Kinematic and diabatic vertical velocity climatologies from a chemistry climate model

Abstract. The representation of vertical velocity in chemistry climate models is a key element for the representation of the large-scale Brewer–Dobson circulation in the stratosphere. Here, we diagnose and compare the kinematic and diabatic vertical velocities in the ECHAM/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model. The calculation of kinematic vertical velocity is based on the continuity equation, whereas diabatic vertical velocity is computed using diabatic heating rates. Annual and monthly zonal mean climatologies of vertical velocity from a 10-year simulation are provided for both kinematic and diabatic vertical velocity representations. In general, both vertical velocity patterns show the main features of the stratospheric circulation, namely, upwelling at low latitudes and downwelling at high latitudes. The main difference in the vertical velocity pattern is a more uniform structure for diabatic and a noisier structure for kinematic vertical velocity. Diabatic vertical velocities show higher absolute values both in the upwelling branch in the inner tropics and in the downwelling regions in the polar vortices. Further, there is a latitudinal shift of the tropical upwelling branch in boreal summer between the two vertical velocity representations with the tropical upwelling region in the diabatic representation shifted southward compared to the kinematic case. Furthermore, we present mean age of air climatologies from two transport schemes in EMAC using these different vertical velocities and analyze the impact of residual circulation and mixing processes on the age of air. The age of air distributions show a hemispheric difference pattern in the stratosphere with younger air in the Southern Hemisphere and older air in the Northern Hemisphere using the transport scheme with diabatic vertical velocities. Further, the age of air climatology from the transport scheme using diabatic vertical velocities shows a younger mean age of air in the inner tropical upwelling branch and an older mean age in the extratropical tropopause region.

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Kinematic and diabatic vertical velocity climatologies from a chemistry climate model

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-15-29939-2015 ◽

2015 ◽

Vol 15 (21) ◽

pp. 29939-29971

Author(s):

C. M. Hoppe ◽

F. Ploeger ◽

P. Konopka ◽

R. Müller

Keyword(s):

Vertical Velocity ◽

Atmospheric Chemistry ◽

Large Scale ◽

Climate Models ◽

Climate Model ◽

Boreal Summer ◽

Uniform Structure ◽

Heating Rates ◽

Tropopause Region ◽

Stratospheric Circulation

Abstract. The representation of vertical velocity in chemistry climate models is a key element for the representation of the large scale Brewer–Dobson-Circulation in the stratosphere. Here, we diagnose and compare the kinematic and diabatic vertical velocities in the ECHAM/Messy Atmospheric Chemistry (EMAC) model. The calculation of kinematic vertical velocity is based on the continuity equation, whereas diabatic vertical velocity is computed using diabatic heating rates. Annual and monthly zonal mean climatologies of vertical velocity from a 10 year simulation are provided for both, kinematic and diabatic vertical velocity representations. In general, both vertical velocity patterns show the main features of the stratospheric circulation, namely upwelling at low latitudes and downwelling at high latitudes. The main difference in the vertical velocity pattern is a more uniform structure for diabatic and a noisier structure for kinematic vertical velocity. Diabatic vertical velocities show higher absolute values both in the upwelling branch in the inner tropics and in the downwelling regions in the polar vortices. Further, there is a latitudinal shift of the tropical upwelling branch in boreal summer between the two vertical velocity representations with the tropical upwelling region in the diabatic representation shifted southward compared to the kinematic case. Furthermore, we present mean age of air climatologies from two transport schemes in EMAC using these different vertical velocities. The age of air distributions show a hemispheric difference pattern in the stratosphere with younger air in the Southern Hemisphere and older air in the Northern Hemisphere using the transport scheme with diabatic vertical velocities. Further, the age of air climatology from the transport scheme using diabatic vertical velocities shows younger mean age of air in the inner tropical upwelling branch and older mean age in the extratopical tropopause region.

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Evaluating the Consistency between Statistically Downscaled and Global Dynamical Model Climate Change Projections

Journal of Climate ◽

10.1175/2008jcli2379.1 ◽

2008 ◽

Vol 21 (22) ◽

pp. 6052-6059 ◽

Cited By ~ 31

Author(s):

B. Timbal ◽

P. Hope ◽

S. Charles

Keyword(s):

Global Warming ◽

Statistical Downscaling ◽

Large Scale ◽

Climate Models ◽

Climate Model ◽

Specific Humidity ◽

Model Output ◽

Winter Rainfall ◽

Direct Model ◽

The Impact

Abstract The consistency between rainfall projections obtained from direct climate model output and statistical downscaling is evaluated. Results are averaged across an area large enough to overcome the difference in spatial scale between these two types of projections and thus make the comparison meaningful. Undertaking the comparison using a suite of state-of-the-art coupled climate models for two forcing scenarios presents a unique opportunity to test whether statistical linkages established between large-scale predictors and local rainfall under current climate remain valid in future climatic conditions. The study focuses on the southwest corner of Western Australia, a region that has experienced recent winter rainfall declines and for which climate models project, with great consistency, further winter rainfall reductions due to global warming. Results show that as a first approximation the magnitude of the modeled rainfall decline in this region is linearly related to the model global warming (a reduction of about 9% per degree), thus linking future rainfall declines to future emission paths. Two statistical downscaling techniques are used to investigate the influence of the choice of technique on projection consistency. In addition, one of the techniques was assessed using different large-scale forcings, to investigate the impact of large-scale predictor selection. Downscaled and direct model projections are consistent across the large number of models and two scenarios considered; that is, there is no tendency for either to be biased; and only a small hint that large rainfall declines are reduced in downscaled projections. Among the two techniques, a nonhomogeneous hidden Markov model provides greater consistency with climate models than an analog approach. Differences were due to the choice of the optimal combination of predictors. Thus statistically downscaled projections require careful choice of large-scale predictors in order to be consistent with physically based rainfall projections. In particular it was noted that a relative humidity moisture predictor, rather than specific humidity, was needed for downscaled projections to be consistent with direct model output projections.

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Impact of Lagrangian transport on lower-stratospheric transport timescales in a climate model

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-15227-2020 ◽

2020 ◽

Vol 20 (23) ◽

pp. 15227-15245

Author(s):

Edward J. Charlesworth ◽

Ann-Kristin Dugstad ◽

Frauke Fritsch ◽

Patrick Jöckel ◽

Felix Plöger

Keyword(s):

Atmospheric Chemistry ◽

Large Scale ◽

Climate Model ◽

Polar Vortex ◽

Transport Processes ◽

Lagrangian Model ◽

Trace Gas ◽

Lagrangian Transport ◽

Transport Barriers ◽

The Impact

Abstract. We investigate the impact of model trace gas transport schemes on the representation of transport processes in the upper troposphere and lower stratosphere. Towards this end, the Chemical Lagrangian Model of the Stratosphere (CLaMS) was coupled to the ECHAM/MESSy Atmospheric Chemistry (EMAC) model and results from the two transport schemes (Lagrangian critical Lyapunov scheme and flux-form semi-Lagrangian, respectively) were compared. Advection in CLaMS was driven by the EMAC simulation winds, and thereby the only differences in transport between the two sets of results were caused by differences in the transport schemes. To analyze the timescales of large-scale transport, multiple tropical-surface-emitted tracer pulses were performed to calculate age of air spectra, while smaller-scale transport was analyzed via idealized, radioactively decaying tracers emitted in smaller regions (nine grid cells) within the stratosphere. The results show that stratospheric transport barriers are significantly stronger for Lagrangian EMAC-CLaMS transport due to reduced numerical diffusion. In particular, stronger tracer gradients emerge around the polar vortex, at the subtropical jets, and at the edge of the tropical pipe. Inside the polar vortex, the more diffusive EMAC flux-form semi-Lagrangian transport scheme results in a substantially higher amount of air with ages from 0 to 2 years (up to a factor of 5 higher). In the lowermost stratosphere, mean age of air is much smaller in EMAC, owing to stronger diffusive cross-tropopause transport. Conversely, EMAC-CLaMS shows a summertime lowermost stratosphere age inversion – a layer of older air residing below younger air (an “eave”). This pattern is caused by strong poleward transport above the subtropical jet and is entirely blurred by diffusive cross-tropopause transport in EMAC. Potential consequences from the choice of the transport scheme on chemistry–climate and geoengineering simulations are discussed.

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Process-oriented analysis of aircraft soot-cirrus interactions constrains the climate impact of aviation

Communications Earth & Environment ◽

10.1038/s43247-021-00175-x ◽

2021 ◽

Vol 2 (1) ◽

Author(s):

Bernd Kärcher ◽

Fabian Mahrt ◽

Claudia Marcolli

Keyword(s):

Radiative Forcing ◽

Large Scale ◽

Climate Models ◽

Climate Model ◽

Global Climate ◽

Climate Impact ◽

Global Climate Models ◽

Soot Particles ◽

Cloud Ice ◽

The Impact

AbstractFully accounting for the climate impact of aviation requires a process-level understanding of the impact of aircraft soot particle emissions on the formation of ice clouds. Assessing this impact with the help of global climate models remains elusive and direct observations are lacking. Here we use a high-resolution cirrus column model to investigate how aircraft-emitted soot particles, released after ice crystals sublimate at the end of the lifetime of contrails and contrail cirrus, perturb the formation of cirrus. By allying cloud simulations with a measurement-based description of soot-induced ice formation, we find that only a small fraction (<1%) of the soot particles succeeds in forming cloud ice alongside homogeneous freezing of liquid aerosol droplets. Thus, soot-perturbed and homogeneously-formed cirrus fundamentally do not differ in optical depth. Our results imply that climate model estimates of global radiative forcing from interactions between aircraft soot and large-scale cirrus may be overestimates. The improved scientific understanding reported here provides a process-based underpinning for improved climate model parametrizations and targeted field observations.

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Representing ozone extremes in European megacities: the importance of resolution in a global chemistry climate model

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-3899-2014 ◽

2014 ◽

Vol 14 (8) ◽

pp. 3899-3912 ◽

Cited By ~ 18

Author(s):

Z. S. Stock ◽

M. R. Russo ◽

J. A. Pyle

Keyword(s):

Atmospheric Chemistry ◽

Urban Areas ◽

Climate Models ◽

Climate Model ◽

Surface Ozone ◽

Global Climate Models ◽

Ozone Concentrations ◽

Coarse Resolution ◽

Highly Nonlinear ◽

The Impact

Abstract. The continuing growth of the world's urban population has led to an increasing number of cities with more than 10 million inhabitants. The higher emissions of pollutants, coupled to higher population density, makes predictions of air quality in these megacities of particular importance from both a science and a policy perspective. Global climate models are typically run at coarse resolution to enable both the efficient running of long time integrations, and the ability to run multiple future climate scenarios. However, when considering surface ozone concentrations at the local scale, coarse resolution can lead to inaccuracies arising from the highly nonlinear ozone chemistry and the sensitivity of ozone to the distribution of its precursors on smaller scales. In this study, we use UM-UKCA, a global atmospheric chemistry model, coupled to the UK Met Office Unified Model, to investigate the impact of model resolution on tropospheric ozone, ranging from global to local scales. We focus on the model's ability to represent the probability of high ozone concentrations in the summer and low ozone concentrations, associated with polluted megacity environments, in the winter, and how this varies with horizontal resolution. We perform time-slice integrations with two model configurations at typical climate resolution (CR, ~150 km) and at a higher resolution (HR, ~40 km). The CR configuration leads to overestimation of ozone concentrations on both regional and local scales, while it gives broadly similar results to the HR configuration on the global scale. The HR configuration is found to produce a more realistic diurnal cycle of ozone concentrations and to give a better representation of the probability density function of ozone values in urban areas such as the megacities of London and Paris. We find the observed differences in model behaviour between CR and HR configurations to be largely caused by chemical differences during the winter and meteorological differences during the summer.

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Summer Season Squall-Line Simulations: Sensitivity of Gravity Waves to Physics Parameterization and Implications for Their Parameterization in Global Climate Models

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-13-0380.1 ◽

2014 ◽

Vol 71 (9) ◽

pp. 3376-3391 ◽

Cited By ~ 14

Author(s):

Claudia Stephan ◽

M. Joan Alexander

Keyword(s):

Gravity Waves ◽

Momentum Flux ◽

Large Scale ◽

Climate Models ◽

Climate Model ◽

Middle Atmosphere ◽

Global Climate ◽

Global Climate Models ◽

Squall Line ◽

The Impact

Abstract Gravity waves have important effects on the middle atmosphere circulation, and those generated by convection are prevalent in the tropics and summer midlatitudes. Numerous case studies have been carried out to investigate their characteristics in high-resolution simulations. Here, the impact of the choice of physics parameterizations on the generation and spectral properties of these waves in models is investigated. Using the Weather Research and Forecasting Model (WRF) a summertime squall line over the Great Plains is simulated in a three-dimensional, nonlinear, and nonhydrostatic mesoscale framework. The distributions of precipitation strength and echo tops in the simulations are compared with radar data. Unsurprisingly, those storm features are most sensitive to the microphysics scheme. However, it is found that these variations in storm morphology have little influence on the simulated stratospheric momentum flux spectra. These results support the fundamental idea behind climate model parameterizations: that the large-scale storm conditions can be used to predict the spectrum of gravity wave momentum flux above the storm irrespective of the convective details that coarse-resolution models cannot capture. The simulated spectra are then contrasted with those obtained from a parameterization used in global climate models. The parameterization reproduces the shape of the spectra reasonably well but their magnitudes remain highly sensitive to the peak heating rate within the convective cells.

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Technical Note: On the use of nudging for aerosol-climate model intercomparison studies

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-14-10311-2014 ◽

2014 ◽

Vol 14 (7) ◽

pp. 10311-10343 ◽

Cited By ~ 1

Author(s):

K. Zhang ◽

H. Wan ◽

X. Liu ◽

S. J. Ghan ◽

G. J. Kooperman ◽

...

Keyword(s):

Large Scale ◽

Climate Models ◽

Climate Model ◽

Cloud Amount ◽

Temperature Fields ◽

Specific Humidity ◽

Radiation Budget ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

The Impact

Abstract. Nudging is an assimilation technique widely used in the development and evaluation of climate models. Constraining the simulated wind and temperature fields using global weather reanalysis facilitates more straightforward comparison between simulation and observation, and reduces uncertainties associated with natural variabilities of the large-scale circulation. On the other hand, the forcing introduced by nudging can be strong enough to change the basic characteristics of the model climate. In the paper we show that for the Community Atmosphere Model version 5, due to the systematic temperature bias in the standard model and the sensitivity of simulated ice formation to anthropogenic aerosol concentration, nudging towards reanalysis results in substantial reductions in the ice cloud amount and the impact of anthropogenic aerosols on longwave cloud forcing. In order to reduce discrepancies between the nudged and unconstrained simulations and meanwhile take the advantages of nudging, two alternative experimentation methods are evaluated. The first one constrains only the horizontal winds. The second method nudges both winds and temperature, but replaces the long-term climatology of the reanalysis by that of the model. Results show that both methods lead to substantially improved agreement with the free-running model in terms of the top-of-atmosphere radiation budget and cloud ice amount. The wind-only nudging is more convenient to apply, and provides higher correlations of the wind fields, geopotential height and specific humidity between simulation and reanalysis. This suggests nudging the horizontal winds but not temperature is a good strategy for the investigation of aerosol indirect effects through ice clouds, since it provides well-constrained meteorology without strongly perturbing the model's mean climate.

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Sensitivity of tropospheric ozone to halogen chemistry in the chemistry-climate model LMDZ-INCA vNMHC

10.5194/gmd-2021-388 ◽

2021 ◽

Author(s):

Cyril Caram ◽

Sophie Szopa ◽

Anne Cozic ◽

Slimane Bekki ◽

Carlos Cuevas ◽

...

Keyword(s):

Atmospheric Chemistry ◽

Tropospheric Ozone ◽

Climate Models ◽

Marine Boundary Layer ◽

Climate Model ◽

Chemical System ◽

Model Studies ◽

Halogen Chemistry ◽

The Impact

Abstract. The atmospheric chemistry of halogenated species (Cl, Br, I) participates in the global chemical sink of tropospheric ozone and perturbs the oxidizing capacity of the troposphere, notably influencing the atmospheric lifetime of methane. Global chemistry-climate models are commonly used to assess the global budget of ozone, its sensitivity to emissions of its precursors, and to project its long-term evolution. Here, we report on the implementation of tropospheric halogens chemistry in the chemistry-climate model LMDZ-INCA and its effects on the tropospheric ozone budget. Overall, the results show that the model simulates satisfactorily the impact of halogens on the photooxidizing system in the troposphere, in particular in the marine boundary layer. To elucidate the mechanisms and quantify the effects, standard metrics representative of the behavior of the tropospheric chemical system (Ox, HOx, NOx, CH4, and NMVOCs) are computed with and without halogen chemistry. Tropospheric halogens in the LMDZ-INCA model lead to a decrease of 22 % in the ozone burden, 8 % in OH, and 33 % in NOx. Additional sensitivity simulations show that the inclusion of halogens chemistry makes ozone more sensitive to perturbations in CH4, NOx, and NMVOCs. Consistent with other global model studies, the sensitivity of the tropospheric ozone burden to changes from pre-industrial to present-day emissions is found to be ~20 % lower when tropospheric halogens are taken into account.

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Evaluation of climate model aerosol seasonal and spatial variability over Africa using AERONET

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-13999-2017 ◽

2017 ◽

Vol 17 (22) ◽

pp. 13999-14023 ◽

Cited By ~ 9

Author(s):

Hannah M. Horowitz ◽

Rebecca M. Garland ◽

Marcus Thatcher ◽

Willem A. Landman ◽

Zane Dedekind ◽

...

Keyword(s):

Biomass Burning ◽

Radiative Forcing ◽

Large Scale ◽

Climate Models ◽

Climate Model ◽

Aerosol Particles ◽

Atmospheric Model ◽

Model Aerosol ◽

The Impact

Abstract. The sensitivity of climate models to the characterization of African aerosol particles is poorly understood. Africa is a major source of dust and biomass burning aerosols and this represents an important research gap in understanding the impact of aerosols on radiative forcing of the climate system. Here we evaluate the current representation of aerosol particles in the Conformal Cubic Atmospheric Model (CCAM) with ground-based remote retrievals across Africa, and additionally provide an analysis of observed aerosol optical depth at 550 nm (AOD550 nm) and Ångström exponent data from 34 Aerosol Robotic Network (AERONET) sites. Analysis of the 34 long-term AERONET sites confirms the importance of dust and biomass burning emissions to the seasonal cycle and magnitude of AOD550 nm across the continent and the transport of these emissions to regions outside of the continent. In general, CCAM captures the seasonality of the AERONET data across the continent. The magnitude of modeled and observed multiyear monthly average AOD550 nm overlap within ±1 standard deviation of each other for at least 7 months at all sites except the Réunion St Denis Island site (Réunion St. Denis). The timing of modeled peak AOD550 nm in southern Africa occurs 1 month prior to the observed peak, which does not align with the timing of maximum fire counts in the region. For the western and northern African sites, it is evident that CCAM currently overestimates dust in some regions while others (e.g., the Arabian Peninsula) are better characterized. This may be due to overestimated dust lifetime, or that the characterization of the soil for these areas needs to be updated with local information. The CCAM simulated AOD550 nm for the global domain is within the spread of previously published results from CMIP5 and AeroCom experiments for black carbon, organic carbon, and sulfate aerosols. The model's performance provides confidence for using the model to estimate large-scale regional impacts of African aerosols on radiative forcing, but local feedbacks between dust aerosols and climate over northern Africa and the Mediterranean may be overestimated.

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Radar observations of winds, waves and tides in the mesosphere and lower thermosphere over South Georgia island (54°S, 36°W) and comparison to WACCM simulations

10.5194/acp-2021-981 ◽

2021 ◽

Author(s):

Neil P. Hindley ◽

Neil Cobbett ◽

David C. Fritts ◽

Diego Janchez ◽

Nicholas J. Mitchell ◽

...

Keyword(s):

Large Scale ◽

Climate Model ◽

Lower Thermosphere ◽

Neutral Atmosphere ◽

Residual Circulation ◽

Radar Observations ◽

South Georgia ◽

Global Circulation Models ◽

Mesosphere And Lower Thermosphere ◽

The Impact

Abstract. The mesosphere and lower thermosphere (MLT) is a dynamic layer of the earth’s atmosphere. This region marks the interface at which neutral atmosphere dynamics begin to influence the ionosphere and space weather. However, our understanding of this region and our ability to accurately simulate it in global circulation models (GCMs) is limited by a lack of observations, especially in remote locations. To this end, a meteor radar was deployed on the remote mountainous island of South Georgia (54° S, 36° W) in the Southern Ocean from 2016 to 2020. The goal of this study is to use these new measurements to characterise the fundamental dynamics of the MLT above South Georgia including large-scale winds, solar tides, planetary waves (PWs) and mesoscale gravity waves (GWs). We first present an improved method for time-height localisation of radar wind measurements and characterise the large-scale MLT winds. We then explore the amplitudes and phases of the diurnal (24 h), semidiurnal (12 h) and terdiurnal (8 h) solar tides at this latitude. We also explore PW activity and find very large amplitudes up to 30 ms−1 for the quasi-2 day wave in summer and show that the dominant modes of the quasi-5, 10 and 16 day waves are westward W1 and W2. We investigate wind variance due to GWs in the MLT and use a new method to show an east-west tendency of GW variance of up to 20 % during summer and a weaker north-south tendency of 0–5 % during winter. This is contrary to the expected tendency of GW directions in the winter stratosphere below, which is a strong suggestion of secondary GW (2GW) observations in the MLT. Lastly, comparison of radar winds to a climatological Whole Atmosphere Community Climate Model (WACCM) simulation reveals a simulated summertime mesopause and zonal wind shear that occur at altitudes around 10 km lower than observed, and southward winds during winter above 90 km altitude in the model that are not seen in observations. Further, wintertime zonal winds above 85 km altitude are eastward in radar observations but in WACCM they are found to weaken and reverse to westward. Recent studies have linked this discrepancy to the impact of 2GWs on the residual circulation which are not included in WACCM. These measurements therefore provide vital constraints that can guide the development of GCMs as they extend upwards into this important region of the atmosphere.

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