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: The photolysis rate constant of dichlorine peroxide (ClOOCl, ClO dimer) JClOOCl is a critical parameter in catalytic cycles destroying ozone (O3) in the polar stratosphere. In the atmospherically relevant wavelength region (λ 〉 310 nm), significant discrepancies between laboratory measurements of ClOOCl absorption cross sections and spectra cause a large uncertainty in JClOOCl. Previous investigations of the consistency of published JClOOCl with atmospheric observations of chlorine monoxide (ClO) and ClOOCl have focused on the photochemical equilibrium between ClOOCl formation and photolysis, and thus could only constrain the ratio of JClOOCl over the ClOOCl formation rate constant krec. Here, we constrain the atmospherically effective JClOOCl independent of krec, using ClO measured in the same air masses before and directly after sunrise during an aircraft flight that was part of the RECONCILE field campaign in the winter 2010 from Kiruna, Sweden. Over sunrise, when the ClO/ClOOCl system comes out of thermal equilibrium and the influence of the ClO recombination reaction is negligible, the increase in ClO concentrations is significantly faster than expected from JClOOCl based on the absorption spectrum proposed by Pope et al. (2007), but does not warrant cross sections larger than recently published values by Papanastasiou et al. (2009). In particular, the existence of a significant ClOOCl absorption band longwards of 420 nm is not supported by our observations. The observed night-time ClO would not be consistent with a ClO/ClOOCl thermal equilibrium constant significantly higher than the one proposed by Plenge et al. (2005).

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 international research project RECONCILE has addressed central questions regarding polar ozone depletion, with the objective to quantify some of the most relevant yet still uncertain physical and chemical processes and thereby improve prognostic modelling capabilities to realistically predict the response of the ozone layer to climate change. This overview paper outlines the scope and the general approach of RECONCILE, and it provides a summary of observations and modelling in 2010 and 2011 that have generated an in many respects unprecedented dataset to study processes in the Arctic winter stratosphere. Principally, it summarises important outcomes of RECONCILE including (i) better constraints and enhanced consistency on the set of parameters governing catalytic ozone destruction cycles, (ii) a better understanding of the role of cold binary aerosols in heterogeneous chlorine activation, (iii) an improved scheme of polar stratospheric cloud (PSC) processes that includes heterogeneous nucleation of nitric acid trihydrate (NAT) and ice on non-volatile background aerosol leading to better model parameterisations with respect to denitrification, and (iv) long transient simulations with a chemistry-climate model (CCM) updated based on the results of RECONCILE that better reproduce past ozone trends in Antarctica and are deemed to produce more reliable predictions of future ozone trends. The process studies and the global simulations conducted in RECONCILE show that in the Arctic, ozone depletion uncertainties in the chemical and microphysical processes are now clearly smaller than the sensitivity to dynamic variability.

Description: The infrared limb spectra of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board the Envisat satellite include detailed information on tropospheric clouds and polar stratospheric clouds (PSC). However, no consolidated cloud product is available for the scientific community. Here we describe a fast prototype processor for cloud parameter retrieval from MIPAS (MIPclouds). Retrieval of parameters such as cloud top height, temperature, and extinction are implemented, as well as retrieval of microphysical parameters, e.g. effective radius and the integrated quantities over the limb path (surface area density and volume density). MIPclouds classifies clouds as either liquid or ice cloud in the upper troposphere and polar stratospheric clouds types in the stratosphere based on statistical combinations of colour ratios and brightness temperature differences. Comparison of limb measurements of clouds with model results or cloud parameters from nadir looking instruments is often difficult due to different observation geometries. We therefore introduce a new concept, the limb-integrated surface area density path (ADP). By means of validation and radiative transfer calculations of realistic 2-D cloud fields as input for a blind test retrieval (BTR), we demonstrate that ADP is an extremely valuable parameter for future comparison with model data of ice water content, when applying limb integration (ray tracing) through the model fields. In addition, ADP is used for a more objective definition of detection thresholds of the applied detection methods. Based on BTR, a detection threshold of ADP = 107 μm2 cm−2 and an ice water content of 10−5 g m−3 is estimated, depending on the horizontal and vertical extent of the cloud. Intensive validation of the cloud detection methods shows that the limb-sounding MIPAS instrument has a sensitivity in detecting stratospheric and tropospheric clouds similar to that of space- and ground-based lidars, with a tendency for higher cloud top heights and consequently higher sensitivity for some of the MIPAS detection methods. For the high cloud amount (HCA, pressure levels below 440 hPa) on global scales the sensitivity of MIPAS is significantly greater than that of passive nadir viewers. This means that the high cloud fraction will be underestimated in the ISCCP dataset compared to the amount of high clouds deduced by MIPAS. Good correspondence in seasonal variability and geographical distribution of cloud occurrence and zonal means of cloud top height is found in a detailed comparison with a climatology for subvisible cirrus clouds from the Stratospheric Aerosol and Gas Experiment II (SAGE II) limb sounder. Overall, validation with various sensors shows the need to consider differences in sensitivity, and especially the viewing geometries and field-of-view size, to make the datasets comparable (e.g. applying integration along the limb path through nadir cloud fields). The simulation of the limb path integration will be an important issue for comparisons with cloud-resolving global circulation or chemical transport models.

Description: Significant reductions in stratospheric ozone occur inside the polar vortices each spring when chlorine radicals produced by heterogeneous reactions on cold particle surfaces in winter destroy ozone mainly in two catalytic cycles, the ClO dimer cycle and the ClO/BrO cycle. Chlorofluorocarbons (CFCs), which are responsible for most of the chlorine currently present in the stratosphere, have been banned by the Montreal Protocol and its amendments, and the ozone layer is predicted to recover to 1980 levels within the next few decades. During the same period, however, climate change is expected to alter the temperature, circulation patterns and chemical composition in the stratosphere, and possible geo-engineering ventures to mitigate climate change may lead to additional changes. To realistically predict the response of the ozone layer to such influences requires the correct representation of all relevant processes. The European project RECONCILE has comprehensively addressed remaining questions in the context of polar ozone depletion, with the objective to quantify the rates of some of the most relevant, yet still uncertain physical and chemical processes. To this end RECONCILE used a broad approach of laboratory experiments, two field missions in the Arctic winter 2009/10 employing the high altitude research aircraft M55-Geophysica and an extensive match ozone sonde campaign, as well as microphysical and chemical transport modelling and data assimilation. Some of the main outcomes of RECONCILE are as follows: (1) vortex meteorology: the 2009/10 Arctic winter was unusually cold at stratospheric levels during the six-week period from mid-December 2009 until the end of January 2010, with reduced transport and mixing across the polar vortex edge; polar vortex stability and how it is influenced by dynamic processes in the troposphere has led to unprecedented, synoptic-scale stratospheric regions with temperatures below the frost point; in these regions stratospheric ice clouds have been observed, extending over 〉106km2 during more than 3 weeks. (2) Particle microphysics: heterogeneous nucleation of nitric acid trihydrate (NAT) particles in the absence of ice has been unambiguously demonstrated; conversely, the synoptic scale ice clouds also appear to nucleate heterogeneously; a variety of possible heterogeneous nuclei has been characterised by chemical analysis of the non-volatile fraction of the background aerosol; substantial formation of solid particles and denitrification via their sedimentation has been observed and model parameterizations have been improved. (3) Chemistry: strong evidence has been found for significant chlorine activation not only on polar stratospheric clouds (PSCs) but also on cold binary aerosol; laboratory experiments and field data on the ClOOCl photolysis rate and other kinetic parameters have been shown to be consistent with an adequate degree of certainty; no evidence has been found that would support the existence of yet unknown chemical mechanisms making a significant contribution to polar ozone loss. (4) Global modelling: results from process studies have been implemented in a prognostic chemistry climate model (CCM); simulations with improved parameterisations of processes relevant for polar ozone depletion are evaluated against satellite data and other long term records using data assimilation and detrended fluctuation analysis. Finally, measurements and process studies within RECONCILE were also applied to the winter 2010/11, when special meteorological conditions led to the highest chemical ozone loss ever observed in the Arctic. In addition to quantifying the 2010/11 ozone loss and to understand its causes including possible connections to climate change, its impacts were addressed, such as changes in surface ultraviolet (UV) radiation in the densely populated northern mid-latitudes.

Description: Here we present observations of the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) of cirrus cloud and water vapour in August 1997 in the upper troposphere and lower stratosphere (UTLS) region. The observations indicate a considerable flux of moisture from the upper tropical troposphere into the extra-tropical lowermost stratosphere (LMS), resulting in the occurrence of high altitude optically thin cirrus clouds in the LMS. The locations of the LMS cloud events observed by CRISTA are consistent with the tropopause height determined from coinciding radiosonde data. For a hemispheric analysis in tropopause relative coordinates an improved tropopause determination has been applied to the ECMWF temperature profiles. We found that a significant fraction of the cloud occurrences in the tropopause region are located in the LMS, even if a conservative overestimate of the cloud top height (CTH) determination by CRISTA of 500 m is assumed. The results show rather high occurrence frequencies (∼5%) up to high northern latitudes (70° N) and altitudes well above the tropopause (〉500 m at ∼350 K and above) in large areas at mid and high latitudes. Comparisons with model runs of the Chemical Lagragian Model of the Stratosphere (CLaMS) over the CRISTA period show a reasonable consistency for the retrieved cloud pattern. For this purpose a limb ray tracing approach was applied through the 3-D model fields to obtain integrated measurement information through the atmosphere along the limb path of the instrument. The simplified cirrus scheme implemented in CLaMS seems to cause a systematic underestimation in the CTH occurrence frequencies in the LMS with respect to the observations. The observations together with the model results demonstrate the importance of isentropic, quasi-horizontal transport of water vapour from the sub-tropics and the potential for the occurrence of cirrus clouds in the lowermost stratosphere and tropopause region.

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: Chlorofluorocarbons (CFCs) play a key role in stratospheric ozone loss and are strong infrared absorbers that contribute to global warming. The stratospheric lifetimes of CFCs are a measure of their global loss rates that are needed to determine global warming and ozone depletion potentials. We applied the tracer-tracer correlation approach to zonal mean climatologies from satellite measurements and model data to assess the lifetimes of CFCl3 (CFC-11) and CF2Cl2 (CFC-12). We present estimates of the CFC-11/CFC-12 lifetime ratio and the absolute lifetime of CFC-12, based on a reference lifetime of 52 yr for CFC-11. We analyzed climatologies from three satellite missions, the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS), the HIgh Resolution Dynamics Limb Sounder (HIRDLS), and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). We found a CFC-11/CFC-12 lifetime ratio of 0.47±0.08 and a CFC-12 lifetime of 111(96–132) yr for ACE-FTS, a ratio of 0.46±0.07 and a lifetime of 112(97–133) yr for HIRDLS, and a ratio of 0.46±0.08 and a lifetime of 112(96–135) yr for MIPAS. The error-weighted, combined CFC-11/CFC-12 lifetime ratio is 0.47±0.04 and the CFC-12 lifetime estimate is 112(102–123) yr. These results agree with the recent Stratosphere-troposphere Processes And their Role in Climate (SPARC) reassessment, which recommends lifetimes of 52(43–67) yr and 102(88–122) yr, respectively. Having smaller uncertainties than the results from other recent studies, our estimates can help to better constrain CFC-11 and CFC-12 lifetime recommendations in future scientific studies and assessments. Furthermore, the satellite observations were used to validate first simulation results from a new coupled model system, which integrates a Lagrangian chemistry transport model into a climate model. For the coupled model we found a CFC-11/CFC-12 lifetime ratio of 0.48±0.07 and a CFC-12 lifetime of 110(95–129) yr, based on a ten-year perpetual run. Closely reproducing the satellite observations, the new model system will likely become a useful tool to assess the impact of advective transport, mixing, and photochemistry as well as climatological variability on the stratospheric lifetimes of long-lived tracers.

Description: Small volcanic ash particles have long residence times in troposphere and stratosphere so that they have impact on the Earth's radiative budget and consequently affect climate. For global long term observations of volcanic aerosol, infrared limb measurements provide excellent coverage, sensitivity to thin aerosol layers, and altitude information. The optical properties of volcanic ash and ice particles, derived from micro-physical properties, have opposing spectral gradients between 700 to 960 cm−1 for small particle sizes. Radiative transfer simulations that account for single scattering showed that the opposing spectral gradients directly transfer to infrared limb spectra. Indeed, we found the characteristic spectral signature, expected for volcanic ash, in measurements of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) after the eruption of the Chilean volcano Puyehue-Cordón Caulle in June 2011. From these measurements we derived an ash detection threshold function. The empirical ash detection threshold was confirmed by simulations covering a wide range of atmospheric conditions, particle sizes, and particle concentrations for ice, volcanic ash, and sulfate aerosol. From the simulations we derived the detectable effective radius range of 0.2 to 3.5 μm and the detectable extinction coefficient range of 5 × 10−3 to 1 × 10−1 km−1. We also showed that this method is only sensitive to volcanic ash particles, but not to volcanic sulfate aerosol. This volcanic ash detection method for infrared limb measurements is a fast and reliable method and provides complementary information to existing satellite aerosol products.