ALBERT

Description: The MIPAS instrument onboard the ESA Envisat satellite operated from July 2002 until April 2012. The infrared limb emission measurements represent a unique dataset of day and night observations of polar stratospheric clouds (PSCs) up to both poles. Cloud detection sensitivity is comparable to spaceborne lidars, and it is possible to classify different cloud types from the spectral measurements in different atmospheric windows regions. Here we present a new PSC classification scheme based on the combination of a well-established two-colour ratio method and multiple 2D brightness temperature difference probability density functions. The method is a simple probabilistic classifier based on Bayes' theorem with a strong independence assumption. The method has been tested in conjunction with a database of radiative transfer model calculations of realistic PSC particle size distributions, geometries, and composition. The Bayesian classifier distinguishes between solid particles of ice and nitric acid trihydrate (NAT), as well as liquid droplets of super-cooled ternary solution (STS). The classification results are compared to coincident measurements from the space borne lidar CALIOP instrument over the temporal overlap of both satellite missions (June 2006 to March 2012). Both datasets show a good agreement for the specific PSC classes, although the viewing geometries, vertical and horizontal resolution are quite different. Discrepancies are observed for the MIPAS ice class. The Bayesian classifier for MIPAS identifies substantially more ice clouds in the southern hemisphere polar vortex than CALIOP. This disagreement is attributed in parts to the difference in the sensitivity on mixed-type clouds. Ice seems to dominate the spectral behaviour in the limb infrared spectra and may cause an overestimation in ice occurrence compared to the real fraction of ice within the PSC area in the polar vortex. The entire MIPAS measurement period was processed with the new classification approach. Examples like the detection of the Antarctic NAT belt during early winter, and its possible link to mountain wave events over the Antarctic Peninsula, which are observed by the AIRS instrument, are highlighting the importance of a climatology of in total 9 southern and 10 northern hemisphere winters. The new dataset is valuable both for detailed process studies, and for comparisons with and improvements of the PSC parameterisations used in chemistry transport and climate models.

Description: Altitude resolved aerosol detection in the upper troposphere and lower stratosphere (UTLS) is a challenging task for remote sensing instruments. Here, we introduce a new method for detecting aerosol in the UTLS based on infrared limb emission measurements. The method applies an improved aerosol-cloud-index that indicates infrared limb spectra affected by aerosol and ice clouds. For the discrimination between aerosol and ice clouds we developed a new method based on brightness temperature difference correlations. The discrimination thresholds for the new method were derived from radiative transfer simulations (including scattering) and Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)/Envisat measurements obtained in 2011. The method not only reliably separates aerosol from ice clouds, but also provides characteristic yet overlapping correlation patterns for volcanic ash and sulfate aerosol. We demonstrate the value of the new approach for volcanic ash and sulfate aerosol originating from the Grímsvötn (Iceland), Puyehue-Cordón Caulle (Chile) and Nabro (Eritrea) eruptions by comparing with Atmospheric Infrared Sounder (AIRS) volcanic ash and SO2 measurements.

Description: The critical fracture toughness is a material parameter describing the resistance of a cracked body to further crack extension. It is an important parameter for simulating and predicting the breakup behavior of ice shelves from the calving of single icebergs to the disintegration of entire ice shelves over a wide range of length scales. The fracture toughness values are calculated with equations that are derived from an elastic stress analysis. Additionally, an X-ray computer tomography (CT scanner) was used to identify the density as a function of depth. The critical fracture toughness of 91 Antarctic bubbly ice samples with densities between 840 and 870 kg m−3 has been determined by applying a four-point bending technique on single-edge v-notched beam samples. The examined ice core was drilled 70 m north of Kohnen Station, Dronnning Maud Land (75°00' S, 00°04' E; 2882 m). Supplementary data are available at doi:10.1594/PANGAEA.835321.

Description: This paper presents a set of observations by the CRISTA infrared limb sounder in low-earth orbit taken in August 1997 and analyses of trace-gases in the Asian Summer Monsoon (ASM) region. The spatially highly-resolved measurements of peroxyacetyl nitrate (PAN) and O3 allow a detailed analysis of an eddy-shedding event of the ASM anticyclone.We identify enhanced PAN volume mixing ratios (VMRs) within the main anticyclone and within the eddy, which are suitable as a tracer for polluted air within the ASM originating in India and China. Comparing the retrieved PAN VMRs with potential vorticity (PV) on isentropes reveals that the PAN VMRs exhibit the strongest decrease at each isentrope for an increasing value of PV which may be used to identify the extent of the ASM. Using temperature values also derived from CRISTA measurements, we also computed the location of the thermal tropopause according to the WMO criterion and find that its location agrees well with the limits of the area of increased PAN VMRs both horizontally on isentropes and vertically within the anticyclone. In contrast, the shed eddy exhibits enhanced PAN VMRs for 1 to 2 km above the thermal tropopause. Using the relationship between PAN as a tropospheric tracer and O3 as a stratospheric tracer to identify mixed air parcels, we further found the anticyclone to contain few such parcels, whereas the region in between the anticyclone and the eddy contains many mixed parcels. In combination, this implies that while the anticyclone confines polluted air masses well, eddy shedding provides a very rapid horizontal transport pathway of Asian pollution into the extratropical lowermost stratosphere with a time scale of only a few days.

Description: The Asian summer monsoon provides an important pathway of tropospheric source gases and pollution into the lower stratosphere. This transport is characterized by deep convection and steady upwelling, combined with confinement inside a large-scale anticyclonic circulation in the upper troposphere and lower stratosphere (UTLS). In this paper, we show that a barrier to horizontal transport along the 380 K isentrope in the monsoon anticyclone can be determined from the potential vorticity (PV) field, following the polar vortex criterion by Nash et al. (1996). Due to large dynamic variability of the anticyclone, the corresponding maximum in the PV gradient is weak and additional constraints are needed (e.g., time averaging). Notwithstanding, PV contours in the monsoon anticyclone agree well with contours of trace gas mixing ratios (CO, O3) and mean age from model simulations with a Lagrangian chemistry transport model (CLaMS) and MLS satellite observations. Hence, the PV-based transport barrier reflects the separation between air inside the anticyclone core and the background atmosphere well. For the summer season 2011 we find an average PV value of 3.6 PVU for the transport barrier in the anticyclone on the 380 K isentrope.

Description: The representation of vertical velocity in chemistry climate models is a key element for the representation of the large scale Brewer–Dobson-Circulation in the stratosphere. Here, we diagnose and compare the kinematic and diabatic vertical velocities in the ECHAM/Messy Atmospheric Chemistry (EMAC) model. The calculation of kinematic vertical velocity is based on the continuity equation, whereas diabatic vertical velocity is computed using diabatic heating rates. Annual and monthly zonal mean climatologies of vertical velocity from a 10 year simulation are provided for both, kinematic and diabatic vertical velocity representations. In general, both vertical velocity patterns show the main features of the stratospheric circulation, namely upwelling at low latitudes and downwelling at high latitudes. The main difference in the vertical velocity pattern is a more uniform structure for diabatic and a noisier structure for kinematic vertical velocity. Diabatic vertical velocities show higher absolute values both in the upwelling branch in the inner tropics and in the downwelling regions in the polar vortices. Further, there is a latitudinal shift of the tropical upwelling branch in boreal summer between the two vertical velocity representations with the tropical upwelling region in the diabatic representation shifted southward compared to the kinematic case. Furthermore, we present mean age of air climatologies from two transport schemes in EMAC using these different vertical velocities. The age of air distributions show a hemispheric difference pattern in the stratosphere with younger air in the Southern Hemisphere and older air in the Northern Hemisphere using the transport scheme with diabatic vertical velocities. Further, the age of air climatology from the transport scheme using diabatic vertical velocities shows younger mean age of air in the inner tropical upwelling branch and older mean age in the extratopical tropopause region.

Description: The impact of different boundary layer source regions in Asia on the chemical composition of the Asian monsoon anticyclone, considering its intraseasonal variability in 2012, is analysed by CLaMS simulations using artificial emission tracers. Our simulations show that the Asian monsoon anticyclone is highly variable in location and shape and oscillates between 2 states: first a symmetric anticyclone and second, an asymmetric anticyclone either elongated or split in two smaller anticyclones. A maximum in the distribution of air originating from Indian/Chinese boundary layer sources is usually found in the core of the symmetric anticyclone, in contrast the asymmetric state is characterised by a double peak structure in the horizontal distribution of air originating from India and China. The simulated horizontal distribution of artificial emission tracers for India/China is in agreement with patterns found in satellite measurements of O3 and CO by the Aura Microwave Limb Sounder (MLS). The contribution of different boundary source regions to the Asian monsoon anticyclone strongly depends on its intraseasonal variability and is therefore more complex than hitherto believed, but in general the highest contributions are from North India and Southeast Asia at 380 K. In the early (June to mid-July) and late (mid-August to October) period of the monsoon 2012, contributions of emissions from Southeast Asia are highest and in the intervening period (≈ mid-July to mid-August) emissions from North India have the largest impact. Further, our simulations confirm that the thermal tropopause above the anticyclone constitutes a vertical transport barrier. Enhanced contributions of emission tracers for Asia are found at the northern flank of the Asian monsoon anticyclone between double tropopauses indicating an isentropic transport from the anticyclone into the lowermost stratosphere. After the breakup of the anticyclone, significant contributions of air masses originating in India/China are sill found over Asia in September/October. In addition, these air masses spread out within the mid-latitudes of the Northern Hemisphere and in the tropics at around 380 K. Moreover, air masses from Southeast Asia experienced diabatic upward transport in the tropics and subsequently isentropic poleward transport occurs at around 380 K with the result that the extratropical lowermost stratosphere is flooded by end of September with air masses originating in Southeast Asia. Our results demonstrate that emissions from Asia have a significant impact on the chemical compositions of the lowermost stratosphere of the Northern Hemisphere in particular after the end of the monsoon season in September/October 2012.

Description: In a case study of a remarkable Major stratospheric sudden Warming (MW) during the boreal winter 2008/09, we investigate how transport and mixing triggered by this event affect the composition of the whole stratosphere in the Northern Hemisphere. We simulate this event with the Chemical Lagrangian Model of the Stratosphere (CLaMS), with optimized mixing parameters and with no mixing, i.e. with transport occurring only along the Lagrangian trajectories. The results are investigated by using the tracer–tracer correlation technique and by applying the Transformed Eulerian Mean formalism. The CLaMS simulation of N2O and O3 with optimized mixing parameters shows good agreement with the Aura Microwave Limb Sounder (MLS) data. The spatial distribution of mixing intensity in CLaMS correlates fairly well with the Eliassen–Palm flux convergence and illustrates how planetary waves drive mixing. By comparing the simulations with and without mixing, we find that after the MW poleward transport of air increases not only across the vortex edge but also across the subtropical transport barrier. Moreover, the MW event also accelerates polar descent and tropical ascent of the Brewer–Dobson circulation. The accelerated ascent in the tropics and descent at high latitudes firstly occurs in the upper stratosphere and then propagates downward to the lower stratosphere. This downward propagation takes over one month from the potential temperature level of 1000 to 400 K.

Description: Fungi are ubiquitous in the atmosphere and may play an important role in atmospheric processes. We investigated the composition and diversity of fungal communities over the Amazon rainforest canopy and compared these communities to fungal communities found in terrestrial environments. We characterized the total fungal community and the metabolically active portion of the community using high-throughout DNA and RNA sequencing and compared these data to predictions generated by a mass-balance model. We found that the total community was primarily comprised of fungi from the phylum Basidiomycota. In contrast, the active community was primarily composed of members of the phylum Ascomycota and included a high relative abundance of lichen fungi, which were not detected in the total community. The relative abundance of Basidiomycota and Ascomycota in the total and active communities was consistent with our model predictions, suggesting that this result was driven by the relative size and number of spores produced by these groups. When compared to other environments, fungal communities in the atmosphere were most similar to communities found in tropical soils and leaf surfaces, suggesting that inputs of fungi to the atmosphere are from local, rather than distant, sources. Our results demonstrate that there are significant differences in the composition of the total and active fungal communities in the atmosphere, and that lichen fungi, which have been shown to be efficient ice nucleators, may be abundant members of active atmospheric fungal communities over the forest canopy.

Description: Fungi are ubiquitous in the atmosphere and may play an important role in atmospheric processes. We investigated the composition and diversity of fungal communities over the Amazon rainforest canopy and compared these communities to fungal communities found in terrestrial environments. We characterized the total fungal community and the metabolically active portion of the community using high-throughput DNA and RNA sequencing and compared these data to predictions generated by a mass-balance model. We found that the total community was primarily comprised of fungi from the phylum Basidiomycota. In contrast, the active community was primarily composed of members of the phylum Ascomycota and included a high relative abundance of lichen fungi, which were not detected in the total community. The relative abundance of Basidiomycota and Ascomycota in the total and active communities was consistent with our model predictions, suggesting that this result was driven by the relative size and number of spores produced by these groups. When compared to other environments, fungal communities in the atmosphere were most similar to communities found in tropical soils and leaf surfaces. Our results demonstrate that there are significant differences in the composition of the total and active fungal communities in the atmosphere, and that lichen fungi, which have been shown to be efficient ice nucleators, may be abundant members of active atmospheric fungal communities over the forest canopy.