation is too CPU time-consuming to be fully incorporated into climate models, and hence it is parameterized. The most used parameterization of the CO 2 15 µm cooling for Earth's middle and upper atmosphere was developed by. The valid range of this parameterization with respect to CO 2 volume mixing ratios (VMRs) is, however, exceeded by the CO 2 of several scenarios considered in the Coupled Climate Model Intercomparison Projects, in particular the abrupt-4 × CO 2 experiment. Therefore, an extension, as well as an update, of that parameterization is both needed and timely. In this work, we present an update of that parameterization that now covers CO 2 volume mixing ratios in the lower atmosphere from ∼0.5 to over 10 times the CO 2 pre-industrial value of 284 ppmv (i.e. 150 to 3000 ppmv). Furthermore, it is improved by using a more contemporary CO 2 line list and the collisional rates that affect the CO 2 cooling rates. Overall, its accuracy is improved when tested for the reference temperature profiles as well as for measured temperature fields covering all expected conditions (latitude and season) of the middle atmosphere. The errors obtained for the reference temperature profiles are below 0.5 K d -1 for the present-day and lower CO 2 VMRs. Those errors increase to ∼1 –2K d -1 at altitudes between 110 and 120 km for CO 2 concentrations of 2 to 3 times the pre-industrial values. For very high CO 2 concentrations (4 to 10 times the pre-industrial abundances), those errors are below ∼1 K d -1 for most regions and conditions, except at 107–135 km, where the parameterization overestimates them by ∼1.2 %. These errors are comparable to the deviation of the non-LTE cooling rates with respect to LTE at about 70 km and below, but they are negligible (several times smaller) above that altitude. When applied to a large dataset of global (pole to pole and four seasons) temperature profiles measured by MIPAS (Michelson Interferometer for Passive Atmospheric Spectroscopy) (middle- and upper-atmosphere mode), the errors of the parameterization for the mean cooling rate (bias) are generally below 0.5 K d -1 , except between 5×10-3 and 3×10-4 hPa (∼85 –98 km), where they can reach biases of 1–2 K d -1. For single-temperature profiles, the cooling rate error (estimated by the root mean square – rms – of a statistically significant sample) is about 1–2 K d -1 below 5×10-3 hPa (∼85 km) and above 2×10-4 hPa (∼102 km). In the intermediate region, however, it is between 2 and 7 K d -1. For elevated stratopause events, the parameterization underestimates the mean cooling rates by 3–7 K d -1 (∼ 10 %) at altitudes of 85–95 km and the individual cooling rates show a significant rms (5–15 K d -1). Further, we have also tested the parameterization for the temperature obtained by a high-resolution version of the Whole Atmosphere Community Climate Model (WACCM-X), which shows a large temperature variability and wave structure in the middle atmosphere. In this case, the mean (bias) error of the parameterization is very small, smaller than 0.5 K d -1 for most atmospheric layers, reaching only maximum values of 2 K d -1 near 5×10-4 hPa (∼ 96 km). The rms has values of 1–2 K d -1 (∼20 %) below ∼2×10-2 hPa (∼80 km) and values smaller than 4 K d -1 (∼2 %) above 10 -4 hPa (∼105 km). In the intermediate region between ∼5×10-3 and ∼2×10-4 hPa (85–102 km), the rms is in the range of 5–12 K d -1. While these values are significant in percentage at ∼5×10-3 – 5×10-4 hPa, they are very small above ∼5×10-4 hPa (96 km). The routine is very fast, taking (1.5–7.5) ×10-5 s, depending on the extension of the atmospheric profile, the processor and the Fortran compiler. [ABSTRACT FROM AUTHOR]

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 modelMesospheric CO2 and temperature are significantly impacted by the additional source of vertical mixingWave‐induced constituent transport largely reconciles the modeled mesospheric Na and Fe layers with the estimated meteoric injection rates [ABSTRACT FROM AUTHOR]

There is considerable academic interest in the potential for air quality improvement as a co-benefit of climate change mitigation. Few studies use regional air quality models for simulating future co-benefits, but many use global chemistry-climate model output. Using regional atmospheric chemistry could provide a better representation of air quality changes than global chemistry-climate models, especially by improving the representation of elevated urban concentrations. We use a detailed regional atmospheric chemistry model (WRF-Chem v 4.2) to model European air quality in 2050 compared to 2014 following three climate change mitigation scenarios. We represent different climate futures by using air pollutant emissions and chemical boundary conditions (from CESM2-WACCM output) for three Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0; a high, medium and low mitigation pathway). We find that in 2050, following SSP1-2.6, mean population-weighted PM2.5 concentrations across European countries reduces by 52 % compared to 2014. Whilst under SSP2-4.5, this average reduction is 34 %. The smallest average reduction was 18 % by following SSP3-7.0. Maximum 6-monthly-mean daily-maximum 8 h (6mDM8h) ozone (O3) is reduced across Europe by 15 % following SSP1-2.6, and 3 % following SSP2-4.5, but increases by 13 % following SSP3-7.0. This demonstrates clear co-benefits of climate mitigation. The additional resolution allows us to analyse regional differences and identify key sectors. We find that mitigation of agricultural emissions will be key for attaining meaningful co-benefits of mitigation policies, evidenced by the importance of changes in NO3 aerosol mass to determining future PM2.5 air quality and changes in CH4 emissions to future O3 air quality. [ABSTRACT FROM AUTHOR]

The radiative infrared cooling of CO2 in the middle atmosphere, where it emits under non-Local Thermodynamic Equilibrium (non-LTE) conditions, is a crucial contribution to the energy balance of this region and hence to establishing its thermal structure. The non-LTE computation is too CPU time-consuming to be fully incorporated in climate models and hence it is parameterized. The most used parameterization of the CO2 15 μ m cooling for the Earth's middle and upper atmosphere was developed by Fomichev et al. (1998). The valid range of this parameterization with respect to CO2 volume mixing ratios (VMR) is, however, exceeded by the CO2 of several scenarios considered in the Coupled Climate Model Intercomparison Projects; in particular, the abrupt-4xCO2 experiment. Therefore, an extension, as well as an update, of that parameterization is both needed and timely. In this work, we present an update of the parameterization developed by Fomichev et al. (1998), which now covers CO2 volume mixing ratios in the lower atmosphere from ~0.5 to over 10 times the CO2 pre-industrial value of 284 ppmv (i.e., 150 ppmv to 3000 ppmv). Furthermore, it is improved by using a more contemporary CO2 line list and collisional rates that affect the CO2 cooling rates. Overall, accuracy is improved when tested against reference line-by-line calculations and by using measured global temperature profiles of the middle atmosphere. On average the errors are below 0.5 K day-1 for the present-day and lower CO2 VMRs. The errors increase to ~1–2 K day-1 at altitudes between 100–120 km for CO2 concentrations of two to three times the preindustrial values. For very high CO2 concentrations (four to ten times the pre-industrial abundances) the errors are below ~1 K day-1 for most regions and conditions, except at 110–120 km where the parameterization overestimates them by ~1.5 %. When applied to a large dataset of global (pole-to-pole and four seasons) measured temperature profiles, the errors of the parameterization are generally below 0.5 K day-1, except between 5⋅10-3 hPa and 3⋅10-4 hPa (~85–95 km), where they can reach biases of 1–2 K day-1. However, for elevated stratopause events, it underestimates the cooling rates by 3–7 K day-1 (~10 %) at altitudes of 80–100 km and the parameterized cooling rates show a large spread when compared to reference calculations. [ABSTRACT FROM AUTHOR]

Abstract. The radiative infrared cooling of CO2 in the middle atmosphere, where it emits under non-Local Thermodynamic Equilibrium (non-LTE) conditions, is a crucial contribution to the energy balance of this region and hence to establishing its thermal structure. The non-LTE computation is too CPU time-consuming to be fully incorporated in climate models and hence it is parameterized. The most used parameterization of the CO2 15 µm cooling for the Earth's middle and upper atmosphere was developed by Fomichev et al. (1998). The valid range of this parameterization with respect to CO2 volume mixing ratios (VMR) is, however, exceeded by the CO2 of several scenarios considered in the Coupled Climate Model Intercomparison Projects; in particular, the abrupt-4xCO2 experiment. Therefore, an extension, as well as an update, of that parameterization is both needed and timely. In this work, we present an update of the parameterization developed by Fomichev et al. (1998), which now covers CO2 volume mixing ratios in the lower atmosphere from -0.5 to over 10 times the CO2 pre-industrial value of 284 ppmv (i.e., 150 ppmv to 3000 ppmv). Furthermore, it is improved by using a more contemporary CO2 line list and collisional rates that affect the CO2 cooling rates. Overall, accuracy is improved when tested against reference lineby- line calculations and by using measured global temperature profiles of the middle atmosphere. On average the errors are below 0.5Kday-1 for the present-day and lower CO2 VMRs. The errors increase to -1-2 Kday1 at altitudes between 100- 120 km for CO2 concentrations of two to three times the preindustrial values. For very high CO2 concentrations (four to ten times the pre-industrial abundances) the errors are below -1Kday-1 for most regions and conditions, except at 110-120 km where the parameterization overestimates them by -1.5%. When applied to a large dataset of global (pole-to-pole and four seasons) measured temperature profiles, the errors of the parameterization are generally below 0.5Kday-1, except between 5x10-3 hPa and 3x10-4 hPa (-85-95 km), where they can reach biases of 1-2Kday-1. However, for elevated stratopause events, it underestimates the cooling rates by 3-7Kday-1 (10%) at altitudes of 80--100 km and the parameterized cooling rates show a large spread when compared to reference calculations. [ABSTRACT FROM AUTHOR]

This Opinion article begins with a review of important advances in the science of the Mesosphere and Lower Thermosphere (MLT) region of the atmosphere that have occurred over the past two decades since the founding of Atmospheric Chemistry and Physics. The emphasis is on chemistry (although, of course, this cannot be decoupled from discussion of atmospheric physics and dynamics), and the primary focus is on work during the past 10 years. Topics that are covered include: observations (satellite, rocket and ground-based techniques); the variability and connectedness of the MLT on various length- and time-scales; airglow emissions; the cosmic dust input and meteoric metal layers; and noctilucent (or polar mesospheric) ice clouds. The paper then concludes with a discussion of important unanswered questions and likely future directions for the field over the next decade. [ABSTRACT FROM AUTHOR]