Detection of Hydrometeor Fall Speed and Vertical Air Velocity in Coherent Doppler Lidar Spectra

Poster PDF

Description

The ARM program currently operates nine coherent Doppler lidars (DLs) at various sites, including five systems at the Southern Great Plains site. These instruments, which operate at a wavelength of 1548nm with low pulse-energy (<100J) and high pulse repetition rate (15kHz), are configured to perform VAD scans once every 10 to 15 minutes, and stare vertically the rest of the time. The lidar’s real-time signal processor provides height- and time-resolved measurements of radial velocity, attenuated aerosol backscatter, and wide-band signal-to-noise ratio (SNR). In addition to these processed data, the systems can also be configured to log raw “spectral” data. The raw data, which are stored as so-called ungated autocovariances (ACF), enable reconstruction of the Doppler power spectra to any range gate size or multiple of the original temporal resolution. In the past, the baseline mode of operation for the ARM DLs has been to collect and archive only the processed data and not to log the raw data because of its large volume (~90 Gb/day). We have recently initiated an effort to begin collection, ingest and archival of raw data from selected DLs during selected Intensive Operational Periods (IOPs). To date, we have collected and archived raw data from the DL at the SGP central facility during the LAFE field campaign (August 2017) and during the SGP3DOBS field project (September 2017). These data are now available to the user community as an a1-level product (e.g. sgpdlacfC1.a1). Although we have just begun exploring the information content in these Doppler spectra, we have so far noted one consistent feature. Doppler lidar spectra acquired during LAFE and SGP3DOBS exhibit distinctive bimodal shapes during rain events. The interpretation is that one mode corresponds to the relative fall speed of the dominate drop size, and the other mode corresponds to the air motion. This clear bimodal structure allows for unambiguous detection of both the clear-air vertical velocity and the relative drop fall speed. In this poster we present several examples showing the Doppler spectra during precipitation events. We also examine the variability in fall speeds during all rain events observed during LAFE and SGP3DOBS.