Abstract
Abstract. Wind profiling by Doppler lidar is common practice and highly useful in a wide range of applications. Airborne Doppler lidar can provide additional insights relative to ground-based systems by allowing for spatially distributed and targeted measurements. Providing a link between theory and measurement, a first large eddy simulation (LES)-based airborne Doppler lidar simulator (ADLS) has been developed. Simulated measurements are conducted based on LES wind fields, considering the coordinate and geometric transformations applicable to real-world measurements. The ADLS provides added value as the input truth used to create the measurements is known exactly, which is nearly impossible in real-world situations. Thus, valuable insight can be gained into measurement system characteristics as well as retrieval strategies. As an example application, airborne Doppler lidar wind profiling is investigated using the ADLS. For commonly used airborne velocity azimuth display (AVAD) techniques, flow homogeneity is assumed throughout the retrieval volume, a condition which is violated in turbulent boundary layer flow. Assuming an ideal measurement system, the ADLS allows to isolate and evaluate the error in wind profiling which occurs due to the violation of the flow homogeneity assumption. Overall, the ADLS demonstrates that wind profiling is possible in turbulent wind field conditions with reasonable errors (root mean squared error of 0.36 m s−1 for wind speed when using a commonly used system setup and retrieval strategy for the conditions investigated). Nevertheless, flow inhomogeneity, e.g., due to boundary layer turbulence, can cause an important contribution to wind profiling error and is non-negligible. Results suggest that airborne Doppler lidar wind profiling at low wind speeds (<5ms-1) can be biased, if conducted in regions of inhomogeneous flow conditions.
Highlights
Doppler lidar has experienced rapidly growing importance and usage in remote sensing of atmospheric winds over the past decades (Weitkamp et al, 2005)
The error observed in the airborne Doppler lidar simulator (ADLS) between input and retrieved wind profile is directly traceable to the violation of the wind field homogeneity assumed in airborne velocity azimuth display (AVAD), e.g., due to boundary layer turbulence
The wind profile retrieval is made assuming an ideal measurement system; thereby all deviations are directly traceable to AVAD assumption violation due to wind field inhomogeneities
Summary
Doppler lidar has experienced rapidly growing importance and usage in remote sensing of atmospheric winds over the past decades (Weitkamp et al, 2005). Gasch et al.: An LES-based airborne Doppler lidar simulator borne Doppler lidar have been reported based on mediumrange aircraft flying in the lower troposphere and at slower speeds (O(50 m s−1)) These systems yield higher spatial resolution (O(1 km) for wind profiling) and can enable boundary layer studies (De Wekker et al, 2012; Godwin et al, 2012; Koch et al, 2014). The error observed in the ADLS between input and retrieved wind profile is directly traceable to the violation of the wind field homogeneity assumed in AVAD, e.g., due to boundary layer turbulence This direct approach is not possible in other measurement system comparisons or simulations so far.
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