Meridional Circulation of Dust and Gas in the Circumstellar Disk: Delivery of Solids onto the Circumplanetary Region

We carried out 3D dust + gas radiative hydrodynamic simulations of forming planets. We investigated a parameter grid of a Neptune-mass, a Saturn-mass, a Jupiter-mass, and a five-Jupiter-mass planet at 5.2, 30, and 50 au distance from their star. We found that the meridional circulation (Szulágyi et al. 2014; Fung & Chiang 2016) drives a strong vertical flow for the dust as well, hence the dust is not settled in the midplane, even for millimeter-sized grains. The meridional circulation will deliver dust and gas vertically onto the circumplanetary region, efficiently bridging over the gap. The Hill-sphere accretion rates for the dust are ∼10−8–10−10 M Jup yr−1, increasing with planet mass. For the gas component, the gain is 10−6–10−8 M Jup yr−1. The difference between the dust and gas-accretion rates is smaller with decreasing planetary mass. In the vicinity of the planet, the millimeter-sized grains can get trapped easier than the gas, which means the circumplanetary disk might be enriched with solids in comparison to the circumstellar disk. We calculated the local dust-to-gas ratio (DTG) everywhere in the circumstellar disk and identified the altitude above the midplane where the DTG is 1, 0.1, 0.01, and 0.001. The larger the planetary mass, the more the millimeter-sized dust is delivered and a larger fraction of the dust disk is lifted by the planet. The stirring of millimeter-sized dust is negligible for Neptune-mass planets or below, but significant above Saturn-mass planets.

Long-Term Variability of Dust Events in Southwestern Iran and Its Relationship with the Drought

Dust storms represent a major environmental challenge in the Middle East. The southwest part of Iran is highly affected by dust events transported from neighboring desert regions, mostly from the Iraqi plains and Saudi Arabia, as well as from local dust storms. This study analyzes the spatio-temporal distribution of dust days at five meteorological stations located in southwestern Iran covering a period of 22 years (from 1997 to 2018). Dust codes (06, 07, 30 to 35) from meteorological observations are analyzed at each station, indicating that 84% of the dust events are not of local origin. The average number of dust days maximizes in June and July (188 and 193, respectively), while the dust activity weakens after August. The dust events exhibit large inter-annual variability, with statistically significant increasing trends in all of five stations. Spatial distributions of the aerosol optical depth (AOD), dust loading, and surface dust concentrations from a moderate resolution imaging spectroradiometer (MODIS) and Modern-Era Retrospective analysis for Research and Applications (MERRA-2) retrievals reveal high dust accumulation over southwest Iran and surrounding regions. Furthermore, the spatial distribution of the (MODIS)-AOD trend (%) over southwest Iran indicates a large spatial heterogeneity during 2000–2018 with trends ranging mostly between −9% and 9% (not statistically significant). 2009 was the most active dust year, followed by 2011 and 2008, due to prolonged drought conditions in the fertile crescent and the enhanced dust emissions in the Iraqi plains during this period. In these years, the AOD was much higher than the 19-year average (2000 to 2018), while July 2009 was the dustiest month with about 25–30 dust days in each station. The years with highest dust activity were associated with less precipitation, negative anomalies of the vegetation health index (VHI) and normalized difference vegetation index (NDVI) over the Iraqi plains and southwest Iran, and favorable meteorological dynamics triggering stronger winds.

The Philae Lander reveals the presence of low strength primitive ice inside cometary boulders

<p>On the 12 November 2014, the Philae lander descended towards comet 67P/Churyumov–Gerasimenko, bounced twice off the surface, finally arriving under an overhanging cliff in the Abydos region. In this study (published in Nature on 28th Oct 2020), we present the results of our investigation of a previously undiscovered site of the second touchdown, where Philae spent almost two minutes of its cross-comet journey, producing four distinct surface contacts on two adjoining cometary boulders. It exposed primitive water ice —that is, water ice from the time of the comet’s formation 4.5 billion years ago — in their interiors while travelling through a crevice between the boulders. Our multi-instrument observations made 19 months later found that this water ice, mixed with ubiquitous dark organic-rich material, has a local dust/ice mass ratio of 2.3 +0.2/−0.16 : 1 , matching values previously observed in freshly exposed water ice from outbursts and water ice in shadow. At the end of the crevice, Philae fell forward and left a 0.25-metre-deep impression in the boulder ice, providing in situ measurements confirming that primitive ice has a very low compressive strength (less than 12 pascals, softer than freshly fallen light snow) and allowing a key estimation to be made of the porosity (75 ± 7 per cent) of the boulders’ icy interiors. Our results provide constraints for cometary landers that seek to access a volatile-rich ice sample. </p>

Saharan dust and giant quartz particle transport towards Iceland

Mineral dust emissions from Saharan sources have an impact on the atmospheric environment and sedimentary units in distant regions. Here, we present the first systematic observations of long-range Saharan dust transport towards Iceland. Fifteen Saharan dust episodes were identified to have occurred between 2008 and 2020 based on aerosol optical depth data, backward trajectories and numerical models. Icelandic samples from the local dust sources were compared with deposited dust from two severe Saharan dust events in terms of their granulometric and mineralogical characteristics. The episodes were associated with enhanced meridional atmospheric flow patterns driven by unusual meandering jets. Strong winds were able to carry large Saharan quartz particles (> 100 µm) towards Iceland. Our results confirm the atmospheric pathways of Saharan dust towards the Arctic, and identify new northward meridional long-ranged transport of giant dust particles from the Sahara, including the first evidence of their deposition in Iceland as previously predicted by models.

Impact of dust in PMIP-CMIP6 mid-Holocene simulations with the IPSL model

We investigate the climate impact of reduced dust during the mid-Holocene using simulations with the IPSL model. We consider simulations where dust is either prescribed from an IPSL PI simulation or from CESM simulations (Albani et al., 2015). In addition, we also consider an extreme mid-Holocene case where dust is suppressed. We focus on the estimation of the dust radiative effects and the relative responses of the African and Indian monsoon, showing how local dust forcing or orography affect atmospheric temperature profiles, humidity and precipitation. The simulated mid-Holocene climate is statistically different in many regions compared to previous mid-Holocene simulations with the IPSL models. However, it translates to only minor improvements compared to palaeoclimate reconstructions, and the effect of dust has little impact on mid-Holocene model skill over large regions. Our analyses confirm the peculiar role of dust radiative effect over bright surfaces such as African deserts compared to other regions, brought about by the change of sign of the dust radiative effect at the top of atmosphere for high surface albedo. We also highlight a strong dependence of results on the dust pattern. In particular, the relative dust forcing between West Africa and the Middle East impacts the relative climate response between India and Africa and between Africa, the western tropical Atlantic and the Atlantic meridional circulation. It also affects the feedback on the Atlantic Ocean thermohaline circulation. Dust patterns should thus be better constrained to fully understand the changes in the dust cycle and forcing during the mid-Holocene, which also informs on the potential changes in key dust feedbacks in the future.

Quantifying past changes in Holocene ultraviolet-absorbing compounds (UACs) from Cedrus pollen in Lake Sidi Ali, Morocco, Africa based on fourier-transform infrared spectroscopy (FTIR)

<div>UV-B radiation may affect atmospheric circulation and influence climate, as well as affecting all biological life on Earth. However, past environmental and climate reconstructions (using various multi-proxy approaches) generally do not focus on UV-B. Ultraviolet-Absorbing Compounds (UACs) in pollen grains are an indicator of the UV-B level received by plants, and it is possible to measure the abundance of UACs in pollen using Fourier Transform Infrared (FTIR) micro-Spectroscopy, providing a promising approach to UV-B reconstruction. This research reconstructed UAC levels in individual <em>Cedrus atlantica</em> (Atlas cedar) pollen grains within sediment samples from a 20 m core at Lake Sidi Ali, Middle Atlas, Morocco, spanning the Holocene. Correlations between UACs and other palaeoenvironmental proxies from the same core are discussed, including, oxygen stable isotopes (<sup>18</sup>O) which reflect winter rainfall, local dust sedimentation, and lake sediment CaCO<sub>3</sub> and Total Organic Carbon (TOC), as well as independent <sup>10</sup>Be-derived Total Solar Irradiance (TSI). Correlations between these proxies may indicate solar-related temperature and humidity fluctuations at Lake Sidi Ali, considering mechanisms of varied ocean and terrestrial circulation under different levels of solar activity, although there is some chronological uncertanty due to different sampling resolutions. A positive correlation between UAC levels and winter rainfall at Morocco implies connections between oceanic circulation and solar activity, while the relationship with local dust sedimentation reveals how solar irradiation may influence the Saharan dry air mass contribution to Morocco. Analysis of single pollen grains using FTIR presents several challenges in obtaining clean spectra, which can be a source of uncertainty with this approach. To minimise noise in FTIR spectra, <em>Cedrus </em>pollen grains should be measured using the same orientation under microscope. As this is not always posssible, we developed a protocol to evaluate spectra quality to filter out spectra that had excessive noise, or were deemed not to be a pollen grain. We also assess the minimum effective number of pollen grains that require measurement to provide a statisitcally significant sample and thereby improve the quality of UAC data. Our protocol represents good practice in developing a robust UAC data set, which can allow for UV-B and solar radiation levels to be inferred. Future work aims to quantitatively reconstruct UV-B levels using a modern UAC calibration.</div><p> </p>

Saharan dust episodes and giant quartz particles in Iceland

<p>Saharan dust has an impact on the atmospheric environment and sedimentary units in distant regions. Although Iceland is located within one of the main atmospheric dust pathways moving towards the Arctic, no evidence of Saharan dust deposition has been provided to date for the region. Here we present the results of fourteen Saharan dust episodes, which were identified in Iceland between 2008 to 2020. Aerosol optical depth data of Terra MODIS, HYSPLIT backward trajectories and numerical simulations of Barcelona Supercomputing Center were used in this work to identify the dust episodes. <br>Grain size and shape of the Saharan mineral material deposited in Iceland during two severe deposition events were investigated in detail. Icelandic dust samples from the most active local dust sources were compared with samples of deposited mineral dust from these two severe Saharan dust events to determine their granulometric (complex grain size and shape parameters) and mineralogical characteristics. An automated static optical image analysis technique was applied to thousands of individual particles, and was completed by Raman spectroscopy to identify external quartz particles. <br>Saharan dust episodes were associated with enhanced meridional atmospheric flow patterns driven by unusual meandering jets. Strong southerly winds were able to carry large Saharan quartz particles (> 100 µm) towards Iceland. Our results confirm the atmospheric pathways of Saharan dust towards the Arctic, and identify new pathways of giant Saharan dust particles in the study region, including the first evidence of their deposition in Iceland as previously predicted by models.<br>The support of the National Research, Development, and Innovation Office (projects NKFIH KH130337 and K120620 (for G. Varga)), Czech Science Foundation (project No. 20-06168Y (for P. Dagsson-Waldhauserova)), and COST inDust Action are gratefully acknowledged.</p>

Impact of dust in PMIP-CMIP6 mid-Holocene simulations with the IPSL model

We investigate the impact of reduced dust during mid-Holocene using simulations with the IPSL model. We consider simulations where dust is either prescribed from an IPSL PI simulation or from CESM simulations (Albani et al., 2015). In addition, we also consider an extreme mid Holocene case where dust is suppressed. We focus on the estimation of the dust radiative effects and the relative responses of the African and Indian monsoon, showing how local dust forcing or orography affect atmospheric temperature profiles, humidity and precipitation. Compared to previous simulations with the IPSL model the results show only minor improvements for the mid Holocene simulation over large regions despite the fact that the IPSLCM6-LR version of the IPSL model is in better agreement with modern observations. The effect of dust has little impact on the model-data comparisons. Our analyses confirm the peculiar role of dust radiative effect over bright surfaces such as African deserts compared to other regions due to the change of sign of the dust radiative effect at the top-of-atmosphere for high surface albedo. We also highlight a strong dependence on the dust pattern. In particular the relative dust forcing between West Africa and the middle east impacts the relative climate response between India and Africa and between Africa, the western tropical Atlantic and the Atlantic meridional circulation. Dust patterns should thus receive a wider attention to fully understand the changes in the dust cycle and forcing during mid Holocene.