Saharan dust transport studied by airborne Doppler wind lidar and numerical models

Every year a large amount of dust is transported over the north Atlantic ocean from North Africa into the Caribbean region. Despite the progress made during the last decades in the understanding and modeling of this process, many uncertainties still remain. In this cumulative thesis, the airborne Doppler wind lidar (DWL) measurements performed during the SALTRACE campaign (June-July 2013), large eddy simulations and the global atmospheric composition model MACC are used to study different aspects of the dust transport process and evaluate the current modeling capabilities. As part of this work, a novel calibration algorithm for the retrieval of quantitative aerosol backscatter and extinction coefficients from the DWL measurements is introduced. This calibration procedure relies on the simultaneous measurement of the same atmospheric volume by the airborne DWL and an aerosol ground-based lidar. This method is then validated with satellite lidar CALIOP and ground-based lidar measurements. Additionally, improvements in vertical wind retrieval algorithms are discussed and their impact on the retrieval accuracy estimated by means of two different methods. Based on this extended airborne DWL dataset, which includes simultaneous backscatter and wind measurements, different processes associated with the Saharan dust transport are investigated. Measurements carried out in the Cabo Verde and Barbados regions revealed the presence of island-induced gravity waves. These measurements are used in combination with in-situ wind and particle number density retrievals, large eddy simulations, and wavelet analysis to determine the main characteristics of the observed waves and their interaction with the Saharan Air Layer. Finally, DWL and CALIOP measurements are used to evaluate the capabilities of the MACC global aerosol model to reproduce the Saharan dust long-range transport process during SALTRACE. A comparison and analysis of the measured and simulated average wind and dust distributions in the West African and Caribbean regions is presented. Three case studies covering different characteristic features, like the African Easterly Jet and the dust transport modulation by the African Easterly Waves, are discussed with an outlook to future campaigns and the assimilation of the data products for the ESA's future satellite missions Aeolus and EarthCARE

Saharan dust transport studied by airborne Doppler wind lidar and numerical models

Every year a large amount of dust is transported over the north Atlantic ocean from North Africa into the Caribbean region. Despite the progress made during the last decades in the understanding and modeling of this process, many uncertainties still remain. In this cumulative thesis, the airborne Doppler wind lidar (DWL) measurements performed during the SALTRACE campaign (June-July 2013), large eddy simulations and the global atmospheric composition model MACC are used to study different aspects of the dust transport process and evaluate the current modeling capabilities. As part of this work, a novel calibration algorithm for the retrieval of quantitative aerosol backscatter and extinction coefficients from the DWL measurements is introduced. This calibration procedure relies on the simultaneous measurement of the same atmospheric volume by the airborne DWL and an aerosol ground-based lidar. This method is then validated with satellite lidar CALIOP and ground-based lidar measurements. Additionally, improvements in vertical wind retrieval algorithms are discussed and their impact on the retrieval accuracy estimated by means of two different methods. Based on this extended airborne DWL dataset, which includes simultaneous backscatter and wind measurements, different processes associated with the Saharan dust transport are investigated. Measurements carried out in the Cabo Verde and Barbados regions revealed the presence of island-induced gravity waves. These measurements are used in combination with in-situ wind and particle number density retrievals, large eddy simulations, and wavelet analysis to determine the main characteristics of the observed waves and their interaction with the Saharan Air Layer. Finally, DWL and CALIOP measurements are used to evaluate the capabilities of the MACC global aerosol model to reproduce the Saharan dust long-range transport process during SALTRACE. A comparison and analysis of the measured and simulated average wind and dust distributions in the West African and Caribbean regions is presented. Three case studies covering different characteristic features, like the African Easterly Jet and the dust transport modulation by the African Easterly Waves, are discussed with an outlook to future campaigns and the assimilation of the data products for the ESA's future satellite missions Aeolus and EarthCARE

ADM-Aeolus pre-launch activities and recent advances in spaceborne and airborne Wind Lidar Systems

The first space-borne wind lidar mission ADM-Aeolus from ESA is currently scheduled for launch by mid-2017. For the preparation of the Aeolus validation, an airborne field experiment was performed during 3 weeks in May 2015 with the DLR Falcon and the NASA DC-8 aircraft. For the first time 4 wind lidars were deployed during an airborne campaign including two coherent and two direct-detection wind lidars at a wavelength of 2μm and 355 nm. A total of 7 coordinated flights of the Falcon and DC-8 yielded an extensive dataset. Additionally, DLR’s airborne coherent Doppler Wind Lidar was recently deployed in 3 coordinated airborne campaigns aiming to investigate the life cycle of gravity waves from ground up to the mesosphere. The horizontal and vertical wind measurements of the lidar provide valuable data for characterizing tropospheric gravity waves and background wind conditions

Retrieval of aerosol backscatter and vertical wind from airborne coherent Doppler wind lidar measurements

This study presents methods for the retrieval of calibrated backscatter and vertical wind speed from an airborne Doppler wind lidar (DWL). The backscatter retrieval relies on sun photometer measurements and the simultaneous retrievals of an airborne DWL at 2 μm and a ground-based aerosol lidar at 532 nm for the retrieval of the calibration constants. A refinement of the vertical wind retrieval algorithms based on speed and distance of ground returns will also be discussed. The derived methods were applied to a set of case studies from the SALTRACE campaign (http://www.pa.op.dlr.de/saltrace), which aimed to characterize the Saharan dust long range transport between Africa and the Caribbean. The calibrated backscatter measurements, first validated with a set of ground based lidar and CALIPSO measurements, were then used in combination with horizontal and vertical wind retrievals for the characterization of island induced gravity waves and the evaluation of an aerosol transport model

Dust transport across the Atlantic studied by airborne Doppler wind lidar during the SALTRACE experiment in 2013

During the SALTRACE field experiment, conducted during June/July 2013, the Saharan dust transport across the Atlantic was analyzed by a set of ground based, in-situ and airborne instruments, including a 2-μm coherent DWL (Doppler wind lidar) mounted onboard the DLR Falcon 20 research aircraft. An overview of the measurements of aerosol backscatter and extinction, horizontal and vertical winds retrieved from the DWL are presented together with a brief description of the applied methods. The retrieved measurements provide direct observation of Saharan dust transport mechanisms across the Atlantic as well as island induced lee waves in the Barbados region

Saharan dust long-range transport across the Atlantic studied by an airborne Doppler wind lidar and the MACC model

A huge amount of dust is transported every year from north Africa into the Caribbean region. This paper presents an investigation of this long-range transport process based on airborne Doppler wind lidar (DWL) measurements conducted during the SALTRACE campaign (June–July 2013), as well as an evaluation of the ability of the MACC (Monitoring Atmospheric Composition and Climate) global aerosol model to reproduce it and its associated features. Although both the modeled winds from MACC and the measurements from the DWL show a generally good agreement, some differences, particularly in the African easterly jet (AEJ) intensity, were noted. The observed differences between modeled and measured wind jet speeds are between 5 and 10 m/s. The vertical aerosol distribution within the Saharan dust plume and the marine boundary layer is investigated during the June–July 2013 period based on the MACC aerosol model results and the CALIOP satellite lidar measurements. While the modeled Saharan dust plume extent shows a good agreement with the measurements, a systematic underestimation of the marine boundary layer extinction is observed. Additionally, three selected case studies covering different aspects of the Saharan dust long-range transport along the west African coast, over the North Atlantic Ocean and the Caribbean are presented. For the first time, DWL measurements are used to investigate the Saharan dust long-range transport. Simultaneous wind and backscatter measurements from the DWL are used, in combination with the MACC model, to analyze different features associated with the long-range transport, including an African easterly wave trough, the AEJ and the intertropical convergence zone

Heterodyne high-spectral-resolution lidar

In this work, a novel lidar technique to perform high-spectral-resolution measurements of the atmospheric backscatter is discussed and the first results are presented. The proposed method, which relies on a heterodyne detection receiver, allows us not only to separate the molecular and the aerosol component of the atmospheric backscatter, but also to investigate the spectral shape of the Rayleigh–Brillouin line. As in the case of the direct-detection high-spectral-resolution lidars, the separation of the different scattering processes would allow an independent system calibration and aerosol extinction measurements. The proposed retrieval technique was successfully tested on the Deutsches Zentrum für Luft- und Raumfahrt airborne Doppler wind lidar system with measurements conducted during different measurement campaigns and under different atmospheric conditions. In light of these results, further ideas for the implementation of a dedicated heterodyne high-spectral-resolution lidar are discussed

Aerosol backscatter and extinction retrieval from airborne coherent Doppler wind lidar measurements

A novel method for coherent Doppler wind lidars (DWLs) calibration is shown in this work. Concurrent measurements of a ground based aerosol lidar operating at 532 nm and an airborne DWL at 2 μm are used in combination with sun photometer measurements for the retrieval of backscatter and extinction profiles. The presented method was successfully applied to the measurements obtained during the Saharan Aerosol Long-range Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE: http://www.pa.op.dlr.de/saltrace), which aimed to characterize the Saharan dust long range transport between Africa and the Caribbean

Investigations of boundary layer structure, cloud characteristics and vertical mixing of aerosols at Barbados with large eddy simulations

Large eddy simulations (LESs) are performed for the area of the Caribbean island Barbados to investigate island effects on boundary layer modification, cloud generation and vertical mixing of aerosols. Due to the presence of a topographically structured island surface in the domain center, the model setup has to be designed with open lateral boundaries. In order to generate inflow turbulence consistent with the upstream marine boundary layer forcing, we use the cell perturbation method based on finite amplitude potential temperature perturbations. In this work, this method is for the first time tested and validated for moist boundary layer simulations with open lateral boundary conditions. Observational data obtained from the SALTRACE field campaign is used for both model initialization and a comparison with Doppler wind and Raman lidar data. Several numerical sensitivity tests are carried out to demonstrate the problems related to “gray zone modeling” when using coarser spatial grid spacings beyond the inertial subrange of three-dimensional turbulence or when the turbulent marine boundary layer flow is replaced by laminar winds. Especially cloud properties in the downwind area west of Barbados are markedly affected in these kinds of simulations. Results of an additional simulation with a strong trade-wind inversion reveal its effect on cloud layer depth and location. Saharan dust layers that reach Barbados via long-range transport over the North Atlantic are included as passive tracers in the model. Effects of layer thinning, subsidence and turbulent downward transport near the layer bottom at z ≈ 1800 m become apparent. The exact position of these layers and strength of downward mixing is found to be mainly controlled atmospheric stability (especially inversion strength) and wind shear. Comparisons of LES model output with wind lidar data show similarities in the downwind vertical wind structure. Additionally, the model results accurately reproduce the development of the daytime convective boundary layer measured by the Raman lidar

Transport of mineral dust derived from airborne wind lidar measurements during SALTRACE

During the SALTRACE field experiment conducted between the 10 of June and the 15 of July 2013, the transport and properties of Saharan dust were characterized by a 2-µm Doppler wind lidar (DWL) deployed on the DLR Falcon 20 research aircraft. Unlike aerosol lidars, the DLW is able to simultaneously measure wind fields and -by means of an adequate calibration- aerosol optical properties, which is more adequate for aerosol transport studies. The retrieved horizontal and vertical wind speed provide a direct observation of dust long range transport mechanisms across the Atlantic (e.g. by the African easterly jet) from Western Africa to the Caribbean. Vertical wind observations revealed the structure of island induced lee waves in the Cape Verde and Barbados regions. A novel method for the calibration of DWLs based on simultaneous measurements with a ground-based aerosol lidar and sun photometer was developed. After being calibrated, the system is able to retrieve quantitative aerosol backscatter and extinction coefficients, which is usually not obtained from coherent lidars. Results from the validation with a ground-based aerosol lidar in Barbados and the CALIPSO satellite instrument will be discussed