Search

Mechanisms behind the phenomenon of Arctic amplification are widely discussed. To contribute to this debate, the (AC)3 project was established in 2016 (www.ac3-tr.de/). It comprises modeling and data analysis efforts as well as observational elements. The project has assembled a wealth of ground-based, airborne, shipborne, and satellite data of physical, chemical, and meteorological properties of the Arctic atmosphere, cryosphere, and upper ocean that are available for the Arctic climate research community. Short-term changes and indications of long-term trends in Arctic climate parameters have been detected using existing and new data. For example, a distinct atmospheric moistening, an increase of regional storm activities, an amplified winter warming in the Svalbard and North Pole regions, and a decrease of sea ice thickness in the Fram Strait and of snow depth on sea ice have been identified. A positive trend of tropospheric bromine monoxide (BrO) column densities during polar spring was verified. Local marine/biogenic sources for cloud condensation nuclei and ice nucleating particles were found. Atmospheric–ocean and radiative transfer models were advanced by applying new parameterizations of surface albedo, cloud droplet activation, convective plumes and related processes over leads, and turbulent transfer coefficients for stable surface layers. Four modes of the surface radiative energy budget were explored and reproduced by simulations. To advance the future synthesis of the results, cross-cutting activities are being developed aiming to answer key questions in four focus areas: lapse rate feedback, surface processes, Arctic mixed-phase clouds, and airmass transport and transformation.

The Hamburg Aerosol Module version 2.3 (HAM2.3) from the ECHAM6.3-HAM2.3 global atmosphere-aerosol model is coupled to the recently developed icosahedral nonhydrostatic ICON-A (icon-aes-1.3.00) global atmosphere model to yield the new ICON-A-HAM2.3 atmosphere-aerosol model. The ICON-A and ECHAM6.3 host models use different dynamical cores, parameterizations of vertical mixing due to sub-grid scale turbulence, and parameter settings for radiation balance tuning. Here, we study the role of the different host models for simulated aerosol optical thickness (AOT) and evaluate impacts of using HAM2.3 and the ECHAM6-HAM2.3 two-moment cloud microphysics scheme on several meteorological variables. Sensitivity runs show that a positive AOT bias over the subtropical oceans is remedied in ICON-A-HAM2.3 because of a different default setting of a parameter in the moist convection parameterization of the host models. The global mean AOT is biased low compared to MODIS satellite instrument retrievals in ICON-A-HAM2.3 and ECHAM6.3-HAM2.3, but the bias is larger in ICON-A-HAM2.3 because negative AOT biases over the Amazon, the African rain forest, and the northern Indian Ocean are no longer compensated by high biases over the sub-tropical oceans. ICON-A-HAM2.3 shows a moderate improvement with respect to AOT observations at AERONET sites. A multivariable bias score combining biases of several meteorological variables into a single number is larger in ICON-A-HAM2.3 compared to standard ICON-A and standard ECHAM6.3. In the tropics, this multivariable bias is of similar magnitude in ICON-A-HAM2.3 and in ECHAM6.3-HAM2.3. In the extra-tropics, a smaller multivariable bias is found for ICON-A-HAM2.3 than for ECHAM6.3-HAM2.3., Journal of Advances in Modeling Earth Systems, 14 (4), ISSN:1942-2466

We introduce and evaluate the aerosol simulations with the global aerosol-climate model ECHAM6.3-HAM2.3, which is the aerosol component of the fully coupled aerosol-chemistry-climate model ECHAM-HAMMOZ. Both the host atmospheric climate model ECHAM6.3 and the aerosol model HAM2.3 were updated from previous versions. The updated version of the HAM aerosol model contains improved parameterizations of aerosol processes such as cloud activation, as well as updated emission fields for anthropogenic aerosol species and modifications in the online computation of sea salt and mineral dust aerosol emissions. Aerosol results from nudged and free running simulations for the 10-year period 2003 to 2012 are compared to various measurements of aerosol properties. While there are regional deviations between model and observations, the model performs well overall in terms of aerosol optical thickness, but may underestimate coarse mode aerosol concentrations to some extent, so that the modeled particles are smaller than indicated by the observations. Sulfate aerosol measurements in the US and Europe are reproduced well by the model, while carbonaceous aerosol species are biased low. Both mineral dust and sea salt aerosol concentrations are improved compared to previous versions of ECHAM-HAM. The evaluation of the simulated aerosol distributions serves as a basis for the suitability of the model for simulating aerosol-climate interactions in a changing climate.

Ice nucleating particles (INPs) increase the temperature at which supercooled droplets start to freeze. They are therefore of particular interest in mixed-phase cloud temperature regimes, where supercooled liquid droplets can persist for extended periods of time in the absence of INPs. When INPs are introduced to such an environment, the cloud can quickly glaciate following ice multiplication processes and the Wegener-Bergeron-Findeisen (WBF) process. The WBF process can also cause the ice to grow to precipitation size and precipitate out. All of these processes alter the radiative properties.Despite their potential influence on climate, the ice nucleation ability and importance of different aerosol species is still not well understood and is a field of active research. In this study, we use the aerosol-climate model ECHAM6-HAM2 to examine the global relevance of marine organic aerosol (MOA), which has drawn much interest in recent years as a potentially important INPs in remote marine regions. We address the uncertainties in emissions and ice nucleation activity of MOA with a range of reasonable set-ups and find a wide range of resulting MOA burdens. The relative importance of MOA as an INP compared to dust is investigated and found to depend strongly on the type of ice nucleation parameterisation scheme chosen. On the zonal mean, freezing due to MOA leads to relative increases in the cloud ice occurrence and in-cloud number concentration close to the surface in the polar regions during summer. Slight but consistent decreases in the in-cloud ice crystal effective radius can also be observed over the same regions during all seasons. Regardless, MOA was not found to affect the radiative balance significantly on the global scale, due to its relatively weak ice activity and a low sensitivity of cloud ice properties to heterogeneous ice nucleation in our model..

[1] This study presents the first climatology for the dust emission amount associated with Nocturnal Low-Level Jets (NLLJs) in North Africa. These wind speed maxima near the top of the nocturnal boundary layer can generate near-surface peak winds due to shear-driven turbulence in the course of the night and the NLLJ breakdown during the following morning. The associated increase in the near-surface wind speed is a driver for mineral dust emission. A new detection algorithm for NLLJs is presented and used for a statistical assessment of NLLJs in 32 years of ERA-Interim reanalysis from the European Centre for Medium-Range Weather Forecasts. NLLJs occur in 29% of the nights in the annual and spatial mean. The NLLJ climatology shows a distinct annual cycle with marked regional differences. Maxima of up to 80% NLLJ frequency are found where low-level baroclinicity and orographic channels cause favorable conditions, e.g., over the Bodélé Depression, Chad, for November–February and along the West Saharan and Mauritanian coast for April–September. Downward mixing of NLLJ momentum to the surface causes 15% of mineral dust emission in the annual and spatial mean and can be associated with up to 60% of the total dust amount in specific areas, e.g., the Bodélé Depression and south of the Hoggar-Tibesti Channel. The sharp diurnal cycle underlines the importance of using wind speed information with high temporal resolution as driving fields for dust emission models. Citation: Fiedler, S., K. Schepanski, B. Heinold, P. Knippertz, and I. Tegen (2013), Climatology of nocturnal low-level jets over North Africa and implications for modeling mineral dust emission, J. Geophys. Res. Atmos., 118, 6100-6121, doi:10.1002/jgrd.50394

We present a study of Saharan dust export towards the tropical North Atlantic using the regional dust emission, transport and deposition model LM-MUSCAT. Horizontal and vertical distribution of dust optical thickness, concentration, and dry and wet deposition rates are used to describe seasonality of dust export and deposition towards the eastern Atlantic for three exemplary months in different seasons. Deposition rates strongly depend on the vertical dust distribution, which differs with seasons. Furthermore the contribution of dust originating from the Bodélé Depression to Saharan dust over the Atlantic is investigated. A maximum contribution of Bodélé dust transported towards the Cape Verde Islands is evident in winter when the Bodélé source area is most active and dominant with regard activation frequency and dust emission. Limitations of using satellite retrievals to estimate dust deposition are highlighted.

The spatio-temporal evolution of the Saharan dust and biomass-burning plume during the SAMUM-2 field campaign in January and February 2008 is simulated at 28 km horizontal resolution with the regional model-system COSMOMUSCAT. The model performance is thoroughly tested using routine ground-based and space-borne remote sensing and local field measurements. Good agreement with the observations is found in many cases regarding transport patterns, aerosol optical thicknesses and the ratio of dust to smoke aerosol. The model also captures major features of the complex aerosol layering. Nevertheless, discrepancies in the modelled aerosol distribution occur, which are analysed in detail. The dry synoptic dynamics controlling dust uplift and transport during the dry season are well described by the model, but surface wind peaks associated with the breakdown of nocturnal low-level jets are not always reproduced. Thus, a strong dust outbreak is underestimated. While dust emission modelling is a priori more challenging, since strength and placement of dust sources depend on on-line computed winds, considerable inaccuracies also arise in observation-based estimates of biomass-burning emissions. They are caused by cloud and spatial errors of satellite fire products and uncertainties in fire emission parameters, and can lead to unrealistic model results of smoke transport.

Current estimates of global dust emission vary by over a factor of two. Here, we use multiple data types and a worldwide array of stations combined with a dust model to constrain the magnitude of the global dust cycle for particles with radii between 0.1 and 8 μm. An optimal value of global emission is calculated by minimizing the difference between the model dust distribution and observations. The optimal global emission is most sensitive to the prescription of the dust source region. Depending upon the assumed source, the agreement with observations is greatest for global, annual emission ranging from 1500 to 2600 Tg. However, global annual emission between 1000 and 3000 Tg remains in agreement with the observations, given small changes in the method of optimization. Both ranges include values that are substantially larger than calculated by current dust models. In contrast, the optimal fraction of clay particles (whose radii are less than 1 μm) is lower than current model estimates. The optimal solution identified by a combination of data sets is different from that identified by any single data set and is more robust. Uncertainty is introduced into the optimal emission by model biases and the uncertain contribution of other aerosol species to the observations.

For absorbing aerosols like soil (or "mineral'') dust, radiative forcing at the surface differs substantially from the value at the top of the atmosphere (TOA). The climate response depends not only upon the TOA forcing, but its difference with respect to the surface value, which represents radiative heating within the atmosphere. Surface forcing alters evaporation and the hydrologic cycle, which feeds back upon the aerosol burden through the efficiency of wet deposition. We calculate the surface forcing by soil dust aerosols and its global sensitivity by varying aspects of the dust distribution that are poorly constrained by observations. Ignorance of the global dust burden corresponds to a forcing uncertainty of over a factor of two, with smaller uncertainties due to imprecise knowledge of particle optical properties and the particle size distribution. While global evaporation and precipitation are reduced in response to surface radiative forcing by dust, precipitation increases locally over desert regions, so that dust emission can act as a negative feedback to desertification. The effect of the global reduction in precipitation is to lengthen the particle lifetime by reducing the efficiency of wet deposition, representing a positive feedback upon the global dust burden. For the current climate, we calculate the reduction in wet deposition by dust radiative forcing and find that the aerosol burden is increased only modestly. However, the global dust burden and associated radiative forcing are substantially higher during glacial climates, so that the amplification of the dust load by this feedback is larger. By extrapolating from its radiative forcing in the current climate, we estimate that dust reduces precipitation during glacial times by as much as half the reduction due to the colder climate alone. [References: 109]

1

[1] We use the observed variations in ocean surface chlorophyll, temperature and height caused by the Antarctic Circumpolar Wave (ACW) as a natural laboratory to determine how marine biology responds to changes in ocean stratification in the Southern Ocean. Interannual variations of surface chlorophyll (+/- 5%) observed by SeaWiFS satellite during 1997- 2001 vary in phase over the entire Southern Ocean in spite of large east-west dipoles in ocean dynamics. We suggest that this behavior is due to the regional predominance of light versus nutrient limitation over the most productive regions of the Southern Ocean.

We present a model of the dust cycle that successfully predicts dust emissions as determined by land surface properties, monthly vegetation and snow cover, and 6-hourly surface wind speeds for the years 1982-1993. The model takes account of the role of dry lake beds as preferential source areas for dust emission. The occurrence of these preferential sources is determined by a water routing and storage model. The dust source scheme also explicitly takes into account the role of vegetation type as well as monthly vegetation cover. Dust transport is computed using assimilated winds for the years 1987-1990. Deposition of dust occurs through dry and wet deposition, where subcloud scavenging is calculated using assimilated precipitation fields. Comparison of simulated patterns of atmospheric dust loading with the Total Ozone Mapping Spectrometer satellite absorbing aerosol index shows that the model produces realistic results from daily to interannual timescales. The magnitude of dust deposition agrees well with sediment flux data from marine sites. Emission of submicron dust from preferential source areas are required for the computation of a realistic dust optical thickness. Sensitivity studies show that Asian dust source strengths are particularly sensitive to the seasonality of vegetation cover.

Despite 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.

[1] The degree to which dust emissions are controlled by vegetation cover and geomorphic setting (specifically closed topographic depressions) was investigated using dust storm frequency (DSF) data based on visibility measurements from >2400 meteorological stations worldwide. Comparisons with distributions of vegetation types suggest that DSF is highest in desert/bare ground (median: 60-80 d/yr) and shrubland (median: 20-30 d/yr) regions, and comparatively low in grassland regions (median: 2-4 d/yr). Average DSF is inversely correlated with leaf area index (an index of vegetation density) and net primary productivity. In non-forested regions, DSF increases as the fraction of closed topographic depressions increases, likely due to the accumulation of fine sediments in these areas. These findings support the importance of incorporating vegetation and geomorphic setting as explicit controls on emissions in global dust cycle models.

Aerosol can affect clouds in various ways. Beside the microphysical impact of aerosol particles on cloud formation, the interference of aerosol with atmospheric radiation leads to changes in local heating, surface fluxes and thus mesoscale circulations, all of which may also modify clouds. Rather little is known about these so‐called semi‐direct effects in realistic settings – a reason why this study investigates the impact of absorbing aerosol particles on cloud and radiation fields over Germany. Using advanced high‐resolution simulations with grid spacings of 312 and 625 m, numerical experiments with different aerosol optical properties are contrasted using purely scattering aerosol as a control case and realistic absorbing aerosol as a perturbation. The combined effect of surface dimming and atmospheric heating induces positive temperature and negative moisture anomalies between 800 and 900 hPa, impacting low‐level cloud formation. Decreased relative humidity as well as increased atmospheric stability below clouds lead to a reduction of low‐level cloud cover, liquid water path and precipitation. It is further found that direct and semi‐direct effects of absorbing aerosol forcing have similar magnitudes and contribute equally to a reduction of net radiation at the top of the atmosphere., Atmospheric aerosol particles can absorb solar radiation, altering the thermal structure of the atmosphere and surface fluxes. Using advanced high‐resolution simulations over Germany with grid spacings of 312 and 625 m, we find that boundary‐layer absorbing aerosol reduces low‐level cloud cover, liquid water path and precipitation. Direct and semi‐direct effects have similar magnitudes and contribute equally to a positive absorbing aerosol forcing., German Ministry for Education and Research EU Horizon 2020 project CONSTRAIN, https://cera-www.dkrz.de/WDCC/ui/cerasearch/entry?acronym=DKRZ_LTA_1174_ds00001