ABSTRACT
A near-infrared radial velocity (RV) survey focusing on the late-M dwarfs started in February 2019 within the framework of the Subaru Strategy Program (SSP). The InfraRed Doppler (IRD) instrument mainly consists of a high-resolution spectrometer and a laser frequency comb (LFC) system as a wavelength reference. Late-M dwarfs emit most of their energy in the near-infrared rather than in the visible. Therefore, to cover the bright absorption lines of M dwarfs, LFC provides a broadband spectrum from 970 nm to 1750 nm with a mode spacing of 12.5 GHz. It has advantages such as simple and robust frequency stabilization, an all-fiber optic configuration, and being observer friendly. The original comb spectrum just generated from highly nonlinear fibers undergoes optical processing such as spectral shaping, depolarization, and mode scrambling in multimode fiber utilization before it is input to the spectrometer. Using the IRD instrument, the IRD-SSP has made more than 100 nights of observations over the last three years. The LFC system operated stably without major trouble during this period, helping to maintain high RV accuracy. Despite the lack of direct maintenance for two and a half years due to Covid-19, the system has continued to operate without any interruption to the observations.
ABSTRACT
Doppler wind lidars (DWLs) have increasingly been used over the last decade to derive the mean wind in the atmospheric boundary layer. DWLs allow the determination of wind vector profiles with high vertical resolution and provide an alternative to classic meteorological tower observations. They also receive signals from altitudes higher than a tower and can be set up flexibly in any power-supplied location. In this work, we address the question of whether and how wind gusts can be derived from DWL observations. The characterization of wind gusts is one central goal of the Field Experiment on Sub-Mesoscale Spatio-Temporal Variability in Lindenberg (FESSTVaL). Obtaining wind gusts from a DWL is not trivial because a monostatic DWL provides only a radial velocity per line of sight, i.e., only one component of a three-dimensional vector, and measurements in at least three linearly independent directions are required to derive the wind vector. Performing them sequentially limits the achievable time resolution, while wind gusts are short-lived phenomena. This study compares different DWL configurations in terms of their potential to derive wind gusts. For this purpose, we develop a new wind retrieval method that is applicable to different scanning configurations and various time resolutions. We test eight configurations with StreamLine DWL systems from HALO Photonics and evaluate gust peaks and mean wind over 10 min at 90 m a.g.l. against a sonic anemometer at the meteorological tower in Falkenberg, Germany. The best-performing configuration for retrieving wind gusts proves to be a fast continuous scanning mode (CSM) that completes a full observation cycle within 3.4 s. During this time interval, about 11 radial Doppler velocities are measured, which are then used to retrieve single gusts. The fast CSM configuration was successfully operated over a 3-month period in summer 2020. The CSM paired with our new retrieval technique provides gust peaks that compare well to classic sonic anemometer measurements from the meteorological tower.