Andreas Fix

Depolarization ratio profiling at several wavelengths in pure Saharan dust during SAMUM 2006

Vertical profiles of the linear particle depolarization ratio of pure dust clouds were measured during the Saharan Mineral Dust Experiment (SAMUM) at Ouarzazate, Morocco (30.9◦N, –6.9◦E), close to source regions in May–June 2006, with four lidar systems at four wavelengths (355, 532, 710 and 1064 nm). The intercomparison of the lidar systems is accompanied by a discussion of the different calibration methods, including a new, advanced method, and a detailed error analysis. Over the whole SAMUM periode pure dust layers show a mean linear particle depolarization ratio at 532 nm of 0.31, in the range between 0.27 and 0.35, with a mean Ångström exponent (AE, 440–870 nm) of 0.18 (range 0.04–0.34) and still high mean linear particle depolarization ratio between 0.21 and 0.25 during periods with aerosol optical thickness less than 0.1, with a mean AE of 0.76 (range 0.65–1.00), which represents a negative correlation of the linear particle depolarization ratio with the AE. A slight decrease of the linear particle depolarization ratio with wavelength was found between 532 and 1064 nm from 0.31 ± 0.03 to 0.27 ± 0.04.

Airborne high spectral resolution lidar for measuring aerosol extinction and backscatter coefficients

An airborne high spectral resolution lidar (HSRL) based on an iodine absorption filter and a high-power frequency-doubled Nd:YAG laser has been developed to measure backscatter and extinction coefficients of aerosols and clouds. The instrument was operated aboard the Falcon 20 research aircraft of the German Aerospace Center (DLR) during the Saharan Mineral Dust Experiment in May-June 2006 to measure optical properties of Saharan dust. A detailed description of the lidar system, the analysis of its data products, and measurements of backscatter and extinction coefficients of Saharan dust are presented. The system errors are discussed and airborne HSRL results are compared to ground-based Raman lidar and sunphotometer measurements.

Tunable β-barium borate optical parametric oscillator: operating characteristics with and without injection seeding

The operating characteristics of a pulsed β-barium borate optical parametric oscillator, pumped at 355 nm by a single-mode Nd:YAG laser, are described. This device operates with high conversion efficiency (up to 61%) and high output energy (>100 mJ) over most of its wide tuning range (0.41–2.7 μm). Continuous narrow-band tuning over as wide a range as 140 cm−1 is achieved by injection seeding with light from a tunable pulsed dye laser. The spectroscopic potential of this narrow-band β-barium borate optical parametric oscillator is demonstrated by recording photoacoustic spectra of acetylene gas in the near-infrared spectral range.

Spatial distribution and optical properties of Saharan dust observed by airborne high spectral resolution lidar during SAMUM 2006

Airborne measurements of pure Saharan dust extinction and backscatter coefficients, the corresponding lidar ratio and the aerosol optical thickness (AOT) have been performed during the Saharan Mineral Dust Experiment 2006, with a high spectral resolution lidar. Dust layers were found to range from ground up to 4–6 km above sea level (asl). Maximum AOT values at 532 nm, encountered within these layers during the DLR Falcon research flights were 0.50–0.55. A significant horizontal variability of the AOT south of the High Atlas mountain range was observed even in cases of a well-mixed dust layer. High vertical variations of the dust lidar ratio of 38–50 sr were observed in cases of stratified dust layers. The variability of the lidar ratio was attributed to dust advection from different source regions. The aerosol depolarization ratio was about 30% at 532 nm during all measurements and showed only marginal vertical variations.

ACRIDICON–CHUVA Campaign: Studying Tropical Deep Convective Clouds and Precipitation over Amazonia Using the New German Research Aircraft HALO

AbstractBetween 1 September and 4 October 2014, a combined airborne and ground-based measurement campaign was conducted to study tropical deep convective clouds over the Brazilian Amazon rain forest. The new German research aircraft, High Altitude and Long Range Research Aircraft (HALO), a modified Gulfstream G550, and extensive ground-based instrumentation were deployed in and near Manaus (State of Amazonas). The campaign was part of the German–Brazilian Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems–Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud Resolving Modeling and to the GPM (Global Precipitation Measurement) (ACRIDICON– CHUVA) venture to quantify aerosol–cloud–precipitation interactions and their thermodynamic, dynamic, and radiative effects by in situ and remote sensing measurements over Amazonia. The ACRIDICON–CHUVA field observations were carried out in cooperation with the second intensive operating period...

ML-CIRRUS - The airborne experiment on natural cirrus and contrail cirrus with the high-altitude long-range research aircraft HALO

AbstractThe Midlatitude Cirrus experiment (ML-CIRRUS) deployed the High Altitude and Long Range Research Aircraft (HALO) to obtain new insights into nucleation, life cycle, and climate impact of natural cirrus and aircraft-induced contrail cirrus. Direct observations of cirrus properties and their variability are still incomplete, currently limiting our understanding of the clouds’ impact on climate. Also, dynamical effects on clouds and feedbacks are not adequately represented in today’s weather prediction models.Here, we present the rationale, objectives, and selected scientific highlights of ML-CIRRUS using the G-550 aircraft of the German atmospheric science community. The first combined in situ–remote sensing cloud mission with HALO united state-of-the-art cloud probes, a lidar and novel ice residual, aerosol, trace gas, and radiation instrumentation. The aircraft observations were accompanied by remote sensing from satellite and ground and by numerical simulations.In spring 2014, HALO performed 16 f...

Latent heat flux profiles from collocated airborne water vapor and wind lidars during IHOP_2002

Abstract Latent heat flux profiles in the convective boundary layer (CBL) are obtained for the first time with the combination of the Deutsches Zentrum fur Luft- und Raumfahrt (DLR) water vapor differential absorption lidar (DIAL) and the NOAA high resolution Doppler wind lidar (HRDL). Both instruments were integrated nadir viewing on board the DLR Falcon research aircraft during the International H2O Project (IHOP_2002) over the U.S. Southern Great Plains. Flux profiles from 300 to 2500 m AGL are computed from high spatial resolution (150 m horizontal and vertical) two-dimensional water vapor and vertical velocity lidar cross sections using the eddy covariance technique. Three flight segments on 7 June 2002 between 1000 and 1300 LT over western Oklahoma and southwestern Kansas are analyzed. On two segments with strong convection, the latent heat flux peaks at (700 ± 200) W m−2 in the entrainment zone and decreases linearly to (200 ± 100) W m−2 in the lower CBL. A water vapor budget analysis reveals that ...

Visible optical parametric oscillation in synchronously pumped beta-barium borate

Abstract A visible BaB 2 O 4 optical parametric oscillator (OPO) synchronously pumped by the 360 nm third harmonic of a pulsed passively modelocked Nd: YAG laser has been demonstrated. The wavelength of the generated 20 ps long pulses is tunable in the range of 406 nm to 3170 nm. The measured total conversion efficiency is as high as 30%; the maximum pulse peak power of the visible output exceeds 15 MW. The spectral width of the generated short pulses has been narrowed to 0.24 nm using a diffraction grating as intracavity wavelength selector. With spectral narrowing and a pump power density of 2 GW/cm 2 the OPO efficiency was about 8.5%.

Spectral properties of pulsed nanosecond optical parametric oscillators: experimental investigation and numerical analysis

We report on an experimental investigation and numerical analysis of the spectral properties of pulsed nanosecond optical parametric oscillation. The optical parametric oscillator (OPO) used in the experimental investigations consisted of a 2.5-mm-long crystal of beta-barium borate (pumped by the third harmonic of a Q-switched Nd:YAG laser) and a 3.5-mm-long optical cavity of two plane mirrors. Despite the short crystal, this OPO provided an efficiency of 26% at pump-pulse intensities of 30 mJ. Owing to the short cavity, the longitudinal mode spacing was ∼1 cm-1. The complete mode spectrum of individual OPO pulses could thus be recorded with a 1-m grating spectrometer having a spectral resolution of 0.25 cm-1. If the OPO was unseeded, the energy distribution in the mode spectrum varied considerably from pulse to pulse. Near threshold, the fluctuations of the energy of individual modes are close to 100%. The origin of these fluctuations are the statistical fluctuations of the vacuum field that initiate the OPO oscillation. The measured mode spectra were numerically simulated by solving the coupled amplitude equations for the signal and the idler modes and the pump field. The numerical results are in good agreement with the measurements. In further investigations, the OPO was seeded with either pulsed or cw laser radiation. The experimental results demonstrate that seed pulse energies of a few nanojoules or cw seed powers of a few milliwatts were sufficient for reliable single-longitudinal-mode operation. The mode spectra of the seeded OPO as well as the spectra recorded for seed powers in the regime between seeded and unseeded OPO operation are in good agreement with the spectra predicted by numerical analysis.

Optical parametric oscillators and amplifiers for airborne and spaceborne active remote sensing of CO2 and CH4

Carbon dioxide (CO2) and methane (CH4) are the most important of the greenhouse gases that are directly influenced by human activities. The Integrated Path Differential Absorption (IPDA) lidar technique using hard target reflection in the near IR (1.57μm and 1.64μm) to measure the column-averaged dry air mixing ratio of CO2 and CH4 with high precision and low bias has the potential to deliver measurements from space and air that are needed to understand the sources and sinks of these greenhouse gases. CO2 and CH4 IPDA require tunable laser sources at 1.57 μm and 1.64 μm that coincide with appropriate absorption lines of these species having high pulse energy and average power as well as excellent spectral and spatial properties. Within this study we have realized more than 50mJ of pulse energy in the near IR coincident with appropriate absorption lines using an injection-seeded optical parametric oscillator-amplifier system pumped at 100 Hz. At the same time this device showed excellent spectral and spatial properties. Bandwidths of less than 100 MHz with a high degree of spectral purity (> 99.9 %) have been achieved. The frequency stability was likewise excellent. The M2-factor was better than 2.3. Owing to these outstanding properties optical parametric devices are currently under investigation for the CH4 lidar instrument on the projected French-German climate satellite MERLIN. A similar device is under development at DLR for the lidar demonstrator CHARM-F which will enable the simultaneous measurement of CO2 and CH4 from an airborne platform.