ALBERT

Beschreibung: Polarization data from SCIAMACHY limb backscatter observations compared to vector radiative transfer model simulations Atmospheric Measurement Techniques Discussions, 5, 2221-2271, 2012 Author(s): P. Liebing, K. Bramstedt, S. Noël, V. Rozanov, H. Bovensmann, and J. P. Burrows SCIAMACHY is a passive imaging spectrometer onboard ENVISAT, designed to obtain trace gas abundances from measured radiances and irradiances in the UV to SWIR range in nadir, limb and occultation viewing modes. Its grating spectrometer introduces a substantial sensitivity to the polarization of the incoming light with nonnegligible effects on the radiometric calibration. To be able to correct for the polarization sensitivity, SCIAMACHY utilizes broadband Polarization Measurement Devices (PMDs). While for the nadir viewing mode the measured atmospheric polarization has been validated against POLDER data (Tilstra and Stammes, 2007, 2010), a similar validation study regarding the limb viewing mode has not yet been performed. This paper aims at an assessment of the quality of the SCIAMACHY limb polarization data. Since limb polarization measurements by other air- or spaceborne instruments in the spectral range of SCIAMACHY are not available, a comparison with radiative transfer simulations by SCIATRAN V3.1(Rozanov et al., 2012) using a wide range of atmospheric parameters is performed. SCIATRAN is a vector radiative transfer model (VRTM) capable of performing calculations of the multiply scattered radiance in a~spherically symmetric atmosphere. The study shows that the limb polarization data exhibit a large systematic bias which is decreasing with wavelength. The most likely reason for this bias is an instrumental phase shift which changes the relative contributions of different Stokes vector components to the PMD signal as compared to on-ground calibration measurements. It is also shown that it is in principle feasible to recalibrate the polarization sensitivity using the in-flight data and the VRTM simulations, enabling also the monitoring of its degradation. Together with an optimization of the algorithm used to calculate the in-flight polarization data an improved polarization correction can increase the radiometric accuracy of SCIAMACHY limb radiance spectra substantially.

Beschreibung: First intercalibration of column-averaged methane from the Total Carbon Column Observing Network and the Network for the Detection of Atmospheric Composition Change Atmospheric Measurement Techniques Discussions, 5, 1355-1379, 2012 Author(s): F. Forster, R. Sussmann, M. Rettinger, N. M. Deutscher, D. W. T. Griffith, N. Jones, and P. K. Patra We present the intercalibration of dry-air column-averaged mole fractions of methane (XCH 4 ) retrieved from solar FTIR measurements of the Network for the Detection of Atmospheric Composition Change (NDACC) in the mid-infrared (MIR) versus near-infrared (NIR) soundings from the Total Carbon Column Observing Network (TCCON). The study uses multi-annual quasi-coincident MIR and NIR measurements from the stations Garmisch, Germany (47.48° N, 11.06° E, 743 m a.s.l.) and Wollongong, Australia (34.41° S, 150.88° E, 30 m a.s.l.). Direct comparison of the retrieved MIR and NIR time series shows a phase shift in XCH 4 seasonality, i.e. a significant time-dependent bias leading to a standard deviation (stdv) of the difference time series (NIR-MIR) of 8.4 ppb. After eliminating differences in a prioris by using ACTM-simulated profiles as a common prior, the seasonalities of the (corrected) MIR and NIR time series agree within the noise (stdv = 5.2 ppb for the difference time series). The difference time series (NIR-MIR) do not show a significant trend. Therefore it is possible to use a simple scaling factor for the intercalibration without a time-dependent linear or seasonal component. Using the Garmisch and Wollongong data together, we obtain an overall calibration factor MIR/NIR = 0.9926(18). The individual calibration factors per station are 0.9940(14) for Garmisch and 0.9893(40) for Wollongong. They agree within their error bars with the overall calibration factor which can therefore be used for both stations. Our results suggest that after applying the proposed intercalibration concept to all stations performing both NIR and MIR measurements, it should be possible to obtain one refined overall intercalibration factor for the two networks. This would allow to set up a harmonized NDACC and TCCON XCH 4 data set which can be exploited for joint trend studies, satellite validation, or the inverse modeling of sources and sinks.

Beschreibung: Characterization and airborne deployment of a new counterflow virtual impactor inlet Atmospheric Measurement Techniques Discussions, 5, 1515-1541, 2012 Author(s): T. Shingler, S. Dey, A. Sorooshian, F. J. Brechtel, Z. Wang, A. Metcalf, M. Coggon, J. Mülmenstädt, L. M. Russell, H. H. Jonsson, and J. H. Seinfeld A new counterflow virtual impactor (CVI) inlet is introduced with details of its design, laboratory characterization tests, and deployment on an aircraft during the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). The CVI inlet addresses three key issues in previous designs; in particular, the inlet operates with: (i) negligible organic contamination; (ii) a significant sample flow rate to downstream instruments (~15 l min −1 ) that reduces the need for dilution; and (iii) a high level of accessibility to the probe interior for cleaning. Wind tunnel experiments characterized the cut size of sampled droplets and the particle size-dependent transmission efficiency in various parts of the probe. For a range of counter-flow rates and air velocities, the measured cut size was between 8.7–13.1 μm. The percentage error between cut size measurements and predictions from aerodynamic drag theory are less than 13%. The CVI was deployed on the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter for thirty flights during E-PEACE to study aerosol-cloud-radiation interactions off the central coast of California between July and August 2011. Results are reported to assess the performance of the inlet including comparisons of particle number concentration downstream of the CVI and cloud drop number concentration measured by two independent aircraft probes. Measurements downstream the CVI are also examined from one representative case flight coordinated with shipboard-emitted smoke that was intercepted in cloud by the Twin Otter.

Beschreibung: Improved cloud screening in MAIAC aerosol retrievals using spectral and spatial analysis Atmospheric Measurement Techniques Discussions, 5, 1575-1595, 2012 Author(s): A. Lyapustin, Y. Wang, I. Laszlo, and S. Korkin An improved cloud/snow screening technique in the Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm is described. It is implemented as part of MAIAC aerosol retrievals based on analysis of spectral residuals and spatial variability. Comparisons with AERONET aerosol measurements and a large-scale MODIS data analysis show strong suppression of aerosol optical depth outliers due to unresolved clouds and snow. At the same time, the developed filter does not reduce the aerosol retrieval capability at high 1 km resolution in strongly inhomogeneous environments, such as near centers of the active fires. Despite significant improvement, the optical depth outliers in high spatial resolution data are and will remain the problem to be addressed by the application-dependent specialized filtering techniques.

Beschreibung: Measuring variations of δ 18 O and δ 2 H in atmospheric water vapour using laser spectroscopy: an instrument characterisation study Atmospheric Measurement Techniques Discussions, 5, 1597-1655, 2012 Author(s): F. Aemisegger, P. Sturm, P. Graf, H. Sodemann, S. Pfahl, A. Knohl, and H. Wernli Variations of stable water isotopes in water vapour have become measurable at a measurement frequency of about 1 Hz in recent years using novel laser spectroscopic techniques. This enables us to perform continuous measurements for process-based investigations of the atmospheric water cycle at the time scales relevant for synoptic meteorology. An important prerequisite for the interpretation of data from automated field measurements lasting for several weeks or months is a detailed knowledge about instrument properties and the sources of measurement uncertainty. We present here a comprehensive characterisation and comparison study of two commercial laser spectroscopic systems based on cavity ring-down spectroscopy (Picarro) and off-axis integrated cavity output spectroscopy (Los Gatos Research). The uncertainty components of the measurements were first assessed in laboratory experiments, focussing on the effects of (i) water vapour mixing ratio, (ii) measurement stability, (iii) uncertainties due to calibration and (iv) response times of the isotope measurements due to adsorption-desorption processes on the tubing and measurement cavity walls. Based on the experience from our laboratory experiments we set up a one-week field campaign for comparing measurements of the ambient isotope signals of the two laser spectroscopic systems. The optimal calibration strategy determined for both instruments was applied as well as the correction functions for water vapour mixing ratio effects. The root mean square difference between the isotope signals from the two instruments during the field deployment was 2.3‰ for δ 2 H, 0.5‰ for δ 18 O and 3.1‰ for deuterium excess. These uncertainty estimates from field measurements compare well to those found in the laboratory experiments. The present quality of measurements from laser spectroscopic instruments combined with a calibration system opens new possibilities for investigating the atmospheric water cycle and the land-atmosphere moisture fluxes.

Beschreibung: Consistency between Fourier transform and small-volume few-wave decomposition for spectral and spatial variability of gravity waves above a typhoon Atmospheric Measurement Techniques Discussions, 5, 1763-1793, 2012 Author(s): C. I. Lehmann, Y.-H. Kim, P. Preusse, H.-Y. Chun, M. Ern, and S.-Y. Kim Convective gravity wave (GW) sources are spatially localized and emit at the same time waves with a wide spectrum of phase speeds. Any wave analysis therefore compromises between spectral and spatial resolution. Future satellite borne limb imagers will for a first time provide real 3d volumes of observations. These volumes will be however limited which will impose further constraints on the analysis technique. In this study a three dimensional few-wave appoach fitting sinusoidal waves to limited 3-D volumes is introduced. The method is applied to simulated GWs above typhoon Ewiniar and GW momentum flux is estimated from temperature fluctuations. Phase speed spectra as well as average profiles of positive, negative and net momentum fluxes are compared to momentum flux estimated by Fourier transform as well as spatial averaging of wind fluctuations. The results agree within 10–20%. The few-wave method can also reveal the spatial orientation of the GWs with respect to the source. The relevance of the results for different types of measurements as well as its applicability to model data is discussed.

Beschreibung: Effect of sampling variation on error of rainfall variables measured by optical disdrometer Atmospheric Measurement Techniques Discussions, 5, 8895-8924, 2012 Author(s): X. C. Liu, T. C. Gao, and L. Liu During the sampling process of precipitation particles by optical disdrometers, the randomness of particles and sampling variability has great impact on the accuracy of precipitation variables. Based on a marked point model of raindrop size distribution, the effect of sampling variation on drop size distribution and velocity distribution measurement using optical disdrometers are analyzed by Monte Carlo simulation. The results show that the samples number, rain rate, drop size distribution, and sampling size have different influences on the accuracy of rainfall variables. The relative errors of rainfall variables caused by sampling variation in a descending order as: water concentration, mean diameter, mass weighed mean diameter, mean volume diameter, radar reflectivity factor, and number density, which are independent with samples number basically; the relative error of rain variables are positively correlated with the margin probability, which is also positively correlated with the rain rate and the mean diameter of raindrops; the sampling size is one of the main factors that influence the margin probability, with the decreasing of sampling area, especially the decreasing of short side of sample size, the probability of margin raindrops is getting greater, hence the error of rain variables are getting greater, and the variables of median size raindrops have the maximum error. To ensure the relative error of rainfall variables measured by optical disdrometer less than 1%, the width of light beam should be at least 40 mm.

Beschreibung: On the effect of moisture on the detection of tropospheric turbulence from in situ measurements Atmospheric Measurement Techniques Discussions, 5, 8223-8240, 2012 Author(s): R. Wilson, H. Luce, H. Hashiguchi, M. Shiotani, and F. Dalaudier The present note addresses the detection of turbulence based on the Thorpe (1977) method applied to an atmosphere where saturation of water vapor occurs. The detection method proposed by Thorpe relies on the reordering in ascending order of a measured profile of a variable conserved through adiabatic processes (e.g. potential temperature). For saturated air, the reordering should be applied to a moist-conservative potential temperature, θ m , which is analogous to potential temperature for a dry (subsaturated) atmosphere. Here, θ m is estimated from the Brunt-Väisälä frequency derived by Lalas and Einaudi (1974) in a saturated atmosphere. The application to balloon data shows that the effective turbulent fraction of the troposphere can dramatically increase when saturation is taken into account. Preliminary results of comparisons with data simultaneously collected from the VHF Middle and Upper atmosphere radar (MUR, Japan) seem to give credence to the proposed approach.

Beschreibung: Calibration and validation of the advanced E-Region Wind Interferometer Atmospheric Measurement Techniques Discussions, 5, 8271-8311, 2012 Author(s): S. K. Kristoffersen, W. E. Ward, S. Brown, and J. R. Drummond The advanced E-Region Wind Interferometer (ERWIN II) combines the imaging capabilities of a CCD detector with the wide field associated with field widened Michelson interferometry. This instrument is capable of simultaneous multi-directional wind observations for three different airglow emissions (oxygen green line (O( 1 S)), the P Q(7) and P P(7) emission lines in the O 2 (0–1) atmospheric band and P 1 (3) emission line in the (6,2) hydroxyl Meinel band) on a three minute cadence. In each direction, for 45 s measurements for typical airglow brightness the instrument is capable of line-of-sight wind precisions of ~ 1 m s −1 for hydroxyl and O( 1 S) and ~ 4 m s −1 for O 2 . This precision is achieved using a new data analysis algorithm which takes advantage of the imaging capabilities of the CCD detector along with knowledge of the instrument phase variation as a function of pixel location across the detector. This instrument is currently located in Eureka, Nunavut as part of the Polar Environment Atmospheric Research Laboratory (PEARL). The details of the physical configuration, the data analysis algorithm, the measurement calibration and validation of the observations are described. Field measurements which demonstrate the capabilities of this instrument are presented. To our knowledge, the wind determinations with this instrument are the most accurate and have the highest observational cadence for airglow wind observations of this region of the atmosphere and match the capabilities of other wind measuring techniques.

Beschreibung: Characterisation of GOME-2 formaldehyde retrieval sensitivity Atmospheric Measurement Techniques Discussions, 5, 7095-7139, 2012 Author(s): W. Hewson, H. Bösch, M. P. Barkley, and I. De Smedt Formaldehyde (HCHO) is an important tracer of tropospheric photochemistry, whose slant column abundance can be retrieved from satellite measurements of solar backscattered UV radiation, using differential absorption retrieval techniques. In this work a spectral fitting sensitivity analysis is conducted on HCHO slant columns retrieved from the Global Ozone Monitoring Experiment 2 (GOME-2) instrument. Despite quite different spectral fitting approaches, the retrieved HCHO slant columns have geographic distributions that generally match expected HCHO sources, though the slant column magnitudes and corresponding uncertainties are particularly sensitive to the retrieval set-up. The choice of spectral fitting window, polynomial order, I 0 correction, and inclusion of minor absorbers tend to have the largest impact on the fit residuals. However, application of a reference sector correction using observations over the remote Pacific Ocean, is shown to largely homogenise the resulting HCHO vertical columns, thereby largely reducing any systematic erroneous spectral fitting.