Sergio Tomás

Atmospheric Boundary Layer Height Monitoring Using a Kalman Filter and Backscatter Lidar Returns

A solution based on a Kalman filter to trace the evolution of the atmospheric boundary layer (ABL) sensed by a ground-based elastic-backscatter tropospheric lidar is presented. An erf-like profile is used to model the mixing-layer top and the entrainment-zone thickness. The extended Kalman filter (EKF) enables to retrieve and track the ABL parameters based on simplified statistics of the ABL dynamics and of the observation noise present in the lidar signal. This adaptive feature permits to analyze atmospheric scenes with low signal-to-noise ratios (SNRs) without the need to resort to long-time averages or range-smoothing techniques, as well as to pave the way for future automated detection solutions. First, EKF results based on oversimplified synthetic and experimental lidar profiles are presented and compared with classic ABL estimation quantifiers for a case study with different SNR scenarios.

Six-channel polychromator design and implementation for the UPC elastic/Raman LIDAR

A 6-channel dichroic-based polychromator is presented as the spectrally selective unit for the U.P.C. elastic/Raman lidar. Light emission is made at 355-nm (ultraviolet, UV), 532-nm (visible, VIS) and 1064-nm (near infrared, NIR) wavelengths. In reception, the polychromator is the spectral separation unit that separates the laser backscattered composite return into 3 elastic (355, 532, 1064-nm wavelengths) and 3 Raman channels (386.7, 607.4 and 407.5-nm (water-vapor) wavelengths). The polychromator houses photo-multiplier tubes (PMT) for all the channels except for the NIR one, which is avalanche photodiode (APD) based. The optomechanical design uses 1-inch optics and Eurorack standards. The APD-based receiver uses a XY-axis translation/elevation micro-positioning stage due to its comparatively small active area and motorised neutral density filters are used in all PMT-based channels to avoid detector saturation. The design has been specially optimized to provide homogeneous spatial light distribution onto the photodetectors and good mechanical repeatability. All channels are acquired in mixed analog and photon-counting mode using Licel® transient recorders, which are controlled by means of a user friendly LabVIEWTM interface. The paper focuses on the main polychromator optical design parameters, that is, light collimation trade-offs, end-to-end transmissivity, net channel responsivity, light distribution and spot size onto the photodetectors. The polychromator along with the rest of the U.P.C. lidar system has successfully been tested during a recent lidar system intercomparison campaign carried out in Madrid (Spain) during Oct. 2010.

Power budget and performance assessment for the RSLAB multispectral elastic/raman lidar system

The need of a multi-spectral lidar has widely been experienced in last few years with a view to invert the optical and microphysical properties of aerosols and their impact on the climate change. As a part of the EARLINET-GALION objectives, a joint effort has already been made by the European Aerosol Research Lidar Network (EARLINET). The EARLINET advanced standard of 3+2-channel configuration for lidar instruments (3+2 standing for 3 elastic channels and 2 respective Raman channels) enables retrieval of aerosol microphysical properties. An overview of the new RSLAB 3+2+1 multispectral lidar system, therefore, is presented in terms of power budget estimation for all the reception channels and overall system performance, that is, signal-to-noise ratio (SNR) and maximum sounding range achieved.

Piece-wise variance method for signal-to-noise ratio estimation in elastic/Raman lidar signals

A straightforward signal-to-noise ratio (SNR) estimator for elastic/Raman lidar channels and related noise-induced errorbars is presented. The estimator is based on piece-wise estimation of the mean signal power and noise variance component under analog detection. The piece-wise estimator results are compared with those obtained from a previously published SNR parametric estimator under high and low SNR scenarios.

Intercomparison of spanish advanced lidars in the framework of EARLINET

To extend and reinforce the action of the EARLINET- ASOS project, a nucleus of Spanish advanced lidars was created. Four systems were intercompared satisfactorily in terms of backscatter coefficients at two elastic wavelengths.

A wind speed and fluctuation simulator for characterizing the wind lidar correlation method

Aerosol distribution in the atmosphere is used as a wind-tracer by lidars since it is drifted by the wind and respond to its changes. Two methods are used: the correlation and the Doppler method. This first one is easier and cheaper to implement than the latter. It makes it competitive for retrieving wind speed profiles or estimating wind turbulence. However, its accuracy decreases significantly as the distance from the optical radiation source increases, hence the need to characterize the method by means of wind field profile simulations and validate it by comparing the retrieval of real wind velocities with that of cooperative instruments such as radiosoundings. In this paper, the bases of a 2D lidar signal simulator are presented. The relationship between wind fields and the aerosol concentration dynamics, and the way they relate to lidar signal returns is explained. The first results of the application of the correlation method for the retrieval of wind velocities from real data at the UPC are presented and compared to radiosoundings measurements.

Engineering of a water-vapour, Raman, elastic-backscatter Lidar at the Technical University of Catalonia (Spain)

Implementation of the pure-vibrational Raman spectra lidar method for simultaneous measurements of atmospheric water-vapour, aerosol extinction and backscatter coefficients is reported. A Q-switched Nd:YAG laser provides the three elastic wavelengths of 1064, 532 and 355 nm while the return signal is collected by a 40-cm aperture telescope. A spot-to-spot fiber bundle conveys the light from the telescope focal plane to a specific polychromator especially simulated and designed with care on minimizing optical losses and physical dimensions. The reception field of view, which is limited by the fiber bundle characteristics, is the same for all wavelengths. By means of four customised dichroic filters and beam splitters, light is separated into the three elastic wavelengths (355, 532, 1064 nm) as well as the 386.7- and 607.4-nm N2-Raman-shifted wavelengths, and the 407.5-nm H2O-Raman-shifted wavelength. Signal detection is achieved by using avalanche photodiodes at 1064 and 532 nm and analog acquisition while photomultiplier tubes and fast photon counting acquisition at the rest of the wavelengths. A specific design of the optoelectronics of the receiving channels is controlled by a distributed CPU thanks to a user-friendly LabViewTM interface. User-configurable scanning tools are built-in, but can also be customized. In this work an overview of the system though particularly geared to the polychromator unit is presented as well as a power link-budget assessment, which is to include simulation of end-to-end transmissivities, will be discussed for the main channels involved. The first measurements have already been made at 1064, 532, and 607.4 nm.

Atmospheric boundary-layer height estimation by adaptive Kalman filtering of lidar data

A solution based on a Kalman filter to trace the evolution of the atmospheric boundary layer (ABL) sensed by an elastic backscatter lidar is presented. An erf-like profile is used to model the mixing layer top and the entrainment zone thickness. The extended Kalman filter (EKF) enables to retrieve and track the ABL parameters based on simplified statistics of the ABL dynamics and of the observation noise present in the lidar signal. This adaptive feature permits to analyze atmospheric scenes with low signal-to-noise ratios without need to resort to long time averages or rangesmoothing techniques, as well as to pave the way for an automated detection method. First EKF results based on synthetic lidar profiles are presented and compared with a typical least-squares inversion for different SNR scenarios.

Overview of Wind Lidar Techniques and Current Related Developments at the Technical University of Catalonia

The Lidar group in the Technical University of Catalonia is actively working in wind lidar techniques. An extensive review of such techniques is presented. A special emphasis is made on the direct Doppler technique and the direct motion technique with a lines-of-sight scanning pattern. Current developments are also mentioned.

Planetary Boundary Layer Height and Wind Field Characterization by Means of a Lidar at the Teide Observatory in the Canary Islands

A lidar field campaign was performed between 30th June and 4th July 2007 in the Teide Observatory in the Canarian island of Tenerife to characterize the atmosphere of this astrophysical observation site in terms of nocturnal boundary layer height and wind fields. The nocturnal boundary layer height was found lower than 810 m in all cases and the aerosol optical thickness lower than 0.005 and 0.03 at 1064 and 532 nm, respectively. Wind fields could hardly be retrieved because of faint signals and very weak wind velocities observed during the campaign.