Tomographic reconstruction of atmospheric volumes from infrared limb-imager measurements

State-of-the art nadir and limb-sounders, but also in situ measurements, do not offer the capability to highly resolve the atmosphere in all three dimensions. This leaves an observational gap with respect to small-scale structures that arise frequently in the atmosphere and that still lack a quantitative understanding. For instance, filaments and tropopause folds in the upper troposphere and lower stratosphere (UTLS) are crucial for its composition and variability. One way to achieve a highly resolved three-dimensional (3-D) picture of the atmosphere is the tomographic evaluation of limb-imager measurements. This thesis presents a methodology for the tomographic reconstruction of atmospheric constituents. To be able to deal with the large increase of observations and unknowns compared to conventional retrievals, great care is taken to reduce memory consumption and processing time. This method is used to evaluate the performance of two upcoming infrared limb-imager instruments and to prepare their missions...

Was macht Düngemittel in 18 km Höhe? – Ammoniak und Ammoniumnitrat in der oberen Troposphäre

Jedes Jahr bildet sich während der Zeit des asiatischen Monsuns eine Aerosolschicht in der oberen Troposphäre in 13–18 km Höhe, die von Ostasien bis über das östliche Mittelmeer reicht, die ATAL (Asian Tropopause Aerosol Layer). Die Partikel der ATAL bestehen zu einem wesentlichen Anteil aus Ammoniumnitrat in fester Phase. Der Ursprung dieser Partikel liegt in Emissionen von Ammoniakgas durch landwirtschaftliche Tätigkeit in Nordindien und Pakistan. NH3 wird durch Konvektion während starker Gewitter in die oberen Luftschichten verbracht und neutralisiert vorhandenes HNO3 unter Bildung von Ammoniumnitrat. Aufgrund ihrer guten Eiskeimfähigkeit können feste Ammoniumnitratpartikel die Bildung und mikrophysikalische Beschaffenheit von Zirren und somit das Klima beeinflussen

A mountain ridge model for quantifying oblique mountain wave propagation and distribution

Following the current understanding of gravity waves (GWs) and especially mountain waves (MWs), they have high potential of horizontal propagation from their source. This horizontal propagation and therefore the transport of energy is usually not well represented in MW parameterizations of numerical weather prediction and general circulation models. The lack thereof possibly leads to shortcomings in the model's prediction as e.g. the cold pole bias in the Southern Hemisphere and the polar vortex breaking down too late. In this study we present a mountain wave model (MWM) for quantification of the horizontal propagation of orographic gravity waves. This model determines MW source location and associates their parameters from a fit of idealized Gaussian shaped mountains to topography data. Propagation and refraction of these MWs in the atmosphere is modeled using the ray-tracer GROGRAT. Ray-tracing each MW individually allows for an estimation of momentum transport due to both vertical and horizontal propagation. This study presents the MWM itself and gives validations of MW induced temperature perturbations to ECMWF IFS numerical weather prediction data and estimations of gravity wave momentum flux (GWMF) compared to HIRDLS satellite observations. The MWM is capable of reproducing the general features and amplitudes of both of these data sets and, in addition, is used to explain some observational features by investigating MW parameters along their trajectories.</p

Observation of cirrus clouds with GLORIA during the WISE campaign: detection methods and cirrus characterization

Cirrus clouds contribute to the general radiation budget of the Earth and play an important role in climate projections. Of special interest are optically thin cirrus clouds close to the tropopause due to the fact that their impact is not yet well understood. Measuring these clouds is challenging as both high spatial resolution as well as a very high detection sensitivity are needed. These criteria are fulfilled by the infrared limb sounder GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere). This study presents a characterization of observed cirrus clouds using the data obtained by GLORIA aboard the German research aircraft HALO during the WISE (Wave-driven ISentropic Exchange) campaign in September and October 2017. We developed an optimized cloud detection method based on the cloud index and the extinction coefficient retrieved at the microwindow 832.4–834.4 cm$^{-1}$. We derived macro-physical characteristics of the detected cirrus clouds such as cloud top height, cloud bottom height, vertical extent and cloud top position with respect to the tropopause. The fraction of cirrus clouds detected above the tropopause is on the order of 13 % to 27 %. In general, good agreement with the clouds predicted by the ERA5 reanalysis dataset is obtained. However, cloud occurrence is ≈ 50 % higher in the observations for the region close to and above the tropopause. Cloud bottom heights are also detected above the tropopause. However, considering the uncertainties, we cannot confirm the formation of unattached cirrus layers above the tropopause

High-resolution optical constants of crystalline ammonium nitrate for infrared remote sensing of the Asian Tropopause Aerosol Layer

Infrared spectroscopic observations have shown that crystalline ammonium nitrate (AN) particles are an abundant constituent of the upper tropospheric aerosol layer which is formed during the Asian summer monsoon period, the so-called Asian Tropopause Aerosol Layer (ATAL). At upper tropospheric temperatures, the thermodynamically stable phase of AN is different from that at 298 K, meaning that presently available room-temperature optical constants of AN, that is, the real and imaginary parts of the complex refractive index, cannot be applied for the quantitative analysis of these infrared measurements. In this work, we have retrieved the first low-temperature data set of optical constants for crystalline AN in the 800–6000 cm−1 wavenumber range with a spectral resolution of 0.5 cm−1. The optical constants were iteratively derived from an infrared extinction spectrum of 1µm sized AN particles suspended in a cloud chamber at 223 K. The uncertainties of the new data set were carefully assessed in a comprehensive sensitivity analysis. We show that our data accurately fit aircraft-borne infrared measurements of ammonium nitrate particles in the ATAL

Superposition of gravity waves with different propagation characteristics observed by airborne and space-borne infrared sounders

Many gravity wave analyses, based on either observations or model simulations, assume the presence of only a single dominant wave. This paper shows that there are much more complex cases with gravity waves from multiple sources crossing each others\u27 paths. A complex gravity wave structure consisting of a superposition of multiple wave packets was observed above southern Scandinavia on 28 January 2016 with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA). The tomographic measurement capability of GLORIA enabled a detailed 3-D reconstruction of the gravity wave field and the identification of multiple wave packets with different horizontal and vertical scales. The larger-scale gravity waves with horizontal wavelengths of around 400 km could be characterised using a 3-D wave-decomposition method. The smaller-scale wave components with horizontal wavelengths below 200 km were discussed by visual inspection. For the larger-scale gravity wave components, a combination of gravity-wave ray-tracing calculations and ERA5 reanalysis fields identified orography as well as a jet-exit region and a low-pressure system as possible sources. All gravity waves are found to propagate upward into the middle stratosphere, but only the orographic waves stay directly above their source. The comparison with ERA5 also shows that ray tracing provides reasonable results even for such complex cases with multiple overlapping wave packets. Despite their coarser vertical resolution compared to GLORIA measurements, co-located AIRS measurements in the middle stratosphere are in good agreement with the ray tracing and ERA5 results, proving once more the validity of simple ray-tracing models. Thus, this paper demonstrates that the high-resolution GLORIA observations in combination with simple ray-tracing calculations can provide an important source of information for enhancing our understanding of gravity wave propagation

Cirrus cloud shape detection by tomographic extinction retrievals from infrared limb emission sounder measurements

An improved cloud-index-based method for the detection of clouds in limb sounder data is presented that exploits the spatial overlap of measurements to more precisely detect the location of (optically thin) clouds. A second method based on a tomographic extinction retrieval is also presented. Using CALIPSO data and a generic advanced infrared limb imaging instrument as examples for a synthetic study, the new cloud index method has a better horizontal resolution in comparison to the traditional cloud index and has a reduction of false positive cloud detection events by about 30 %. The results for the extinction retrieval even show an improvement of 60 %. In a second step, the extinction retrieval is applied to real 3-D measurements of the airborne Gimballed Limb Observer for Radiance Imaging in the Atmosphere (GLORIA) taken during the Wave-driven ISentropic Exchange (WISE) campaign to retrieve small-scale cirrus clouds with high spatial accuracy

Quantification and mitigation of the instrument effects and uncertainties of the airborne limb imaging FTIR GLORIA

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) is an infrared imaging FTS (Fourier transform spectrometer) with a 2-D infrared detector that is operated on two high-flying research aircraft. It has flown on eight campaigns and measured along more than 300 000 km of flight track.This paper details our instrument calibration and characterization efforts, which, in particular, almost exclusively leverage in-flight data. First, we present the framework of our new calibration scheme, which uses information from all three available calibration sources (two blackbodies and upward-pointing “deep space” measurements). Part of this scheme is a new algorithm for correcting the erratically changing nonlinearity of a subset of detector pixels and the identification of the remaining bad pixels.Using this new calibration, we derive a 1σ bound of 1 % on the instrument gain error and a bound of 30 nW cm−2 sr−1 cm on the instrument offset error. We show how we can examine the noise and spectral accuracy for all measured atmospheric spectra and derive a spectral accuracy of 5 ppm on average. All these errors are compliant with the initial instrument requirements.We also discuss, for the first time, the pointing system of the GLORIA instrument. Combining laboratory calibration efforts with the measurement of astronomical bodies during the flight, we can achieve a pointing accuracy of 0.032∘, which corresponds to one detector pixel.The paper concludes with a brief study of how these newly characterized instrument parameters affect temperature and ozone retrievals. We find that the pointing uncertainty and, to a lesser extent, the instrument gain uncertainty are the main contributors to the error in the result

Technical note: Lowermost-stratosphere moist bias in ECMWF IFS model diagnosed from airborne GLORIA observations during winter–spring 2016

Numerical weather forecast systems like the ECMWF IFS (European Centre for Medium-Range Weather Forecasts – Integrated Forecasting System) are known to be affected by a moist bias in the extratropical lowermost stratosphere (LMS) which results in a systematic cold bias there. We use high-spatial-resolution water vapor measurements by the airborne infrared limb-imager GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) during the PGS (POLSTRACC/GW-LCYCLE-II/SALSA) campaign to study the LMS moist bias in ECMWF analyses and 12 h forecasts from January to March 2016. Thereby, we exploit the two-dimensional observational capabilities of GLORIA, when compared to in situ observations, and the higher vertical and horizontal resolution, when compared to satellite observations. Using GLORIA observations taken during five flights in the polar sub-vortex region around Scandinavia and Greenland, we diagnose a systematic moist bias in the LMS exceeding +50 % (January) to +30 % (March) at potential vorticity levels from 10 PVU (∼ highest level accessed with suitable coverage) to 7 PVU. In the diagnosed time period, the moist bias decreases at the highest and driest air masses observed but clearly persists at lower levels until mid-March. Sensitivity experiments with more frequent temporal output, and lower or higher horizontal and vertical resolution, show the short-term forecasts to be practically insensitive to these parameters on timescales of < 12 h. Our results confirm that the diagnosed moist bias is already present in the initial conditions (i.e., the analysis) and thus support the hypothesis that the cold bias develops as a result of forecast initialization. The moist bias in the analysis might be explained by a model bias together with the lack of water vapor observations suitable for assimilation above the tropopause