Abstract
Abstract. Shipborne wind observations by a coherent Doppler lidar (CDL) have been conducted to study the structure of the marine atmospheric boundary layer (MABL) during the 2014 Yellow Sea campaign. This paper evaluates uncertainties associated with the ship motion and presents the correction methodology regarding lidar velocity measurement based on modified 4-Doppler beam swing (DBS) solution. The errors of calibrated measurement, both for the anchored and the cruising shipborne observations, are comparable to those of ground-based measurements. The comparison between the lidar and radiosonde results in a bias of −0.23 ms−1 and a standard deviation of 0.87 ms−1 for the wind speed measurement, and 2.48, 8.84∘ for the wind direction. The biases of horizontal wind speed and random errors of vertical velocity are also estimated using the error propagation theory and frequency spectrum analysis, respectively. The results show that the biases are mainly related to the measuring error of the ship velocity and lidar pointing error, and the random errors are mainly determined by the signal-to-noise ratio (SNR) of the lidar backscattering spectrum signal. It allows for the retrieval of vertical wind, based on one measurement, with random error below 0.15 ms−1 for an appropriate SNR threshold and bias below 0.02 ms−1. The combination of the CDL attitude correction system and the accurate motion correction process has the potential of continuous long-term high temporal and spatial resolution measurement for the MABL thermodynamic and turbulence process.
Highlights
The vertical structure of atmospheric variables in the marine atmospheric boundary layer (MABL) plays an important role in the earth’s climate system, governing exchanges of energy, sensible heat, water vapour, and momentum between the ocean and the overlying atmosphere (Rocers et al, 1995; Wulfmeyer and Janjic, 2005)
One of the most common direct techniques for measuring surface fluxes is eddy correlation, which utilizes the covariance of mixing ratios and vertical wind velocity (Lenschow et al, 1981; Anctil et al, 1994; Fairaill et al, 2000), but wind velocity retrieval is complicated by contamination due to platform motion, representing a major source of uncertainty in measurement of turbulence and air–sea interaction
For a shipborne coherent Doppler lidar (CDL), the recorded velocity corresponds to the relative velocity along the laser beam direction between the ship and the atmospheric target, where the ship platform motion will add to the measured line of sight (LOS) velocity in ship coordinate system (SCS)
Summary
The vertical structure of atmospheric variables in the marine atmospheric boundary layer (MABL) plays an important role in the earth’s climate system, governing exchanges of energy, sensible heat, water vapour, and momentum between the ocean and the overlying atmosphere (Rocers et al, 1995; Wulfmeyer and Janjic, 2005). More complicated attitude corrections need to be considered when CDL is carried out on a moving platform such as a ship or aircraft, since the orientation of the transmitting laser beam is not fixed and the speed of the ship itself and ocean wave will be stacked to the LOS velocity. This has a more serious detrimental effect on vertical velocity. In order to simplify the mechanical structure and to ease installation of the CDL on the ship platform, an algorithm-based attitude correction method was developed for relaxing the requirements for mechanical stability and active compensation mechanisms.
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