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- ItemThe influence of dust optical properties on the colour of simulated MSG-SEVIRI Desert Dust infrared imagery(Katlenburg-Lindau : EGU, 2018) Banks, Jamie R.; Schepanski, Kerstin; Heinold, Bernd; Hünerbein, Anja; Brindley, Helen E.Satellite imagery of atmospheric mineral dust is sensitive to the optical properties of the dust, governed by the mineral refractive indices, particle size, and particle shape. In infrared channels the imagery is also sensitive to the dust layer height and to the surface and atmospheric environment. Simulations of mineral dust in infrared "Desert Dust" imagery from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) have been performed, using the COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model) dust transport model and the Radiative Transfer for TOVS (RTTOV) program, in order to investigate the sensitivity of the imagery to assumed dust properties. This paper introduces the technique and performs initial validation and comparisons with SEVIRI measurements over North Africa for daytime hours during 6 months covering June and July of 2011–2013. Using T-matrix scattering theory and assuming the dust particles to be spherical or spheroidal, wavelength- and size-dependent dust extinction values are calculated for a number of different dust refractive index databases, along with several values of the particle aspect ratio, denoting the particle shape. The consequences for the infrared extinction values of both the particle shape and the particle orientation are explored: this analysis shows that as the particle asphericity increases, the extinctions increase if the particles are aligned horizontally, and decrease if they are aligned vertically. Randomly oriented spheroidal particles have very similar infrared extinction properties as spherical particles, whereas the horizontally and vertically aligned particles can be considered to be the upper and lower bounds on the extinction values. Inputting these values into COSMO-MUSCAT-RTTOV, it is found that spherical particles do not appear to be sufficient to describe fully the resultant colour of the dust in the infrared imagery. Comparisons of SEVIRI and simulation colours indicate that of the dust types tested, the dust refractive index dataset produced by Volz (1973) shows the most similarity in the colour response to dust in the SEVIRI imagery, although the simulations have a smaller range of colour than do the observations. It is also found that the thermal imagery is most sensitive to intermediately sized particles (radii between 0.9 and 2.6 µm): larger particles are present in too small a concentration in the simulations, as well as with insufficient contrast in extinction between wavelength channels, to have much ability to perturb the resultant colour in the SEVIRI dust imagery.
- ItemThe importance of the representation of air pollution emissions for the modeled distribution and radiative effects of black carbon in the Arctic(Katlenburg-Lindau : EGU, 2019) Schacht, Jacob; Heinold, Bernd; Quaas, Johannes; Backman, John; Cherian, Ribu; Ehrlich, Andre; Herber, Andreas; Huang, Wan Ting Katty; Kondo, Yutaka; Massling, Andreas; Sinha, P.R.; Weinzierl, Bernadett; Zanatta, Marco; Tegen, InaAerosol particles can contribute to the Arctic amplification (AA) by direct and indirect radiative effects. Specifically, black carbon (BC) in the atmosphere, and when deposited on snow and sea ice, has a positive warming effect on the top-of-atmosphere (TOA) radiation balance during the polar day. Current climate models, however, are still struggling to reproduce Arctic aerosol conditions.We present an evaluation study with the global aerosol-climate model ECHAM6.3-HAM2.3 to examine emission-related uncertainties in the BC distribution and the direct radiative effect of BC. The model results are comprehensively compared against the latest ground and airborne aerosol observations for the period 2005-2017, with a focus on BC. Four different setups of air pollution emissions are tested. The simulations in general match well with the observed amount and temporal variability in near-surface BC in the Arctic. Using actual daily instead of fixed biomass burning emissions is crucial for reproducing individual pollution events but has only a small influence on the seasonal cycle of BC. Compared with commonly used fixed anthropogenic emissions for the year 2000, an up-to-date inventory with transient air pollution emissions results in up to a 30% higher annual BC burden locally. This causes a higher annual mean all-sky net direct radiative effect of BC of over 0.1Wm-2 at the top of the atmosphere over the Arctic region (60-90° N), being locally more than 0.2Wm-2 over the eastern Arctic Ocean. We estimate BC in the Arctic as leading to an annual net gain of 0.5Wm-2 averaged over the Arctic region but to a local gain of up to 0.8Wm-2 by the direct radiative effect of atmospheric BC plus the effect by the BC-in-snow albedo reduction. Long-range transport is identified as one of the main sources of uncertainties for ECHAM6.3-HAM2.3, leading to an overestimation of BC in atmospheric layers above 500 hPa, especially in summer. This is related to a misrepresentation in wet removal in one identified case at least, which was observed during the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) summer aircraft campaign. Overall, the current model version has significantly improved since previous intercomparison studies and now performs better than the multi-model average in the Aerosol Comparisons between Observation and Models (AEROCOM) initiative in terms of the spatial and temporal distribution of Arctic BC. © Author(s) 2019.
- ItemMeteorological conditions during the ACLOUD/PASCAL field campaign near Svalbard in early summer 2017(Katlenburg-Lindau : EGU, 2018) Knudsen, Erlend M.; Heinold, Bernd; Dahlke, Sandro; Bozem, Heiko; Crewell, Susanne; Gorodetskaya, Irina V.; Heygster, Georg; Kunkel, Daniel; Maturilli, Marion; Mech, Mario; Viceto, Carolina; Rinke, Annette; Schmithüsen, Holger; Ehrlich, André; Macke, Andreas; Lüpkes, Christof; Wendisch, ManfredThe two concerted field campaigns, Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) and the Physical feedbacks of Arctic planetary boundary level Sea ice, Cloud and AerosoL (PASCAL), took place near Svalbard from 23 May to 26 June 2017. They were focused on studying Arctic mixed-phase clouds and involved observations from two airplanes (ACLOUD), an icebreaker (PASCAL) and a tethered balloon, as well as ground-based stations. Here, we present the synoptic development during the
- ItemMineral dust in central Asia: 18-month lidar measurements in Tajikistan during the central Asian dust experiment (CADEX)(Les Ulis : EDP Sciences, 2018) Hofer, Julian; Althausen, Dietrich; Abdullaev, Sabur F.; Makhmudov, Abduvosit; Nazarov, Bakhron I.; Schettler, Georg; Fomba, K.Wadinga; Müller, Konrad; Heinold, Bernd; Baars, Holger; Engelmann, Ronny; Ansmann, Albert; Nicolae, D.; Makoto, A.; Vassilis, A.; Balis, D.; Behrendt, A.; Comeron, A.; Gibert, F.; Landulfo, E.; McCormick, M.P.; Senff, C.; Veselovskii, I.; Wandinger, U.Tajikistan is often affected by atmospheric mineral dust. The direct and indirect radiative effects of dust play a sensitive role in the climate system in Central Asia. The Central Asian Dust Experiment (CADEX) provides first lidar measurements in Tajikistan. The autonomous multiwavelength polarization Raman lidar PollyXT was operated for 1.5 years (2015/16) in Dushanbe. In spring, lofted layers of long-range transported dust and in summer/ autumn, lower laying dust from local or regional sources with large optical thicknesses occurred.
- ItemA process-based evaluation of dust-emitting winds in the CMIP5 simulation of HadGEM2-ES(Heidelberg : Springer, 2016) Fiedler, Stephanie; Knippertz, Peter; Woodward, Stephanie; Martin, Gill M.; Bellouin, Nicolas; Ross, Andrew N.; Heinold, Bernd; Schepanski, Kerstin; Birch, Cathryn E.; Tegen, InaDespite the importance of dust aerosol in the Earth system, state-of-the-art models show a large variety for North African dust emission. This study presents a systematic evaluation of dust emitting-winds in 30 years of the historical model simulation with the UK Met Office Earth-system model HadGEM2-ES for the Coupled Model Intercomparison Project Phase 5. Isolating the effect of winds on dust emission and using an automated detection for nocturnal low-level jets (NLLJs) allow an in-depth evaluation of the model performance for dust emission from a meteorological perspective. The findings highlight that NLLJs are a key driver for dust emission in HadGEM2-ES in terms of occurrence frequency and strength. The annually and spatially averaged occurrence frequency of NLLJs is similar in HadGEM2-ES and ERA-Interim from the European Centre for Medium-Range Weather Forecasts. Compared to ERA-Interim, a stronger pressure ridge over northern Africa in winter and the southward displaced heat low in summer result in differences in location and strength of NLLJs. Particularly the larger geostrophic winds associated with the stronger ridge have a strengthening effect on NLLJs over parts of West Africa in winter. Stronger NLLJs in summer may rather result from an artificially increased mixing coefficient under stable stratification that is weaker in HadGEM2-ES. NLLJs in the Bodélé Depression are affected by stronger synoptic-scale pressure gradients in HadGEM2-ES. Wintertime geostrophic winds can even be so strong that the associated vertical wind shear prevents the formation of NLLJs. These results call for further model improvements in the synoptic-scale dynamics and the physical parametrization of the nocturnal stable boundary layer to better represent dust-emitting processes in the atmospheric model. The new approach could be used for identifying systematic behavior in other models with respect to meteorological processes for dust emission. This would help to improve dust emission simulations and contribute to decreasing the currently large uncertainty in climate change projections with respect to dust aerosol.
- ItemDust impacts on radiative effects of black carbon aerosol in Central Asia(Les Ulis : EDP Sciences, 2019) Tegen, Ina; Heinold, BerndThe radiative effect of mineral dust and black carbon aerosol are investigated with aerosolclimate model simulations with fixed sea surface temperatures as boundary condition. The semi-direct effects of the absorbing aerosol are assessed as the residual between the total direct radiative effect and the instantaneous direct radiative effect of the aerosol species. For Central Asia the presence of mineral dust aerosol below a black carbon aerosol layer enhances the positive radiative effect of the black carbon aerosol. © 2019 The Authors, published by EDP Sciences.
- ItemModelling mineral dust in the Central Asian region(Les Ulis : EDP Sciences, 2019) Heinold, Bernd; Tegen, InaIn Central Asia, climate and air quality are largely affected by local and long-travelled mineral dust. For the last century, the area has experienced severe land-use changes and water exploitation producing new dust sources. Today global warming causes rapid shrinking of mountain glaciers with yet unknow consequences for dust and its climate effects. Despite the importance for a growing population, only little is known about sources, transport pathways and properties of Central Asian dust. A transport study with a global aerosol-climate model is undertaken to investigate the life cycle of mineral dust in Central Asia for the period of a remote-sensing campaign in Tajikistan in 2015-2016. An initial evaluation with sun photometer measurements shows reasonable agreement for the average amount of dust, but a significant weakness of the model in reproducing the seasonality of local dust with maximum activity in summer. Source apportionment reveals a major contribution from Arabia throughout the year in accordance with observations. In the model, local sources mainly contribute in spring and autumn while summer-time dust production is underestimated. The results underline the importance of considering long-range transport and, locally, a detailed representation of atmospheric dynamics and surface characteristics for modelling dust in Central Asia. © 2019 The Authors, published by EDP Sciences.
- ItemImplementation of aerosol-cloud interactions in the regional atmosphere-aerosol model COSMO-Muscat(5.0) and evaluation using satellite data(Katlenburg-Lindau : Copernicus, 2017) Dipu, Sudhakar; Quaas, Johannes; Wolke, Ralf; Stoll, Jens; Mühlbauer, Andreas; Sourdeval, Odran; Salzmann, Marc; Heinold, Bernd; Tegen, InaThe regional atmospheric model Consortium for Small-scale Modeling (COSMO) coupled to the Multi-Scale Chemistry Aerosol Transport model (Muscat) is extended in this work to represent aerosol-cloud interactions. Previously, only one-way interactions (scavenging of aerosol and in-cloud chemistry) and aerosol-radiation interactions were included in this model. The new version allows for a microphysical aerosol effect on clouds. For this, we use the optional two-moment cloud microphysical scheme in COSMO and the online-computed aerosol information for cloud condensation nuclei concentrations (Cccn), replacing the constant Cccn profile. In the radiation scheme, we have implemented a droplet-size-dependent cloud optical depth, allowing now for aerosol-cloud-radiation interactions. To evaluate the models with satellite data, the Cloud Feedback Model Intercomparison Project Observation Simulator Package (COSP) has been implemented. A case study has been carried out to understand the effects of the modifications, where the modified modeling system is applied over the European domain with a horizontal resolution of 0.25°g × g0.25°. To reduce the complexity in aerosol-cloud interactions, only warm-phase clouds are considered. We found that the online-coupled aerosol introduces significant changes for some cloud microphysical properties. The cloud effective radius shows an increase of 9.5g%, and the cloud droplet number concentration is reduced by 21.5g%.
- ItemLong-term profiling of mineral dust and pollution aerosol with multiwavelength polarization Raman lidar at the Central Asian site of Dushanbe, Tajikistan: Case studies(München : European Geopyhsical Union, 2017) Hofer, Julian; Althausen, Dietrich; Abdullaev, Sabur F.; Makhmudov, Abduvosit N.; Nazarov, Bakhron I.; Schettler, Georg; Engelmann, Ronny; Baars, Holger; Fomba, K.Wadinga; Müller, Konrad; Heinold, Bernd; Kandler, Konrad; Ansmann, AlbertFor the first time, continuous vertically resolved aerosol measurements were performed by lidar in Tajikistan, Central Asia. Observations with the multiwavelength polarization Raman lidar PollyXT were conducted during CADEX (Central Asian Dust EXperiment) in Dushanbe, Tajikistan, from March 2015 to August 2016. Co-located with the lidar, a sun photometer was also operated. The goal of CADEX is to provide an unprecedented data set on vertically resolved aerosol optical properties in Central Asia, an area highly affected by climate change but largely missing vertically resolved aerosol measurements. During the 18-month measurement campaign, mineral dust was detected frequently from ground to the cirrus level height. In this study, an overview of the measurement period is given and four typical but different example measurement cases are discussed in detail. Three of them are dust cases and one is a contrasting pollution aerosol case. Vertical profiles of the measured optical properties and the calculated dust and non-dust mass concentrations are presented. Dust source regions were identified by means of backward trajectory analyses. A lofted layer of Middle Eastern dust with an aerosol optical thickness (AOT) of 0.4 and an extinction-related Ångström exponent of 0.41 was measured. In comparison, two near-ground dust cases have Central Asian sources. One is an extreme dust event with an AOT of 1.5 and Ångström exponent of 0.12 and the other one is a most extreme dust event with an AOT of above 4 (measured by sun photometer) and an Ångström exponent of −0.08. The observed lidar ratios (and particle linear depolarization ratios) in the presented dust cases range from 40.3 to 46.9sr (and 0.18–0.29) at 355nm and from 35.7 to 42.9sr (0.31–0.35) at 532nm wavelength. The particle linear depolarization ratios indicate almost unpolluted dust in the case of a lofted dust layer and pure dust in the near-ground dust cases. The lidar ratio values are lower than typical lidar ratio values for Saharan dust (50–60sr) and comparable to Middle Eastern or west-Asian dust lidar ratios (35–45sr). In contrast, the presented case of pollution aerosol of local origin has an Ångström exponent of 2.07 and a lidar ratio (particle linear depolarization ratio) of 55.8sr (0.03) at 355nm and 32.8sr (0.08) at 532nm wavelength.
- ItemHarmattan, Saharan heat low, and West African monsoon circulation: modulations on the Saharan dust outflow towards the North Atlantic(München : European Geopyhsical Union, 2017) Schepanski, Kerstin; Heinold, Bernd; Tegen, InaThe outflow of dust from the northern African continent towards the North Atlantic is stimulated by the atmospheric circulation over North Africa, which modulates the spatio-temporal distribution of dust source activation and consequently the entrainment of mineral dust into the boundary layer, as well as the transport of dust out of the source regions. The atmospheric circulation over the North African dust source regions, predominantly the Sahara and the Sahel, is characterized by three major circulation regimes: (1) the harmattan (trade winds), (2) the Saharan heat low (SHL), and (3) the West African monsoon circulation. The strength of the individual regimes controls the Saharan dust outflow by affecting the spatio-temporal distribution of dust emission, transport pathways, and deposition fluxes. This study aims at investigating the atmospheric circulation pattern over North Africa with regard to its role favouring dust emission and dust export towards the tropical North Atlantic. The focus of the study is on summer 2013 (June to August), during which the SALTRACE (Saharan Aerosol Long-range TRansport and Aerosol-Cloud interaction Experiment) field campaign also took place. It involves satellite observations by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) flying on board the geostationary Meteosat Second Generation (MSG) satellite, which are analysed and used to infer a data set of active dust sources. The spatio-temporal distribution of dust source activation frequencies (DSAFs) allows for linking the diurnal cycle of dust source activations to dominant meteorological controls on dust emission. In summer, Saharan dust source activations clearly differ from dust source activations over the Sahel regarding the time of day when dust emission begins. The Sahara is dominated by morning dust source activations predominantly driven by the breakdown of the nocturnal low-level jet. In contrast, dust source activations in the Sahel are predominantly activated during the second half of the day, when downdrafts associated with deep moist convection are the major atmospheric driver. Complementary to the satellite-based analysis on dust source activations and implications from their diurnal cycle, simulations on atmosphere and dust life cycle were performed using the mesoscale atmosphere–dust model system COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model). Fields from this simulation were analysed regarding the variability of the harmattan, the Saharan heat low, and the monsoon circulation as well as their impact on the variability of the Saharan dust outflow towards the North Atlantic. This study illustrates the complexity of the interaction among the three major circulation regimes and their modulation of the North African dust outflow. Enhanced westward dust fluxes frequently appear following a phase characterized by a deep SHL. Ultimately, findings from this study contribute to the quantification of the interannual variability of the atmospheric dust burden.