The need for high temporally and spatially resolved wind measurements was well realized, but the current ground-based and airborne measurements are difficult to provide (Geerts et al. 2017b; NAS 2018a, b). Current airborne Doppler lidars use the scanning and starring observation mode with exact scanning patterns depending on observation systems. Figure 1a shows a five-direction scanning pattern with a nadir direction. Assuming that stopping at each angle for one second, a scanning cycle could take 15 to 40 seconds, which corresponds to horizontal resolutions ~2 km or coarse. One basic assumption of wind retrievals is homogeneous flow. Thus, inhomogeneous flow associated with complex terrain, surface heterogeneity, local meteorology, and circulations presents a great challenge for scanning Doppler lidar data processing.   Therefore, ADL implemented a five-fixed beam design as schematically illustrated in Fig. 1b with actual system assembly given in Fig. 1c. Each beam is an independent Doppler lidar to provide five simultaneous radial velocity measurements , providing fine-scale (sub-kilometer) horizontal and vertical wind measurements without scanning.

Figure 1. A schematic comparison of a scanning five-beam (a) and a fixed-five-beam (b) systems. (c) ADL five-beam telescope assembly with the nadir beam mounted on a stabilized platform (in yellow color).

ADL is designed to meet a wide range of platform and atmospheric conditions. To cover aircraft speeds of 50—160 m/s and ground-relative atmospheric wind speeds of ±80 m/s, the data system needs a bandwidth of 30-300 MHz and a sampling rate of 1G/s. The ADL real-time data processing system performs 4096-point FFT over 200 range gates. Full power spectra are saved for post-data processing. The range gates are 18 to 90 m selectable with selectable gate overlaps. ADL lasers run at 10 KHz with a pulse width of 200 ns or 300 ns, allowing a 12-km measurement range. The data system can provide up to 10 Hz profiles with no dead time between profiles.

The ADL individual beam was well tested on the ground and compared to commercial systems. Figure 2 presents 8-hour measurements at Boulder, CO, showing a typical late fall convective mixing layer daily cycle. We operated a two-beam ADL system (nadir and zenith beams) on NPS Twin Otter aircraft for ~ 70 hours in the summer of 2023 in Barbados, and  the five-beam ADL will be tested on NASA P-3 in summer 2024 during the ARCSIX.

Figure 2. ADL ground-based measurement example (pointing vertically) showing local convective mixing layer development.