Impact of increased vertical resolution on simulation of tropical climate

Summary The aim of this study is to describe the behaviour of tropical dynamics in the ECHAM4 model when increased vertical resolution around the tropopause and in the planetary boundary layer is used. In this work we perform experiments with the ECHAM4 model using T30 horizontal resolution and 19 and 42 vertical levels. The impact of the increased vertical resolution on the simulation of tropical clouds and precipitation has been investigated. Therefore, the dynamic fields related to tropical convection have been analyzed. The results suggest a beneficial effect of the increased number of vertical levels on the convective scheme performance and on the related dynamic fields over the Tropics. The improvement of the rainfall climatologies in the 42-level model has been explained via the impact of vertical resolution on the cloud structure. In the cloud spectrum of the L42 simulation, a third peak appears around 600 hPa, revealing that when using higher vertical resolution the convective parametrization starts to represent cumulus congestus clouds.     

Meteorological responses to Mt. Baekdu volcanic eruption over east asia in an offline global climate-chemistry model: A pilot study

We examine the meteorological responses due to the probable eruption of Mt. Baekdu using an off-line Climate-Chemistry model that is composed of the National Center for Atmospheric Research (NCAR) Climate Atmosphere Model version 3 (CAM3) and a global chemistry transport model (GEOS-Chem). Using the aerosol dataset from the GEOS-Chem driven by GEOS-5 meteorology, experiment and control simulations of the climate model are performed and their meteorological differences between the two simulations are analyzed. The magnitudes of volcanic eruption and column injection height were presumably set to 1/200 of the Mt. Pinatubo eruption and 9 km, respectively. Significant temperature drop in the lower troposphere (850 hPa), which is mainly due to a direct effect of prescribed volcanic aerosols from Mt. Baekdu, has been simulated up to about ?4 K. The upper atmosphere (150 hPa) right above the volcano, however, shows significant warming due to the absorption of the infrared radiation by volcanic aerosols. As a result of the volcanic eruption in the climate model, wave-like patterns are shown in both the geopotential height and horizontal wind. The changes in the lower atmospheric temperature are well associated with the modification of the atmospheric circulation through the hydrostatic balance. In spite of limitations in our current simulations due to several underlying assumptions, our results could give a clue to understanding the meteorological impacts from Mt. Baekdu eruptions that are currently attracting considerable public attention.     

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.     

菲律宾皮纳图博火山爆发的卫星探测分析

应用气象卫星的探测处理资料，揭示和探讨了1991年6月15日菲律宾皮纳图博火山爆发后火山灰和火山尘云的演变和漂移中的一些重要特征，为研究这次火山爆发对天气和气候的影响提供了卫星观测分析信息。     

Pinatubo eruption winter climate effects: model versus observations

Large volcanic eruptions, in addition to the well-known effect of producing global cooling for a year or two, have been observed to produce shorterterm responses in the climate system involving non-linear dynamical processes. In this study, we use the ECHAM2 general circulation model forced with stratospheric aerosols to test some of these ideas. Run in a perpetual-January mode, with tropical stratospheric heating from the volcanic aerosols typical of the 1982 El Chichón eruption or the 1991 Pinatubo eruption, we find a dynamical response with an increased polar night jet in the Northern Hemisphere (NH) and stronger zonal winds which extend down into the troposphere. The Azores High shifts northward with increased tropospheric westerlies at 60°N and increased easterlies at 30°N. Surface temperatures are higher both in northern Eurasia and North America, in agreement with observations for the NH winters of 1982–83 and 1991–92 as well as the winters following the other 10 largest volcanic eruptions since 1883.This paper was presented at the Second International Conference on Modelling of Global Climate Variability, held in Hamburg 7–11 September 1992 under the auspices of the Max Planck Institute for Meteorology. Guest Editor for these papers is L. Dümenil     

Response to the eruption of Mount Pinatubo in relation to climate sensitivity in the CMIP3 models

The radiative flux perturbations and subsequent temperature responses in relation to the eruption of Mount Pinatubo in 1991 are studied in the ten general circulation models incorporated in the Coupled Model Intercomparison Project, phase 3 (CMIP3), that include a parameterization of volcanic aerosol. Models and observations show decreases in global mean temperature of up to 0.5 K, in response to radiative perturbations of up to 10 W m⁻², averaged over the tropics. The time scale representing the delay between radiative perturbation and temperature response is determined by the slow ocean response, and is estimated to be centered around 4 months in the models. Although the magniude of the temperature response to a volcanic eruption has previously been used as an indicator of equilibrium climate sensitivity in models, we find these two quantities to be only weakly correlated. This may partly be due to the fact that the size of the volcano-induced radiative perturbation varies among the models. It is found that the magnitude of the modelled radiative perturbation increases with decreasing climate sensitivity, with the exception of one outlying model. Therefore, we scale the temperature perturbation by the radiative perturbation in each model, and use the ratio between the integrated temperature perturbation and the integrated radiative perturbation as a measure of sensitivity to volcanic forcing. This ratio is found to be well correlated with the model climate sensitivity, more sensitive models having a larger ratio. Further, if this correspondence between “volcanic sensitivity” and sensitivity to CO₂ forcing is a feature not only among the models, but also of the real climate system, the alleged linear relation can be used to estimate the real climate sensitivity. The observational value of the ratio signifying volcanic sensitivity is hereby estimated to correspond to an equilibrium climate sensitivity, i.e. equilibrium temperature increase due to a doubling of the CO₂ concentration, between 1.7 and 4.1 K. Several sources of uncertainty reside in the method applied, and it is pointed out that additional model output, related to ocean heat storage and radiative forcing, could refine the analysis, as could reduced uncertainty in the observational record, of temperature as well as forcing.     

Wavelength dependence of aerosol optical depth and the fit of the Ångström law

Summary ?The dependence of aerosol optical depth on wavelength as well as the fit of the ?ngstr?m approximation have been investigated under different air masses at a sub-Arctic location (Abisko, Sweden; 68° 21′ N, 18° 49′ E) and a tropical environment (Ife, Nigeria; 7° 30′ N, 4° 31′ E). The study is based on spectral data acquired with a high resolution spectral radiometer (spectral range: 300–1100 nm) in absorption-free regions. The wavelength dependence of the aerosols under different air mass conditions at the sub-arctic location offer significant contrasts to aerosols of Saharan origin at Ife. A general characteristic of the aerosol optical depth spectra after the Pinatubo volcanic eruption was a much weaker wavelength dependence relative to pre-Pinatubo conditions. Categorising the features of the optical depth spectra according to their wavelength dependence, three main groups were observed at Abisko, while two main classes have been discussed for the harmattan season in the tropical climate of Ife and environs. For the first two groups in Abisko (and the first group at Ife), aerosol optical depth generally decreased with wavelength while the third group (second group at Ife) exhibited strong curvatures. The correlation coefficient obtained from the regression equation of the ?ngstr?m equation, has been shown to be a good index of the general fit of the ?ngstr?m approximation for the three groups at Abisko, but much weaker for the harmattan conditions at the tropical location. Although the probability of systematic deviations from the ?ngstr?m law is highest under intense harmattan conditions with considerably high β and low α, it has been observed that the ?ngstr?m fit was good in many highly turbid conditions at the tropical site. Hence, apart from the level of turbidity, the applicability of the ?ngstr?m approximation is strongly dependent on aerosol characteristics and source region. Formerly Adeyefa. Received May 18, 2001; revised June 20, 2002; accepted August 5, 2002     

Internal Variability as a Cause of Qualitative Intermodel Disagreement on Anthropogenic Climate Changes

Summary The qualitative agreement of two climate models, HADCM2 and ECHAM3, on the response of surface climate to anthropogenic climate forcing in the period 2020 – 2049 is studied. Special attention is paid to the role of internal climate variability as a source of intermodel disagreement. After illustrating the methods in an intermodel comparison of simulated changes in June–August mean precipitation, some global statistics are presented. Excluding surface air temperature, the four-season mean proportion of areas in which the two models agree on the sign of the climatic response is only 53 – 60% both for increases in CO₂ alone and for increases in CO₂ together with direct radiative forcing by sulphate aerosols, but somewhat larger, 59 – 70% for the separate aerosol effect. In areas where the response is strong (at least twice the standard error associated with internal variability) in both models, the agreement is better and the contrast between the different forcings becomes more marked. The proportion of agreement in such areas is 57 – 75% for the response to increases in CO₂ alone, 64 – 84% for the response to combined CO₂ and aerosol forcing, and as high as 88 – 94% for the separate aerosol effect. The relatively good intermodel agreement for aerosol-induced climate changes is suggested to be associated with the uneven horizontal distribution of aerosol forcing. Received December 2, 1998 Revised May 5, 1999     

Mid-Holocene greening of the Sahara: first results of the GAIM 6000 year BP Experiment with two asynchronously coupled atmosphere/biome models

 The mid-Holocene `green' Sahara represents the largest anomaly of the atmosphere-biosphere system during the last 12 000 years. Although this anomaly is attributed to precessional forcing leading to a strong enhancement of the African monsoon, no climate model so far has been able to simulate the full extent of vegetation in the Sahara region 6000 years ago. Here two atmospheric general circulation models (LMD 5.3 and ECHAM 3) are asynchronously coupled to an equilibrium biogeography model to give steady-state simulations of climate and vegetation 6000 years ago, including biogeophysical feedback. The two model results are surprisingly different, and neither is fully realistic. ECHAM shows a large northward extension of vegetation in the western part of the Sahara only. LMD shows a much smaller and more zonal vegetation shift. These results are unaffected by the choice of `green' or modern initial conditions. The inability of LMD to sustain a `green' Sahara 6000 years ago is linked to the simulated strength of the tropical summer circulation. During the northern summer monsoon season, the meridional gradient of sea-level pressure and subsidence over the western part of northern Africa are both much weaker in ECHAM than in LMD in the present as well as the mid-Holocene. These features allow the surface moist air flux to penetrate further into northern Africa in ECHAM than in LMD. This comparison illustrates the importance of correct simulation of atmospheric circulation features for the sensitivity of climate models to changes in radiative forcing, particularly for regional climates where atmospheric changes are amplified by biosphere-atmosphere feedbacks. Received: 20 April 1999 / Accepted: 20 January 2000     

The Effect of Super Volcanic Eruptions on Ozone Depletion in a Chemistry–Climate Model

With the gradual yet unequivocal phasing out of ozone depleting substances(ODSs), the environmental crisis caused by the discovery of an ozone hole over the Antarctic has lessened in severity and a promising recovery of the ozone layer is predicted in this century. However, strong volcanic activity can also cause ozone depletion that might be severe enough to threaten the existence of life on Earth. In this study, a transport model and a coupled chemistry–climate model were used to simulate the impacts of super volcanoes on ozone depletion. The volcanic eruptions in the experiments were the 1991 Mount Pinatubo eruption and a 100 × Pinatubo size eruption. The results show that the percentage of global mean total column ozone depletion in the 2050 RCP8.5 100 × Pinatubo scenario is approximately 6% compared to two years before the eruption and 6.4% in tropics. An identical simulation, 100 × Pinatubo eruption only with natural source ODSs, produces an ozone depletion of 2.5% compared to two years before the eruption, and with 4.4% loss in the tropics. Based on the model results,the reduced ODSs and stratospheric cooling lighten the ozone depletion after super volcanic eruption.     

青藏高原上空气溶胶含量的分布特征及其与臭氧的关系

采用1991年10月—2005年11月的HALOE资料,分析了青藏高原(27°~40°N,75°~105°E)上空气溶胶数密度、体积密度、面积密度的分布和变化特征,探讨了它们与臭氧的关系,并且与同纬度带中国东部地区(107°~122°E,27°~40°N)、北太平洋(170°E~170°W,27°~40°N)上空进行了对比。结果表明:高原上空气溶胶的体积密度、面积密度受Pinatubo火山喷发的影响主要发生在1991—1995年,然而气溶胶数密度受火山影响则不如前二者明显;高原上空气溶胶在对流层顶附近存在一个极大值区,在夏季该极大值区位于对流层顶下方(约120 hPa),而其他季节则位于对流层顶上方(约100hPa);青藏高原、中国东部地区、北太平洋三地上空气溶胶数密度的差异主要出现在60 hPa以下的气层,夏季差异最突出,高原上120 hPa附近的气溶胶数密度约为平原上的1.8倍,约为海洋上的5.5倍;在高原上空对流层顶附近以及平流层低层,气溶胶数密度与臭氧体积混合比呈很好的负相关关系,而在20 hPa以上则有明显的正相关关系;对比三地上空气溶胶与臭氧的关系,得到在对流层顶附近及平流层低层气溶胶在高原和平原上空与臭氧的变化呈很好的负相关,其中以高原上空的负相关关系更好,但是在海洋上空气溶胶和臭氧的相关不明显。而在20 hPa以上气层中,三地上空的气溶胶与臭氧的变化都具有很好的正相关关系。     

Response of the Western European climate to a collapse of the thermohaline circulation

Two ensemble simulations with the ECHAM5/MPI-OM climate model have been investigated for the atmospheric response to a thermohaline circulation (THC) collapse. The model forcing was specified from observations between 1950 and 2000 and it followed a rising greenhouse gases emission scenario from 2001 to 2100. In one ensemble, a THC collapse was induced by adding freshwater in the northern North Atlantic, from 2001 onwards. After about 20 years, an almost stationary response pattern develops, that is, after the THC collapse, global mean temperature rises equally fast in both ensembles with the hosing ensemble displaying a constant offset. The atmospheric response to the freshwater hosing features a strong zonal gradient in the anomalous 2-m air temperature over Western Europe, associated with a strong land–sea contrast. Since Western Europe climate features a strong marine impact due to the prevailing westerlies, the question arises how such a strong land–sea contrast can be maintained. We show that a strong secondary cloud response is set up with increased cloud cover over sea and decreased cloud cover over land. Also, the marine impact on Western European climate decreases, which results from a reduced transport of moist static energy from sea to land. As a result, the change in lapse rate over the cold sea surface temperature (SST) anomalies west of the continent is much larger than over land, dominated by changes in moisture content rather than temperature.     

A time-slice experiment with the ECHAM4 AGCM at high resolution: the impact of horizontal resolution on annual mean climate change

 The climate response to increasing levels of atmospheric greenhouse gases, prescribed according to the International Panel of Climate Change (IPCC) scenario IS92a, is studied in two model simulations. The reference simulation is a transient response experiment performed with a medium-resolution (T42) coupled general circulation model of the atmosphere and ocean (ECHAM4/OPYC) developed at the Max-Planck-Institute for Meteorology. For two 30-year “time slices”, representing the present-day climate and the future climate at the time of effective CO₂ doubling, the annual mean climate states are compared with those obtained from the high-resolution (T106) ECHAM4 model forced with monthly sea surface temperatures and sea-ice from the coupled model. The large-scale changes in temperature, zonal wind, sea-level pressure and precipitation are broadly similar. This applies, in particular, to the respective zonal means. In general, except for precipitation, the responses in the time-slice experiments are slightly weaker than those simulated in the coupled model due to a smaller effect of the horizontal resolution on the simulations of the future (warmer) period than on the simulations of the present period. On a regional scale, the impact of horizontal resolution is smaller in the Southern than in the Northern Hemisphere, where the response differences are caused mainly by changes in the positions of the stationary waves. Although the precipitation responses are broadly similar, there are few notable exceptions such as a more pronounced maximum over the equatorial oceans in the T106 experiment but a weaker response over low-latitude land areas. Differences in precipitation response are found especially in areas with strong topographical control such as South America, for example. Received: 17 January 2000 / Accepted: 7 July 2000     

Design and performance of a new cloud microphysics scheme developed for the ECHAM general circulation model

A new cloud microphysics scheme including a prognostic treatment of cloud ice (PCI) is developed to yield a more physically based representation of the components of the atmospheric moisture budget in the general circulation model ECHAM. The new approach considers cloud water and cloud ice as separate prognostic variables. The precipitation formation scheme for warm clouds distinguishes between maritime and continental clouds by considering the cloud droplet number concentration, in addition to the liquid water content. Based on several observational data sets, the cloud droplet number concentration is derived from the sulfate aerosol mass concentration as given from the sulfur cycle simulated by ECHAM. Results obtained with the new scheme are compared to satellite observations and in situ measurements of cloud physical and radiative properties. In general, the standard model ECHAM4 and also PCI capture the overall features, and the simulated results usually lie within the range of observed uncertainty. As compared to ECHAM4, only slight improvements are achieved with the new scheme. For example, the overestimated liquid water path and total cloud cover over convectively active regions are reduced in PCI. On the other hand, some shortcomings of the standard model such as underestimated shortwave cloud forcing over the extratropical oceans of the respective summer hemisphere are more pronounced in PCI.This paper was presented at the Third International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 4–9 Sept. 1995 under the auspices of the Max Planck Institute for Meteorology, Hamburg. Editor for these papers is L. Dümenil.     

Lagrangian transport of water vapor and cloud water in the ECHAM4 GCM and its impact on the cold bias

The Lagrangian advection scheme ATTILA has been applied for the transport of water vapor and cloud water in the general circulation model (GCM) ECHAM4.L39(DLR) (E39) instead of the operational semi-Lagrangian transport scheme (SLT). ATTILA is a purely Lagrangian scheme that is numerically non-diffusive, while the operational semi-Lagrangian scheme exhibits a considerable numerical diffusion in the presence of sharp gradients. The model version E39/SLT significantly overestimates the water vapor mixing ratio in the extratropical lowermost stratosphere (wet bias) by a factor of 3–5 compared to HALOE observations. Compared to E39/SLT, E39/ATTILA shows substantially reduced water vapor mixing ratios in the extratropical lowermost stratosphere up to 70%, and a steeper meridional water vapor gradient in the subtropics which is in better agreement with observations. Furthermore, the temperature distribution as simulated with E39/SLT is characterized by a pronounced cold temperature bias in the extratropical lowermost stratosphere (cold bias) and in the polar stratosphere above 50 hPa in winter (cold pole). The improvements concerning the water vapor distribution in E39/ATTILA lead to a substantial reduction of the simulated cold bias by approximately 5–7 K which also results in a better representation of the modeled tropopause, especially in the extratropics. Sensitivity studies indicate that the warming of the extratropical lowermost stratosphere in E39/ATTILA is directly related to the reduced wet bias resulting in a less infrared radiative cooling. Additionally, the cold pole problem is also slightly reduced in E39/ATTILA by approximately 2–5 K.     

Future impact of anthropogenic sulfate aerosol on North Atlantic climate

We examine the simulated future change of the North Atlantic winter climate influenced by anthropogenic greenhouses gases and sulfate aerosol. Two simulations performed with the climate model ECHAM4/OPYC3 are investigated: a simulation forced by greenhouse gases and a simulation forced by greenhouse gases and sulfate aerosol. Only the direct aerosol effect on the clear-sky radiative fluxes is considered. The sulfate aerosol has a significant impact on temperature, radiative quantities, precipitation and atmospheric dynamics. Generally, we find a similar, but weaker future climate response if sulfate aerosol is considered additionally. Due to the induced negative top-of-the-atmosphere radiative forcing, the future warming is attenuated. We find no significant future trends in North Atlantic Oscillation (NAO) index in both simulations. However, the aerosol seems to have a balancing effect on the occurence of extreme NAO events. The simulated correlation patterns of the NAO index with temperature and precipitation, respectively, agree well with observations up to the present. The extent of the regions influenced by the NAO tends to be reduced under strong greenhouse gas forcing. If sulfate is included and the warming is smaller, this tendency is reversed. Also, the future decrease in baroclinicity is smaller due to the aerosols’ cooling effect and the poleward shift in track density is partly offset. Our findings imply that in simulations where aerosol cooling is neglected, the magnitude of the future warming over the North Atlantic region is overestimated, and correlation patterns differ from those based on the future simulation including aerosols.     

Radiative effects of Pinatubo aerosols in the middle latitudes of the Northern Hemisphere

Summary Using scattering coefficient profiles of the Pinatubo aerosols derived from the observation of skylight polarization and lidar backscattering ratio in Beijing, the radiative effect of Pinatubo aerosols in middle latitudes is assessed by a delta-four-stream radiative transfer model. It is shown that the Pinatubo aerosols significantly change the radiation field. Due to the presence of the volcanic aerosols, the downward short wave flux at the surface decreases with a maximum of 8 W/m² while the upward short wave flux at the top of the atmosphere increases with a maximum of 6.5 W/m². The volcanic aerosols are injected into the region bounded below by the tropopause and up by the 25 km level. The upward and downward radiative fluxes are changed in opposite directions at those two boundaries. Downward short wave fluxes below the tropopause are 7–9 W/m² less than background values and downward long wave fluxes below the tropopause are 2 W/m² more than background values. Upward short wave fluxes above 25 km level is 5–7 W/m² more and upward fluxes above there are about 3 W/m² less.The effects of the Pinatubo aerosols on heating rates are also significant. The maximum increase in the short wave heating rate can be as large as 0.2 K/day at 22 km. The increase in the long wave heating rate is less with a maximum amplitude of about 0.15 K/day. The maximum increase of the total heating rate is about 0.35 K/day, which is comparable with the heating rate caused by the ozone 9.6 µm band in this region.Results of this study are compared with studies of Lacis et al. (1992) and Russell et al. (1993) as well as ERBE measurements. The results generally agree well. Causes for the differences are analyzed.Based on the numerical study, it is also found that the LOWTRAN fresh volcanic model is not representative for the Pinatubo aerosols.With 9 Figures     

Validation of general circulation climate models and projectionsof temperature and rainfall changes in Cameroon and someof its neighbouring areas

Summary  Four coupled atmosphere-ocean general circulation models were examined for the ability of their control runs to simulate present climate given present forcings. The area of study is mainly Cameroon and some of its surrounding areas (0–25° E, 5° S-30° N). These models are from the UK Meteorological Office Hadley Centre (HadCM2), the German Max-Planck-Institut für Meteorologie (ECHAM4), the Canadian Centre for Climate Modelling and Analysis (CGCM1) and the Australian Commonwealth Science and Industrial Research Organisation (CSIRO-Mk2). The ability of the models to reproduce the observed spatial and temporal patterns was studied. ECHAM4 and HadCM2 were found to reproduce the spatial pattern well, with a correlation of more than 90%. They also simulated the main annual features of both temperature and rainfall. The CSIRO-Mk2 model was slightly less successful and the CGCM1 had the worst results for the area, especially as concern rainfall. In view of these results, ECHAM4 and HADCM2 were used to evaluate projected changes in rainfall and temperature resulting from increased concentration of greenhouse gases in the atmosphere for the 30 year period 2040 to 2070. Received February 15, 1999/Revised March 10, 2000     

Optimal detection and attribution of climate change: sensitivity of results to climate model differences

Fingerprint techniques for the detection of anthropogenic climate change aim to distinguish the climate response to anthropogenic forcing from responses to other external influences and from internal climate variability. All these responses and the characteristics of internal variability are typically estimated from climate model data. We evaluate the sensitivity of detection and attribution results to the use of response and variability estimates from two different coupled ocean atmosphere general circulation models (HadCM2, developed at the Hadley Centre, and ECHAM3/LSG from the MPI für Meteorologie and Deutsches Klimarechenzentrum). The models differ in their response to greenhouse gas and direct sulfate aerosol forcing and also in the structure of their internal variability. This leads to differences in the estimated amplitude and the significance level of anthropogenic signals in observed 50-year summer (June, July, August) surface temperature trends. While the detection of anthropogenic influence on climate is robust to intermodel differences, our ability to discriminate between the greenhouse gas and the sulfate aerosol signals is not. An analysis of the recent warming, and the warming that occurred in the first half of the twentieth century, suggests that simulations forced with combined changes in natural (solar and volcanic) and anthropogenic (greenhouse gas and sulfate aerosol) forcings agree best with the observations.     

A review of global stratospheric aerosol: Measurements, importance, life cycle, and local stratospheric aerosol

Stratospheric aerosol, noted after large volcanic eruptions since at least the late 1800s, were first measured in the late 1950s, with the modern continuous record beginning in the 1970s. Stratospheric aerosol, both volcanic and non-volcanic are sulfuric acid droplets with radii (concentrations) on the order of 0.1–0.5 µm (0.5–0.005 cm^− 3), increasing by factors of 2–4 (10–10³) after large volcanic eruptions. The source of the sulfur for the aerosol is either through direct injection from sulfur-rich volcanic eruptions, or from tropical injection of tropospheric air containing OCS, SO₂, and sulfate particles. The life cycle of non-volcanic stratospheric aerosol, consisting of photo-dissociation and oxidation of sulfur source gases, nucleation/condensation in the tropics, transport pole-ward and downward in the global planetary wave driven tropical pump, leads to a quasi steady state relative maximum in particle number concentration at around 20 km in the mid latitudes. Stratospheric aerosol have significant impacts on the Earth's radiation balance for several years following volcanic eruptions. Away from large eruptions, the direct radiation impact is small and well characterized; however, these particles also may play a role in the nucleation of near tropopause cirrus, and thus indirectly affect radiation. Stratospheric aerosol play a larger role in the chemical, particularly ozone, balance of the stratosphere. In the mid latitudes they interact with both nitrous oxides and chlorine reservoirs, thus indirectly affecting ozone. In the polar regions they provide condensation sites for polar stratospheric clouds which then provide the surfaces necessary to convert inactive to active chlorine leading to polar ozone loss. Until the mid 1990s the modern record has been dominated by three large sulfur-rich eruptions: Fuego (1974), El Chichón (1982) and Pinatubo (1991), thus definitive conclusions concerning the trend of non-volcanic stratospheric aerosol could only recently be made. Although anthropogenic emissions of SO₂ have changed somewhat over the past 30 years, the measurements during volcanically quiescent periods indicate no long term trend in non-volcanic stratospheric aerosol.