Hans-F. Graf

Radiative forcing from the 1991 Mount Pinatubo volcanic eruption

Volcanic sulfate aerosols in the stratosphere produce significant long-term solar and infrared radiative perturbations in the Earths atmosphere and at the surface, which cause a response of the climate system. Here we study the fundamental process of the development of this volcanic radiative forcing, focusing on the eruption of Mount Pinatubo in the Philippines on June 15, 1991. We develop a spectral-, space-, and time-dependent set of aerosol parameters for 2 years after the Pinatubo eruption using a combination of SAGE II aerosol extinctions and UARS-retrieved effective radii, supported by SAM II, AVHRR, lidar and balloon observations. Using these data, we calculate the aerosol radiative forcing with the ECHAM4 general circulation model (GCM) for cases with climatological and observed sea surface temperature (SST), as well as with and without climate response. We find that the aerosol radiative forcing is not sensitive to the climate variations caused by SST or the atmospheric response to the aerosols, except in regions with varying dense cloudiness. The solar forcing in the near infrared contributes substantially to the total stratospheric heating. A complete formulation of radiative forcing should include not only changes of net fluxes at the tropopause but also the vertical distribution of atmospheric heating rates and the change of downward thermal and net solar radiative fluxes at the surface. These forcing and aerosol data are available for GCM experiments with any spatial and spectral resolution.

Arctic Oscillation response to volcanic eruptions in the IPCC AR4 climate models

[1]xa0Stratospheric sulfate aerosol particles from strong volcanic eruptions produce significant transient cooling of the troposphere and warming of the lower stratosphere. The radiative impact of volcanic aerosols also produces a response that generally includes an anomalously positive phase of the Arctic Oscillation (AO) that is most pronounced in the boreal winter. The main atmospheric thermal and dynamical effects of eruptions typical of the past century persist for about two years after each eruption. In this paper we evaluate the volcanic responses in simulations produced by seven of the climate models included in the model intercomparison conducted as part of the preparation of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). We consider global effects as well as the regional circulation effects in the extratropical Northern Hemisphere focusing on the AO responses forced by volcanic eruptions. Specifically we analyze results from the IPCC historical runs that simulate the evolution of the circulation over the last part of the 19th century and the entire 20th century using a realistic time series of atmospheric composition (greenhouse gases and aerosols). In particular, composite anomalies over the two boreal winters following each of the nine largest low-latitude eruptions during the period 1860–1999 are computed for various tropospheric and stratospheric fields. These are compared when possible with observational data. The seven IPCC models we analyzed use similar assumptions about the amount of volcanic aerosols formed in the lower stratosphere following the volcanic eruptions that have occurred since 1860. All models produce tropospheric cooling and stratospheric warming as in observations. However, they display a considerable range of dynamic responses to volcanic aerosols. Nevertheless, some general conclusions can be drawn. The IPCC models tend to simulate a positive phase of the Arctic Oscillation in response to volcanic forcing similar to that typically observed. However, the associated dynamic perturbations and winter surface warming over Northern Europe and Asia in the post-volcano winters is much weaker in the models than in observations. The AR4 models also underestimate the variability and long-term trend of the AO. This deficiency affects high-latitude model predictions and may have a similar origin. This analysis allows us to better evaluate volcanic impacts in up-to-date climate models and to better quantify the model Arctic Oscillation sensitivity to external forcing. This potentially could lead to improving model climate predictions in the extratropical latitudes of the Northern Hemisphere.

Climate model simulation of winter warming and summer cooling following the 1991 Mount Pinatubo volcanic eruption

We simulate climate change for the 2-year period following the eruption of Mount Pinatubo in the Philippines on June 15, 1991, with the ECHAM4 general circulation model (GCM). The model was forced by realistic aerosol spatial-time distributions and spectral radiative characteristics calculated using Stratospheric Aerosol and Gas Experiment II extinctions and Upper Atmosphere Research Satellite-retrieved effective radii. We calculate statistical ensembles of GCM simulations with and without volcanic aerosols for 2 years after the eruption for three different sea surface temperatures (SSTs): climatological SST, El Nino-type SST of 1991–1993, and La Nina-type SST of 1984–1986. We performed detailed comparisons of calculated fields with observations. We analyzed the atmospheric response to Pinatubo radiative forcing and the ability of the GCM to reproduce it with different SSTs. The temperature of the tropical lower stratosphere increased by 4 K because of aerosol absorption of terrestrial longwave and solar near-infrared radiation. The heating is larger than observed, but that is because in this simulation we did not account for quasi-biennial oscillation (QBO) cooling and the cooling effects of volcanically induced ozone depletion. We estimated that both QBO and ozone depletion decrease the stratospheric temperature by about 2 K. The remaining 2 K stratospheric warming is in good agreement with observations. By comparing the runs with the Pinatubo aerosol forcing with those with no aerosols, we find that the model calculates a general cooling of the global troposphere, but with a clear winter warming pattern of surface air temperature over Northern Hemisphere continents. This pattern is consistent with the observed temperature patterns. The stratospheric heating and tropospheric summer cooling are directly caused by aerosol radiative effects, but the winter warming is indirect, produced by dynamical responses to the enhanced stratospheric latitudinal temperature gradient. The aerosol radiative forcing, stratospheric thermal response, and summer tropospheric cooling do not depend significantly on SST. The stratosphere-troposphere dynamic interactions and tropospheric climate response in winter are sensitive to SST.

Injection of gases into the stratosphere by explosive volcanic eruptions

[1]xa0Explosive eruptions can inject large amounts of volcanic gases into the stratosphere. These gases may be scavenged by hydrometeors within the eruption column, and high uncertainties remain regarding the proportion of volcanic gases, which eventually reach the stratosphere. These are caused by the difficulties of directly sampling explosive volcanic eruption columns and by the lack of laboratory studies in the extreme parameter regime characterizing them. Using the nonhydrostatic nonsteady state plume model Active Tracer High Resolution Atmospheric Model (ATHAM), we simulated an explosive volcanic eruption. We examined the scavenging efficiency for the climatically relevant gases within the eruption column. The low concentration of water in the plume results in the formation of relatively dry aggregates. More than 99% of these are frozen because of their fast ascent to low-temperature regions. Consideration of the salinity effect increases the amount of liquid water by one order of magnitude, but the ice phase is still highly dominant. Consequently, the scavenging efficiency for HCl is very low, and only 1% is dissolved in liquid water. However, scavenging by ice particles via direct gas incorporation during diffusional growth is a significant process. The salinity effect increases the total scavenging efficiency for HCl from about 50% to about 90%. The sulfur-containing gases SO2 and H2S are only slightly soluble in liquid water; however, these gases are incorporated into ice particles with an efficiency of 10 to 30%. Despite scavenging, more than 25% of the HCl and 80% of the sulfur gases reach the stratosphere because most of the particles containing these species are lifted there. Sedimentation of the particles would remove the volcanic gases from the stratosphere. Hence the final quantity of volcanic gases injected in a particular eruption depends on the fate of the particles containing them, which is in turn dependent on the volcanic and environmental conditions.

Volcanic plume simulation on large scales

Abstract The plume model ATHAM (Active Tracer High Resolution Atmospheric Model) is designed to simulate explosive volcanic eruptions for a given mass flux of pyroclastic material under realistic atmospheric background conditions. Based on the assumption that all particles are small the models equations are simplified such that, besides equations for gaseous, liquid and solid constituents of arbitrary concentrations, only the volume means of momentum and heat are predicted. The exchange of momentum and heat between the fluids constituents are treated diagnostically. A prognostic turbulence closure scheme describing the entrainment of ambient air into the plume takes into account the anisotropy of the horizontal and vertical components of turbulence. Its length scale is assumed to be isotropic. Microphysical processes such as the exchange of heat and momentum between dry air, water vapor, cloud water, precipitable water, ice crystals and graupel are parameterized. Ash and lapilli represent the spectrum of silicate particles. A diagnostic sedimentation velocity allows for the separation of gas and particles. The model is formulated with an implicit time stepping scheme. The equations of motion and the transport equations for tracers are formulated in flux form in order to guarantee the conservation of momentum and all tracer masses. The heat transport equation is in advective form. The wave equation and the equations for the transport of momentum, heat and tracers are solved using a combined line-relaxation successive overrelaxation scheme. Two-dimensional experiments for symmetric cases with cylindrical coordinates yield qualitatively similar results to other dynamic–thermodynamic models. However, entrainment processes are now computed quantitatively through the turbulence closure and condensed matter has a sophisticated description. In order to study the transferability of results from computationally cheap two-dimensional experiments to costly three-dimensional simulations of a realistic plume, comparisons between calculations with and without cylindrical coordinates are performed. Finally, experiments for different atmospheric background conditions allow investigation of plume development on the influence of cross wind effects, and temperature and humidity profiles.

The effect of phase changes of water on the development of volcanic plumes

Abstract A complex thermodynamic–microphysical package has been formulated that is able to deal with the microphysical processes of condensed water vapour in a volcanic plume. The microphysics follows a prognostic bulk approach for cloud water, cloud ice, rain and graupel and the interaction between them. In a standard experiment, this module, applied within a new nonhydrostatic volcano plume model, Active Tracer High Resolution Atmospheric Model (ATHAM), produces reasonable concentrations of different types of hydrometeors. Under tropical conditions, the plume gains three times as much water from the environment through entrainment as from the volcanic source. The formed hydrometeors are dominated by the ice phase. Thermodynamic effects of phase changes contribute about 13% to the plumes total thermal energy and therefore have a considerable effect on the vertical development of the plume.

Central Pacific El Niño, the “subtropical bridge,” and Eurasian climate

[1]xa0This study contributes to the discussion on possible effects of El Nino on North Atlantic/European regional climates. We use NCEP/NCAR reanalysis data to show how the two different types of El Ninos (the central Pacific, or CP, and the east Pacific, or EP) result in remarkably different European winter temperature anomalies, specifically weak warming during EP and significant cooling during CP El Ninos, the latter being associated with a negative phase of the winter North Atlantic Oscillation (NAO). Our results diverge from former suggestions addressing the weakened stratospheric polar vortex as the dominant factor contributing to the El Nino/NAO teleconnection. We propose a tropospheric bridge as the mechanism primarily responsible for the establishment of a negative NAO phase and of associated cold European winters. This mechanism includes the subtropical jet (STJ) waveguide being activated only during CP El Ninos, when anomalous convective heating occurs near the edge of the Pacific warm pool. Under these conditions the STJ is enhanced by planetary wave flux divergence in the subtropical upper troposphere, providing favorable conditions for the propagation of a wave number 5 disturbance around the subtropical Northern Hemisphere. This wave contributes to weakening of the Azores High and, hence, to the negative NAO phase. As global warming scenarios project an increase in the frequency of CP El Nino events, the distinctive nature of this mechanism implies that the probability of cold European winters may increase as well in future decades.

Simulation of a biomass‐burning plume: Comparison of model results with observations

[1]xa0We have simulated the dynamical evolution of the plume from a prescribed biomass fire, using the active tracer high-resolution atmospheric model (ATHAM). Initialization parameters were set to reflect the conditions during the fire. The model results are compared with airborne remote-sensing and in situ measurements of the plume. ATHAM reproduces the injection height (250–600 m) and the horizontal extent of the plume (∼4 km) with good accuracy. The aerosol mass concentrations are underestimated but still in the range of the observations. Remaining differences between the model results and the measurements are attributed to limited meteorological and fire emission information. Additionally, radiative transfer simulations using in situ measurements of the aerosol properties are performed. A comparison of the measured and simulated reflected solar flux shows an underestimation by the model over the ocean surface, which is most likely due to an underestimation of the aerosol optical depth in the model. The instantaneous radiative forcing was calculated to −36 W m−2 over land and −58 W m−2 over the ocean for a solar zenith angle of 47° and an optical depth of unity, consistent with previous studies. Overall, it appears that ATHAM is a valuable tool for the examination of transport processes within biomass-burning plumes and, together with a chemical and microphysical module, is suitable for studies of the interaction between transport, chemistry, and microphysics within such plumes.

Sensitivity of the global circulation to the suppression of precipitation by anthropogenic aerosols

Abstract From recent satellite observations, it is evident that an increase in cloud condensation nuclei—for instance, due to biomass burning—can substantially reduce rain efficiency of convective clouds. This is potentially important for the global climate since the release of latent heat due to condensation of water vapour and fallout of rain from cumulus convection is the most important source for available potential energy in the free troposphere. Beyond this, cumulus convection is a key process in controlling the water vapour content of the atmosphere. The sensitivity of the global climate to modification in rain efficiency of convective clouds due to the suppression of drop coalescence by anthropogenic aerosols is studied by using the atmospheric general circulation model (A-GCM), ECHAM4. This paper presents results from a 15-year sensitivity study, which considers the aerosol effect on warm precipitation formation. Effects on ice processes are not included yet, and therefore the results likely underestimate the magnitude of the full effects due to suppression of precipitation. Instantaneous forcing is large locally (up to 100% reduction of precipitation and the related latent heat release) but confined to small areas, leading to small large-scale mean anomalies in the convective heating and therefore the vertical temperature gradient. We found a definite perturbation of the global circulation, showing distinct sensitivity to the impact of aerosols on suppressing rainfall.

Effect of environmental conditions on volcanic plume rise

Sensitivity studies were performed with a complex nonhydrostatic volcano plume model that explicitly treats turbulence and microphysics. The impact of environmental conditions such as wind, temperature and humidity profiles was studied for standard observational data. To investigate the wind effects, a two-dimensional Cartesian formulation of the model was used, while for the temperature and humidity effects a cylindrical coordinate system had to be applied, since this treats the entrainment process more realistically. It was found that horizontal wind generally reduces the height of the ash plume. The gaseous part of the plume sometimes may rise higher than without wind owing to the more effective separation between gas and solid material. Besides reduced static stability, the absolute temperature and humidity also increase the plume height. All environmental impacts strongly depend on the strength of entrainment and thus on the quality of the prognostic turbulence.