Correcting winds measured with a Rayleigh Doppler lidar from pressure and temperature effects

Abstract

The molecular channel of the space-based Doppler lidar ADM-Aeolus relies on a double Fabry-Perot (FP) interferometer. The difference in photon numbers transmitted by the two FPs divided by their sum-the so-called Rayleigh response-is a function of the central frequency of the spectrum of the laser light backscattered by the atmosphere, so that a proper inversion enables the measurement of Doppler shifts and line-of-sight wind velocities. In this paper, it is shown that the relationship between the Rayleigh response and the Doppler shift does not depend on the sole characteristics of the instrument, but also on the atmospheric pressure and temperature (through the Rayleigh-Brillouin effect), and the likely presence of a narrow-band radiation due to particle scattering. The impact of these on the precision of inverted Doppler shifts (or line-of-sight winds) is assessed showing that a correction is needed. As they are lacking the appropriate precision, climatology profiles of pressure, temperature or aerosols cannot be used as an input. It is proposed to use data predicted by a numerical weather prediction system instead. A possible correction scheme is proposed. Its implication on the quality of retrieved Rayleigh winds is discussed.

The prospects for increasing the horizontal resolution of the Aeolus horizontal line‐of‐sight wind profiles

“…The Rayleigh‐clear HLOS wind error standard deviation (Figure a) follows well a dependency of , which can be explained by the Poisson noise as a main source of the random errors (Dabas et al ., ). At L = 90 km the HLOS wind error standard deviation is about 2.5 m/s in the layer between 5 km and 15 km.…”

Horizontal lidar measurements for the proof of spontaneous Rayleigh–Brillouin scattering in the atmosphere

“…As already mentioned in Section 1, the spectrum Sf of molecular scattered light can be approximated by a Gaussian line shape due to the thermal motion of the molecules when the effect of Brillouin scattering is neglected [4]:…”

“…Atmospheric lidar techniques for the measurement of wind [1][2][3][4][5][6], temperature [7,8], and aerosols [9][10][11][12] rely on the exact knowledge of the spectral line shape of light scattered on air molecules such as N 2 and O 2 . The underlying concept of these measurement techniques is based on the idea of resolving the spectral line shape with high-resolution interferometers and relating it to theoretical line-shape models.…”

“…For instance, for Doppler wind lidar (DWL) using directdetection techniques, a systematic, wind-and pressure-dependent error in the retrieved wind speed occurs if the SRB spectrum is assumed to be Gaussian. For the particular DWL on ADM-Aeolus, the wind speed is overestimated by 10% at sea level (p ≈ 1013 hPa) and still by 3% for 10 km altitude (p ≈ 300 hPa) [4,27].…”

Airborne direct-detection and coherent wind lidar measurements along the east coast of Greenland in 2009 supporting ESA's Aeolus mission

“…a sequential arrangement of a Fizeau-and a double-edge Fabry-Perot spectrometer. Extensive research has been dedicated to describe the interaction of this optical arrangement with atmospheric signals [6][7][8] [9]. The A2D is working in the ultraviolet region at 355 nm analysing narrowband Mie backscatter from aerosols and clouds and broadband Rayleigh backscatter from molecules, respectively, with its two dedicated interferometers.…”