Thermally generated convection and precipitation over volcanoes: Numerical modelling of flow over Montserrat

Atmospheric flow over orography is a classic research area, while the atmospheric response to surface heating has become a focus more recently in the context of solar heating and forest fires. Here, for the first time, these forcing mechanisms are superposed to examine atmospheric flow over a mountain with a heated summit, i.e. an active volcano. Intense rainfall over active volcanoes is known to trigger dangerous volcanic hazards, from remobilising loose surface material into lahars or mudflows, to initiating explosive activity such as pyroclastic flows. The effect of a heated volcanic surface on the atmospheric circulation is investigated here – including examining the triggering of precipitation over the volcano. Recent activity at the Soufri`ere Hills Volcano (SHV), Montserrat, Eastern Caribbean, is a well-documented example of such rainfall–volcano interactions. Hence, Montserrat is used as a template for the experiments, although the experimental setup is general so the results will have applicability for other tropical island volcanoes. The Weather Research and Forecasting (WRF) atmospheric model has been used for the study, run in an idealised configuration with horizontal grid sizes down to 100 m. Initially, the effect of the heated surface is studied through idealised simulations over a Gaussian mountain with an imposed surface temperature anomaly on the volcano summit. Subsequently, a digital elevation model (DEM) of Montserrat is used to study the effects over this specific island. The atmospheric structure in most simulations is that of a typical tropical setting – easterly TradeWinds, capped by a temperature inversion. In these cases, localised convection triggered by the heat source can overcome convective inhibition and force deep convection, if there is sufficient convective available potential energy. A significant increase in precipitation over the volcano covering a 4 km2 area is consistently simulated for surface temperature anomalies above 40�C, an area-average value that is exceeded at the SHV. For a range of realistic atmospheric conditions, covering up to 18% of days in a relevant climatological study in the Caribbean, the precipitation increase is well above the observed threshold (5–10 mm h

Orographic effects on the transport and deposition of volcanic ash: A case study of Mt. Sakurajima, Japan

Volcanic ash is a major atmospheric hazard that has a significant impact on local populations and international aviation. The topography surrounding a volcano affects the transport and deposition of volcanic ash, but these effects have not been studied in depth. Here we investigate orographic impacts on ash transport and deposition in the context of the Sakurajima volcano in Japan, using the chemistry-resolving version of the Weather Research and Forecasting model. Sakurajima is an ideal location for such a study because of the surrounding mountainous topography, frequent eruptions, and comprehensive observing network. At Sakurajima, numerical experiments reveal that across the 2–8ϕ grain size range, the deposition of “medium-sized” ash (3–5ϕ) is most readily affected by orographic flows. The direct effects of resolving fine-scale orographic phenomena are counteracting: mountain-induced atmospheric gravity waves can keep ash afloat, while enhanced downslope winds in the lee of mountains (up to 50% stronger) can force the ash downward. Gravity waves and downslope winds were seen to have an effect along the dispersal path, in the vicinity of both the volcano and other mountains. Depending on the atmospheric conditions, resolving these orographic effects means that ash can be transported higher than the initial injection height (especially for ash finer than 2ϕ), shortly after the eruption (within 20 min) and close to the vent (within the first 10 km), effectively modifying the input plume height used in an ash dispersal model—an effect that should be taken into account when initializing simulations

Chemical and dynamical identification of emission outflows during the HALO campaign EMeRGe in Europe and Asia

The number of large urban agglomerations is steadily increasing worldwide. At a local scale, their emissions lead to air pollution, directly affecting people\u27s health. On a global scale, their emissions lead to an increase of greenhouse gases, affecting climate. In this context, in 2017 and 2018, the airborne campaign EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales) investigated emissions of European and Asian major population centres (MPCs) to improve the understanding and predictability of pollution outflows. Here, we present two methods to identify and characterise pollution outflows probed during EMeRGe. First, we use a set of volatile organic compounds (VOCs) as chemical tracers to characterise air masses by specific source signals, i.e. benzene from anthropogenic pollution of targeted regions, acetonitrile from biomass burning (BB, primarily during EMeRGe-Asia), and isoprene from fresh biogenic signals (primarily during EMeRGe-Europe. Second, we attribute probed air masses to source regions and estimate their individual contribution by constructing and applying a simple emission uptake scheme for the boundary layer which combines FLEXTRA back trajectories and EDGAR carbon monoxide (CO) emission rates (acronyms are provided in the Appendix). During EMeRGe-Europe, we identified anthropogenic pollution outflows from northern Italy, southern Great Britain, the Belgium–Netherlands–Ruhr (BNR) area and the Iberian Peninsula. Additionally, our uptake scheme indicates significant long-range transport of pollution from the USA and Canada. During EMeRGe-Asia, the pollution outflow is dominated by sources in China and Taiwan, but BB signals from Southeast Asia and India contribute as well. Outflows of pre-selected MPC targets are identified in less than 20 % of the sampling time, due to restrictions in flight planning and constraints of the measurement platform itself. Still, EMeRGe combines in a unique way near- and far-field measurements, which show signatures of local and distant sources, transport and conversion fingerprints, and complex air mass compositions. Our approach provides a valuable classification and characterisation of the EMeRGe dataset, e.g. for BB and anthropogenic influence of potential source regions and paves the way for a more comprehensive analysis and various model studies

Chemical and dynamical identification of emission outflows during the HALO campaign EMeRGe in Europe and Asia

The airborne megacity campaign EMeRGe provided an unprecedented amount of trace gas measurements. We combine measured volatile organic compounds (VOCs) with trajectory-modelled emission uptakes to identify potential source regions of pollution. We also characterise the chemical fingerprints (e.g. biomass burning and anthropogenic signatures) of the probed air masses to corroborate the contributing source regions. Our approach is the first large-scale study of VOCs originating from megacities

Experimental high resolution forecasting of volcanic ash hazard at Sakurajima, Japan

A high-resolution forecast methodology for the ash hazard at Sakurajima volcano, Japan, is presented. The methodology employs a combined modeling approach and utilizes eruption source parameters estimated by geophysical observations from Sakurajima, allowing for a proactive approach in forecasting. The Weather Research and Forecasting (WRF) model is used to downscale Japan Meteorological Agency (JMA) forecast data over the area of interest. The high-resolution meteorological data are then used in FALL3D model to provide a forecast for the ash dispersal and deposition. The methodology is applied for an eruption that occurred on June 16, 2018. Disdrometer observations of ashfall are used along with ash dispersal modeling to inform the choice of the total grain size distribution (TGSD). A series of pseudo-forecast ash dispersal simulations are then carried out using the proposed methodology and estimated TGSD, initialized with meteorological forecast data released up to ∼13 hours before the eruption, with results showing surprising consistency up to ∼10 hours before the eruption. Using forecast data up to 4 hours before the eruption was seen to constrain observation to model ratios within a factor of 2–4 depending on the timing of simulation and location. A number of key future improvements for the methodology are also highlighted

New insights into real-time detection of tephra grainsize, settling velocity and sedimentation rate

Characterizing the size and settling velocity of pyroclastic fragments injected into the atmosphere during volcanic eruptions (i.e., tephra) is crucial to the forecasting of plume and cloud dispersal. Optical disdrometers have been integrated into volcano monitoring networks worldwide in order to best constrain these parameters in real time. Nonetheless, their accuracy during tephra fallout still needs to be assessed. A significant complication is the occurrence of particle aggregates that modify size and velocity distributions of falling tephra. We made the first use of the Thies Clima Laser Precipitation Monitor (LPM) for tephra-fallout detection at Sakurajima volcano (Japan), which is characterized by a lower size detection window with respect to more commonly used disdrometers (e.g., Parsivel 2 ) and can more easily distinguish different falling objects. For the first time, individual particles have been distinguished from most aggregates based on disdrometer data, with the potential to provide useful grain-size information in real time. In case of negligible aggregation, LPM and collected sample-based estimates are in agreement for both grain-size and sedimentation rate. In case of significant aggregation, particle shape analyses and a dedicated drag equation are used to filter out aggregates from LPM data that also provide good agreement with collected tephra samples. </p

Aerodynamic characteristics and genesis of aggregates at Sakurajima Volcano, Japan

Aggregation of volcanic ash is known to significantly impact sedimentation from volcanic plumes. The study of particle aggregates during tephra fallout is crucial to increase our understanding of both ash aggregation and sedimentation. In this work, we describe key features of ash aggregates and ash sedimentation associated with eleven Vulcanian explosions at Sakurajima Volcano (Japan) based on state-of-the-art sampling techniques. We identified five types of aggregates of both Particle Cluster (PC) and Accretionary Pellet (AP) categories. In particular, we found that PCs and the first and third type of APs can coexist within the same eruption in rainy conditions. We also found that the aerodynamic properties of aggregates (e.g., terminal velocity and density) depend on their type. In addition, grainsize analysis revealed that characteristics of the grainsize distributions (GSDs) of tephra samples correlate with the typology of the aggregates identified. In fact, bimodal GSDs correlate with the presence of cored clusters (PC3) and liquid pellets (AP3), while unimodal GSDs correlate either with the occurrence of ash clusters (PC1) or with the large particles (coarse ash) coated by fine ash (PC2)