Abstract
Abstract. Airborne lidar forward sensing along the flight direction can serve for notification of clear air turbulence (CAT) and help to prevent injuries or fatal air accidents. The validation of this concept was presented in the framework of the DELICAT (DEmonstration of LIdar-based CAT detection) project. However, the strong variations in signal level, which were observed during the DELICAT measurements but not explained, sometimes indicated the need of a better understanding the observational errors due to geometrical factors. In this paper, we discuss possible error sources pertinent to this technique, related to fluctuations of the flight parameters, which may lead to strong signal variations caused by the random deviations of the sensing beam from the forward flight trajectory. We analyze the variations in backscattered lidar signal caused by fluctuations of the most important forward-sensing flight parameter, the pitch angle. The fluctuation values considered in the paper correspond to the error limits of the compensational gyro platform used in civil aviation. The part of the pitch angle fluctuations not compensated for by the beam-steering device in the presence of aerosol concentration variations can lead to noticeable signal variations that can be mistakenly attributed to wind shear, turbulence, or fast evolution of the aerosol layer. We formulate the criteria that allow the recognition of signal variations caused by pitch angle fluctuations. Influence of these fluctuations is shown to be stronger for aerosol variations on smaller vertical scales. An example of DELICAT observations indicating a noticeable pitch angle fluctuation impact is presented.
Highlights
Airborne lidar systems (Fukuchi and Shiina, 2012; Weitkamp, 2006) may play a significant role in warning about, preventing, and compensating for problems caused by atmospheric turbulence
We discuss the examples of strong backscattered signal variations caused by pitch angle fluctuations which were sometimes observed during the experiments
It is shown that the pitch angle fluctuations are the important parameter for the airborne lidar sensing ahead in the flight direction in the case when their uncompensated for values result in the sensing beam shift about the vertical size of the aerosol clusters
Summary
Airborne lidar systems (Fukuchi and Shiina, 2012; Weitkamp, 2006) may play a significant role in warning about, preventing, and compensating for problems caused by atmospheric turbulence. Changes in the aerosol layer density during the observation time and the experimental noise, which can affect signal in both polarizations simultaneously, could be a problem for backscattered signal analysis. There is another technique of CAT detection based on the backscattering enhancement (BSE) effect, which was initially found in theoretical research (Vinogradov et al, 1973) and experimentally confirmed (Gurvich and Kashkarov, 1977). There are many experimental observations of variations in aerosol and water vapor concentrations on small vertical (about 100 m) and horizontal (several km) scales in the lower atmosphere. We discuss the impact of pitch angle fluctuations on both simulated and measured lidar signal in the presence of aerosol clusters with different sizes monitored by an airborne lidar.
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