Impact of Holuhraun volcano aerosol on clouds in cloud-system resolving simulations

<p>Since increased anthropogenic aerosol result in an enhancement in cloud droplet number concentration, cloud and precipitation process are modified. It is unclear how exactly cloud liquid water path (LWP) and cloud fraction respond to aerosol perturbations. A large volcanic eruption may help to better understand and quantify the cloud response to external perturbations, with a focus on the short-term cloud adjustments . Volcloud is one of the research projects in the Vollmpact collaborative German research unit which aims to the improve understanding of how the climate system responds to volcanic eruptions. This includes skills in satellite remote sensing of atmospheric composition, stratospheric aerosol parameters and clouds as well as in modelling of aerosol microphysical and cloud processes, and in climate modelling. The goal of VolCloud is to understand and quantify the response of clouds to volcanic eruptions and to thereby advance the fundamental understanding of the cloud response to external forcing, particularly aerosol-cloud interactions. In this study we used ICON-NWP atmospheric model at a cloud-system-resolving resolution of 2.5 km horizontally, to simulate the region around the Holuhraun volcano for the duration of one week (1 – 7 September 2014). The pair of simulations, with and without the volcanic aerosol emissions allowed us to assess the simulated effective radiative forcing and its mechanisms as well as its impact on adjustments of cloud liquid water path and cloud fraction to the perturbations of cloud droplet number concentration. In this case studies liquid water path positively correlates with enhanced cloud droplet concentration.</p>

Rock mass deterioration model of bank slope based on high-precision 3D multiperiod point clouds in the Three Gorges Reservoir, China

Karst bank slopes are widely developed in the Three Gorges Reservoir area. Potential bank slope instability caused by the deterioration of rock masses has aroused considerable attention. This paper describes an attempt to use high-precision 3D laser scanning point cloud technology to reveal the deterioration law of a rock mass from a meso-/macroscale perspective and to preliminarily establish the relationship between the characteristics of the rock mass deterioration and 3D point cloud response. The main conclusions are as follows. The point cloud response characteristics of rock mass deterioration can be divided into three types: crack manifestation–extension, collapse-block falling, and translation-rotation. The crack manifestation–extension type can be further subdivided into three subcategories: crack extension, crack widening and crack deepening. These three subclasses of point cloud response characteristics of crack manifestation–extension all exhibit strip-like erosion-induced extensions, but the deformation forms and locations are slightly different. The point cloud response of collapse-block falling shows irregular sheet erosion. The point cloud response characteristics of translational rotation are similar, but associated erosion deformation often presents regular isosurface deformation characteristics. The C2C and M3C2 algorithms are used to calculate the deterioration and deformation, and the difference between recording points at the same location is small, except where the local normal vector N→ changes greatly or the point cloud is sparse, where there are large differences. Based on 3D point clouds, the deterioration rate formula De = n/N for a rock mass is proposed for the first time, where n is the number of degraded and deformed point clouds, and N is the total number of point clouds in the test area. According to the deterioration rates De corresponding to different deformation thresholds, the deterioration degree and state of a rock mass can be described and judged.

Polluted cloud lines in satellite snapshots and satellite climatologies

<p>It is challenging to separate the cause from effect in aerosol-cloud interactions. Anomalous cloud lines polluted by anthropogenic aerosols help distinguish the cause from effect as properties of polluted clouds can be directly compared to nearby unpolluted clouds’ properties. Pollution tracks in clouds induced by localised aerosol emissions (Toll et al. 2019, Nature, https://doi.org/10.1038/s41586-019-1423-9)  are visually detectable ship-track-like quasi-linear polluted cloud features in satellite snapshots. We detected similar anomalous polluted cloud lines in the long-term average satellite data, where cloud response to aerosol over a long time is recorded. Polluted cloud tracks are induced by various aerosol sources like oil refineries, smelters, coal-fired power plants, smaller industry towns, ships, and volcanoes. We detected polluted cloud tracks at spatial scales varying from tens of kilometres to thousands of kilometres (Trofimov et al. 2020; JGR Atmospheres, https://doi.org/10.1029/2020JD032575).  </p><p> </p><p>Polluted cloud tracks detected in satellite snapshots are excellent for the process-level understanding of aerosol-cloud interactions. Polluted cloud tracks recorded in satellite climatologies are great for estimating the average cloud response to aerosols. MODIS snapshots of polluted cloud tracks show relatively weak cloud water response to aerosols at various spatial scales. High-resolution analysis of South-East Atlantic shipping corridor shows partial off-set of the Twomey effect by decreased cloud water. Cloud fraction sometimes increases in the polluted cloud tracks and sometimes decreases compared to the nearby unpolluted clouds. The temporal evolution of cloud responses in pollution tracks estimated from geostationary SEVIRI data and meteorological conditions favourable for pollution track occurrence is presented. We expect that the utilisation of these real-world laboratories of aerosol impacts on clouds helps to improve global climate models’ physical parameterisations.</p>

Inter-calibration of nine UV sensing instruments over Antarctica and Greenland since 1980

Nadir-viewed intensities (radiances) from nine UV sensing satellite instruments are calibrated over the East Antarctic Plateau and Greenland during summer. The calibrated radiances from these UV instruments ultimately will provide a global long-term record of cloud trends and cloud response from ENSO events since 1980. We first remove the strong solar zenith angle dependence from the intensities using an empirical approach rather than a radiative transfer model. Then small multiplicative adjustments are made to these solar zenith angle normalized intensities in order to minimize differences when two or more instruments temporally overlap. While the calibrated intensities show a negligible long-term trend over Antarctica and a statistically insignificant UV albedo trend of −0.05 % per decade over the interior of Greenland, there are small episodic reductions in intensities which are often seen by multiple instruments. Three of these darkening events are explained by boreal forest. Other events are caused by surface melting or volcanoes. We estimate a 2-sigma uncertainty of 0.35 % for the calibrated radiances.

Low-level cloud feedbacks in CMIP6 models and EUREC4A observations

<p>How will low-level clouds respond to global warming? We approach this question by first investigating the spread of climate sensitivity and cloud feedbacks in CMIP6 models. We stratify the cloud response by circulation regime and focus in greater detail on the cloud response in tropical regimes of subsidence and weak ascent  (i.e., their vertical structure in the present-day and future climate, how cloud profile changes relate to changes in cloud-controlling factors). This CMIP6 model analysis dovetails with an observational analysis of low cloud responses from the EUREC4A field campaign. We seek to employ a simple model of low cloud behavior, constrained with observations from EUREC4A and longer time series from the Barbados Cloud Observatory, to better constrain the range of low cloud behavior spanned by CMIP6 models. </p>

Diurnal cycle of the semi-direct effect from a persistent absorbing aerosol layer over marine stratocumulus in large-eddy simulations

The rapid adjustment, or semi-direct effect, of marine stratocumulus clouds to elevated layers of absorbing aerosols may enhance or dampen the radiative effect of aerosol–radiation interactions. Here we use large-eddy simulations to investigate the sensitivity of stratocumulus clouds to the properties of an absorbing aerosol layer located above the inversion layer, with a focus on the location, timing, and strength of the radiative heat perturbation. The sign of the daily mean semi-direct effect depends on the properties and duration of the aerosol layer, the properties of the boundary layer, and the model setup. Our results suggest that the daily mean semi-direct effect is more elusive than previously assessed. We find that the daily mean semi-direct effect is dominated by the distance between the cloud and absorbing aerosol layer. Within the first 24 h the semi-direct effect is positive but remains under 2 W m−2 unless the aerosol layer is directly above the cloud. For longer durations, the daily mean semi-direct effect is consistently negative but weakens by 30 %, 60 %, and 95 % when the distance between the cloud and aerosol layer is 100, 250, and 500 m, respectively. Both the cloud response and semi-direct effect increase for thinner and denser layers of absorbing aerosol. Considerable diurnal variations in the cloud response mean that an instantaneous semi-direct effect is unrepresentative of the daily mean and that observational studies may underestimate or overestimate semi-direct effects depending on the observed time of day. The cloud response is particularly sensitive to the mixing state of the boundary layer: well-mixed boundary layers generally result in a negative daily mean semi-direct effect, and poorly mixed boundary layers result in a positive daily mean semi-direct effect. The properties of the boundary layer and model setup, particularly the sea surface temperature, precipitation, and properties of the air entrained from the free troposphere, also impact the magnitude of the semi-direct effect and the timescale of adjustment. These results suggest that the semi-direct effect simulated by coarse-resolution models may be erroneous because the cloud response is sensitive to small-scale processes, especially the sources and sinks of buoyancy.

Diurnal cycle of the semi–direct effect over marine stratocumulus in large–eddy simulations

The rapid adjustment, or semi–direct effect, of marine stratocumulus clouds to elevated layers of absorbing aerosols may enhance or dampen the radiative effect of aerosol–radiation interactions. Here we use large eddy simulations to investigate the sensitivity of stratocumulus clouds to the properties of an absorbing aerosol layer located above the inversion layer. The sign of the daily mean semi–direct effect depends on the properties of the aerosol layer, the properties of the boundary layer, and the model setup. Diurnal variations in the cloud response mean that an instantaneous semi–direct effect is unrepresentative of the daily mean, and that observational studies may under– or over–estimate semi–direct effects depending on the observed time of day. The observed role of the distance between the cloud top and the absorbing layer in modulating the strength of the cloud and radiative response is reproduced by the large eddy simulations. Both cloud response and semi–direct effect increase for thinner, denser, layers of absorbing aerosol located nearer the cloud layer. The cloud response is particularly sensitive to the mixing state of the boundary layer: well-mixed boundary layers generally result in a negative daily mean semi–direct effect, and poorly mixed boundary layers result in a positive daily mean semi–direct effect. Properties of the boundary layer and model setup, particularly the sea surface temperature, precipitation, and properties of the air entrained from the free troposphere, also impact the magnitude of the semi–direct effect and the timescale of adjustment. These results suggest that the semi–direct effect simulated by coarse-resolution models may be erroneous because the cloud response is sensitive to small-scale processes, especially the sources and sinks of buoyancy.