ALBERT

Description: Limb sounding instruments play an important role in the monitoring of climate trends, as they provide a good vertical resolution. Traceability to the International System of Units (SI) via onboard reference or transfer standards is needed to compare trend estimates from multiple instruments. This study investigates the required uncertainty of these radiation standards to properly resolve decadal trends of climate-relevant trace species like ozone, water vapor, and temperature distribution for the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA). Temperature nonuniformities of the onboard reference blackbodies, used for radiometric calibration, have an impact on the calibration uncertainty. The propagation of these nonuniformities through the retrieval is analyzed. A threshold for the maximum tolerable uncertainty of the blackbody temperature is derived, so that climate trends can be significantly identified with GLORIA.

Description: When computing climatological averages of atmospheric trace-gas mixing ratios obtained from satellite-based measurements, sampling biases arise if data coverage is not uniform in space and time. Homogeneous spatiotemporal coverage is essentially impossible to achieve. Solar occultation measurements, by virtue of satellite orbit and the requirement of direct observation of the sun through the atmosphere, result in particularly sparse spatial coverage. In this proof-of-concept study, a method is presented to adjust for such sampling biases when calculating climatological means. The method is demonstrated using carbonyl sulfide (OCS) measurements at 16 km altitude from the ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer). At this altitude, OCS mixing ratios show a steep gradient between the poles and Equator. ACE-FTS measurements, which are provided as vertically resolved profiles, and integrated stratospheric OCS columns are used in this study. The bias adjustment procedure requires no additional information other than the satellite data product itself. In particular, the method does not rely on atmospheric models with potentially unreliable transport or chemistry parameterizations, and the results can be used uncompromised to test and validate such models. It is expected to be generally applicable when constructing climatologies of long-lived tracers from sparsely and heterogeneously sampled satellite measurements. In the first step of the adjustment procedure, a regression model is used to fit a 2-D surface to all available ACE-FTS OCS measurements as a function of day-of-year and latitude. The regression model fit is used to calculate an adjustment factor that is then used to adjust each measurement individually. The mean of the adjusted measurement points of a chosen latitude range and season is then used as the bias-free climatological value. When applying the adjustment factor to seasonal averages in 30∘ zones, the maximum spatiotemporal sampling bias adjustment was 11 % for OCS mixing ratios at 16 km and 5 % for the stratospheric OCS column. The adjustments were validated against the much denser and more homogeneous OCS data product from the limb-sounding MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) instrument, and both the direction and magnitude of the adjustments were in agreement with the adjustment of the ACE-FTS data.

Description: We report airborne remote-sensing observations of a tropopause fold during two crossings of the polar front jet over northern Italy on 12 January 2016. The GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) observations allowed for a simultaneous mapping of temperature, water vapour, and ozone. They revealed deep, dry, and ozone-rich intrusions into the troposphere. The mesoscale fine structures of dry filaments at the cyclonic shear side north of the jet and tongues of moist air entraining tropospheric air into the stratosphere along the anticyclonic shear side south of the jet were clearly resolved by GLORIA observations. Vertically propagating mountain waves with recorded temperature residuals exceeding ±3 K were detected above the Apennines. Their presence enhanced gradients of all variables locally in the vicinity of the tropopause. The combination of H2O−O3 correlations with potential temperature reveals an active mixing region and shows clear evidence of troposphere-to-stratosphere and stratosphere-to-troposphere exchange. High-resolution short-term deterministic forecasts of ECMWF's integrated forecast system (IFS) applying GLORIA's observational filter reproduce location, shape, and depth of the tropopause fold very well. The fine structure of the mixing region, however, cannot be reproduced even with the 9 km horizontal resolution of the IFS, used here. This case study demonstrates convincingly the capabilities of linear limb-imaging observations to resolve mesoscale fine structures in the upper troposphere and lower stratosphere, validates the high quality of the IFS data, and suggests that mountain wave perturbations have the potential to modulate exchange processes in the vicinity of tropopause folds.

Description: The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) was operated on board the German High Altitude and Long Range Research Aircraft (HALO) during the PGS (POLSTRACC/GW-LCYCLE/SALSA) aircraft campaigns in the Arctic winter 2015/2016. Research flights were conducted from 17 December 2015 until 18 March 2016 within 25–87∘ N, 80∘ W–30∘ E. From the GLORIA infrared limb-emission measurements, two-dimensional cross sections of temperature, HNO3, O3, ClONO2, H2O and CFC-12 are retrieved. During 15 scientific flights of the PGS campaigns the GLORIA instrument measured more than 15 000 atmospheric profiles at high spectral resolution. Dependent on flight altitude and tropospheric cloud cover, the profiles retrieved from the measurements typically range between 5 and 14 km, and vertical resolutions between 400 and 1000 m are achieved. The estimated total (random and systematic) 1σ errors are in the range of 1 to 2 K for temperature and 10 % to 20 % relative error for the discussed trace gases. Comparisons to in situ instruments deployed on board HALO have been performed. Over all flights of this campaign the median differences and median absolute deviations between in situ and GLORIA observations are -0.75K±0.88 K for temperature, -0.03ppbv±0.85 ppbv for HNO3, -3.5ppbv±116.8 ppbv for O3, -15.4pptv±102.8 pptv for ClONO2, -0.13ppmv±0.63 ppmv for H2O and -19.8pptv±46.9 pptv for CFC-12. Seventy-three percent of these differences are within twice the combined estimated errors of the cross-compared instruments. Events with larger deviations are explained by atmospheric variability and different sampling characteristics of the instruments. Additionally, comparisons of GLORIA HNO3 and O3 with measurements of the Aura Microwave Limb Sounder (MLS) instrument show highly consistent structures in trace gas distributions and illustrate the potential of the high-spectral-resolution limb-imaging GLORIA observations for resolving narrow mesoscale structures in the upper troposphere and lower stratosphere (UTLS).

Description: Gravity waves (GWs) play an important role in the dynamics of the mesosphere and lower thermosphere (MLT). Therefore, global observations of GWs in the MLT region are of particular interest. The small scales of GWs, however, pose a major problem for the observation of GWs from space. We propose a new observation strategy for GWs in the mesopause region by combining limb and sub-limb satellite-borne remote sensing measurements for improving the spatial resolution of temperatures that are retrieved from atmospheric soundings. In our study, we simulate satellite observations of the rotational structure of the O2 A-band nightglow. A key element of the new method is the ability of the instrument or the satellite to operate in so-called target mode, i.e. to point at a particular point in the atmosphere and collect radiances at different viewing angles. These multi-angle measurements of a selected region allow for tomographic 2-D reconstruction of the atmospheric state, in particular of GW structures. The feasibility of this tomographic retrieval approach is assessed using simulated measurements. It shows that one major advantage of this observation strategy is that GWs can be observed on a much smaller scale than conventional observations. We derive a GW sensitivity function, and it is shown that target mode observations are able to capture GWs with horizontal wavelengths as short as  ∼ 50 km for a large range of vertical wavelengths. This is far better than the horizontal wavelength limit of 100–200 km obtained from conventional limb sounding.

Description: The 2015/2016 Arctic winter was one of the coldest stratospheric winters in recent years. A stable vortex formed by early December and the early winter was exceptionally cold. Cold pool temperatures dropped below the nitric acid trihydrate (NAT) existence temperature of about 195 K, thus allowing polar stratospheric clouds (PSCs) to form. The low temperatures in the polar stratosphere persisted until early March, allowing chlorine activation and catalytic ozone destruction. Satellite observations indicate that sedimentation of PSC particles led to denitrification as well as dehydration of stratospheric layers. Model simulations of the 2015/2016 Arctic winter nudged toward European Centre for Medium-Range Weather Forecasts (ECMWF) analysis data were performed with the atmospheric chemistry–climate model ECHAM5/MESSy Atmospheric Chemistry (EMAC) for the Polar Stratosphere in a Changing Climate (POLSTRACC) campaign. POLSTRACC is a High Altitude and Long Range Research Aircraft (HALO) mission aimed at the investigation of the structure, composition and evolution of the Arctic upper troposphere and lower stratosphere (UTLS). The chemical and physical processes involved in Arctic stratospheric ozone depletion, transport and mixing processes in the UTLS at high latitudes, PSCs and cirrus clouds are investigated. In this study, an overview of the chemistry and dynamics of the 2015/2016 Arctic winter as simulated with EMAC is given. Further, chemical–dynamical processes such as denitrification, dehydration and ozone loss during the 2015/2016 Arctic winter are investigated. Comparisons to satellite observations by the Aura Microwave Limb Sounder (Aura/MLS) as well as to airborne measurements with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) performed aboard HALO during the POLSTRACC campaign show that the EMAC simulations nudged toward ECMWF analysis generally agree well with observations. We derive a maximum polar stratospheric O3 loss of ∼ 2 ppmv or 117 DU in terms of column ozone in mid-March. The stratosphere was denitrified by about 4–8 ppbv HNO3 and dehydrated by about 0.6–1 ppmv H2O from the middle to the end of February. While ozone loss was quite strong, but not as strong as in 2010/2011, denitrification and dehydration were so far the strongest observed in the Arctic stratosphere in at least the past 10 years.

Description: Gravity waves (GWs) have been intensively studied over recent decades because of their dominant role in the dynamics of the mesosphere and lower thermosphere (MLT). The momentum deposition caused by breaking GWs determines the basic structure and drives the large-scale circulation in the MLT. Satellite observations provide a way to qualify the properties and effects of GWs on a global scale. As GWs can propagate vertically and horizontally in the atmosphere, resolving both horizontal and vertical wavelengths is important for the quantification of a wave. However, this can hardly be achieved by one instrument with a good spatial coverage and resolution. In this paper, we propose a new observation strategy, called “sweep mode”, for a real three-dimensional (3-D) tomographic reconstruction of GWs in the MLT by modifying the observation geometry of conventional limb sounding measurements. It enhances the horizontal resolution that typical limb sounders can achieve, while at the same time retaining the good vertical resolution they have. This observation strategy is simulated for retrieving temperatures from measurements of the rotational structure of the O2 A-band airglow. The idea of this observation strategy is to sweep the line of sight (LOS) of the limb sounder horizontally across the orbital track during the flight. Therefore, two-dimensional (2-D) slices, i.e., vertical planes, that reveal the projection of GWs can be observed in the direction along and across the orbital track, respectively. The 3-D wave vector is then reproduced by combining the projected 2-D wave slices in the two directions. The feasibility of this sweep-mode tomographic retrieval approach is assessed using simulated measurements. It shows that the horizontal resolution in both along- and across-track directions is affected by an adjustable turning angle, which also determines the spatial coverage of this observation mode. The retrieval results can reduce the errors in deducing momentum flux substantially by providing an unbiased estimation of the real horizontal wavelength of a wave.

Description: Three-dimensional measurements of gravity waves are required in order to quantify their direction-resolved momentum fluxes and obtain a better understanding of their propagation characteristics. Such 3-D measurements of gravity waves in the lowermost stratosphere have been provided by the airborne Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) using full angle tomography. Closed flight patterns of sufficient size are needed to acquire the full set of angular measurements for full angle tomography. These take about 2 h and are not feasible everywhere due to scientific reasons or air traffic control restrictions. Hence, this paper investigates the usability of limited angle tomography for gravity wave research based on synthetic observations. Limited angle tomography uses only a limited set of angles for tomographic reconstruction and can be applied to linear flight patterns. A synthetic end-to-end simulation has been performed to investigate the sensitivity of limited angle tomography to gravity waves with different wavelengths and orientations with respect to the flight path. For waves with wavefronts roughly perpendicular to the flight path, limited angle tomography and full angle tomography can derive wave parameters like wavelength, amplitude, and wave orientation with similar accuracy. For waves with a horizontal wavelength above 200 km and vertical wavelength above 3 km, the wavelengths can be retrieved with less than 10 % error, the amplitude with less than 20 % error, and the horizontal wave direction with an error below 10∘. This is confirmed by a comparison of results obtained from full angle tomography and limited angle tomography for real measurements taken on 25 January 2016 over Iceland. The reproduction quality of gravity wave parameters with limited angle tomography, however, depends strongly on the orientation of the waves with respect to the flight path. Thus, full angle tomography might be preferable in cases in which the orientation of the wave cannot be predicted or waves with different orientations exist in the same volume and thus the flight path cannot be adjusted accordingly. Also, for low-amplitude waves and short-scale waves full angle tomography has advantages due to its slightly higher resolution and accuracy.

Description: Atmospheric gravity waves are a major cause of uncertainty in atmosphere general circulation models. This uncertainty affects regional climate projections and seasonal weather predictions. Improving the representation of gravity waves in general circulation models is therefore of primary interest. In this regard, measurements providing an accurate 3-D characterization of gravity waves are needed. Using the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA), the first airborne implementation of a novel infrared limb imaging technique, a gravity wave event over Iceland was observed. An air volume disturbed by this gravity wave was investigated from different angles by encircling the volume with a closed flight pattern. Using a tomographic retrieval approach, the measurements of this air mass at different angles allowed for a 3-D reconstruction of the temperature and trace gas structure. The temperature measurements were used to derive gravity wave amplitudes, 3-D wave vectors, and direction-resolved momentum fluxes. These parameters facilitated the backtracing of the waves to their sources on the southern coast of Iceland. Two wave packets are distinguished, one stemming from the main mountain ridge in the south of Iceland and the other from the smaller mountains in the north. The total area-integrated fluxes of these two wave packets are determined. Forward ray tracing reveals that the waves propagate laterally more than 2000 km away from their source region. A comparison of a 3-D ray-tracing version to solely column-based propagation showed that lateral propagation can help the waves to avoid critical layers and propagate to higher altitudes. Thus, the implementation of oblique gravity wave propagation into general circulation models may improve their predictive skills.