Pierre H. Flamant

Urban boundary-layer height determination from lidar measurements over the paris area.

The Paris area is strongly urbanized and is exposed to atmospheric pollution events. To understand the chemical and physical processes that are taking place in this area it is necessary to describe correctly the atmospheric boundary-layer (ABL) dynamics and the ABL height evolution. During the winter of 1994-1995, within the framework of the Etude de la Couche Limite Atmosphérique en Agglomération Parisienne (ECLAP) experiment, the vertical structure of the ABL over Paris and its immediate suburbs was extensively documented by means of lidar measurements. We present methods suited for precise determination of the ABL structures temporal evolution in a dynamic environment as complex as the Paris area. The purpose is to identify a method that can be used on a large set of lidar data. We compare commonly used methods that permit ABL height retrievals from backscatter lidar signals under different meteorological conditions. Incorrect tracking of the ABL depths diurnal cycle caused by limitations in the methods is analyzed. The study uses four days of the ECLAP experiment characterized by different meteorological and synoptic conditions.

Simulation in the time domain for heterodyne coherent laser radar.

A feuilleté model for assessment of the performance of heterodyne coherent laser radar (HCLR) under nonstationary atmospheric conditions such as wind shear and a fine layering of scattering properties is proposed. The model is an end-to-end signal simulation in the time domain at intermediate frequency from beam propagations to signal processing. The model takes into account the full transversedimension of the problem of laser-atmosphere interaction. The scatterers are grouped into slices, then an integration is performed over the slices that contribute to the HCLR signal. Two applications are presented: wind-shear detection for 10-µm HCLR and wind-vortex detection for 2-µm HCLR.

Lidar effective multiple-scattering coefficients in cirrus clouds.

We delimit a regime, valid for most ground-based lidar probings of cirrus clouds, in which the field-of-view dependence of multiple scattering reaches a plateau. In this regime and assuming the phase function to be constant around pi, we formally demonstrate Platts modification of the single-scattering lidar equation, with a parameter eta(P) accounting for the reduction of the effective scattering coefficient defined so that (1 - eta(P)) is the amount of energy scattered in the forward peak. Then, to cope with nonconstant backscattering functions, we discuss the introduction of an effective backscattering coefficient that is an average of the scattering probabilities around pi.

Characterization of pulsed coherent Doppler LIDAR with the speckle effect

The relationships among heterodyne efficiency γ, number of speckle cells M, and the ratio of receiver area to coherence area S(R)/S(C) for a pulsed coherent laser radar (CLR) are written through the use of mutual coherence functions. It is shown that numerical values for S(R)/S(C) that follow Goodmans definition [J. W. Goodman, in Laser Speckles and Related Phenomena, J. C. Dainty, ed. (Springer-Verlag, Berlin, 1975), pp. 9-75] or that are obtained through the use of a transverse-field coherence length agree. In the frame of the Gaussian model proposed by Frehlich and Kavaya [Appl. Opt. 30, 5325 (1991)] a new equation is derived: M = (1 + S(R)/S(C)). This equation agrees with our experimental results. Our theoretical analysis shows that the number of speckle cells for an optimal monostatic CLR system is M ~ 4. An experiment has been conducted with a ground-based pulsed CO(2) LIDAR and remote hard targets to study the probability density function of LIDAR returns as a function of M and to study the dependence of M on S(R)/S(C). An assessment of CLR performance through the use of M or the collecting aperture S(R) is discussed.

Spectral diversity technique for heterodyne Doppler lidar that uses hard target returns

A two-mode CO(2) laser is used as transmitter in a 10-microm heterodyne Doppler lidar (HDL) to take advantage of a spectral diversity technique, i.e., independent realizations obtained with different spectral components. The objective is to improve the properties (i.e., less variance) of power returns from a hard target. The statistical properties are presented first for a broad-spectrum laser transmitter and then for a two-mode laser transmitter. The experimental results for a cooperative diffuse hard target show that the return signals for a frequency separation Deltaf = 15 MHz can be decorrelated, depending on the angle of incidence and the target roughness. The experimental results show that the spectral diversity technique improves the performance of the HDL.

Four-element receiver for pulsed 10-µm heterodyne Doppler lidar

A four-element photomixer receiver has been tested in a 10-mum heterodyne Doppler lidar. It addresses a reduction of the variance of the power scattered off distributed aerosols targets at ranges as long as 8 km. An improvement in performance is expected when the four independent signals recorded on every single shot are combined. Two summation techniques of the four signals have been implemented: a coherent summation of signal amplitude and an incoherent summation of intensities. A phasing technique for the four signals is proposed. It is based on a more suitable correlation time with discernible self-consistent packets (SCPs). The SCP technique has been successfully tested, and the results obtained with a coherent summation of the four signals, i.e., variance reduction, carrier-to-noise ratio improvement, and velocity accuracy improvement, are in agreement with theory.

Wind velocity, water vapor, and temperature measurements from space using 2-μm Tm:Ho;YAG laser

In meteorological and climatological fields, the scientific community will increasingly need global measurements of key atmospheric parameters with high spatial resolution (horizontal as well as vertical): the spaceborne lidars are the most suitable instruments for those missions. While backscatter lidar (ATLID, currently studied as ESA) is presently first candidate for space deployment, the next generation of lidars will be DIAL and Doppler wind lidars, presenting a higher level of complexity, mainly due to the large power and complex signal processing required. The present considered wind lidars are based on CO2 lasers, whose space compliance still needs confirmation, while alexandrite lasers are considered for water vapor and temperature measurements, but they need flashlamp pumping which poses a lot of several thermal constraints and lifetime problems: on the other side, the recent developments achieved in solid-state technology allow to envisage diode pumping as most promising possibility for both previous applications.

Radiative Impact of Cirrus at Meso-scale Using a 1-D Radiative Transfer Model and Ground-Based Lidar, Ground-Based Radiometers, and Satellite Data

Several cirrus cases were observed near Paris, by a ground-based lidar and radiometers. In order to study the impact of cirrus on the earth radiative budget, we derive and compare the radiative fluxes (at the surface and at the top of the atmosphere) from ground-based radiometers, a radiative transfer model and Meteosat-5. The preliminary analysis show a fair agreement especially for shortwave fluxes.

ALADIN: an atmospheric laser Doppler wind lidar instrument for wind velocity measurements from space

AEROSPATIALE, leading a European team, has just conducted a successful study, under ESA contract, to demonstrate the feasibility of a spaceborne Doppler wind lidar instrument meeting the scientific requirements of wind velocity measurements from space with high spatial resolution. A first parametric investigation, based upon the initial set of mission requirements, and supported by dedicated models and detailed trade-off studies, took account of capabilities of most promising signal processing algorithms and calibration/validation constraints: it yielded a large conically scanned instrument deemed technologically risky. A risk analysis was then carried out to propose a less challenging instrument meeting most key mission requirements. The fixed line-of-sight concept with return signal accumulation appeared as most attractive. A second set of requirements agreed upon by scientific users was therefore issued, with relaxed constraints mainly on horizontal resolution, keeping roughly the same level of wind velocity measurement accuracy. A second instrument and subsystem trade-off was then performed to eventually produce an attractive instrument concept based upon a pair of small diameter telescopes each one associated to one scanning mirror rotating stepwise around the telescope axis, which drastically reduces the detection bandwidth. Following the main contract, studies of accommodation on the International Space Station have been performed, confirming the interest of such an instrument for wind measurements from space.

ALADIN: an atmosphere laser doppler wind lidar instrument for wind velocity measurements from space

AEROSPATIALE, leading a European team, has just conducted a successful study, under ESA contract, to demonstrate the feasibility of a spaceborne Doppler wind lidar instrument meeting the scientific requirements of wind velocity measurements from space with high spatial resolution. A first parametric investigation, based upon the initial set of mission requirements, and supported by dedicated models and detailed trade-off studies, took account of capabilities of the most promising signal processing algorithms and calibration/validation constrains: it yielded a large conically scanned instrument deemed technologically risky. A risk analysis was then carried out to propose a less challenging instrument meeting most key mission requirements. The fixed line-of-sight concept with return signal accumulation appeared as most attractive. A second set of requirements agreed upon by scientific users was therefore issued, with relaxed constraints mainly on horizontal resolution, keeping roughly the same level of wind velocity measurement accuracy. A second instrument and subsystem trade- off was then performed to eventually produce an attractive instrument concept based upon a pair of small diameter telescopes each one associated to one scanning mirror rotating stepwise around the telescope axis, which drastically reduces the detection bandwidth. Following the main contract, studies of accommodation on the International Space Station have been performed, confirming the interest of such an instrument for wind measurements from space.