Development of a high spatial and temporal resolution Raman lidar for turbulent observations

Water vapor plays an important role in weather, global climate processes and atmospheric chemistry. It is the most significant greenhouse gas and affects the planet's radiative and non-radiative energy balance. The distribution of water vapor in the atmosphere is quite variable both horizontally and vertically and has a significant influence on the atmospheric circulation and temperature structure. Accurate data about water vapor and temperature is needed for weather forecasting, weather and climate research, boundary layer and cloud process studies, and atmospheric chemistry. Despite the need for these data, obtaining accurate water vapor and temperature measurements with high temporal and spatial resolution has remained an only partially solved problem. The Raman lidar technique for water vapor and temperature measurements is a straightforward method and is based on well-known physical principles. It can supply accurate data with high spatial and temporal resolution for weather, climate, atmospheric boundary layer (ABL) studies and other atmospheric research. One of the two goals of this thesis is to develop and construct a Raman lidar instrument with high spatial (1.5 m) and temporal (1 s) resolution and an operational range of 15 – 500 m for systematic observations of water vapor profiles. The measured profiles together with a new generation of Large Eddy Simulations (LES) will be used to attain an improved understanding of the complex linkages between the land surface and the overlying atmospheric boundary layer. The second goal is to develop and test a new method for the determination of the atmospheric transmission correction factor for non solar-blind water vapor Raman lidars based exclusively on the use of Raman lidar signals without needing a priori information about the aerosol optical properties.

Ozone and water vapor measurements by Raman lidar in the planetary boundary layer

The temporal and spatial retrieve of ozone (O3) concentration and water vapor (H2O) mixing ratio in the troposphere is of essential interest. Contrary to the stratospheric case, the tropospheric ozone can have a harmful impact, with its toxic effect, on humans and vegetation, accelerating the degradation of the minerals and participating in the green-house problem. Concerning the water vapor, knowledge of its highly variable concentration is essential to both the chemistry of the troposphere (O(1D) + H2O —> 2 OH) where it participates, among others, in the generation of the hydroxyl radical (OH) and to the meteorology. Water vapor is the dominant green-house gas, it plays an important role in the atmospheric chemistry. The conversion and transport of water in the atmosphere is the essential point in the earth's radiation budget. Due to the complexity and the non-linearity of the air pollution system including emissions, chemistry, thermal radiation, transport and deposition, pollution abatement strategies can only be designed rightly by the use of a three-dimensional mesoscale Eulerian photochemical transport model. To check such models, measurement campaigns are undertaken, in which many physical (wind, temperature, H2O, etc.) and chemical parameters (emissions and imissions) are measured at different parts of the atmosphere. LIDAR (LIght Detection And Ranging), which is a real-time method for measuring air pollutants in situ, is one of the best tools to make 3-D measurements of gases concentrations like O3, H2O and others. Contrary to the ground based measurements that are highly sensitive to the very local conditions, lidar sensitivity and resolution in space and time is optimal to obtain measurements and to compare or give some input data for the models. During the last thirty years, (elastic backscatter) Differential Absorption Lidar (DIAL) has been established as a convenient tool for the monitoring of the three dimensional real time concentrations of air pollutants [Measures, 1992], [Schoulepnikoff et al., 1998]. But the DIAL apparatus has shown limitations: — the operation in layers with high aerosol loading like in the Planetary Boundary Layer (PBL) where they are highly variable — the simultaneous detection of several atmospheric components or pollutants is impossible [Bösenberg, 1996] — the detection at short range is difficult due to the high dynamics. Furthermore, due to its spectrum and the strong influence from other elements, the water vapor can not be easily measured in the UV with classical DIAL systems. The goal of this work was to develop a method to simultaneously measure the ozone absolute concentration and the water vapor mixing ratio in the PBL. Experiments with Nd : YAG and KrF lasers were made and utilization of both analog and photon counting techniques, increasing the dynamic range, were investigated. To retrieve the ozone concentration profile, we take advantage of the simultaneous spontaneous Raman backscattering on the molecules of nitrogen (N2) and oxygen (O2) that have different ozone absorption cross-sections. Thus with a modified DIAL technique, the ozone concentration can be measured without most of the interference from poorly known backscatter by particles. Water vapor mixing ratio profile can also be obtained with a set of three Raman backscattered signals, simultaneously detected, from the molecules of H2O, N2 and O2. The main advantage of this Raman system is its essential independence to the wavelength dependent backscatter problems as induced by aerosols, and the fact that the N2 and O2 concentrations are well known as well as the Raman cross-sections of interest. Although the Raman cross-sections are two or three orders of magnitude lower than the elastic backscattering cross-sections, they are compensated by the proportionally much higher concentrations of O2, N2 and H2O compared to trace gases like O3. The development of the Raman — DIAL method for atmospheric measurements in the PBL presents several challenges. One is the development of highly-sensitive lidar systems, in particular the optical receiver, the spectrometer and the signal acquisition for the Raman part of the Raman-DIAL system. Also the data processing procedure for simultaneous evaluation of the ozone and the water vapor profiles. Both of these challenges present a number of issues, theoretical and practical, that are investigated in the frame of this work.

EPFL2001

Measurement and analysis of aerosols, cirrus-contrails, water vapor and temperature in the upper troposphere with the Jungfraujoch LIDAR system

Ioan Balin

The impact of human activities on the global climate may lead to large disruptions of the economic, social and political status quo in the middle and long term. Understanding the dynamics of the Earth's climate is thus of paramount importance and one of the major scientific challenges of our time. The estimation of the relative contribution of the many components (interacting each other) of the Earth's climate system requires observation and continuous monitoring of various atmospheric physical and chemical parameters. Temperature, water vapor and greenhouse gases concentration, aerosol and clouds loads, and atmospheric dynamics are parameters of particular importance in this respect. The quantification of the anthropogenic influence on the dynamics of these above-mentioned parameters is of crucial importance nowadays but still affected by significant uncertainties. In the present context of these huge uncertainties in our understanding of how these different atmospheric compounds contribute to the radiative forcing, the research presented in this report is related to the following topics: Development of lidar-based remote sensing techniques for monitoring atmospheric compounds and processes Aerosols – cirrus – contrails optical properties up to the tropopause Water vapor mixing ratio and relative humidity estimation in the upper troposphere Temperature profiling in the upper troposphere-lower stratosphere Characterization of the long-range transported mineral aerosols (i.e. Saharan dust outbreaks) Planetary boundary layer-upper troposphere exchanges (i.e. August 2003 heatwave effect) In the above research frames, the development and application of measurement techniques for the monitoring of climate-change parameters, this work refers to the implementation of a multi-wavelength LIDAR1 system (JFJ - LIDAR)2 at the International Scientific Station of Jungfraujoch (ISSJ, 46°33' N, 7°59' E, at 3580 m ASL- above sea level). The JFJ3 station is situated above the planetary boundary layer (PBL) almost all year long and is located in a mountain pass linking the Swiss plateau to the North with the Rhone Valley to the South through the Aletsch glacier corridor. Measurements with the JFF-LIDAR system provide regular vertical and horizontal remote sensing of water vapor, temperature, and optical properties (backscatter and extinction coefficients) of aerosols, cirrus clouds and contrails in the upper troposphere (UT)4. The lidar system is based on the laser emission at 355, 532 and 1064 nm and on subsequent detection of both elastic (Mie) and inelastic (Raman) atmospheric backscatter light. The backscattering collected radiation is precisely: the elastic at 355, 532 and 1064 nm; the rotational-vibrational Raman radiation from nitrogen at ~ 387 nm, and from water vapor at ~ 407 nm as well as the pure rotational nitrogen/oxygen Raman excited at ~ 532 nm. The depolarization of the initially linearly polarized radiation was also detected at 532 nm and it was use to distinguish between water and ice contents in cirrus clouds, but also it may reveal long-range transported mineral aerosols such as Saharan dust. Profiles of backscatter and extinction coefficients of aerosols-cirrus-contrails, needed for estimation of the radiative balance of the atmosphere, are derived from elastic and Raman light scattering processes, or through a combination of both, using devoted algorithms and software developed within this research. Data gathered from routine measurements are statistically analyzed and interpreted in comparison with similar measurements obtained from colocated techniques. Optical and microphysical properties of a typical contrail were studied. The UT water vapor mixing ratio profiles are estimated from the ratio of ~ 407 nm and ~387 nm Raman radiation excited by 355 nm. Upon appropriate calibration, real time water vapor mixing ratio profiles derived from LIDAR measurements are found in good agreement with the closest radiosounding techniques, and co-located measurements such as the GPS5 and sun photometer based measurements. The water vapor profiles, combined with simultaneous temperature profiles taken from atmospheric models, radiosounding or, more realistically, based on the pure rotational Raman technique, were used for the estimation of relative humidity profiles which allow the identification of UT super-saturation regions. Air temperature profiles were obtained up to the lower stratosphere using the backscatter of pure rotational Raman radiation excited by 532 nm. These first results compare well to simultaneous regional radiosounding measurements, and follow standard atmospheric models. The pure rotational Raman backscatter was also used for determining absolute extinction and the lidar ratio for cirrus clouds. Based on the JFJ-LIDAR measurements, supported by co-located and regional measurements, the research presents also in detail two case studies related to climate problematic: The first concerns the tracking of a Saharan dust outbreak (SDO) and the derivation of its optical properties. The second study refers to the analysis of the evolution and consequences of the high altitudes planetary boundary layer (PBL)6 convection during the August 2003 heat - wave episode. The results presented within this research provide a promising basis for extending these JFJ-LIDAR observations from the upper troposphere into the stratosphere by using the existent astronomic telescope (~15 times increased sensitivity) and a new (~ 3 times more powerful) laser source. Consequently DIAL7 technique for measuring the stratospheric ozone will be developed and implemented in the near future at JFJ. Future challenges include also JFJ-LIDAR remote control operation and the ability of real time obtained atmospheric calibrated profiles (i.e. optical properties of aerosols-cirruscontrails, water vapor, temperature and ozone). --------------------1 LIDAR – LIght Detection And Ranging 2 JFJ-LIDAR is the acronym used here for Jungfraujoch multi-wavelength LIDAR system 3 JFJ is the abbreviation for Jungfraujoch 4 UT will be used as abbreviation for upper troposphere (from ~ 3600 m ASL to the tropopause atmospheric region) 5 GPS is the acronym for Global Positioning System 6 PBL - planetary boundary layer – its top is usually situated under the altitude of the JFJ station (i.e. 3600m ASL) 7 DIAL - is the acronym coming from DIfferential Absorption Lidar

EPFL2004

Theoretical and numerical studies of atmospheric boundary-layer flows over complex terrain

Marco Giovanni Giometto

Atmospheric boundary-layer (ABL) flows over complex terrain have been the focus of active research, given their impact on weather and climate variability. Surface complexity is understood in a broad sense and includes variation in roughness properties, inclination of the underlying surface, presence of heterogeneous forcing mechanisms (e.g., buoyancy, humidity), to name but a few. Most assumptions of classical boundary-layer similarity theory do not hold under such conditions, complicating matters from both a measurement and modeling perspective. Here, a combination of analytical and numerical approaches are used to address two among the most relevant problems: turbulent slope flows, and ABL flows over multi-scale rough surfaces. The first part of the thesis focuses on slope flows: the building blocks of local weather in mountainous regions. To understand the system conceptually, a closed-form analytic solution to the Prandtl slope flow model is first derived, prescribing transfer coefficients in accordance to the O'Brien K-theory model. Profiles are characterized by stark variations in both phase and amplitude of extrema compared to the classic constant-K and a more recent solution, valid within the Wentzel-Kramers-Brillouin theory, shedding new light on this long-standing geophysical problem. In addition, direct numerical simulation is used to study the turbulent structure of anabatic and katabatic flows, and to describe the sensitivity of the solution to variations in the parameter space, within the conceptual framework of the Prandtl model. Variations in the sloping angle from the vertical wall setup are shown to induce a progressive departure of averaged profiles between the two flow regimes, ultimately resulting in stark differences at gentle sloping angles. The thermodynamical mechanisms responsible for sustaining mean and turbulent kinetic energy are used to further distinguish between flow regimes, and to propose a qualitative partition of the boundary layer in slope flows. The DNS setup is additionally adopted to identify coherent structures in katabatic flows over steep slopes. Coherent motions are responsible for the maintenance of turbulence in the ABL, hence their characterization is of fundamental importance toward a better understanding of boundary-layer dynamics. Packets of hairpins are found to connect in the streamwise direction to form large-scale motions (LSMs). For the lower sloping angles that are considered, it is then shown how LSMs further align to form very-large-scale motions (VLSMs). LSMs and VLSMs are found to be dominant contributors to streamwise momentum variance and turbulent momentum transfer in the above-jet regions. Next, drag properties of fractal-like sea ice surface morphologies are examined within the large-eddy simulation framework. The effects of large-scale surface features on wind flow are accounted for by an immersed boundary method. Conversely, the drag forces caused by subgrid-scale features are modeled through a novel dynamic roughness approach, in which the hydrodynamic roughness length parameter is determined using the first-principles based constraint that the total momentum flux (drag) must be independent of the grid-filter scale. This approach leads to accurate flow predictions, and provides an estimate of the otherwise unknown roughness parameter for sea ice surfaces, of use in climate, weather prediction and scalar transport models to evaluate the hydrodynamic roughness length.

EPFL2016