Land-Atmosphere Interactions Measured with Raman Lidar

The EPFL water vapor and temperature scanning solar blind Raman lidar is a research tool developed for atmospheric boundary layer studies. The lidar has raw spatial and temporal resolutions of 1.25 meters and 1 second respectively and offers a new vision of the atmosphere. This study is dedicated to the validation of its design and investigates some of the potential applications and capacities of such a remote sensing technique. The use of four telescopes is evaluated with preliminary indoor experiments. The calibration of the system, obtained with point sensors along the beam is necessary to retrieve the absolute values of water vapor mixing ratio and temperature. The accuracy of water vapor measurement is below 0.32 g/kg with 2 min by 3.75 m averaged signals, from 15 up to 500 meters. The temperature measurements needs more work to be fully satisfying. The quality of the lidar data is verified with a signal to noise ratio threshold method. In order to test the lidar ability to detect various atmospheric boundary layer phenomena, three sites are studied with different scanning strategies. Vertical soundings and scans allow the measurement of stable atmospheric situations as well as low level clouds, front passages, convective cells, nocturnal jets and internal boundary layers. Street canyon effect and coherent patterns of humidity are studied with horizontal soundings. The different analysis performed support the hypothesis that coherent structures in the humidity time-space field are signatures of low-speed fluid cores induced by nested packets of hairpin vortices. It is the first time that such observations are reported with lidar measurements. Comparing the energy spectra from lidar and eddy covariance system measurements allows for the definition of the smallest turbulent scales that the lidar can confidently detect in a static orientation. Such scales range from 14 to 25 seconds below 150 meters from the light source and support the use of lidar measurements to retrieve fluxes.

Development of the Jungfraujoch multiwavelength lidar system for continuous observations of the aerosol optical properties in the free troposphere

Gilles Larchevêque

Climate changes and global warming are generally associated with the enhanced greenhouse effect, but aerosols can induce a cooling effect and thus regionally mask this warming effect. Unfortunately, the strong variability both in space and in time of the aerosols and thus the difficulty to characterize their global basic properties induce large uncertainties in the predictions of the numerical models. Those uncertainties are as high as the absolute level of the enhanced greenhouse forcing. To solve this problem it is necessary to improve the set of well-calibrated instruments (both in situ and remote sensing) with the ability to measure the changes in stratospheric and tropospheric aerosols amounts and their radiative properties, changes in atmospheric water vapor and temperature distributions, and changes in clouds cover and cloud radiative properties. The quantity used to assess the importance of one compound (greenhouse gases, aerosols) to the variation of the radiative budget of the Earth is the radiative forcing. One of those forcings is the direct aerosol radiative forcing and it depends on the optical depths and the upscatter fraction of the aerosols. Those two parameters depend on the chemical composition and size distribution of the aerosols. Thus the key parameters of this radiative forcing are the chemical composition through its refractive index and the size distribution of the aerosols. This thesis deals with the design and the implementation of one multi-wavelength lidar system at the Jungfraujoch Alpine Research Station (Alt. 3580m asl). This lidar system is a combination of one standard backscatter lidar and one Raman lidar. Its design have been supported by a ray tracing analysis of the receiver part. The laser transmitter is based on a tripled Nd:YAG laser and the backscattered light is collected by one Newtonian telescope for the tropospheric measurements and by one Cassegrain telescope for the future stratospheric measurements. The received wavelengths for each telescope include three elastically scattered wavelengths (355, 532 and 1064nm), two spontaneous Raman signals from nitrogen (387 and 607nm) and one spontaneous Raman signal from the water vapor (408nm). The optical signals received by each of the telescopes are separated spectrally by two filter polychromators. They are build up around a set of beamsplitters and custom design thin band pass filters with high out-of-band rejection. On the visible channel, the adds of a Wollaston prism separates the parallel polarized backscattered signal (532(p)nm) of the perpendicular polarized one (532(c)nm). Photomultiplier tubes perform the detection of the signals for the UV and visible wavelengths and by Si-avalanche photodiodes for the near-infrared signal. The acquisition of the signals is performed by seven transient recorders in analog and in photon counting modes. Within the frame of the EARLINET (European Aerosol Research Lidar Network), hardware and software intercomparisons have been done. The software intercomparison has been divided into the validation of the elastic algorithm and the Raman algorithm. Those intercomparisons of the inversions of the lidar signals have been performed using synthetic data for a number of situations of different complexity. The hardware intercomparison have been achieved with the mobile micro-lidar of the Observatoire Cantonal de Neuchâtel. The present lidar system provides independent aerosol extinction and backscatter profiles, depolarization ratio and water vapor mixing ratio up to the tropopause. Their uncertainties could be smaller than 20% and thus make possible the retrieval of the microphysical aerosol parameters like the volume concentration distribution and the mean and integral parameters of the particle size distribution, (effective radius, total surface-area concentration, total volume concentration and number concentration of particles). This retrieval is performed by one algorithm of the Institute of Mathematic of the University of Postdam based on the hybrid regularization method. The first results of the retrieval of the volume concentration distribution with three backscatter (355, 532 and 1064nm) and one extinction (355nm) profiles has demonstrated promising results. Future upgrades of the system will add ozone concentration and temperature profile up to the stratopause.

EPFL2002

Une première mesure lidar combinée d'ozone et de vent, à partir d'une instrumentation et d'une méthodologie coup par coup

During these last decades, air pollution has drawn the attention of scientists because of the deterioration of the human environment that it has caused. Unfortunately, the study of this dynamic phenomenon is very complex and requires the use of models that compute the evolution of the physico-chemical parameters of the atmosphere. Moreover, the predictions of the numerical simulations have to be validated by range- and time-resolved measurements, carried out continuously up to the top of the planetary boundary layer. Only the optical radar or lidar satisfies these specifications: for this reason the Laboratory for Air and Soil Pollution (LPAS) of the Swiss Federal Institute of Technology (EPFL) has developed such a system in order to provide the modellers with information complementary to that given by the ground measurements. This instrument, after mounting on a mobile platform, has determined the concentration of the key molecule ozone during several field campaigns, demonstrating its reliability and precision. Moreover, the EPFL system is a shot per shot lidar, able to record each detected signal and not only its average in a time interval. This feature is innovative to lidar determinations of tropospheric ozone and allows one to measure accurately the statistical error, to correct for the effect of systematic biases and to observe the dynamic behaviour of the atmosphere: the measurement of the statistical error has been used to build a model of the signal and of its noise, useful in the design of future lidar systems and for the comparison of the various data processing algorithms. the correction of the bias caused by the artificial deformation of the statistical distribution of the signal at long distance has led to a considerable increase of the range of the retrieved ozone profiles, the observation of the dynamic behaviour of the atmosphere has resulted in the first simultaneous measurement of a pollutant concentration and of the wind velocity with the same data set recorded by a lidar.

EPFL1996

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

Pablo Roberto Ristori

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.

EPFL2008

Une première mesure lidar combinée d'ozone et de vent, à partir d'une instrumentation et d'une méthodologie coup par coup

EPFL1996

Development of the Jungfraujoch multiwavelength lidar system for continuous observations of the aerosol optical properties in the free troposphere

Gilles Larchevêque

EPFL2002