Search

Dusty cirrus clouds are extended optically thick cirrocumulus decks that occur during strong mineral dust events. So far they have been mostly documented over Europe associated with dust-infused baroclinic storms. Since today's numerical weather prediction models neither predict mineral dust distributions nor consider the interaction of dust with cloud microphysics, they cannot simulate this phenomenon. We postulate that the dusty cirrus forms through a mixing instability of moist clean air with drier dusty air. A corresponding sub-grid parameterization is suggested and tested in the ICON-ART model. Only 5 with help of this parameterization ICON-ART is able to simulate the formation of the dusty cirrus, which leads to substantial improvements in cloud cover and radiative fluxes compared to simulations without this parameterization. A statistical evaluation over six Saharan dust events with and without observed dusty cirrus shows robust improvements in cloud and radiation scores. The ability to simulate dusty cirrus formation removes the linear dependency on mineral dust aerosol optical depth from the bias of the radiative fluxes. This suggests that the formation of dusty cirrus clouds is the dominant aerosol-cloud-radiation 10 effect of mineral dust over Europe. [ABSTRACT FROM AUTHOR]

The evolution and the properties of a Saharan dust plume were studied near the city of Karlsruhe in south-west Germany (8.4298 °E, 49.0953 °N) from April 7 to 9, 2018 combining a scanning lidar (90°, 30°), a vertically pointing lidar (90°), a sun photometer, and the transport model ICOsahedral Nonhydrostatic model - Aerosols and Reactive Trace gases (ICON-ART). The lidar measurements show that the dust particles had backscatter coefficients of 0.86 ± 0.14 Mm-1 sr -1, an extinction coefficient of 40 ± 0.8 Mm-1, a lidar ratio of 46 ± 5 sr, and a particle depolarization ratio of 0.33 ± 0.07. These values are in good agreement with those obtained in previous studies of Saharan dust plumes in Western Europe. Compared to the remote sensing measurements, the model simulation predicts the plume arrival time, its layer height, and structure very well but overestimates the backscatter coefficient. In this manuscript, we discuss the complementarity and advantages of the different measurement methods as well as model simulations to predict Saharan dust plumes. Main conclusions are that the ICON-ART model can predict the structure of Saharan dust plumes very well but overestimates the backscatter coefficients by a factor of 2.2 ± 0.16 at 355 nm and underestimates the aerosol optical depth (AOD) by a factor of 1.5 ± 0.11 at 340 nm for this Saharan dust plume event. Employing a scanning aerosol lidar allows determining backscatter coefficient, particle depolarization ratio and especially lidar ratio of Saharan dust both for daytime and nighttime independently. Combining lidar with sun photometer data allows constraining aerosol optical depth in different ways and determining column integrated lidar ratios. These comprehensive datasets allow for a better understanding of Saharan dust plumes in Western Europe. [ABSTRACT FROM AUTHOR]

In June 2019, the Raikoke volcano, Kuril Islands, emitted 0.4-1.8 x 109 kg of very fine ash and 1-2 x 109 kg of SO2 up to 14 km into the atmosphere. The eruption was characterized by several phases or puffs of different duration and eruption heights. Resolving such complex eruption dynamics is required for precise volcanic plume dispersion forecasts. To address this issue, we coupled the atmospheric model system ICON-ART (ICOsahedral Nonhydrostatic – Aerosols and Reactive Trace gases) with the 1-D plume model FPlume to calculate the eruption source parameters (ESPs) online. The main inputs are the plume heights for the different eruption phases that are geometrically derived from satellite data. An empirical relationship is used to derive the amount of very fine ash (particles <32µm), which is relevant for long range transport in the atmosphere. On the first day after the onset of the eruption, the modeled ash loading agrees very well with the ash loading estimated from AHI (Advanced Himawari Imager) observations due to the resolution of the eruption phases and the online treatment of the ESPs. In later hours, aerosol dynamical processes (nucleation, condensation, coagulation) explain the loss of ash in the atmosphere in agreement with the observations. However, a direct comparison is partly hampered by water and ice clouds overlapping the ash cloud in the observations. We compared 6-hourly means of model and AHI data with respect to the structure, amplitude, and location (SAL-method) to further validate the simulated dispersion of SO² and ash. In the beginning, the structure and amplitude values differed largely because the dense ash cloud leads to an underestimation of the SO2 amount in the satellite data. On the second and third day, the SAL values are close to zero for all parameters indicating a very good agreement of model and observations. Furthermore, we found a separation of the ash and SO² plume after one day due to particle sedimentation, chemistry, and aerosol-radiation interaction. The results confirm that coupling the atmospheric model system and plume model enables detailed treatment of the plume dynamics (phases and ESPs) and leads to significant improvement of the ash and SO[sub 2] dispersion forecast. This approach can benefit the operational forecast of ash and SO2 especially in case of complex and non-continuous volcanic eruptions like the Raikoke 2019. [ABSTRACT FROM AUTHOR]

A correct and reliable forecast of volcanic plume dispersion is vital for aviation safety. This can only be achieved by representing all responsible physical and chemical processes (sources, sinks, and interactions) in the forecast models. The representation of the sources has been enhanced over the last decade, while the sinks and interactions have received less attention. In particular, aerosol dynamic processes and aerosol-radiation interaction are neglected so far. Here we address this gap by further developing the ICON-ART (ICOsahedral Nonhydrostatic - Aerosols and Reactive Trace gases) global modelling system to account for these processes. We use this extended model for the simulation of volcanic aerosol dispersion after the Raikoke eruption in June 2019. Additionally, we validate the simulation results with measurements from AHI (Advanced Himawari Imager), CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization), and OMPS-LP (Ozone Mapping and Profiling Suite - Limb Profiler). Our results show that around 50 % of very fine volcanic ash mass (particles with diameter d < 30 µm) is removed due to particle growth and aging. Furthermore, the maximum volcanic cloud top height rises more than 6 km over the course of 4 days after the eruption due to aerosol-radiation interaction. This is the first direct evidence that shows how cumulative effects of aerosol dynamics and aerosol-radiation interaction lead to a more precise forecast of very fine ash lifetime in volcanic clouds. [ABSTRACT FROM AUTHOR]

Aeolian dust, the most dominant atmospheric aerosol by mass, influences the weather and climate directly through absorbing and scattering the radiation. Particle size distribution (PSD) controls the magnitude of these impacts through modulating the dust residence time in the atmosphere and optical properties. Although the dust PSD varies during the atmospheric transport, current models usually neglect the effect of these variations on dust optical properties. This study investigates the impact of variable PSD on dust optical properties and radiative impacts using the next-generation atmospheric modeling system ICON-ART (ICOsahedral Nonhydrostatic with Aerosols and Reactive Trace gases). Two sets of numerical experiments are conducted assuming fixed and variable PSD. A parameterization is developed to account for the effect of variable PSD on dust optical properties in the model. This parameterization is then implemented in ICON-ART to simulate a period of 1 month on a global grid with 80 km horizontal resolution. Results show that the consideration of a variable PSD increases the dust AOD. However, the magnitude of the changes depends on the dominant size modes. Although the optical properties of the fine mode show the highest sensitivity to the variable PSD, the median diameter of this mode hardly changes during transport. Thus, the new parametrization does not have a significant impact on AOD where fine mode is dominant. In contrast, when coarse mode is prevailing, the increase of AOD is more pronounced. This makes the surface slightly warmer (+0.2 K) in the source regions. [ABSTRACT FROM AUTHOR]