Retrieval of vertical profiles of cloud droplet effective radius using solar reflectance from cloud sides

Convective clouds play an essential role for Earth’s climate as well as for regional weather events since they have a large influence on the global radiation budget and the global water cycle. In particular, cloud albedo and the formation of precipitation are influenced by aerosol particles within clouds. In order to improve the understanding of processes from aerosol activation, over cloud droplet growth to changes in cloud properties, remote sensing techniques to monitor these microphysical processes throughout the cloud are becoming more and more important. While passive retrievals for spaceborne observations have become sophisticated and commonplace to infer cloud optical thickness and droplet size from cloud tops, cloud sides have remained largely uncharted territory for passive remote sensing. Faced with the small-scale structure of cloud sides, ‘classical’ passive remote sensing techniques, like Nakajima-King, are rendered inappropriate. The aim of this work is to test the theoretical and practical feasibility to gain new insights into the vertical evolution of cloud droplet effective radius by using reflected solar radiation from cloud sides. A central aspect of this study was the close analysis of the impact unknown cloud surface geometry has on effective radius retrievals. In order to handle spatially highly resolved measurements from cloud sides, this work therefore rethought the Nakajima-King approach in the context of a unknown cloud surface geometry. Using extensive Monte-Carlo calculations to explore 3D-effects at convective cloud sides, the sensitivity of reflected solar radiation to cloud droplet size was examined. Furthermore, a method was established to resolve ambiguous radiance regions and thus enhance this sensitivity. Influencing factors were identified and masked out like shadows, ground reflections and cloud ice phase. Based on these findings, a statistical approach was used to develop an effective radius retrieval. Putting the method into practice, the new hyperspectral cloud and sky imager specMACS (Spectrometer of the Munich Aerosol Cloud Scanner) was developed and thoroughly characterized in this work. Additionally, the instrument was applied to convective cloud sides from a ground-based perspective as well as on board the new German research aircraft HALO (High Altitude and LOng Range Research Aircraft). In order to validate this approach, the retrieval was compared to aircraft in situ measurements made during the ACRIDICON-CHUVA experiment conducted over the Brazilian rain forest. The present thesis demonstrates the feasibility to retrieve cloud particle size profiles from cloud sides and thus marks a further important step towards an operational application of this technique

Assessment of low-frequency aeroacoustic emissions of a wind turbine under rapidly changing wind conditions based on an aero-servo-elastic CFD simulation

A meteorologically challenging situation that represents a demanding control task (rotational speed, pitch and yaw) for a wind turbine is presented and its implementation in a simulation is described. A high-fidelity numerical process chain, consisting of the computational fluid dynamics (CFD) solver FLOWer, the multi-body system (MBS) software SIMPACK and the Ffowcs Williams-Hawkings code ACCO, is used. With it, the aerodynamic, servoelastic and aeroacoustic (<20 Hz) behaviour of a generic wind turbine during a meteorological event with strong and rapid changes in wind speed and direction is investigated. A precursor simulation with the meteorological model system PALM is deployed to generate realistic inflow data. The simulated strong controller response of the wind turbine and the resulting aeroelastic behaviour are analysed. Finally, the low-frequency sound emissions are evaluated and the influence of the different operating and flow parameters during the variable inflow is assessed. It is observed that the wind speed and, linked to it, the rotational speed as well as the turbulence intensity are the main influencing factors for the emitted low-frequency sound power of the wind turbine. Yawed inflow, on the other hand, has little effect unless it changes the operational mode to load reduction, resulting in a swap of the main emitter from the blades to the tower

Ground-based imaging remote sensing of ice clouds: uncertainties caused by sensor, method and atmosphere

In this study a method is introduced for the retrieval of optical thickness and effective particle size of ice clouds over a wide range of optical thickness from ground-based transmitted radiance measurements. Low optical thickness of cirrus clouds and their complex microphysics present a challenge for cloud remote sensing. In transmittance, the relationship between optical depth and radiance is ambiguous. To resolve this ambiguity the retrieval utilizes the spectral slope of radiance between 485 and 560aEuro-nm in addition to the commonly employed combination of a visible and a short-wave infrared wavelength. An extensive test of retrieval sensitivity was conducted using synthetic test spectra in which all parameters introducing uncertainty into the retrieval were varied systematically: ice crystal habit and aerosol properties, instrument noise, calibration uncertainty and the interpolation in the lookup table required by the retrieval process. The most important source of errors identified are uncertainties due to habit assumption: Averaged over all test spectra, systematic biases in the effective radius retrieval of several micrometre can arise. The statistical uncertainties of any individual retrieval can easily exceed 10aEuro-A mu m. Optical thickness biases are mostly below 1, while statistical uncertainties are in the range of 1 to 2.5. For demonstration and comparison to satellite data the retrieval is applied to observations by the Munich hyperspectral imager specMACS (spectrometer of the Munich Aerosol and Cloud Scanner) at the Schneefernerhaus observatory (2650aEuro-maEuro-a.s.l.) during the ACRIDICON-Zugspitze campaign in September and October 2012. Results are compared to MODIS and SEVIRI satellite-based cirrus retrievals (ACRIDICON - Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems;MODIS - Moderate Resolution Imaging Spectroradiometer;SEVIRI - Spinning Enhanced Visible and Infrared Imager). Considering the identified uncertainties for our ground-based approach and for the satellite retrievals, the comparison shows good agreement within the range of natural variability of the cloud situation in the direct surrounding

Evaluation of convective cloud microphysics in numerical weather prediction models with dual-wavelength polarimetric radar observations: methods and examples

The representation of cloud microphysical processes contributes substantially to the uncertainty of numerical weather simulations. In part, this is owed to some fundamental knowledge gaps in the underlying processes due to the difficulty of observing them directly. On the path to closing these gaps, we present a setup for the systematic characterization of differences between numerical weather model and radar observations for convective weather situations. Radar observations are introduced which provide targeted dual-wavelength and polarimetric measurements of convective clouds with the potential to provide more detailed information about hydrometeor shapes and sizes. A convection-permitting regional weather model setup is established using five different microphysics schemes (double-moment, spectral bin ("Fast Spectral Bin Microphysics", FSBM), and particle property prediction (P3)). Observations are compared to hindcasts which are created with a polarimetric radar forward simulator for all measurement days. A cell-tracking algorithm applied to radar and model data facilitates comparison on a cell object basis. Statistical comparisons of radar observations and numerical weather model runs are presented on a data set of 30 convection days. In general, simulations show too few weak and small-scale convective cells. Contoured frequency by altitude diagrams of radar signatures reveal deviations between the schemes and observations in ice and liquid phase. Apart from the P3 scheme, high reflectivities in the ice phase are simulated too frequently. Dual-wavelength signatures demonstrate issues of most schemes to correctly represent ice particle size distributions, producing too large or too dense graupel particles. Comparison of polarimetric radar signatures reveals issues of all schemes except the FSBM to correctly represent rain particle size distributions

A reference specimen for compaction tests of fiber reinforcements

Compaction behavior of textiles has a major influence on the outcome of various manufacturing processes for fiber reinforced polymer composites. Nevertheless, no standard exists up to date which specifies test methods or test rigs. A recent international benchmark revealed high variation associated with the result data. This work is a very first step towards a reference specimen, allowing for an isolated view on variations attributed to the test rig mechanics. A specimen design is proposed, intended to show compaction characteristics similar to technical textiles in terms of transverse compaction pressure and corresponding displacement. The reference specimen was tested in a round-robin study comprising test rigs at four different European research institutions. While reproducibility of the compaction behavior on each of the test rigs was high, clear variations between the results gained with different test rigs were observed

Core-Collapse Supernovae: Explosion Dynamics, Neutrinos and Gravitational Waves

The quest for the supernova explosion mechanism has been one of the outstanding challenges in computational astrophysics for several decades. Simulations have now progressed to a stage at which the solution appears close and neutrino and gravitational wave signals from self-consistent explosion models are becoming available. Here we focus one of the recent advances in supernova modeling, the inclusion of general relativity in multi-dimensional neutrino hydrodynamics simulations, and present the latest simulation results for an 11.2 and a 15 solar mass progenitor. We also mention 3D effects as another aspect in supernova physics awaiting further, more thorough investigation.Comment: Contribution to the Proceedings of HANSE 2011 workshop, 8 pages, 4 figure

Laser Powder Bed Fusion Processing of Fe-Mn-Al-Ni Shape Memory Alloy - On the Effect of Elevated Platform Temperatures

In order to overcome constraints related to crack formation during additive processing (laser powder bed fusion, L-BPF) of Fe-Mn-Al-Ni, the potential of high-temperature L-PBF processing was investigated in the present study. The effect of the process parameters on crack formation, grain structure, and phase distribution in the as-built condition, as well as in the course of cyclic heat treatment was examined by microstructural analysis. Optimized processing parameters were applied to fabricate cylindrical samples featuring a crack-free and columnar grained microstructure. In the course of cyclic heat treatment, abnormal grain growth (AGG) sets in, eventually promoting the evolution of a bamboo like microstructure. Testing under tensile load revealed a well-defined stress plateau and reversible strains of up to 4%