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The outgoing longwave radiation (OLR) emitted to space is a fundamental component of the Earth’s energy budget. There are numerous, entangled physical processes that contribute to OLR and that are responsible for driving, and responding to, climate change. Spectrally resolved observations can disentangle these processes, but technical limitations have precluded accurate space-based spectral measurements covering the far infrared (FIR) from 100 to 667 cm−1 (wavelengths between 15 and 100 µm). The Earth’s FIR spectrum is thus essentially unmeasured even though at least half of the OLR arises from this spectral range. The region is strongly influenced by upper-tropospheric–lower-stratospheric water vapor, temperature lapse rate, ice cloud distribution, and microphysics, all critical parameters in the climate system that are highly variable and still poorly observed and understood. To cover this uncharted territory in Earth observations, the Far-Infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission has recently been selected as ESA’s ninth Earth Explorer mission for launch in 2026. The primary goal of FORUM is to measure, with high absolute accuracy, the FIR component of the spectrally resolved OLR for the first time with high spectral resolution and radiometric accuracy. The mission will provide a benchmark dataset of global observations which will significantly enhance our understanding of key forcing and feedback processes of the Earth’s atmosphere to enable more stringent evaluation of climate models. This paper describes the motivation for the mission, highlighting the scientific advances that are expected from the new measurements.

Absolute values of gravity wave momentum flux (GWMF) deduced from satellite measurements by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument and the High Resolution Dynamics Limb Sounder (HIRDLS) are correlated with sea surface temperature (SST) with the aim of identifying those oceanic regions for which convection is a major source of gravity waves (GWs). Our study identifies those latitude bands where high correlation coefficients indicate convective excitation with confidence. This is based on a global ray-tracing simulation, which is used to delineate the source and wind-filtering effects. Convective GWs are identified at the eastern coasts of the continents and over the warm water regions formed by the warm ocean currents, in particular the Gulf Stream and the Kuroshio. Potential contributions of tropical cyclones to the excitation of the GWs are discussed. Convective excitation can be identified well into the mid-mesosphere. In propagating upward, the centers of GWMF formed by convection shift poleward. Some indications of the main forcing regions are even shown for the upper mesosphere/lower thermosphere (MLT).

International audience; The stratospheric circulation is an important element of climate as it determines the concentration of radiatively active species likewater vapor and aerosol above the tropopause. Climate models predict that increasing greenhouse gas levels speed up the stratospheric circulation. However, these results have been challenged by observational estimates of the circulation strength, constitutingan uncertainty in current climate simulations. Here, we quantify the effect of volcanic aerosol on the stratospheric circulation focusing on the Mount Pinatubo eruption and discussing further the minor extratropical volcanic eruptions after 2008. We show that the observed pattern of decadal circulation change over the past decades is substantially driven by volcanic aerosol injections. Thus, climate model simulations need to realistically take into account the effect of volcanic eruptions, including the minor eruptions after 2008, for a reliable reproduction of observed stratospheric circulation changes.

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) presents new opportunities for the retrieval of trace gases in the upper troposphere and lower stratosphere. The radiometric calibration of the measured signal is achieved using in-flight measurements of reference blackbody and upward-pointing "deep space" scenes. In this paper, we present techniques developed specifically to calibrate GLORIA data exploiting the instrument's imaging capability. The algorithms discussed here make use of the spatial correlation of parameters across GLORIA's detector pixels in order to mitigate the noise levels and artefacts in the calibration measurements. This is achieved by combining a priori and empirical knowledge about the instrument background radiation with noise-mitigating compression methods, specifically low-pass filtering and principal component analysis (PCA). In addition, a new software package for the processing of GLORIA data is introduced which allows us to generate calibrated spectra from raw measurements in a semi-automated data processing chain.

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.

During the TACTS aircraft campaign enhanced tropospheric trace gases such as CO, CH4, and H2O and reduced stratospheric O3 were measured in situ in the lowermost stratosphere over Northern Europe on 26 September 2012. The measurements indicate that these air masses differ from the stratospheric background. The calculation of 40 day backward trajectories with the trajectory module of the CLaMS model shows that these air masses are affected by the Asian monsoon anticyclone. Some air masses originate from the boundary layer in Southeast Asia/West Pacific and are rapidly lifted (1–2 days) within a typhoon. Afterwards they are injected directly into the anticyclonic circulation of the Asian monsoon. The subsequent long-range transport (8–14 days) of enhanced water vapour and pollutants to the lowermost stratosphere in Northern Europe is driven by eastward transport of tropospheric air from the Asian monsoon anticyclone caused by an eddy shedding event. We find that the combination of rapid uplift by a typhoon and eastward eddy shedding from the Asian monsoon anticyclone is an additional fast transport pathway that, in this study, carries boundary emissions from Southeast Asia/West Pacific within approximately 5 weeks to the lowermost stratosphere in Northern Europe.

A new data set of atomic oxygen abundance in the upper mesosphere and lower thermosphere is presented. The data are derived from the nighttime atomic oxygen green line limb emission measurements of the SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) instrument on the European Environmental Satellite. The temporal coverage is October 2002 until April 2012, and the latitudinal extent is 50°S to 80°N at 10 P.M. local time. This data set is compared to other satellite data sets, in particular to recently published data of SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) and the Mass Spectrometer and Incoherent Scatter model. SCIAMACHY atomic oxygen peak abundances are typically 3–6×1011 mol/cm3 at the atomic oxygen maximum region, depending on latitude and season. These values are similar to previous values based on chemiluminescence measurements of the atomic oxygen three-body recombination reaction but at least 30% lower than atomic oxygen abundances obtained from SABER.

Thirty years of balloon-borne measurements over Boulder (40°N, 105°W) are used to investigate the water vapor trend in the tropopause region. This analysis extends previously published trends, usually focusing on altitudes greater than 16 km, to lower altitudes. Two new concepts are applied: (1) Trends are presented in a thermal tropopause (TP) relative coordinate system from –2 km below to 10 km above the TP, and (2) sonde profiles are selected according to TP height. Tropical (TPz > 14 km), extratropical (TPz < 12 km), and transitional air mass types (12 km < TPz < 14 km) reveal three different water vapor reservoirs. The analysis based on these concepts reduces the dynamically induced water vapor variability at the TP and principally favors refined water vapor trend studies in the upper troposphere and lower stratosphere. Nonetheless, this study shows how uncertain trends are at altitudes −2 to +4 km around the TP. This uncertainty in turn has an influence on the uncertainty and interpretation of water vapor radiative effects at the TP, which are locally estimated for the 30 year period to be of uncertain sign. The much discussed decrease in water vapor at the beginning of 2001 is not detectable between −2 and 2 km around the TP. On lower stratospheric isentropes, the water vapor change at the beginning of 2001 is more intense for extratropical than for tropical air mass types. This suggests a possible link with changing dynamics above the jet stream such as changes in the shallow branch of the Brewer-Dobson circulation.

The upper troposphere/lower stratosphere (UTLS) region plays an important role in the climate system. Changes in the structure and chemical composition of this region result in particularly large changes in radiative forcings of the atmosphere. Quantifying the processes that control UTLS composition (e.g., stratosphere-troposphere exchange) therefore represents a crucial task. We assess the influence of uncertainties in the atmospheric mixing strength on global UTLS distributions of greenhouse gases (water vapor, ozone, methane, and nitrous oxide) and associated radiative effects. The study is based on multiannual simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by ERA-Interim meteorological data and on a state-of-the-art radiance code. Mixing, the irreversible part of transport, is controlled by the local horizontal strain and vertical shear of the atmospheric flow. We find that simulated radiative effects of water vapor and ozone, both characterized by steep gradients in the UTLS, are particularly sensitive to uncertainties of the atmospheric mixing strength. Globally averaged radiative effects are about 0.72 and 0.17 W/m2 for water vapor and ozone, respectively. For ozone, the largest impact of mixing uncertainties is observed in the extra-tropical lower stratosphere.

Variations in the mixing ratio of long-lived trace gases entering the stratosphere in the tropics are carried upward with the rising air with the signal being observable throughout the tropical lower stratosphere. This phenomenon, referred to as "atmospheric tape recorder" has previously been observed for water vapor, CO2, and CO which exhibit an annual cycle. Recently, based on Microwave Limb Sounder (MLS) and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) satellite measurements, the tape recorder signal has been observed for hydrogen cyanide (HCN) but with an approximately two-year period. Here we report on a model simulation of the HCN tape recorder for the time period 2002-2008 using the Chemical Lagrangian Model of the Stratosphere (CLaMS). The model can reproduce the observed pattern of the HCN tape recorder signal if time-resolved emissions from fires in Indonesia are used as lower boundary condition. This finding indicates that inter-annual variations in biomass burning in Indonesia, which are strongly influenced by El Nino events, control the HCN tape recorder signal. A longer time series of tropical HCN data will probably exhibit an irregular cycle rather than a regular biannual cycle. Citation: Pommrich, R., R. Muller, J.-U. Grooss, G. Gunther, P. Konopka, M. Riese, A. Heil, M. Schultz, H.-C. Pumphrey, and K. A. Walker (2010), What causes the irregular cycle of the atmospheric tape recorder signal in HCN?, Geophys. Res. Lett., 37, L16805, doi:10.1029/2010GL044056.

Satellite observations show that the enormous solar proton events (SPEs) in October-November 2003 had significant effects on the composition of the stratosphere and mesosphere in the polar regions. After the October-November 2003 SPEs and in early 2004, significant enhancements of NOx(=NO+NO2) in the upper stratosphere and lower mesosphere in the Northern Hemisphere were observed by several satellite instruments. Here we present global full chemistry calculations performed with the CLaMS model to study the impact of mesospheric NOx intrusions on Arctic polar ozone loss processes in the stratosphere. Several model simulations are preformed with different upper boundary conditions for NOx at 2000 K potential temperature (approximate to 50 km altitude). In our study we focus on the impact of the non-local production of NOx, which means the downward transport of enhanced NOx from the mesosphere to the stratosphere. The local production of NOx in the stratosphere is neglected. Our findings show that intrusions of mesospheric air into the stratosphere, transporting high burdens of NOx, affect the composition of the Arctic polar region down to about 400 K (approximate to 17-18 km). We compare our simulated NOx and O-3 mixing ratios with satellite observations by ACE-FTS and MIPAS processed at IMK/IAA and derive an upper limit for the ozone loss caused by enhanced mesospheric NOx. Our findings show that in the Arctic polar vortex (equivalent lat. > 70 degrees N) the accumulated column ozone loss between 350-2000 K potential temperature (approximate to 14-50 km altitude) caused by the SPEs in October-November 2003 in the stratosphere is up to 3.3 DU with an upper limit of 5.5 DU until end of November. Further, we found that about 10 DU, but in any case lower than 18 DU, accumulated ozone loss additionally occurred until end of March 2004 caused by the transport of mesospheric NOx-rich air in early 2004. The solar-proton-produced NOx above 55 km due to the SPEs of October-November 2003 had a negligibly small impact on ozone loss processes through the end of November in the lower stratosphere (350-700 K approximate to 14-27 km). The mesospheric NOx intrusions in early 2004 yielded a lower stratospheric ozone loss of about 3.5 DU, and clearly lower than 6.5 DU through the end of March. Overall, the non-local production of NOx is an additional variability in the existing variations of the ozone loss observed in the Arctic.

Atmospheric Chemistry and Physics, 6 (1), ISSN:1680-7375, ISSN:1680-7367

During the CRISTA-1 mission three pronounced fingerlike structures reaching from the lower latitudes to the mid-latitudes, so-called streamers, were observed in the measurements of several trace gases in early November 1994. A simulation of these streamers in previous studies employing the KASIMA (Karlsruhe Simulation Model of the Middle Atmosphere) and ROSE (Research on Ozone in the Stratosphere and its Evolution) model, both being Eulerian models, show that their formation is due to adiabatic transport processes. Here, the impact of mixing on the development of these streamers is investigated. These streamers were simulated with the CLaMS model (Chemical Lagrangian Model of the Stratosphere), a Lagrangian model, using N2O as long-lived tracer. Using several different initialisations the results were compared to the KASIMA simulations and CRISTA (Cryogenic Infrared Spectrometer and Telescope for the Atmosphere) observations. Further, since the KASIMA model was employed to derive a 9-year climatology, the quality of the reproduction of streamers from such a study was tested by the comparison of the KASIMA results with CLaMS and CRISTA. The streamers are reproduced well for the Northern Hemisphere in the simulations of CLaMS and KASIMA for the 6 November 1994. However, in the CLaMS simulation a stronger filamentation is found while larger discrepancies between KASIMA and CRISTA were found especially for the Southern Hemisphere. Further, compared to the CRISTA observations the mixing ratios of N2O are in general underestimated in the KASIMA simulations. An improvement of the simulations with KASIMA was obtained for a simulation time according to the length of the CLaMS simulation. To quantify the differences between the simulations with CLaMS and KASIMA, and the CRISTA observations, the probability density function technique (PDF) is used to interpret the tracer distributions. While in the PDF of the KASIMA simulation the small scale structures observed by CRISTA are smoothed out due to the numerical diffusion in the model, the PDFs derived from CRISTA observations can be reproduced by CLaMS by optimizing the mixing parameterisation. Further, this procedure gives information on small-scale variabilities not resolved by the CRISTA observations.

[1] The second mission of the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment took place in August 1997. The experiment was flown aboard the ASTROnomical Shuttle PAllet Satellite (ASTRO-SPAS) free-flying platform launched by the NASA space shuttle. CRISTA analyzes the infrared radiation emitted by trace gases from the Earth limb in the altitude regime from the upper troposphere to the lower thermosphere. The main aim of CRISTA is to detect small-scale dynamically induced structures in the distribution of trace constituents in the middle atmosphere. The instrument is therefore equipped with three telescopes that simultaneously collect the infrared radiation from three different air volumes. The high spatial density of the measurement grid obtained during the first CRISTA mission in November 1994, as well as the latitudinal coverage, was considerably improved by making use of newly developed satellite pointing and maneuvering capabilities. The altitude coverage was extended to include the upper troposphere where water vapor distributions are analyzed. Dynamically induced features are observed in practically all trace gases and at various spatial scales. The smallest scales that could be analyzed on the basis of the CRISTA data set are well below 100 km. Compared to the first mission, much more emphasis was laid on measurements in the upper mesosphere and lower thermosphere-this was possible because of higher radiometric sensitivities in some channels. Atomic oxygen, carbon dioxide, and ozone densities are derived in the upper mesosphere and lower thermosphere. The mission conditions allowed the study of polar stratospheric clouds (PSC) over the Antarctic and of polar mesospheric clouds (PMC) at high northern latitudes. For the first time, summer high latitude mesopause temperatures were retrieved from CO2 15-mum spectra using a nonlocal thermodynamic equilibrium model. The derived temperatures compare well with a temperature climatology based on rocket soundings.

[1] Planetary waves drive the mean meridional circulation of the stratosphere and at the same time facilitate quasi-horizontal mixing of trace gases. This paper presents significant day-to-day variability of stratospheric trace gas fields associated with large planetary wave activity observed during the second mission of the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment. Geopotential height data of the UK Met Office show that the CRISTA-2 observations in the Southern Hemisphere winter were made during a period of extremely large amplitudes of both wave-1 and wave-2. The planetary wave-1, usually a quasi-stationary feature, moved eastward with the traveling planetary wave-2. The large amplitudes of both wave-1 and wave-2 led to a significant displacement of the edge of the polar vortex toward the tropics (down to 30 S). As a result of the large wave amplitudes and favorable phase alignment, the anticyclone drawing up tropical air was unusually strong, and thus considerable wave-induced trace gas flux from the tropics toward midlatitudes was observed, mainly in the form of a pronounced planetary-scale tongue advected out of the tropics around the vortex and into the anticyclone. Quantitative transport calculations based on a sequential data assimilation system highlight the importance of such transport events for trace gas eddy-flux in the Southern Hemisphere winter stratosphere.

[1] Water vapor mixing ratios at the tropopause are derived as a new Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) data product from limb scan measurements of the second mission. Global maps are obtained on a daily basis. Data loss due to high clouds is found to be moderate. Good agreement with in situ airplane measurements (Fast In Situ Stratospheric Hygrometer (FISH)) is obtained for these Version 1 data. A number of different analyses are performed to show the research potential of the data product: the CRISTA data are compared to measurements of the Microwave Limb Sounder (MLS) instrument on the Upper Atmosphere Research Satellite (UARS). Version 4.9 climatology data and Version 5 coincident measurements are used. Good agreement of CRISTA and Version 4.9 data is obtained, whereas there are differences with respect to the Version 5.0 data. CRISTA finds vapor mixing ratios to be highly variable. Only a small part of this is instrumental. Variability is structured, and a scaling behavior is observed. Relation to convectively generated gravity waves is discussed. Relative humidity (RH) is determined on the basis of the CRISTA data. Suitability for supersaturation statistics is discussed and appears to be limited. CRISTA water vapor data are assimilated into a 3D transport model driven by UK Meteorological Office (UKMO) winds. Results are discussed in terms of meridional transports and atmospheric diffusivities. Diffusivities appear to be connected with the water vapor variances in a simple manner.

[1] The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) instrument measured stratospheric temperatures and trace species concentrations with high precision and spatial resolution during two missions. The measuring technique is infrared limb-sounding of optically thin emissions. In a general approach, we investigate the applicability of the technique to measure gravity waves (GWs) in the retrieved temperature data. It is shown that GWs with wavelengths of the order of 100-200 km horizontally can be detected. The results are applicable to any instrument using the same technique. We discuss additional constraints inherent to the CRISTA instrument. The vertical field of view and the influence of the sampling and retrieval imply that waves with vertical wavelengths similar to3-5 km or larger can be retrieved. Global distributions of GW fluctuations were extracted from temperature data measured by CRISTA using Maximum Entropy Method (MEM) and Harmonic Analysis (HA), yielding height profiles of vertical wavelength and peak amplitude for fluctuations in each scanned profile. The method is discussed and compared to Fourier transform analyses and standard deviations. Analysis of data from the first mission reveals large GW amplitudes in the stratosphere over southernmost South America. These waves obey the dispersion relation for linear two-dimensional mountain waves (MWs). The horizontal structure on 6 November 1994 is compared to temperature fields calculated by the Pennsylvania State University (PSU)/ National Center for Atmospheric Research (NCAR) mesoscale model (MM5). It is demonstrated that precise knowledge of the instrument's sensitivity is essential. Particularly good agreement is found at the southern tip of South America where the MM5 accurately reproduces the amplitudes and phases of a large-scale wave with 400 km horizontal wavelength. Targeted ray-tracing simulations allow us to interpret some of the observed wave features. A companion paper will discuss MWs on a global scale and estimates the fraction that MWs contribute to the total GWenergy (Preusse et al., in preparation, 2002).

[1] The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) instrument measured a variety of trace gases globally with high spatial resolution during two Space Shuttle missions. This paper concentrates on members of the NOy family and highlights differences between CRISTA 1 (November 1994) and CRISTA 2 (August 1997). A sequential assimilation technique is used to combine the CRISTA measurements of total NOy fields with corresponding model forecasts based on the National Center for Atmospheric Research Research for Ozone in the Stratosphere and its Evolution (ROSE) model. For this study we use a model version driven by wind and temperature data provided by the UK Met Office. NO2 and N2O show large- and medium-scale structures caused by dynamical processes. N2O5 shows a strong dependence on the aerosol load and solar zenith angles. N2O5 and NO2 changes from CRISTA 1 to CRISTA 2 are consistent with a reduction of aerosol concentrations in the Southern Hemisphere and minor aerosol changes in the Northern Hemisphere. For both missions the model reproduces well the measured diurnal cycles of the NOy family members. Measured diurnal variations of N2O5 and NO2 are consistent with the nighttime production of N2O5 from NO2. Compared to the effect of heterogeneous chemistry, the influence of ozone and temperature changes on the NOy partitioning is rather small. A model run based on a three-dimensional aerosol field derived from CRISTA observations indicates that zonal asymmetries in the background aerosol have strong local effects on the N2O5 and NO2 distribution.

[1] The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) instrument observed thin cirrus clouds at and above the tropopause during its two missions in November 1994 and August 1997. A simple cloud detection scheme was developed for extinctions greater than 2 x 10(-3) km(-1) through analysis of the measured infrared spectra in the 12-mum range. Horizontal and vertical distributions of cloud occurrence frequencies are in good agreement with the Stratospheric Aerosol and Gas Experiment (SAGE) II subvisual cirrus cloud (SVC) climatology as well as SAGE measurements for the 1997 period. Seasonal variations, strong longitudinal variability, and indications of enhanced cloud occurrence frequencies in separated regions caused by El Nino events were detected in the CRISTA data set. A substantial day-to-day variability could be found throughout the tropics, and several regions with enhanced variability have been identified. In addition, a significant amount of cloud was found above the midlatitude tropopause. Backward trajectories in relation to outgoing longwave radiation (OLR) measurements and cloud observation in the troposphere by meteorological satellites suggest that about three fourths of the high clouds (> 15 km) observed by CRISTA in the tropics stem from deep convection systems and the outflow of these systems. This would imply that on the order of at least one fourth of the observed cloud events are originated by other mechanisms, such as in situ formation due to cooling events on synoptic and/or gravity wave scales. For the convective generated cirrus clouds, a maximum lifetime of around 3-4 days was estimated over a wide range of latitudes. Such a long lifetime could be important for modeling the impact of cirrus clouds on radiation budget (climate) and heterogeneous chemical processes around the tropopause.