Active Tracer High Resolution Atmospheric Model Research Articles

Two-Dimensional Evaluation of ATHAM-Fluidity, a Nonhydrostatic Atmospheric Model Using Mixed Continuous/Discontinuous Finite Elements and Anisotropic Grid Optimization

AbstractThis paper presents the first attempt to apply the compressible nonhydrostatic Active Tracer High-Resolution Atmospheric Model–Fluidity (ATHAM-Fluidity) solver to a series of idealized atmospheric test cases. ATHAM-Fluidity uses a hybrid finite-element discretization where pressure is solved on a continuous second-order grid while momentum and scalars are computed on a first-order discontinuous grid (also known as ). ATHAM-Fluidity operates on two- and three-dimensional unstructured meshes, using triangular or tetrahedral elements, respectively, with the possibility to employ an anisotropic mesh optimization algorithm for automatic grid refinement and coarsening during run time. The solver is evaluated using two-dimensional-only dry idealized test cases covering a wide range of atmospheric applications. The first three cases, representative of atmospheric convection, reveal the ability of ATHAM-Fluidity to accurately simulate the evolution of large-scale flow features in neutral atmospheres at rest. Grid convergence without adaptivity as well as the performances of the Hermite–Weighted Essentially Nonoscillatory (Hermite-WENO) slope limiter are discussed. These cases are also used to test the grid optimization algorithm implemented in ATHAM-Fluidity. Adaptivity can result in up to a sixfold decrease in computational time and a fivefold decrease in total element number for the same finest resolution. However, substantial discrepancies are found between the uniform and adapted grid results, thus suggesting the necessity to improve the reliability of the approach. In the last three cases, corresponding to atmospheric gravity waves with and without orography, the model ability to capture the amplitude and propagation of weak stationary waves is demonstrated. This work constitutes the first step toward the development of a new comprehensive limited area atmospheric model.

Comprehensive mapping and characteristic regimes of aerosol effects on the formation and evolution of pyro-convective clouds

Abstract. A recent parcel model study (Reutter et al., 2009) showed three deterministic regimes of initial cloud droplet formation, characterized by different ratios of aerosol concentrations (NCN) to updraft velocities. This analysis, however, did not reveal how these regimes evolve during the subsequent cloud development. To address this issue, we employed the Active Tracer High Resolution Atmospheric Model (ATHAM) with full microphysics and extended the model simulation from the cloud base to the entire column of a single pyro-convective mixed-phase cloud. A series of 2-D simulations (over 1000) were performed over a wide range of NCN and dynamic conditions. The integrated concentration of hydrometeors over the full spatial and temporal scales was used to evaluate the aerosol and dynamic effects. The results show the following. (1) The three regimes for cloud condensation nuclei (CCN) activation in the parcel model (namely aerosol-limited, updraft-limited, and transitional regimes) still exist within our simulations, but net production of raindrops and frozen particles occurs mostly within the updraft-limited regime. (2) Generally, elevated aerosols enhance the formation of cloud droplets and frozen particles. The response of raindrops and precipitation to aerosols is more complex and can be either positive or negative as a function of aerosol concentrations. The most negative effect was found for values of NCN of ~ 1000 to 3000 cm−3. (3) The nonlinear properties of aerosol–cloud interactions challenge the conclusions drawn from limited case studies in terms of their representativeness, and ensemble studies over a wide range of aerosol concentrations and other influencing factors are strongly recommended for a more robust assessment of the aerosol effects.

3-D model simulations of dynamical and microphysical interactions in pyroconvective clouds under idealized conditions

Abstract. Dynamical and microphysical processes in pyroconvective clouds in mid-latitude conditions are investigated using idealized three-dimensional simulations with the Active Tracer High resolution Atmospheric Model (ATHAM). A state-of-the-art two-moment microphysical scheme building upon a realistic parameterization of cloud condensation nuclei (CCN) activation has been implemented in order to study the influence of aerosol concentration on cloud development. The results show that aerosol concentration influences the formation of precipitation. For low aerosol concentrations (NCN = 200 cm−3), rain droplets are rapidly formed by autoconversion of cloud droplets. This also triggers the formation of large graupel and hail particles, resulting in an early onset of precipitation. With increasing aerosol concentration (NCN = 1000 cm−3 and NCN = 20 000 cm−3) the formation of rain droplets is delayed due to more but smaller cloud droplets. Therefore, the formation of ice crystals and snowflakes becomes more important for the eventual formation of graupel and hail, which is delayed at higher aerosol concentrations. This results in a delay of the onset of precipitation and a reduction of its intensity with increasing aerosol concentration. This study is the first detailed investigation of the interaction between cloud microphysics and the dynamics of a pyroconvective cloud using the combination of a high-resolution atmospheric model and a detailed microphysical scheme.

Microwave Radiometric Remote Sensing of Volcanic Ash Clouds From Space: Model and Data Analysis

The potential of satellite passive microwave sensors to provide quantitative information about near-source volcanic ash cloud parameters is assessed. To this aim, ground-based microwave weather radar and spaceborne microwave radiometer observations are used together with forward-model simulations. The latter are based on 2-D simulations with the numerical plume model Active Tracer High-Resolution Atmospheric Model (ATHAM), in conjunction with the radiative transfer model Satellite Data Simulator Unit (SDSU) that is based on the deltaEddington approximation and includes Mie scattering. The study area is the Icelandic subglacial volcanic region. The analyzed case study is that of the Grimsvotn eruption in May 2011. ATHAM input parameters are adjusted using available ground data, and sensitivity tests are conducted to investigate the observed brightness temperatures and their variance. The tests are based on the variation of environmental conditions like the terrain emissivity, water vapor, and ice in the volcanic plume. Quantitative correlation analysis between ATHAM/SDSU forward-model columnar content simulations and available microwave radiometric brightness temperature measurements, derived from the Special Sensor Microwave Imager/Sounder (SSMIS), are encouraging in terms of both dynamic range and correlation coefficient. The correlation coefficients are found to vary from -0.37 to -0.63 for SSMIS channels from 91 to 183 ± 1 GHz, respectively. The larger sensitivity of the brightness temperature at 183 ± 1 GHz to the columnar content, with respect to other channels, allowed us to consider this channel as the basis for a model-based polynomial relationship of volcanic plume height as a function of the measured SSMIS brightness temperature.

A modelling investigation into lake-breeze development and convection triggering in the Nam Co Lake basin, Tibetan Plateau

This paper uses the cloud resolving Active Tracer High-resolution Atmospheric Model coupled to the interactive surface model Hybrid in order to investigate the diurnal development of a lake-breeze system at the Nam Co Lake on the Tibetan Plateau. Simulations with several background wind speeds are conducted, and the interaction of the lake breeze with topography and background wind in triggering moist and deep convection is studied. The model is able to adequately simulate the systems most important dynamical features such as turbulent surface fluxes and the development of a lake breeze for the different wind conditions. We identify two different mechanisms for convection triggering that are dependent on the direction of the background wind: triggering over topography, when the background wind and the lake breeze have the same flow direction, and triggering due to convergence between the lake-breeze front and the background wind. Our research also suggests that precipitation measurements at the centre of the basins on the Tibetan Plateau are not representative for the basin as a whole as precipitation is expected to occur mainly in the vicinity of the topography.

Ascent dynamics of large phreatomagmatic eruption clouds: The role of microphysics

We examine the impact of abundant surface water interaction on the development of volcanic clouds from large‐scale (>108 kg s−1 magma) phreatomagmatic eruptions, presenting the first 2‐D large‐eddy simulations of “wet” volcanic plumes that incorporate the effects of microphysics. The Active Tracer High‐Resolution Atmospheric Model was forced with field‐derived inputs from an exceptionally large phreatomagmatic eruption: the 27 ka Oruanui supereruption (Taupo volcano, New Zealand). Surface water contents were varied from 0 to 40 wt% for eruptions with equivalent magma eruption rates of ∼1.3 × 108 and 1.1 × 109 kg s−1. Our findings confirm that increased surface water has a pronounced impact on column stability, leading to transitional column behavior and hybrid clouds resulting from simultaneous ascent of material from stable columns and pyroclastic density currents (PDCs). Contrary to the suggestion of previous workers, however, abundant surface water does not systematically lower the spreading level or maximum height of volcanic clouds, owing to vigorous microphysics‐assisted lofting of PDCs. The simulated behavior and ash cloud dimensions provide a close match to field evidence from the Oruanui case study. Cloud heights from the collapsing eruption columns also show a notable sensitivity to changes in the altitude of the tropopause, while ambient humidity primarily impacts the abundance of airborne hydrometeors (particularly ice) associated with the volcanic clouds. General relationships between eruption style, meteorological conditions, and the resulting vertical profiles of volcanic emissions outlined here could also be adapted for use in operational volcanic ash forecasting and deposit reconstruction techniques already in existence.

Model-Based Weather Radar Remote Sensing of Explosive Volcanic Ash Eruption

Microphysical and dynamical features of volcanic ash clouds can be quantitatively monitored by using ground-based microwave weather radars. These systems can provide data for determining the ash volume, total mass, and height of eruption clouds. In order to demonstrate the unique potential of this microwave active remote-sensing technique, the case study of the eruption of Augustine Volcano in Alaska in January 2006 is described and analyzed. Volume scan data, acquired by a NEXRAD WSR-88D S-band ground-based weather radar, are processed to automatically classify and estimate eruptive cloud particle concentration. The numerical results of the coupled model Z-reflectivity from Active Tracer High resolution Atmospheric Model (ATHAM), including particle aggregation processes and simulation of radar reflectivity from the ATHAM microphysical model, are exploited to train the inversion algorithm. The volcanic ash radar retrieval based on the ATHAM algorithm is a physical-statistical approach based on the backscattering microphysical model of volcanic cloud particles (hydrometeors, ash, and aggregates), used within a Bayesian classification and optimal regression algorithm. A sensitivity analysis is carried out to evaluate the overall error budget. The evolution of the Augustine eruption is discussed in terms of radar measurements and products, pointing out the unique features, the current limitations, and future improvements of radar remote sensing of volcanic plumes.

Applying the three‐dimensional model ATHAM to volcanic plumes: Dynamic of large co‐ignimbrite eruptions and associated injection heights for volcanic gases

Many of the past large volcanic eruptions like Tambora in 1815, Krakatau in 1835, and Pinatubo in 1991 were secondary so called co‐ignimbrite eruptions that were forced over a large area instead of a point source as in the Plinian case. Previous modeling studies were based on one‐dimensional plume models. We used the fully three‐dimensional plume model ATHAM (Active Tracer High‐Resolution Atmospheric Model) to investigate the dynamics and the resulting plume heights of co‐ignimbrite eruptions in an idealized setup. Ash particles as well as a sulfur dioxide (SO2) tracer are included in the model. In the analysis we focus on the behavior of SO2 since the neutral buoyancy height is an ill‐defined parameter for gravitationally settling particles. In contrast to Plinian plumes the co‐ignimbrite plumes develop from multiple updrafts resulting in significantly reduced neutral buoyancy heights. At least a two‐dimensional modeling framework is necessary to capture the relevant dynamical features.

Technical Note: Sensitivity of 1-D smoke plume rise models to the inclusion of environmental wind drag

Abstract. Vegetation fires emit hot gases and particles which are rapidly transported upward by the positive buoyancy generated by the combustion process. In general, the final vertical height that the smoke plumes reach is controlled by the thermodynamic stability of the atmospheric environment and the surface heat flux released by the fire. However, the presence of a strong horizontal wind can enhance the lateral entrainment and induce additional drag, particularly for small fires, impacting the smoke injection height. In this paper, we revisit the parameterization of the vertical transport of hot gases and particles emitted from vegetation fires, described in Freitas et al. (2007), to include the effects of environmental wind on transport and dilution of the smoke plume at its scale. This process is quantitatively represented by introducing an additional entrainment term to account for organized inflow of a mass of cooler and drier ambient air into the plume and its drag by momentum transfer. An extended set of equations including the horizontal motion of the plume and the additional increase of the plume radius is solved to simulate the time evolution of the plume rise and the smoke injection height. One-dimensional (1-D) model results are presented for two deforestation fires in the Amazon basin with sizes of 10 and 50 ha under calm and windy atmospheric environments. The results are compared to corresponding simulations generated by the complex non-hydrostatic three-dimensional (3-D) Active Tracer High resolution Atmospheric Model (ATHAM). We show that the 1-D model results compare well with the full 3-D simulations. The 1-D model may thus be used in field situations where extensive computing facilities are not available, especially under conditions for which several optional cases must be studied.

Volcanic plumes and wind: Jetstream interaction examples and implications for air traffic

Volcanic plumes interact with the wind at all scales. On smaller scales, wind affects local eddy structure; on larger scales, wind shapes the entire plume trajectory. The polar jets or jetstreams are regions of high [generally eastbound] winds that span the globe from 30 to 60° in latitude, centered at an altitude of about 10 km. They can be hundreds of kilometers wide, but as little as 1 km in thickness. Core windspeeds are up to 130 m/s. Modern transcontinental and transoceanic air routes are configured to take advantage of the jetstream. Eastbound commercial jets can save both time and fuel by flying within it; westbound aircraft generally seek to avoid it. Using both an integral model of plume motion that is formulated within a plume-centered coordinate system (BENT) as well as the Active Tracer High-resolution Atmospheric Model (ATHAM), we have calculated plume trajectories and rise heights under different wind conditions. Model plume trajectories compare well with the observed plume trajectory of the Sept 30/Oct 1, 1994, eruption of Kliuchevskoi Volcano, Kamchatka, Russia, for which measured maximum windspeed was 30–40 m/s at about 12 km. Tephra fall patterns for some prehistoric eruptions of Avachinsky Volcano, Kamchatka, and Inyo Craters, CA, USA, are anomalously elongated and inconsistent with simple models of tephra dispersal in a constant windfield. The Avachinsky deposit is modeled well by BENT using a windspeed that varies with height. Two potentially useful conclusions can be made about air routes and volcanic eruption plumes under jetstream conditions. The first is that by taking advantage of the jetstream, aircraft are flying within an airspace that is also preferentially occupied by volcanic eruption clouds and particles. The second is that, because eruptions with highly variable mass eruption rate pump volcanic particles into the jetstream under these conditions, it is difficult to constrain the tephra grain size distribution and mass loading present within a downwind volcanic plume or cloud that has interacted with the jetstream. Furthermore, anomalously large particles and high mass loadings could be present within the cloud, if it was in fact formed by an eruption with a high mass eruption rate. In terms of interpretation of tephra dispersal patterns, the results suggest that extremely elongated isopach or isopleth patterns may often be the result of eruption into the jetstream, and that estimation of the mass eruption rate from these elongated patterns should be considered cautiously.

Examination of the aerosol indirect effect under contrasting environments during the ACE-2 experiment

Abstract. The Active Tracer High-resolution Atmospheric Model (ATHAM) has been adopted to examine the aerosol indirect effect in contrasting clean and polluted cloudy boundary layers during the Second Aerosol Characterization Experiment (ACE-2). Model results are in good agreement with available in-situ observations, which provides confidence in the results of ATHAM. Sensitivity tests have been conducted to examine the response of the cloud fraction (CF), cloud liquid water path (LWP), and cloud optical depth (COD) to changes in aerosols in the clean and polluted cases. It is shown for two cases that CF and LWP would decrease or remain nearly constant with an increase in aerosols, a result which shows that the second aerosol indirect effect is positive or negligibly small in these cases. Further investigation indicates that the background meteorological conditions play a critical role in the response of CF and LWP to aerosols. When large-scale subsidence is weak as in the clean case, the dry overlying air above the cloud is more efficiently entrained into the cloud, and in so doing, removes cloud water more efficiently, and results in lower CF and LWP when aerosol burden increases. However, when the large-scale subsidence is strong as in the polluted case, the growth of the cloud top is suppressed and the entrainment drying makes no significant difference when aerosol burden increases. Therefore, the CF and LWP remain nearly constant. In both the clean and polluted cases, the COD tends to increase with aerosols, and the total aerosol indirect effect (AIE) is negative even when the CF and LWP decrease with an increase in aerosols. Therefore, the first AIE dominates the response of the cloud to aerosols.

Modeling of biomass smoke injection into the lower stratosphere by a large forest fire (Part I): reference simulation

Abstract. Wildland fires in boreal regions have the potential to initiate deep convection, so-called pyro-convection, due to their release of sensible heat. Under favorable atmospheric conditions, large fires can result in pyro-convection that transports the emissions into the upper troposphere and the lower stratosphere. Here, we present three-dimensional model simulations of the injection of fire emissions into the lower stratosphere by pyro-convection. These model simulations are constrained and evaluated with observations obtained from the Chisholm fire in Alberta, Canada, in 2001. The active tracer high resolution atmospheric model (ATHAM) is initialized with observations obtained by radiosonde. Information on the fire forcing is obtained from ground-based observations of the mass and moisture of the burned fuel. Based on radar observations, the pyro-convection reached an altitude of about 13 km, well above the tropopause, which was located at about 11.2 km. The model simulation yields a similarly strong convection with an overshoot of the convection above the tropopause. The main outflow from the pyro-convection occurs at about 10.6 km, but a significant fraction (about 8%) of the emitted mass of the smoke aerosol is transported above the tropopause. In contrast to regular convection, the region with maximum updraft velocity in the pyro-convection is located close to the surface above the fire. This results in high updraft velocities >10 m s−1 at cloud base. The temperature anomaly in the plume decreases rapidly with height from values above 50 K at the fire to about 5 K at about 3000 m above the fire. While the sensible heat released from the fire is responsible for the initiation of convection in the model, the release of latent heat from condensation and freezing dominates the overall energy budget. Emissions of water vapor from the fire do not significantly contribute to the energy budget of the convection.

Modeling of biomass smoke injection into the lower stratosphere by a large forest fire (Part II): sensitivity studies

Abstract. The Chisholm forest fire that burned in Alberta, Canada, in May 2001 resulted in injection of substantial amounts of smoke into the lower stratosphere. We used the cloud-resolving plume model ATHAM (Active Tracer High resolution Atmospheric Model) to investigate the importance of different contributing factors to the severe intensification of the convection induced by the Chisholm fire and the subsequent injection of biomass smoke into the lower stratosphere. The simulations show strong sensitivity of the pyro-convection to background meteorology. This explains the observed coincidence of the convective blow-up of the fire plume and the passage of a synoptic cold front. Furthermore, we performed model sensitivity studies to the rate of release of sensible heat and water vapor from the fire. The release of sensible heat by the fire plays a dominant role for the dynamic development of the pyro-cumulonimbus cloud (pyroCb) and the height to which smoke is transported. The convection is very sensitive to the heat flux from the fire. The emissions of water vapor play a less significant role for the injection height but enhance the amount of smoke transported beyond the tropopause level. The aerosol burden in the plume has a strong impact on the microphysical structure of the resulting convective cloud. The dynamic evolution of the pyroCb, however, is only weakly sensitive to the abundance of cloud condensation nuclei (CCN) from the fire. In contrast to previous findings by other studies of convective clouds, we found that fire CCN have a negative effect on the convection dynamics because they give rise to a delay in the freezing of cloud droplets. Even in a simulation without fire CCN, there is no precipitation formation within the updraft region of the pyroCb. Enhancement of convection by aerosols as reported from studies of other cases of convection is therefore not found in our study.

Volcanic particle aggregation in explosive eruption columns. Part II: Numerical experiments

The goal of this paper is to determine the parameters that control the aggregation efficiency and the growth rate of volcanic particles within the eruption column. Numerical experiments are performed with the plume model ATHAM (Active Tracer High resolution Atmospheric Model). In this study we employ the parameterizations described in a companion paper (this issue). The presence of hydrometeors promotes the aggregation of ash particles, which strongly increases their fall velocities and thus their environmental impact. The tephra mass is about two orders of magnitude greater than that of hydrometeors during typical Plinian eruptions without interaction of external water. Ice is highly dominant in comparison to liquid water (> 99% by mass). This is caused by the fast column rise (> 100 m s − 1 on average) to very cold altitudes. Most particles occur in the form of frozen aggregates with low ice content. The collection efficiency is governed by the availability of hydrometeors acting as adhesives at the particles’ surface in our study, and wet ash particles have a higher sticking capacity than icy ones. Therefore, aggregation is fastest during the eruption within the column when limited regions of liquid water exist and when particle concentrations are very high (of the order of 10 5 cm − 3 ). Increased humidity in the background atmosphere generally leads to enhanced ice formation, but shows only a weak influence on the aggregation process. First sensitivity studies showed, however, a significant increase of the liquid water fraction when considering salinity effects. The availability of water or ice at the particles' surfaces is also governed by the surface properties, the porosity and permeability of ash, which are not well established to date. Particle growth is significantly enhanced for greater differences in the sizes and fall velocities among particles, as gravitational capture becomes more efficient. Our experiments indicate a major influence of the erupted particle size distribution. First sensitivity studies show that electrostatic forces result in a significant enhancement of aggregated particles. The present exploratory study provides new insights into the sensitivity of the ash aggregation process to a number of key parameters. Our results indicate the need of further constraining particle composition, size, porosity, permeability, and surface properties at low temperatures by in situ observations in the laboratory and in the field. In addition further research on electrostatic aggregation would be desirable.

Volcanic particle aggregation in explosive eruption columns. Part I: Parameterization of the microphysics of hydrometeors and ash

The aggregation of volcanic ash particles within the eruption column of explosive eruptions has been observed at many volcanoes. It influences the residence time of ash in the atmosphere and the radiative properties of the umbrella cloud. However, the information on the processes leading to aggregate formation are still either lacking or very incomplete. We examine the fate of ash particles through numerical experiments with the plume model ATHAM (Active Tracer High resolution Atmospheric Model) in order to determine the conditions that promote ash particle aggregation. In this paper we describe the microphysics and parameterization of ash and hydrometeors. In a companion paper (this issue) we use this information in a series of numerical experiments. The parameterization includes the condensation of water vapor in the rising eruption column. The formation of liquid and solid hydrometeors and the effect of latent heat release on the eruption column dynamics are considered. The interactions of hydrometeors and volcanic ash within the eruption column that lead to aggregate formation are simulated for the first time within a rising eruption column. The microphysical parameterization follows a modal approach. The hydrometeors are described by two size classes, each of which is divided into a liquid and a frozen category. By analogy with the hydrometeor classification, we specify four categories of volcanic ash particles. We imply that volcanic particles are active as condensation nuclei for water and ice formation. Ash can be contained in all categories of hydrometeors, thus forming mixed particles of any composition reaching from mud rain to accretionary lapilli. Collisions are caused by gravitational capture of particles with different fall velocity. Coalescence of hydrometeor–ash aggregates is assumed to be a function of the hydrometeor mass fraction within the mixed particles. The parameterization also includes simplified descriptions of electrostatics and salinity effects.

A Prognostic Turbulence Scheme for the Nonhydrostatic Plume Model ATHAM

The presented turbulence scheme was developed for the Active Tracer High-Resolution Atmospheric Model (ATHAM) to parameterize the effect of subgrid-scale turbulence. In contrast to the commonly used assumption of local isotropy in high-resolution atmospheric modeling, this scheme differentiates between horizontal and vertical turbulent exchange to represent the strong influence of buoyancy forces and vertical transports. Its computational efficiency is similar to classical turbulent kinetic energy approaches while preserving one of the main feature of higher-order schemes. The present extensions to include anisotropic effects in a turbulent kinetic energy approach do not need any ad hoc assumptions and are equivalent to the classical formulation in the isotropic limit. The presence of high tracer concentrations in a particle-laden plume is taken into account, as well as supersonic effects at low Mach numbers. The turbulent exchange coefficients used in the equations of motion are derived from a set of three coupled differential equations for the horizontal and vertical turbulent energy and the turbulent length scale. No turbulent equilibrium is assumed. All turbulent quantities are treated prognostically. Numerical simulations of convective plumes of a typical Plinian volcanic eruption with the nonhydrostatic plume model ATHAM reveal that a complex treatment of turbulent quantities is necessary in order to capture the bulk characteristics of the plume, such as the plume height, the horizontal extent, and plume development in time. Anisotropic effects of turbulence have a significant impact on the stability and internal structure of the plume. For the first time, results from a fully three-dimensional simulation of a volcanic plume are presented. Because of its general formulation the presented turbulence scheme is suitable for a wide range of atmospheric applications.

Injection of gases into the stratosphere by explosive volcanic eruptions

Explosive 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.

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

We 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−2over land and −58 W m−2over 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.

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

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 plume's total thermal energy and therefore have a considerable effect on the vertical development of the plume.

Volcanic plume simulation on large scales

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 model's 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 fluid's 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.