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Alain Dabas, Julie Périn, and Pierre H. Flamant

Abstract

An airborne pulsed Doppler lidar implementing a downlooking conical scan rotating around the vertical axis is under development. The information contained in the measured radial velocities is studied to assess the capacity to retrieve the 3D wind field at mesoscale. First, in the frame of a variational analysis, it is shown that the observations cannot resolve horizontal scales shorter than approximately 5 km. Then, constraints of mass conservation and regularity as well as various boundary conditions are added to improve the resolution. The various constraints are tested on synthetic data. Further, a full analysis scheme is proposed that combines a preanalysis at low resolution followed by a finer resolution analysis. The achievable performances are discussed as well as the main limitations.

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Robert T. Menzies, David M. Tratt, and Pierre H. Flamant

Abstract

The use of an airborne C02 lidar to obtain cloud backscatter and extinction data at a thermal infrared wavelength is described. The extinction in this spectral region is proportional to the cloud liquid water content. The use of coherent detection results in high sensitivity and narrow field of view, the latter property greatly reducing multiple-scattering effects. Backscatter measurements in absolute units are obtained through a hard target calibration methodology. For clouds of low to moderate optical thickness at the lidar wavelength, both geometric thickness and optical thickness can be measured. The sea surface reflectance signal is used to obtain estimates of the cloud optical thickness. The utility of this technique results from studies that indicate that the spatial scale of variability of the sea surface reflectance is generally large compared with that of cloud option thickness. Selected results are presented from data taken during flights over the Pacific Ocean.

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Alain M. Dabas, Philippe Drobinski, and Pierre H. Flamant

Abstract

Frequency estimates by heterodyne Doppler lidar (HDL) may result in velocity bias due to the atmospheric speckle effect and an asymmetrical power spectrum of the probing pulse, as discussed in a previous paper by Dabas et al. In this paper, it has been shown that the velocity bias can be accounted for and corrected on a single measurement basis for a mean frequency estimator (e.g., pulse pair). In the present paper, a new procedure is proposed and validated for adaptive filters (e.g., Levin, notch, etc.), which accounts for nonstationary conditions such as wind turbulence, wind shear, and backscattered power gradient. The present study is conducted using both numerical simulations and actual data taken by a 10-μm HDL.

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Alain M. Dabas, Philippe Drobinski, and Pierre H. Flamant

Abstract

Unreliable frequency estimates at low signal-to-noise ratios, provided by a heterodyne Doppler lidar (HDL), undermines any data analysis scheme requiring high-accuracy wind fields retrievals. To meet demanding specifications, that is, high accuracy associated with high reliability on radial velocity components, a quality control (QC) procedure has to be implemented at signal processing level. The authors propose use of a recursive implementation of an adaptive filter for frequency estimate coupled with a QC that combines a statistical test on the signal energy filtered out as proposed by Rye and Hardesty and a persistency criterion (PC). The PC leads to improved performance with respect to the percentage of data finally accepted and frequency accuracy. The performance is validated using simulated signals and HDL actual data.

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Philippe Drobinski, Alain M. Dabas, and Pierre H. Flamant

Abstract

Heterodyne Doppler lidars (HDLs) are used to monitor atmospheric wind field and wind turbulence at remote distance. This last application calls for the derivation of wind spectra, which can be characterized by the dissipation rate and the κ-spectral peak (or outer scale of turbulence). However, the HDL technique may suffer two problems. First, HDL measurements result in spatial averaging of the true wind velocity along the line of sight, because of the laser pulse duration and windowing effect on processed signals. Second, even at high signal-to-noise ratio, the retrieved turbulent velocity field may be contaminated by errors due to speckle fluctuations. It is shown that both spatial averaging and error contribution to the wind spectra can be modeled starting from the transmitted laser pulse characteristics and signal processing parameters, so that their effect can be predicted. The rms difference between the estimated and predicted turbulent spectra is minimized in order to infer the turbulence parameters. This procedure is tested on simulated signals and validated on actual data taken by a 10-μm HDL during a field campaign in 1995.

The data collected during two periods of two consecutive days (9 and 10 March and 13 and 14 March 1995) are analyzed. On these days, moderate to light winds prevailed. The stability parameter z i/L MO indicated slightly unstable conditions with sometimes probable convection. The HDL measured energy dissipation rates ranging between 0.7 × 10−3 and 8 × 10−3 m2 s−3 in good agreement with sonic anemometer measurements. The κ-spectral peak ranged between 200 and 600 m.

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Fabien Gibert, Pierre H. Flamant, Juan Cuesta, and Didier Bruneau

Abstract

Vertical mean CO2 mixing ratio measurements are reported in the atmospheric boundary layer (ABL) and in the lower free troposphere (FT), using a 2-μm heterodyne differential absorption lidar (HDIAL). The mean CO2 mixing ratio in the ABL is determined using 1) aerosol backscatter signal and a mean derivative of the increasing optical depth as a function of altitude and 2) optical depth measurements from cloud target returns. For a 1-km vertical long path in the ABL, 2% measurement precision with a time resolution of 30 min is demonstrated for the retrieved mean CO2 absorption. Spectroscopic calculations are reported in details using new spectroscopic data in the 2-μm domain and the outputs of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Then, using both aerosols in the ABL and midaltitude dense clouds in the free troposphere, preliminary HDIAL measurements of mean CO2 mixing ratio in the free troposphere are also presented. The 2-μm HDIAL vertical measurements are compared to ground-based and airborne in situ CO2 mixing ratio measurements and discussed with the atmospheric synoptic conditions.

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Alain M. Dabas, Philippe Drobinski, and Pierre H. Flamant

Abstract

Radial wind velocity measurements by a pulsed CO2 Doppler lidar may be biased even in stationary atmospheric conditions. The authors show it is due to random speckle fluctuations of the backscattered signal and is related to the dissymmetry of the transmitted laser pulse periodogram. A procedure is proposed to correct for the bias on a shot-to-shot basis when a pulse-pair mean-frequency estimator is used for processing. The procedure is validated on simulated and actual lidar signals.

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Jean-Luc Zarader, Gérard Ancellet, Alain Dabas, Nacer K. M'Sirdi, and Pierre H. Flamant

Abstract

An adaptive notch filter (ANF) is proposed for range-resolved frequency estimates of Doppler lidar atmospheric returns. The ANF is based on the spectral filtering of lidar return to remove the atmospheric contribution from noise. An adaptive algorithm is used to retrieve the filter parameters at a time k knowing both the input signal and filter output at times ki, where i = [1, k]. It is shown that ANF performs well at low SNR (−5 dB) compared to the poly-pulse-pair (PPP) estimator currently used for Doppler lidar signal processing. The standard deviation of frequency estimates is 0.01F s − 0.02F s (F s is the sampling frequency) at SNR = −5 dB, depending on the signal spectral width. It corresponds to a wind velocity uncertainty of 2–4 m s−1 for F s = 40 MHz and a laser wavelength λ = 10 µm. The ANF also proved to perform better than PPP in tracking a time-varying frequency, and in the presence of a colored noise.

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Laurent Sauvage, Pierre H. Flamant, Hélène Chepfer, Gérard Brogniez, Vincent Trouillet, Jacques Pelon, and Franck Albers

Abstract

During the intensive European Cloud and Radiation Experiment 1994 (EUCREX’94) conducted off the coast of Brittany (France) over the Atlantic Ocean during April 1994, natural cirrus have been analyzed from in situ and remote sensing measurements. The authors have particularly studied the case of 17 April 1994. For this day a cirrus bank is described by a complete dataset, that is, classic airborne thermodynamical measurements, microphysical (forward scattering spectrometer probe) and OAP-2D2-C (optical array probe-cloud) probes manufactured by Particle Measuring System, and radiative (Barnes Precision Radiation Thermometer, Eppley pyranometers, and upward- and downward-looking pyrgeometers) measurements above and below the cloud. More specific airborne instruments were used such as upward backscatter lidar with polarization capabilities (LEANDRE) on board the Avion de Recherches Atmosphériques et Télédétection and the Polarization and Directionality of the Earth’s Reflectances (POLDER) radiometer on board the Falcon for measurement of bidirectional and polarized reflectances. The scene was also documented by NOAA-12/Advanced Very High Resolution Radiometer data. However, the nonsphericity of cirrus ice crystals is clearly demonstrated by the lidar backscattering depolarization ratio measurements (Δp = 24%) and by the absence of any rainbow in POLDER bidirectional reflectances. A specular reflection of the solar light observed on POLDER images indicates the presence of horizontally oriented ice particles in the cloud. All these optical properties will be studied in a companion paper (Part II) and compared with optical properties derived from microphysical models in order to evaluate the radiative impact of natural cirrus clouds.

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Hélène Chepfer, Gérard Brogniez, Laurent Sauvage, Pierre H. Flamant, Vincent Trouillet, and Jacques Pelon

Abstract

In this paper, a quantitative analysis of in situ and radiative measurements concerning cirrus clouds is presented. These measurements were performed during the European Cloud and Radiative Experiment 1994 (EUCREX’94) as discussed in an earlier paper (Part I). The analyses are expressed in terms of cirrus microphysics structure. The complex microphysical structure of cirrus cloud is approximated by simple hexagonal monocrystalline particles (columns and plates) and by polycrystalline particles (randomized triadic Koch fractals of second generation) both arbitrarily oriented in space (3D). The authors have also considered hexagonal plates randomly oriented in horizontal planes with a tilted angle of 15° (2D). Radiative properties of cirrus cloud are analyzed, assuming that the cloud is composed of 3D ice crystals, by way of an adding–doubling code. For the hypothesis of 2D ice crystals, a modified successive order of scattering code has been used. The first order of scattering is calculated exactly using the scattering phase function of 2D crystals; for the higher orders, it is assumed that the same particles are 3D oriented. To explain the whole dataset, the most appropriate microphysics, in terms of radiative properties of cirrus clouds, is that of the 2D hexagonal plates whose aspect ratio (length divided by diameter) is 0.05.

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