Distribution Of Stratospheric Aerosols Research Articles

Climate response to latitudinal and altitudinal distribution of stratospheric sulfate aerosols

Climate response to latitudinal and altitudinal distribution of stratospheric sulfate aerosols

Climate response to latitudinal and altitudinal distribution of stratospheric sulfate aerosols

Climate response to latitudinal and altitudinal distribution of stratospheric sulfate aerosols

Quantifying CanESM5 and EAMv1 sensitivities to Mt. Pinatubo volcanic forcing for the CMIP6 historical experiment

Abstract. Large volcanic eruptions reaching the stratosphere have caused marked perturbations to the global climate including cooling at the Earth's surface, changes in large-scale circulation and precipitation patterns and marked temporary reductions in global ocean heat content. Many studies have investigated these effects using climate models; however, uncertainties remain in the modelled response to these eruptions. This is due in part to the diversity of forcing datasets that are used to prescribe the distribution of stratospheric aerosols resulting from these volcanic eruptions, as well as uncertainties in optical property derivations from these datasets. To improve this situation for the sixth phase of the Coupled Model Intercomparison Project (CMIP6), a two-step process was undertaken. First, a combined stratospheric aerosol dataset, the Global Space-based Stratospheric Aerosol Climatology (GloSSAC; 1979–2016), was constructed. Next, GloSSAC, along with information from ice cores and Sun photometers, was used to generate aerosol distributions, characteristics and optical properties to construct a more consistent stratospheric aerosol forcing dataset for models participating in CMIP6. This “version 3” of the stratospheric aerosol forcing has been endorsed for use in all contributing CMIP6 simulations. Recent updates to the underlying GloSSAC from version 1 to version 1.1 affected the 1991–1994 period and necessitated an update to the stratospheric aerosol forcing from version 3 to version 4. As version 3 remains the official CMIP6 input, quantification of the impact on radiative forcing and climate is both relevant and timely for interpreting results from experiments such as the CMIP6 historical simulations. This study uses two models, the Canadian Earth System Model version 5 (CanESM5) and the Energy Exascale Earth System Model (E3SM) Atmosphere Model version 1 (EAMv1), to estimate the difference in instantaneous radiative forcing in simulated post-Pinatubo climate response when using version 4 instead of version 3. Differences in temperature, precipitation and radiative forcings are generally found to be small compared to internal variability. An exception to this is differences in monthly temperature anomalies near 24 km altitude in the tropics, which can be as large as 3 ∘C following the eruption of Mt. Pinatubo.

Issues related to the retrieval of stratospheric-aerosol particle size information based on optical measurements

Abstract. Stratospheric-sulfate aerosols play an important role in the physics and chemistry of the atmosphere. The radiative and chemical effects of stratospheric-sulfate aerosols depend critically on the aerosol particle size distribution and its variability. Despite extensive research spanning several decades, the scientific understanding of the particle size distribution of stratospheric aerosols is still incomplete. Particle size estimates (often represented by the median radius of an assumed monomodal log-normal distribution with a fixed width or by the effective radius) reported in different studies cover a wide range, even under background stratospheric conditions, and particle size estimates retrieved from satellite solar-occultation measurements in the optical spectral range show a tendency to be systematically larger than retrievals based on other optical methods. In this contribution we suggest a potential reason for these systematic differences. Differences between the actual aerosol particle size distribution and the size distribution function assumed for aerosol size retrievals may lead to systematic differences in retrieved aerosol size estimates. We demonstrate that these systematic differences may differ significantly for different measurement techniques, which is related to the different sensitivities of these measurement techniques to specific parts of the aerosol particle population. In particular, stratospheric-aerosol size retrievals based on solar-occultation observations may yield systematically larger particle size estimates (median or effective radii) compared to, e.g., lidar backscatter measurements. Aerosol concentration, on the other hand, may be systematically smaller in retrievals based on occultation measurements compared to lidar measurements. The results indicate that stratospheric-aerosol size retrievals based on occultation or lidar measurements have to be interpreted with caution, as long as the actual aerosol particle size distribution is not well known.

Aerosol particle size distribution in the stratosphere retrieved from SCIAMACHY limb measurements

Abstract. Information about aerosols in the Earth's atmosphere is of a great importance in the scientific community. While tropospheric aerosol influences the radiative balance of the troposphere and affects human health, stratospheric aerosol plays an important role in atmospheric chemistry and climate change. In particular, information about the amount and distribution of stratospheric aerosols is required to initialize climate models, as well as validate aerosol microphysics models and investigate geoengineering. In addition, good knowledge of stratospheric aerosol loading is needed to increase the retrieval accuracy of key trace gases (e.g. ozone or water vapour) when interpreting remote sensing measurements of the scattered solar light. The most commonly used characteristics to describe stratospheric aerosols are the aerosol extinction coefficient and Ångström coefficient. However, the use of particle size distribution parameters along with the aerosol number density is a more optimal approach. In this paper we present a new retrieval algorithm to obtain the particle size distribution of stratospheric aerosol from space-borne observations of the scattered solar light in the limb-viewing geometry. While the mode radius and width of the aerosol particle size distribution are retrieved, the aerosol particle number density profile remains unchanged. The latter is justified by a lower sensitivity of the limb-scattering measurements to changes in this parameter. To our knowledge this is the first data set providing two parameters of the particle size distribution of stratospheric aerosol from space-borne measurements of scattered solar light. Typically, the mode radius and w can be retrieved with an uncertainty of less than 20 %. The algorithm was successfully applied to the tropical region (20° N–20° S) for 10 years (2002–2012) of SCIAMACHY observations in limb-viewing geometry, establishing a unique data set. Analysis of this new climatology for the particle size distribution parameters showed clear increases in the mode radius after the tropical volcanic eruptions, whereas no distinct behaviour of the absolute distribution width could be identified. A tape recorder, which describes the time lag as the perturbation propagates to higher altitudes, was identified for both parameters after the volcanic eruptions. A quasi-biannual oscillation (QBO) pattern at upper altitudes (28–32 km) is prominent in the anomalies of the analysed parameters. A comparison of the aerosol effective radii derived from SCIAMACHY and SAGE II data was performed. The average difference is found to be around 30 % at the lower altitudes, decreasing with increasing height to almost zero around 30 km. The data sample available for the comparison is, however, relatively small.

Divergent responses of tropical cyclone genesis factors to strong volcanic eruptions at different latitudes

To understand the behaviors of tropical cyclones (TCs), it is very important to explore how TCs respond to anthropogenic greenhouse gases and natural forcings. Volcanic eruptions are a major natural forcing mechanism because they inject sulphate aerosols into the stratosphere, which modulate the global climate by absorbing and scattering solar radiation. The number of Atlantic hurricanes is thought to be reduced following strong tropical eruptions, but whether the response of TCs varies with the locations of the volcanoes and the different ocean basins remains unknown. Here, we use the Community Earth System Model-Last Millennium Ensemble to investigate the response of the large-scale environmental factors that spawn TCs to strong volcanic eruptions at different latitudes. A composite analysis indicates that tropical and northern hemisphere volcanic eruptions lead to significantly unfavorable conditions for TC genesis over the whole Pacific basin and the North Atlantic during the 3 years post-eruption, relative to the preceding 3 years. Southern hemisphere volcanic eruptions result in obviously unfavorable conditions for TC formation over the southwestern Pacific, but more favorable conditions over the North Atlantic. The mean response over the Indian Ocean is generally muted and insignificant. It should be noted that volcanic eruptions impact on environmental conditions through both the direct effect (i.e. on radiative forcing) and the indirect effect (i.e. on El Nino-Southern Oscillation), which is not differentiated in this study. In addition, the spread of the TC genesis response is considerably large for each category of eruptions over each ocean basin, which is also seen in the observational/proxy-based records. This large spread is attributed to the differences in stratospheric aerosol distributions, initial states and eruption intensities, and makes the short-term forecast of TC activity following the next large eruption challenging.

The Aerosol Limb Imager: acousto-optic imaging of limb-scattered sunlight for stratospheric aerosol profiling

Abstract. The Aerosol Limb Imager (ALI) is an optical remote sensing instrument designed to image scattered sunlight from the atmospheric limb. These measurements are used to retrieve spatially resolved information of the stratospheric aerosol distribution, including spectral extinction coefficient and particle size. Here we present the design, development and test results of an ALI prototype instrument. The long-term goal of this work is the eventual realization of ALI on a satellite platform in low earth orbit, where it can provide high spatial resolution observations, both in the vertical and cross-track. The instrument design uses a large-aperture acousto-optic tunable filter (AOTF) to image the sunlit stratospheric limb in a selectable narrow wavelength band ranging from the visible to the near infrared. The ALI prototype was tested on a stratospheric balloon flight from the Canadian Space Agency (CSA) launch facility in Timmins, Canada, in September 2014. Preliminary analysis of the hyperspectral images indicates that the radiance measurements are of high quality, and we have used these to retrieve vertical profiles of stratospheric aerosol extinction coefficient from 650 to 1000 nm, along with one moment of the particle size distribution. Those preliminary results are promising and development of a satellite prototype of ALI within the Canadian Space Agency is ongoing.

Long-Term Variation of Stratospheric Aerosols Observed With Lidar from 1982 to 2014 Over Tsukuba, Japan

The vertical distribution of stratospheric aerosols has been measured with lidars at the Meteorological Research Institute (MRI) over Tsukuba since 1982. After two major volcanic eruptions (Mt. El Chichon in 1982 and Mt. Pinatubo in 1991), stratospheric aerosol loading increased about 50–100 times compared with the background level which was observed for 1997-2000. From 2000 to 2012, a slight increase (5.3% year–1) was observed by some volcanic eruptions. This long-term lidar data have been used for assessing of impact of the stratospheric aerosols on climate and the ozone layer.

A new Geoengineering Model Intercomparison Project (GeoMIP) experiment designed for climate and chemistry models

Abstract. A new Geoengineering Model Intercomparison Project (GeoMIP) experiment "G4 specified stratospheric aerosols" (short name: G4SSA) is proposed to investigate the impact of stratospheric aerosol geoengineering on atmosphere, chemistry, dynamics, climate, and the environment. In contrast to the earlier G4 GeoMIP experiment, which requires an emission of sulfur dioxide (SO2) into the model, a prescribed aerosol forcing file is provided to the community, to be consistently applied to future model experiments between 2020 and 2100. This stratospheric aerosol distribution, with a total burden of about 2 Tg S has been derived using the ECHAM5-HAM microphysical model, based on a continuous annual tropical emission of 8 Tg SO2 yr−1. A ramp-up of geoengineering in 2020 and a ramp-down in 2070 over a period of 2 years are included in the distribution, while a background aerosol burden should be used for the last 3 decades of the experiment. The performance of this experiment using climate and chemistry models in a multi-model comparison framework will allow us to better understand the impact of geoengineering and its abrupt termination after 50 years in a changing environment. The zonal and monthly mean stratospheric aerosol input data set is available at https://www2.acd.ucar.edu/gcm/geomip-g4-specified-stratospheric-aerosol-data-set.

Characterization of Stratospheric Aerosol Distributions during the Volcanically Quiescent Period of 1998-2004

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Decadal prediction skill in a multi-model ensemble

Decadal climate predictions may have skill due to predictable components in boundary conditions (mainly greenhouse gas concentrations but also tropospheric and stratospheric aerosol distributions) and initial conditions (mainly the ocean state). We investigate the skill of temperature and precipitation hindcasts from a multi-model ensemble of four climate forecast systems based on coupled ocean-atmosphere models. Regional variations in skill with and without trend are compared with similarly analysed uninitialised experiments to separate the trend due to monotonically increasing forcings from fluctuations around the trend due to the ocean initial state and aerosol forcings. In temperature most of the skill in both multi-model ensembles comes from the externally forced trends. The rise of the global mean temperature is represented well in the initialised hindcasts, but variations around the trend show little skill beyond the first year due to the absence of volcanic aerosols in the hindcasts and the unpredictability of ENSO. The models have non-trivial skill in hindcasts of North Atlantic sea surface temperature beyond the trend. This skill is highest in the northern North Atlantic in initialised experiments and in the subtropical North Atlantic in uninitialised simulations. A similar result is found in the Pacific Ocean, although the signal is less clear. The uninitialised simulations have good skill beyond the trend in the western North Pacific. The initialised experiments show some skill in the decadal ENSO region in the eastern Pacific, in agreement with previous studies. However, the results in this study are not statistically significant (p ≈ 0.1) by themselves. The initialised models also show some skill in forecasting 4-year mean Sahel rainfall at lead times of 1 and 5 years, in agreement with the observed teleconnection from the Atlantic Ocean. Again, the skill is not statistically significant (p ≈ 0.2). Furthermore, uninitialised simulations that include volcanic aerosols have similar skill. It is therefore still an open question whether initialisation improves predictions of Sahel rainfall. We conclude that the main source of skill in forecasting temperature is the trend forced by rising greenhouse gas concentrations. The ocean initial state contributes to skill in some regions, but variations in boundary forcings such as aerosols are as important in decadal forecasting.

Impact of the ENSO Cycle on the Stratospheric Aerosol Distribution

AbstractIn this paper the Oceanic Nino Index and the HALOE dataset were used to study the impact of the ENSO cycle on the stratospheric aerosols by analyzing the lag correlation. The comparison of aerosol surface area after El Nino and the La Nina images the impact clearly and the change can be explained according to the transporting velocity and stream situation. The result displays that ENSO significantly affects the distribution of stratospheric aerosols, especially above the equator and low latitude. Within six months after El Nino, the aerosol surface density in the tropical lower‐stratosphere layer is larger than the average while it's smaller in the middle stratosphere; however, the situation was contrary after La Nina. El Nino and La Nina had significantly different effects, which made the differences in aerosol content in 60 hPa as high as 45%. The change of aerosol surface density in the six months lag could be up to 16% as the temperature changed 1 K. The strong influence of ENSO could last for about six months and faded out in 2 years. Changes in the tropics were stronger than in the middle and high latitudes, while the characteristics of northern and southern hemispheres are different. ENSO lead to a change in the aerosol delivery by affecting the residual circulation, and in turn lead to these differences of the aerosol distribution mentioned above.

The global middle-atmosphere aerosol model MAECHAM5-SAM2: comparison with satellite and in-situ observations

Abstract. In this paper we investigate results from a three-dimensional middle-atmosphere aerosol-climate model which has been developed to study the evolution of stratospheric aerosols. Here we focus on the stratospheric background period and evaluate several key quantities of the global distribution of stratospheric aerosols and their precursors with observations and other model studies. It is shown that the model fairly well reproduces in situ observations of the aerosol size and number concentrations in the upper troposphere and lower stratosphere (UT/LS). Compared to measurements from the limb-sounding SAGE II satellite instrument, modelled integrated aerosol quantities are more biased the lower the moment of the aerosol population is. Both findings are consistent with earlier work analysing the quality of SAGE II retrieved e.g. aerosol surface area densities in the volcanically unperturbed stratosphere (SPARC/ASAP, 2006; Thomason et al., 2008; Wurl et al., 2010). The model suggests that new particles are formed over large areas of the LS, albeit nucleation rates in the upper troposphere are at least one order of magnitude larger than those in the stratosphere. Hence, we suggest that both, tropospheric sulphate aerosols and particles formed in situ in the LS are maintaining the stability of the stratospheric aerosol layer in the absence of direct stratospheric emissions from volcanoes. Particle size distributions are clearly bimodal, except in the upper branches of the stratospheric aerosol layer where aerosols evaporate. Modelled concentrations of condensation nuclei (CN) are smaller than measured in regions of the aerosol layer where aerosol mixing ratios are largest. This points to an overestimated particle growth by coagulation. Transport regimes of tropical stratospheric aerosol have been identified from modelled aerosol mixing ratios and correspond to those deduced from satellite extinction measurements. We found that convective updraft in the Asian Monsoon region significantly contributes to both stratospheric aerosol load and size. The timing of formation and descend of layers of fine mode particles in the winter and spring polar stratosphere (CN layer) are well reproduced by the model. Where temperatures in the stratosphere increase with altitude, nucleation is unlikely to occur. Nevertheless, in these regions we find a significant concentration of fine mode aerosols. The place of origin of these particles is in the polar stratosphere. They are mixed into the mid-latitudes by planetary waves. There enhanced condensation rates of sulphuric acid vapour counteract evaporation and extend aerosol lifetime in the upper branches of the stratospheric aerosol layer. Measured aerosol precursors concentrations, SO2 and sulphuric acid vapour, are fairly well reproduced by the model throughout the stratosphere.

Global warming mitigation by means of controlled aerosol emissions into the stratosphere: Global and regional peculiarities of temperature response as estimated in IAP RAS CM simulations

The problem of climate warming mitigation by means of controlled sulphur emissions into the stratosphere has become of growing interest in recent years. Using the IAP RAS global climate model with uniform horizontal distribution of stratospheric aerosols, it has been shown that a complete compensation of global warming, realizable under the SRES A1B scenario of the anthropogenic impact on climate, requires stratospheric sulfate emissions of 5–16 TgS/yr (depending on the chosen values of stratospheric aerosol parameters) in the middle and of 10–30 TgS/yr at the end of the 21st century. Such emissions will result in the essential additional aerosol pollution of the troposphere due to the sedimentation of stratospheric aerosol particles there. Significant-in-magnitude regional anomalies of the surface air temperature of different signs occur in global warming compensation in different regions. Warming compensation in the most sensitive to climate forcing Earth regions (in particular, Siberia), additionally increases the required emissions of stratospheric aerosols by about 10%. In addition, in the case of ceasing such controlled climate forcing, its temperature effect vanishes in one to two decades with a sharp acceleration of global and regional near-surface warming in this period. Thus, the rate of regional temperature changes will attain 3–4 K/decade if the compensating action ceases in 2075.

Simulating the effects of the 1991 Mount Pinatubo volcanic eruption using the ARPEGE atmosphere general circulation model

The climate changes that occured following the volcanic eruption of Mount Pinatubo in the Phillippines on 15 June 1991 have been simulated using the ARPEGE atmosphere general circulation model (AGCM). The model was forced by a reconstructed spatial-time distribution of stratospheric aerosols intended for use in long climate simulations. Four statistical ensembles of the AGCM simulations with and without volcanic aerosols over a period of 5 years following the eruption have been made, and the calculated fields have been compared to available observations. The model is able to reproduce some of the observed features after the eruption, such as the winter warming pattern that was observed over the Northern Hemisphere (NH) during the following winters. This pattern was caused by an enhanced Equator-to-pole temperature gradient in the stratosphere that developed due to aerosol heating of the tropics. This in turn led to a strengthening of the polar vortex, which tends to modulate the planetary wave field in such a way that an anomalously positive Arctic Oscillation pattern is produced in the troposphere and at the surface, favouring warm conditions over the NH. During the summer, the model produced a more uniform cooling over the NH.

CALIPSO observations of stratospheric aerosols: a preliminary assessment

Abstract. We have examined the 532-nm aerosol backscatter coefficient measurements by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) for their use in the monitoring of stratospheric aerosol. CALIPSO makes observations that span from 82° S to 82° N each day and, for each profile, backscatter coefficient values reported up to ~40 km. The possibility of using CALIPSO for stratospheric aerosol monitoring is demonstrated by the clear observation of the 20 May 2006 eruption of Montserrat in the earliest CALIPSO data in early June as well as by observations showing the 7 October 2006 eruption of Tavurvur (Rabaul). However, the very low aerosol loading within the stratosphere makes routine observations of the stratospheric aerosol far more difficult than relatively dense volcanic plumes. Nonetheless, we found that averaging a complete days worth of nighttime-only data into 5-deg latitude by 1-km vertical bins yields a stratospheric aerosol distribution that is fairly consistent with past measurements by spaceborne instruments. Based on comparisons with 2004 data from the Stratospheric Aerosol and Gas Experiment, the derived values are close to expectation except in the tropics where they are larger by about a factor of 2. The cause of the difference in the tropics is not readily apparent but is most likely related to difficulties in the lidar calibration process currently found in the CALIOP data at tropical latitudes.

A global climatology of stratospheric aerosol size distribution parameters derived from SAGE II data over the period 1984–2000: 2. Reference data

In a companion paper, a global climatology was presented for different parameters characterizing the size distribution of stratospheric aerosols; the particle number density, median radius, and mode width of the particle size distribution were assumed to be lognormal. Those aerosol parameters were retrieved by optical inversion of Stratospheric Aerosol and Gas Experiment (SAGE) II aerosol extinction profiles. The data cover the time period 1984–2000. In this second part of the study, we present the climatology in a more quantitative way, and starting from the case of the Mount Pinatubo eruption in June 1991, we try to derive reference data for the three considered aerosol parameters. Using four time intervals representative of the different phases of the aerosol relaxation after a major eruption, we investigate the data set spanning the period June 1991 to mid‐1999 and present a spatiotemporal description of the aerosol distribution for the whole relaxation period. The range of the data dispersion due to seasonal effects and random variations is also estimated. Finally, a study of the error on the three parameters is presented for the whole data set, as well as an overview of our validation efforts.

A global climatology of stratospheric aerosol size distribution parameters derived from SAGE II data over the period 1984–2000: 1. Methodology and climatological observations

A global climatology has been derived for different parameters characterizing the size distribution of stratospheric aerosols; the particle number density, median radius, and mode width of the particle distribution were assumed to be lognormal. Those parameters were retrieved by applying an original optical inversion scheme on Stratospheric Aerosol and Gas Experiment (SAGE) II aerosol extinction profiles spanning from 1984 to 2000. The specificity of the inversion algorithm resides in the use of a continuity constraint that allows one to partly get rid of the ill‐posedness of the inversion problem and to greatly improve the quality and the stability of the solution. The long time period covered by the SAGE II experiment allows us to observe very different situations of volcanic load, from highly volcanic periods following major eruptions (mainly after the Pinatubo eruption of June 1991), to periods of very low aerosol load. Beyond the effects of volcanism on the aerosol parameters, other influences are observed such as seasonal effects, more particularly at midlatitude. Also the quasi‐biennial oscillation (QBO) shows a clear influence on the size distribution. In this paper, we present the general features of the different aerosol profiles, and we investigate the effects of these three main influences affecting the size distribution. We present the climatology in a more quantitative way in a companion paper, by proposing reference data for the three considered aerosol parameters.

Infrared Satellite Retrievals in the Presence of Stratospheric Aerosol

The presence of stratospheric aerosol can bias the results of infrared satellite retrievals of sea surface temperature (SST) and total precipitable water (TPW). In the case of linear SST retrieval using the Along Track Scanning Radiometer (ATSR), on the ESA European remote-sensing satellites, constant coefficients can be found that give negligible bias (less than 0.1 K) over a wide range of aerosol amount (11-μm optical thickness from 0.0 to 0.022). For TPW retrieval, in contrast, the biases associated with stratospheric aerosol are less satisfactory (2 kg m−2 or greater across a range of 11-μm optical thickness of 0.0–0.01). However, the authors show how to find optimal aerosol-dependent retrieval coefficients for any stratospheric aerosol distribution from knowledge of the mean and variance of that aerosol distribution. Examples of SST and TPW retrieval using simulated ATSR brightness temperature data are given.

Role of the quasi‐biennial oscillation in the transport of aerosols from the tropical stratospheric reservoir to midlatitudes

The temporal evolution of the stratospheric aerosol distribution in the tropical stratospheric reservoir after the eruption of Mount Pinatubo was observed from 1992 to 1995 by the HALOE instrument on the UARS satellite. Since the spatial gradient of aerosol loading is large at the boundaries of the tropical stratospheric reservoir due to the volcanic aerosols, the effect of the meridional circulation on the distribution is seen clearly. The mechanism for dispersal of aerosol in the lower stratosphere from the tropics into midlatitudes strongly depends on the phase of the equatorial zonal wind. The time‐latitude crosssections of the normalized distribution of aerosol on isentropic surfaces are used to observe the equatorial variation as well as change in meridional dispersal during the quasibiennial period. Observed tropical stratospheric winds are used with a simple analytical dynamical model to examine transport processes of tracers from the tropics during several phases of the quasibiennial oscillation (QBO) from 1992 to 1995. The Lagrangian meridional circulation in the tropics is consistent with the vertical and meridional velocities correlated with the QBO in the zonal wind. We find that vertical motion plays a crucial role in vertical and subsequent meridional transport. The pattern of meridional divergence derived from the vertical velocity is closely related to the observed HALOE aerosol distributions and their temporal development in the equatorial region. The westerly (easterly) shear phase of the QBO is associated with sinking (rising) motions at the equator and subsequent poleward (equatorward) transport in the lower stratosphere.