Browsing by Subject "Aerosol"

Now showing 1 - 2 of 2

A backscatter lidar forward operator for aerosol-representing atmospheric chemistry models
(2020) Geisinger, Armin; Wulfmeyer, Volker
State-of-the-art atmospheric chemistry models are capable of simulating the transport and evolution of aerosols and trace gases but there is a lack of reliable methods for model validation and data assimilation. Networks of automated ceilometer lidars (ACLs) could be used to fill this gap. These networks are already used for the detection of clouds and aerosols, providing a 3D dataset of atmospheric backscatter profiles. But as the aerosol number concentration cannot be obtained from the ACL data alone; one needs a backscatter-lidar forward model to simulate lidar profiles from the model variables. Such an operator allows then for a qualitative and quantitative model validation based on ACL data. In this work, a newly developed backscatter-lidar forward operator and the related sensitivity studies are presented and results of the forward operator applied on model output data are compared to measured ACL profiles in the frame of a case study. As case study, the eruption of the Icelandic volcano Eyjafjallajökull in 2010 was chosen and extensively analyzed. The Consortium for Small-scale Modeling - Aerosols and Reactive Trace gases (COSMO-ART) model of DWD (Deutscher Wetterdienst) was operated during this event for ash-transport simulations over Europe. For the forward model, the attenuated backscatter coefficient is used as lidar-independent variable, which only relies on the laser wavelength. To calculate the attenuated backscatter coefficient, the size-dependent aerosol number concentration and the scattering properties of each aerosol type and size have to be simulated. While the aerosol number concentration is a model output variable, the scattering properties were determined by extensive scattering calculations. As these scattering calculations require assumptions about the aerosol refractive indices and shapes, sensitivity studies were performed to estimate the uncertainties related to the particle properties as represented by the model system. An analysis of the particle shape effect for the extinction and backscatter coefficients resulted in huge differences of the scattering properties between spherical, ellipsoidal and cylindrical particle shapes. Due to a particle shape mixture in typical volcanic ash plumes, the application of non-spherical scattering calculation methods for estimating the effective optical properties requires more information related to the particle shape distribution (specifically: a particle size and shape distribution). As such information was not available for the present case study, it was necessary to assume spherical shaped volcanic ash particles but estimate the uncertainty related to this assumption within the frame of additional sensitivity studies. Finally, the forward modeled lidar profiles were compared to ACL measurements from stations of the German ACL network. The comparison required an extraction of common time and height intervals of the ACL and forward modeled COMSO-ART data as well as reshaping the datasets to the same vertical and temporal resolution. Significant differences between ACL profiles and the output of the forward operator applied to the COSMO-ART data were found. Some ash layer structures were at similar coordinates which is remarkable due to the uncertainties related to the model dynamics and the limited amount of measurement data that could be used for model validation. In detail, however, the major fraction of the compared time and height interval differed both in the relative signal intensity and the layer structures of the volcanic ash plume. Based on such quantitative comparison, a future data assimilation system could correct the model prediction of the forward modeled attenuated backscatter coefficient, the time of arrival, as well as the vertical structure of the volcanic ash plume. In summary, the continuous and distributed data stream provided by ACL stations was found to deliver valuable verification information for dispersion simulations of aerosol events. But major issues have been determined which limit current realizations of backscatter-lidar forward operators for aerosol transport simulations: First, it is suggested that the ACL systems improve their dynamic range and perform automatic calibration to increase the precision of ACL data and for calculating the measured attenuated backscatter coefficient with a minimum leftover of uncertainties. This will allow for the calculation of the attenuated backscatter coefficient in the presence of clouds as well as of faint aerosol signals. Second, the aerosols scattering properties have to be analyzed even more extensively which includes both the variety of aerosol sizes or types as well as the size distribution information. From the findings within this study, the particle size distribution was indentified to be a critical component when using monodisperse size classes.
Development of an eye-safe solid-state tunable laser transmitter around 1.45 my m based on Cr 4+:YAG crystal for lidar applications
(2008) Petrova-Mayor, Anna; Wulfmeyer, Volker
A gain switched tunable Cr4+:YAG laser was developed using a Q-switched flashlamp?pumped Nd:YAG pump laser at 10 Hz. A vacuum spatial filter (VSF) was designed in order to filter the ?hot spots? of the pump beam profile. As a result of applying the VSF, a nearly Gaussian-shaped beam profile was achieved which enabled safe pumping of the Cr4+:YAG crystal with pulse energies in excess of 100 mJ. An extensive experimental optimization of the efficiency of the wavelength converter was performed. A maximum output energy of ~7 mJ at 1430?1450 nm, corresponding to ~7% conversion efficiency (with regard to absorbed pump energy), and a pulse duration of 30?35 ns were obtained with a 25-cm-long stable resonator. Tunability in the range 1350?1500 nm and spectral linewidth of ~200G Hz were demonstrated using a 3-plate birefringent filter. The laser was multimode with a flat-top profile and sufficiently good M2~4. The performance and size of the laser are acceptable for use in a laboratory based non-scanning lidar system if a narrow-band birefringent filter is installed. In order to employ a scanning mobile lidar, high pulse frequency (>100 Hz) of the pump laser for the Cr4+:YAG laser is required. The tunability permits the improvement of the laser transmitter for water-vapor DIAL measurements at on-line wavelengths of approximately 1459 nm or 1484 nm if injection-seeding is applied.