Spatial distributions of aerosol at civil aviation flight altitudes according to the DELICAT project

In 2009–2015, the DELICAT (DEmonstration of LIdar based Clear Air Turbulence detection) project was implemented with the support of the Commission of the European Communities as part of the 7th Framework Program, with the participation of the Obukhov Institute of Atmospheric Physics of the Russian Academy of Sciences. During the project, an airborne ultraviolet lidar was developed and manufactured for the early detection of clear-air turbulence (CAT) ahead based on the backscatter signal. The backscattered radiation was observed in two polarizations in order to isolate signal fluctuations from aerosol, which can lead to depolarization. From 17.07–12.08.2013, 11 flights with a total length of 33 hours were conducted to test the lidar. The flights took place over Western Europe, the British Isles and adjacent sea areas in clear weather conditions. Aerosol and/or CAT clusters were recorded in each flight. The task of identifying the aerosol type is not set in this work. As a result, a unique array of observations was obtained.
Aerosol plays two roles:
1) it is a noise that distorts the information about possible CAT areas,
2) it is an object of study, in particular, the study of the size and spatiotemporal evolution of its clusters.
In this work, an analysis of the spatial spectra of aerosol density fluctuations was carried out. For this, flight segments with constant altitude, direction and speed of the aircraft were selected. A multi-hour record of the scattered radiation intensity was divided into segments lasting 1 minute. During this time, the aircraft shifted 9-11 km along the flight path. The signal is considered in two coordinates: the distance from the aircraft to the scattering volume and the path of the aircraft relative to the air mass. A new method of signal processing based on a 2-dimensional Fourier transform was proposed, which allows identifying aerosol clouds. The following approximations are made: 1) stationary clouds on a time scale of about 1 minute, 2) small variations in the direction of the sounding line. In the coordinates introduced above, aerosol clouds appear as bands inclined at an angle of 45 degrees. Two-dimensional Fourier spectra of the signal were calculated for the selected flight zones: a) without aerosol, which allows one to estimate the noise level; b) with aerosol clouds. For zones where aerosol is observed, the spectral density has a sharp peak in the vicinity of the main diagonal of the frequency plane. The useful signal in the specified coordinates is a plane wave in the direction of the diagonal, and, therefore, the Fourier decomposition of such a signal will contain only harmonic waves along the diagonal. The diagonal sections of the obtained 2-dimensional spectra are equal to the 1-dimensional spatial spectra of aerosol fluctuations. The 1-dimensional spectra of the co-polarized signal and the cross-polarized signal obey a power law with an exponent from 2 to 2.5. Aerosol effects dominate the fluctuations of the observed signal.